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Premium Member
2003 Cadillac Seville STS
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76 Posts
Discussion Starter #1
Right now I am ripping my Seville STS 2003 apart in trying to find a elusive misfire but upon putting it back together hopefully issue solved, I’m gonna be deleting the cat and installing a 3” pipe back to some muffler brand not sure yet. I’m gonna do my best to follow the stock exhaust path keeping it as oem fit as possible. I’ll be using T304 stainless vibrant performance mandrel bent tubing and probably a few flex connectors. I’m sorry I just don’t believe that this exhaust is perfect for this engine no matter what GM or experts say. I’m building a 2.4 K24 Honda and they thrive on 3 inch exhaust and anything less and you’re leaving HP on the table. That’s a 4 banger so no way a 4.6 is breathing its best on this tiny exhaust that tapers down to less than 2 inches lol. Intake wise the stock intake should be perfect it’s got a big throttle body for stock as well as tubing so I’m gonna leave that. But as far as exhaust it could use a bigger diameter. It’s too bad no one makes headers for these cars this thing would sound insane. Any tips for DIY stuff I can do with the harness and radiator etc ripped out? I’ll have the injectors and such out as well along with the intake manifold.
 

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Premium Member
2003 Cadillac Seville STS
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76 Posts
Discussion Starter #2
Right now I am ripping my Seville STS 2003 apart in trying to find a elusive misfire but upon putting it back together hopefully issue solved, I’m gonna be deleting the cat and installing a 3” pipe back to some muffler brand not sure yet. I’m gonna do my best to follow the stock exhaust path keeping it as oem fit as possible. I’ll be using T304 stainless vibrant performance mandrel bent tubing and probably a few flex connectors. I’m sorry I just don’t believe that this exhaust is perfect for this engine no matter what GM or experts say. I’m building a 2.4 K24 Honda and they thrive on 3 inch exhaust and anything less and you’re leaving HP on the table. That’s a 4 banger so no way a 4.6 is breathing its best on this tiny exhaust that tapers down to less than 2 inches lol. Intake wise the stock intake should be perfect it’s got a big throttle body for stock as well as tubing so I’m gonna leave that. But as far as exhaust it could use a bigger diameter. It’s too bad no one makes headers for these cars this thing would sound insane. Any tips for DIY stuff I can do with the harness and radiator etc ripped out? I’ll have the injectors and such out as well along with the intake manifold.
I’m gonna do my best to trick this touchy ECU to make it think the cat is still there and functioning properly
 

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Administrator
2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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69,366 Posts
You really need to read all the sticky threads at the top of this Northstar main page - they give you a good idea of what can and can't be done to/for the engine.

No one has hacked the Northstar FWD 2000 - 2004 PCM yet. Your VIN 9 STS has all the performance that was set at the factory. No way to "trick" it for any perceived performance gain.

Forget hacking up the original exhaust system and simply do a good install of a CORSA system. #14150. Quiet in town, nice idle, no highway drone, and will wake the dead when you romp on it. You'll NEED a good lift and as always, measure twice, cut once. It WILL give your STS 16 extra HP up at full throttle redline - nothing but bling anywhere else in the power curve. See the link to dyno runs below the pictures ........


3.OEM exhaust top-replaced at 35450 mi..jpg
CORSA on bench.jpg
CORSA 13.jpg


Find the before and after 2001 STS dyno runs by MoistCabbage - read the whole thread, but the dyno runs are at about Post #48.........

 

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Premium Member
2003 Cadillac Seville STS
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76 Posts
Discussion Starter #4
You really need to read all the sticky threads at the top of this Northstar main page - they give you a good idea of what can and can't be done to/for the engine.

No one has hacked the Northstar FWD 2000 - 2004 PCM yet. Your VIN 9 STS has all the performance that was set at the factory. No way to "trick" it for any perceived performance gain.

Forget hacking up the original exhaust system and simply do a good install of a CORSA system. #14150. Quiet in town, nice idle, no highway drone, and will wake the dead when you romp on it. You'll NEED a good lift and as always, measure twice, cut once. It WILL give your STS 16 extra HP up at full throttle redline - nothing but bling anywhere else in the power curve. See the link to dyno runs below the pictures ........


View attachment 582348 View attachment 582349 View attachment 582350

Find the before and after 2001 STS dyno runs by MoistCabbage - read the whole thread, but the dyno runs are at about Post #48.........

