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Premium Member
2003 Cadillac Seville STS
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Discussion Starter #1
Hello! My car has a misfire on cylinders 7 and 3 mostly 7 it seems. The misfire completely goes away when I warm the car up to operating temperature. I’ve swapped injectors around, replaced spark plugs with denso double platinums, changed intake plenum boot and intake manifold gaskets, I sprayed everywhere trying to find a vacuum leak. I replaced the back coil cassette and ignition control module, all to amount to nothing. It’s still misfiring and nothing changed. I just did a compression test on those cylinders on a cold engine and they both read around 170psi which is pretty good to me especially on a cold engine. I’m out of ideas. What would cause this that I haven’t already checked?? Why is it worse when I first start the car? I have an Autel MK908 scan tablet and all the sensor signals seem normal to me but then again I’m not an expert. Please help!!!
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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69,052 Posts
EDIT: Welcome to CF !!!!

P 0300 is the generic code for "Miss Detected'. You probably got the actual cylinder numbers from your tablet scanner.

Did you check each plug boot/spring for carbon tracking ? A faulty boot will cause a miss that is murder to find. Also, check the inner connector for the removable ICM in each coil cassette.Tricky.

AC Delco (DENSO) 41-987 plugs only.

A miss that clears up after warmup MIGHT be due to condensation affecting one or more spark plugs by grounding some of the spark. Use a tad of silicone dielectric ignition grease in each boot top and tip. Any parts store, dirt cheap.

Also check the cassette coil connector area for cracks or discoloration - a cracked coil/tower WILL cause a miss. Both cassettes and their ICMs are identical except for color. The cassette ground springs must be clean and "springy".

coil-boot-plug 1.JPG
2011-07-09_100054_carbon_tracking.jpg
Spark plug boot and spring.JPG
DSCN0197.jpg
coil cassette  upside down.JPG
 

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Premium Member
2003 Cadillac Seville STS
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60 Posts
Discussion Starter #4
Hello thank you for the welcome! Yea the boots looked good to me I didn’t see any tracking. I believe I actually swapped the boots around for that cylinder with a known good one as well. I do know my spark plug tube seals are leaking on both sides and when I pull the boots out I can see condensation for sure around the boot and when I take the spark plug out their is oil all over the threads. Not sure how likely that would be to cause my misfire. I’m afraid to attempt to replace the valves cover gaskets on this engine lol. I have not checked the inner connector for the ICM closely yet I will have to do that. I also found the engine ground right next to the coolant temp sensor but it looks like hell to get too. It is covered in oil from the leaking valve covers it looks like. I also did a combustion leak test on the surge take and the fluid never changed color after several minutes so it looks like I’m good there too
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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Many Northstars develop oil in the plug wells - not a problem because oil is an insulator.

There are several ground connections on the engine heads/block. As long as they are tight should be OK.

Good on the combustion gas test.
 

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Premium Member
2003 Cadillac Seville STS
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60 Posts
Discussion Starter #6
Oh okay so I should be good to leave the valve cover gaskets alone then? Is doable to change the rear with good tools? I did out dielectric on the tips the last time I assembled. Idk man this problem has me thrown for sure lol. Is their anyway an idle air control valves issue could cause this because it seems to have started after I messed with that and the MAF. I cleaned the MAF and just checked it again and it looks good except a few bent metal fins. Maybe I didn’t seat the IACV right lol I’m not sure. I’ll check those connectors for the module and see as well. Is a 2003 northstar with 130k miles likely to run into the headgasket problem? I know the 2000 and later are better. I’m hoping the transmission holds up as well as it is perfect right now. I had a shop replace the coolant but when I got it back they only used one 50/50 gallon jug.... I replaced the atf with Amsoil signature ATF and am using AMSOIL signature series oil. I change the oil every 3k miles regardless I know it’s overkill but still
 

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Premium Member
2003 Cadillac Seville STS
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60 Posts
Discussion Starter #7
Many Northstars develop oil in the plug wells - not a problem because oil is an insulator.

There are several ground connections on the engine heads/block. As long as they are tight should be OK.

Good on the combustion gas test.
Another weird thing is that the cassette in the rear towards the firewall is brand new and has no cracks or discoloring on the electrode area and neither does the old one that it replaced but the front cassette has some hairline cracks around a couple electrodes but according to my scanner those cylinders never misfire. Because cylinders 1,3,5, and 7 is the bank by the firewall correct? Wouldn’t that be something if I had that backwards LOL
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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1-3-5-7 is the right (rear) bank
2-4-6-8 is the left (front) bank

The right cam cover gasket is murder to change - AIR chek valve and parts to be removed.
The left cover is even worse - special tools to remove and replace the main water pump drive pulley, hub TIGHTLY pressed onto the intake cam extension. That pulley may still have a black plastic thread protector screwed into the end - Torx bit.

Cam cover gasket set -

247_VICTOR REINZ_VS50331_1__ra_p.jpg
water pump drive pulleys.gif
cam extension.gif
 

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Premium Member
2003 Cadillac Seville STS
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60 Posts
Discussion Starter #9
Do you think it would be worth the effort to replace the valve cover gaskets? I have a decent variety of tools and can purchase whatever I need. This car has 133k miles on it and it was babied for a long time I think. The owners I bought it from were very old and had it since 43k miles. I got a scope down in cylinder 7 and it looks like the piston has a good layer of carbon completely covering it lol. How long do you think this engine and tranny can last with regular fluid changes? Supposedly the last owners went by the book and took it to firestone to get fluid changes and followed the book. I noticed a somewhat noticeable ticking on cold starts is that normal for the Northstar? It’s a very nice well kept car I just don’t wanna put a lot of money into it if it’s gonna break soon. If it were to break I would consider getting a bulletproof rebuild though as I really like the car. It pains me because the car could even have more potential with a better more performance geared transmission. The final drive ratio is good but having only 4 gears kills it a bit
 

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Premium Member
2003 Cadillac Seville STS
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60 Posts
Discussion Starter #10
Here are my compression numbers. Cylinders 7 and 3 are the ones with the cold start misfire. I can definitely feel a shake on cold startup and engine light flashes. What do you think of the numbers
581274
 

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Administrator
2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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69,052 Posts
165 is still good. All the cylinders are within about 10% of each other.