I’ll check out the thread. Thank you. I was just thinking about making my own full 3 inch. Even without changing the pcm tuning my own made exhaust should yield a few ponies in the mid and high end. Probably lose a little low end. The corsa system is nice for sure. Thinking about my other build I just figured that no way the stock exhaust isn’t a bottomless neck. You got like a 3 inch intake and some parts of the exhaust are was than 2 inches lol. That’s like what you find on a stock Honda Civic from the 90s lol. Vibrant makes nice T304 tuning mandre bent and very nice mufflers. I could prob piece together a very nice system for under 500
 

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Premium Member
2003 Cadillac Seville STS
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76 Posts
Discussion Starter #5
You really need to read all the sticky threads at the top of this Northstar main page - they give you a good idea of what can and can't be done to/for the engine.

No one has hacked the Northstar FWD 2000 - 2004 PCM yet. Your VIN 9 STS has all the performance that was set at the factory. No way to "trick" it for any perceived performance gain.

Forget hacking up the original exhaust system and simply do a good install of a CORSA system. #14150. Quiet in town, nice idle, no highway drone, and will wake the dead when you romp on it. You'll NEED a good lift and as always, measure twice, cut once. It WILL give your STS 16 extra HP up at full throttle redline - nothing but bling anywhere else in the power curve. See the link to dyno runs below the pictures ........


View attachment 582348 View attachment 582349 View attachment 582350

Find the before and after 2001 STS dyno runs by MoistCabbage - read the whole thread, but the dyno runs are at about Post #48.........

Is the pipe going to the CAT 2.5 inch? I’ll be turning it into 3 inch from there and deleting the cat. No need for it where I live lol. I’ll see if a Jbung will make the computer happy if not I have a special connection with a filter. If that doesn’t work oh well lol. Figure I can make a really nice system for around 500 bucks. The y pipe area in the stock system is definitely restrictive no doubt being smaller than 2 inches lol. I see Northstar performance is coming out with a tune for our 200-2003 pcms as well as badass cams. Says will give you 70-100 hp. Those must be at the crank no way it’s whp but still that’s insane. If that’s true then GM really did leave a good amount to play with
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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If you want a couple of hp up at redline WOT shift, remove the exhaust Y-pipe at the crossunder and cat flange. Use a Dremel cutoff wheel to remove the excess pipe flash in the larger pipe. No real ponies in the midrange - look at the CORSA dyno runs again.

Remember; the throttle blade is open only 7% - 9% at highway cruise speeds - very, very little air enters the engine and practically no exhaust exits. When you climb on the yeehaaaa!!!!! pedal is when things happen.

The engine is a little 279 cu.in. powerplant - there's a finite size for the exhaust pipe(s) and 2.5" is plenty for the produced exhaust flow - larger pipe IDs are NOT always more "power".

Remove the cat and probably get P codes and maybe a reduced engine power fault.

Yes, Jake is working on a PCM tune for us. It has been a while ............

crossover pipe flash.jpg


Did you ever study this ???? Written by a CF member who was a GM Northstar Systems powertrain engineer. Applies to both the pre-2000 and the 2000 and later engines. If you're guessing and using a butt dyno on your efforts you'll spend a lot of money for nothing.

 

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Premium Member
2003 Cadillac Seville STS
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76 Posts
Discussion Starter #7
If you want a couple of hp up at redline WOT shift, remove the exhaust Y-pipe at the crossunder and cat flange. Use a Dremel cutoff wheel to remove the excess pipe flash in the larger pipe. No real ponies in the midrange - look at the CORSA dyno runs again.

Remember; the throttle blade is open only 7% - 9% at highway cruise speeds - very, very little air enters the engine and practically no exhaust exits. When you climb on the yeehaaaa!!!!! pedal is when things happen.

The engine is a little 279 cu.in. powerplant - there's a finite size for the exhaust pipe(s) and 2.5" is plenty for the produced exhaust flow - larger pipe IDs are NOT always more "power".

Remove the cat and probably get P codes and maybe a reduced engine power fault.

Yes, Jake is working on a PCM tune for us. It has been a while ............

View attachment 582573

Did you ever study this ???? Written by a CF member who was a GM Northstar Systems powertrain engineer. Applies to both the pre-2000 and the 2000 and later engines. If you're guessing and using a butt dyno on your efforts you'll spend a lot of money for nothing.

I realize that this car is not a race car and I don’t need it to be. I’m more so just looking for sound but the extra ponies is a bonus. Yes I have researched but the Northstar is no out of the ordinary engine you know what I mean? The way I’m thinking is that if my 2.4 liter naturally aspirated K24A2 engine benefits mid range and top end from going from a 2.5 diameter to a 3 inch then the same must be true for an almost twice as big 4.6 both using a single exhaust. Now my K24 is modded to hell and makes 290whp naturally aspirated on E85 or should be around there but still the stock exhaust has got to be the biggest restriction on this car easily. I’m assuming a healthy Northstar with a 4t80e will make around 220-240whp but I bet with a decent exhaust it could easily make 250-260whp and I bet a decent chunk around 4K rpms too! I’m not saying you don’t know what you’re talking about I am just telling you what I was thinking. I’m no expert at all I’m just a 21 year old who loves cars and modding them. And although all my friends say what a waste of money and time this Cadillac is I live it. If I can make it run good and function properly I just got a 300hp luxury car with an awesome chassis for 3k plus whatever I put into it. Do you kinda follow me on my comparison to a 2.4L even benefiting from a bigger exhaust? I know the Northstar is a well built engine with just one major flaw and I know it must have quite a bit of potentiometer hidden behind the pcm that hasn’t been unlocked and may never be. But I hope so. Also I plan to get their stage 4 tune and camshaft package so a bigger exhaust is a must. As long as I can figure out this damn misfire issue. I’m really started to feel like giving up on finding the cause but I don’t want to. I’m gonna try the original plugs once I put it back together
 

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Premium Member
2003 Cadillac Seville STS
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76 Posts
Discussion Starter #8
If you want a couple of hp up at redline WOT shift, remove the exhaust Y-pipe at the crossunder and cat flange. Use a Dremel cutoff wheel to remove the excess pipe flash in the larger pipe. No real ponies in the midrange - look at the CORSA dyno runs again.

Remember; the throttle blade is open only 7% - 9% at highway cruise speeds - very, very little air enters the engine and practically no exhaust exits. When you climb on the yeehaaaa!!!!! pedal is when things happen.

The engine is a little 279 cu.in. powerplant - there's a finite size for the exhaust pipe(s) and 2.5" is plenty for the produced exhaust flow - larger pipe IDs are NOT always more "power".

Remove the cat and probably get P codes and maybe a reduced engine power fault.

Yes, Jake is working on a PCM tune for us. It has been a while ............

View attachment 582573

Did you ever study this ???? Written by a CF member who was a GM Northstar Systems powertrain engineer. Applies to both the pre-2000 and the 2000 and later engines. If you're guessing and using a butt dyno on your efforts you'll spend a lot of money for nothing.

When you say remove the Y pipe at the crossover are you talking about turning it into a single exit exhaust instead of two or what do you mean? Sorry I didn’t follow. I was gonna make it 3 inch from right before the cat all the way back. Possibly turning into 2.5 after the Y pipe.
 

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Premium Member
2003 Cadillac Seville STS
Joined
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76 Posts
Discussion Starter #9
If you want a couple of hp up at redline WOT shift, remove the exhaust Y-pipe at the crossunder and cat flange. Use a Dremel cutoff wheel to remove the excess pipe flash in the larger pipe. No real ponies in the midrange - look at the CORSA dyno runs again.

Remember; the throttle blade is open only 7% - 9% at highway cruise speeds - very, very little air enters the engine and practically no exhaust exits. When you climb on the yeehaaaa!!!!! pedal is when things happen.

The engine is a little 279 cu.in. powerplant - there's a finite size for the exhaust pipe(s) and 2.5" is plenty for the produced exhaust flow - larger pipe IDs are NOT always more "power".

Remove the cat and probably get P codes and maybe a reduced engine power fault.

Yes, Jake is working on a PCM tune for us. It has been a while ............

View attachment 582573

Did you ever study this ???? Written by a CF member who was a GM Northstar Systems powertrain engineer. Applies to both the pre-2000 and the 2000 and later engines. If you're guessing and using a butt dyno on your efforts you'll spend a lot of money for nothing.

Also, I noticed when removing the radiator it has tranny cooler lines but not oil. I was under the impression that it had both tranny oil and engine oil coolers because both sides had the black tank
 

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Super Moderator
White Diamond '03 DHS (with DTS floor shift)
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87,076 Posts
It has side tanks for coolant flow., whether or not it has a trans cooler or an engine oil cooler.
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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69,366 Posts
HD cooling (includes an engine oil cooler in the passenger (right) side radiator tank) was an option for the trailer towing package. If installed it would show code V03 on the RPO sticker under the spare tire cover. Totally unnecessary for a daily driver or even a modified Northstar.

The front Y-pipe connects the two exhaust manifold pipes together - in front of the flex coupling. The excess flash is in there. 2.5" is plenty large for our exhaust piping. The cat is a honeycomb high performance unit.

If you are going to perform modifications to the engine for more power you need to do before and after dyno runs at each step to make sure that you are really gaining instead of losing. Dead serious - I've been in the engine building part of this hobby for over 60 years - very easy to "upgrade" and yet go backwards in the power output department.
 

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Premium Member
2003 Cadillac Seville STS
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76 Posts
Discussion Starter #12
HD cooling (includes an engine oil cooler in the passenger (right) side radiator tank) was an option for the trailer towing package. If installed it would show code V03 on the RPO sticker under the spare tire cover. Totally unnecessary for a daily driver or even a modified Northstar.

The front Y-pipe connects the two exhaust manifold pipes together - in front of the flex coupling. The excess flash is in there. 2.5" is plenty large for our exhaust piping. The cat is a honeycomb high performance unit.

If you are going to perform modifications to the engine for more power you need to do before and after dyno runs at each step to make sure that you are really gaining instead of losing. Dead serious - I've been in the engine building part of this hobby for over 60 years - very easy to "upgrade" and yet go backwards in the power output department.
I get what you mean man thank you. Mine does not have the engine oil cooler then. I was originally assuming it was standard. The reason I am kinda guessing the Northstar has a lot more potential that GM says is because of jakes claims of his new stage 4 package supposedly 75-100hp!? That’s crazy even if it’s not to the wheels. For most other cars that would be like FBO and maybe cams as well I would believe. Even the tune alone he does for other years says something like 30-50 hp. If those numbers are true then the pcm oem tube must be insanely conservative. I don’t understand why this pcm is so hard to tune compared to other GM products. Like why would GM do that? I would think they would know people tuning their cars would probably increase sales. Idk lol. Anyways I’ve already ordered my 3 inch exhaust materials so hopefully it isn’t terrible. If it is I’ll take it off and do 2.5 lol. I could maybe reduce it down to 2.5 after the y or something.
 

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Premium Member
2003 Cadillac Seville STS
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76 Posts
Discussion Starter #13
HD cooling (includes an engine oil cooler in the passenger (right) side radiator tank) was an option for the trailer towing package. If installed it would show code V03 on the RPO sticker under the spare tire cover. Totally unnecessary for a daily driver or even a modified Northstar.

The front Y-pipe connects the two exhaust manifold pipes together - in front of the flex coupling. The excess flash is in there. 2.5" is plenty large for our exhaust piping. The cat is a honeycomb high performance unit.

If you are going to perform modifications to the engine for more power you need to do before and after dyno runs at each step to make sure that you are really gaining instead of losing. Dead serious - I've been in the engine building part of this hobby for over 60 years - very easy to "upgrade" and yet go backwards in the power output department.
Do you think the Northstar would benefit from an upgraded radiator? Assuming I had a separate cooler unit for the tranny
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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ANY internal combustion engine can be tweaked and tuned to produce more power - but how much and for how long and how reliable? The ONLY time you use extra power is at full throttle upper rpm range operation - 1/10 of 1% of the engine operating time. A 1,000 hp Northstar needs to put out only about 60 hp to move the car down the highway at 65 mph. Yes, extra horsepower is fun and exciting - but it seldom gets used and is not needed for the average daily driver. Yes, horsepower comparisons and brand name comparisons have been around since Day One. My Dad (r.i.p.) drove a 1930 Ford Model A that had a Pep Boys high compression head.

You don't need a separate transmission fluid cooler -- the existing one is designed to keep the fluid at the design temperature range from 180 to 220.

The Northstar was designed and tested to run for 50 miles with NO coolant. Camel Mode. A larger radiator is unneeded and do not try to use a lower temp thermostat. It's set to run the engine between 190 and 210 degrees. Go lower than the original opening of 188 and you run into emissions and calibration problems, and the engine oil doesn't heat up enough to flow and lubricate properly.

The engine oil is supposed to run at 190 - 220 in order to boil off blowby acids and condensation moisture for scavenging by the PCV system.

Run the oil too cold and you get the old 1950s "sludge -

sludge.jpg
 

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Premium Member
2003 Cadillac Seville STS
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76 Posts
Discussion Starter #15 (Edited by Moderator)
ANY internal combustion engine can be tweaked and tuned to produce more power - but how much and for how long and how reliable? The ONLY time you use extra power is at full throttle upper rpm range operation - 1/10 of 1% of the engine operating time. A 1,000 hp Northstar needs to put out only about 60 hp to move the car down the highway at 65 mph. Yes, extra horsepower is fun and exciting - but it seldom gets used and is not needed for the average daily driver. Yes, horsepower comparisons and brand name comparisons have been around since Day One. My Dad (r.i.p.) drove a 1930 Ford Model A that had a Pep Boys high compression head.

You don't need a separate transmission fluid cooler -- the existing one is designed to keep the fluid at the design temperature range from 180 to 220.

The Northstar was designed and tested to run for 50 miles with NO coolant. Camel Mode. A larger radiator is unneeded and do not try to use a lower temp thermostat. It's set to run the engine between 190 and 210 degrees. Go lower than the original opening of 188 and you run into emissions and calibration problems, and the engine oil doesn't heat up enough to flow and lubricate properly.

The engine oil is supposed to run at 190 - 220 in order to boil off blowby acids and condensation moisture for scavenging by the PCV system.

Run the oil too cold and you get the old 1950s "sludge -

View attachment 582686
I understand all of that man I do and I’m sure you understand exactly what I mean by adding power the way I’m talking about. I don’t really care much about the peak HP but more a combination of midrange up to the peak. I know with a bigger exhaust the low end will likely suffer a little but I don’t care lol. It’s a small V8 not a 7.0 haha. I understand that a properly operating Northstar coolant system will not overheat but what I was curious about is if a dual core or even dual pass radiator would benefit the engine and help it operate on the LOWER side of the normal operating temp. Meaning around like 190-205 or so. Like keep the average temp down overall. The trick of course is making sure whatever else I use to cool the tranny better be good enough for factory spec which I should be able to tell with my mode 6 scan tablet monitoring the tranny temp

Either way it seems like the Northstar for some reason runs on the hot side constantly when atleast from what I’ve seen most cars operate like 170-200. I wonder what the reason is for the Northstar running sometimes in the 230s. That doesn’t seem right to me but then again I haven’t compared it to a lot of engines just a few. One of them being a performance engine as well. It just seems the cooling system even when operating 100% might be lacking a bit
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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A Northstar with the cooling system in good condition runs 24/7/365 at the design temps of 190 - 210 whether in Alaska or the Sahara. For our 2000 - 2005 series the fans don't even go to SLOW until 224 degrees and FAST at 236. If the fans come on the engine cools to 213, fans shut off. That cycle occurs in heavy traffic or low speed summer driving with no HVAC compressor function set. If you use A/C or DEFROST the fans run in SLOW all the time to insure airflow through the condenser, not for cooling. Fans don't run at over about 30 mph - not needed.

The cooling system is not lacking one iota - but it must be maintained, just like the rest of the car. Most Cadillac owners out there operate under the assumption of "It's a Cadillac. It don't need no maintenance!".

Don't worry about transmission temperatures - you're obsessing over nothing.

Bury your (apparently) preconceived and sometimes incorrect assumptions about a Northstar engine - you're making mountains out of molehills.

Here's your temp gauge (I may have posted to one of your threads before) - it's heavily damped at 12 o'clock because owners get squirrely when a gauge actually moves in response to changing engine conditions.

The white dot at about 160 degrees is loop change, depending on ambient temp.

Temp gauge - my numbers.jpg
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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Here you go ........... last 2 posts......... #13 and #14 .......

 

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Premium Member
2003 Cadillac Seville STS
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76 Posts
Discussion Starter #18
A Northstar with the cooling system in good condition runs 24/7/365 at the design temps of 190 - 210 whether in Alaska or the Sahara. For our 2000 - 2005 series the fans don't even go to SLOW until 224 degrees and FAST at 236. If the fans come on the engine cools to 213, fans shut off. That cycle occurs in heavy traffic or low speed summer driving with no HVAC compressor function set. If you use A/C or DEFROST the fans run in SLOW all the time to insure airflow through the condenser, not for cooling. Fans don't run at over about 30 mph - not needed.

The cooling system is not lacking one iota - but it must be maintained, just like the rest of the car. Most Cadillac owners out there operate under the assumption of "It's a Cadillac. It don't need no maintenance!".

Don't worry about transmission temperatures - you're obsessing over nothing.

Bury your (apparently) preconceived and sometimes incorrect assumptions about a Northstar engine - you're making mountains out of molehills.

Here's your temp gauge (I may have posted to one of your threads before) - it's heavily damped at 12 o'clock because owners get squirrely when a gauge actually moves in response to changing engine conditions.

The white dot at about 160 degrees is loop change, depending on ambient temp.

View attachment 582692
I know I way over think and analyze things which can sometimes be a bad thing lol. I just want to experiment I guess and see if I can improve a few things. Time and effort probably is better served at this point in replacing needed parts and solving my misfire. Don’t need it to be a race car just want it to be as efficient and powerful as possible without sacrificing reliability. I’ll be happy with a good running engine and exhaust for now until Jake releases his stage 4 tune and cams
 
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