Your 2003 STS has a final drive ratio of 3.71:1. 3rd gear is the direct drive ratio at 1.00:1. 4th is OD at 0.68:1. Above 43 mph, road load steady speeds the TCC operates to remove the last 300 or so rpm of converter slip. Using the recommended 87 octane fuels (NO need to run higher octanes as the 2000 - 2005 PCM WILL NOT advance timing to compensate for the higher octane). Like my STS, yours should get about 25 mpg long-term open highway. 11 - 15 in town, 16 - 20 suburban/near rural.

Learn to do a WOT - see if a few of those will firm up the compression numbers by shedding carbon buildup in the ring groves due to babying the engine. These cars need the flying dog snot run out of them once in a while. Did you know that you can take the car to its 130 mph speed limiter with the shift stick in 1 ?

Study this - all of it. Although written for the pre-2000 cars, it's all good info. The 4T80E won't let you break anything - full power upshifts or full vacuum downshifts.


TCC speed/rpm tables. Your engine is the 300 hp VIN 9 -

Final drive speed graph.jpg
 

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Premium Member
2003 Cadillac Seville STS
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60 Posts
Discussion Starter #12
165 is still good. All the cylinders are within about 10% of each other.

Your 2003 STS has a final drive ratio of 3.71:1. 3rd gear is the direct drive ratio at 1.00:1. 4th is OD at 0.68:1. Above 43 mph, road load steady speeds the TCC operates to remove the last 300 or so rpm of converter slip. Using the recommended 87 octane fuels (NO need to run higher octanes as the 2000 - 2005 PCM WILL NOT advance timing to compensate for the higher octane). Like my STS, yours should get about 25 mpg long-term open highway. 11 - 15 in town, 16 - 20 suburban/near rural.

Learn to do a WOT - see if a few of those will firm up the compression numbers by shedding carbon buildup in the ring groves due to babying the engine. These cars need the flying dog snot run out of them once in a while. Did you know that you can take the car to its 130 mph speed limiter with the shift stick in 1 ?

Study this - all of it. Although written for the pre-2000 cars, it's all good info. The 4T80E won't let you break anything - full power upshifts or full vacuum downshifts.


TCC speed/rpm tables. Your engine is the 300 hp VIN 9 -

View attachment 581282
Hey man thank you. I just tested wires on my ignition coil connector to my pcm for cylinder 7 and 3 as well as the ground and low reference and they all have a good connection even wiggling the harness. Checked the injector as well and it’s got a good connection both ways as well. I’m kinda lost at this point as I don’t know what else it could be other than the computer. I don’t know why it would do this only when cold. Really a weird issue. Hopefully it’s not some sensor somewhere but I don’t see any abnormal readings on my scan tablet but then again I’m not experienced enough to know exactly what some of them should read. Thank you for the info. What would you suggest I do now? I’m worried the misfire is gonna eventually ruin my compression from soaking the cylinder with gas or something. I don’t care about the cat I’ll just stick a pipe there lol if it goes bad
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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Whatever the problem is, it isn't the "computer". The PCM (Powertrain Control Module) in the air filter box is basically bulletproof - and no one has hacked into the 2000 - 2005 FWD Northstar PCM yet.

Remove the cat and you'll get all sorts of emissions and powertrain codes. It is a honeycomb high flow cat.

Way back in this thread - oil in the plug wells and a statement about "condensation" on/in plug boots. If it's water/moisture condensation, that's most likely your problem. Oil is an insulator, so oil-wetted boots should not cause a miss.

Rent a fuel pressure tester from a parts store. The test port is on the corner of the fuel rail, passenger firewall corner - #2, 3. Spec pressure at Key: ON and at idle is 41 - 47 psi.

Fuel rail - metal.gif
 

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Premium Member
2003 Cadillac Seville STS
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60 Posts
Discussion Starter #14
Whatever the problem is, it isn't the "computer". The PCM (Powertrain Control Module) in the air filter box is basically bulletproof - and no one has hacked into the 2000 - 2005 FWD Northstar PCM yet.

Remove the cat and you'll get all sorts of emissions and powertrain codes. It is a honeycomb high flow cat.

Way back in this thread - oil in the plug wells and a statement about "condensation" on/in plug boots. If it's water/moisture condensation, that's most likely your problem. Oil is an insulator, so oil-wetted boots should not cause a miss.

Rent a fuel pressure tester from a parts store. The test port is on the corner of the fuel rail, passenger firewall corner - #2, 3. Spec pressure at Key: ON and at idle is 41 - 47 psi.

View attachment 581297
Well it definitely very well could be the pcm I can’t count that out. My friend a couple years ago had to replace his because of leaking capacitors. It definitely most likely isn’t the problem but I’m not 100% counting it out. I own a fuel pressure tester I’ll have to check it out. The fitting I had screwed on perfectly but it didn’t press that pintle thing in the middle so it didn’t read any pressure :/. You’re talking about the one with the black screw on cap right? I have noticed my fuel pump is noisy and you hear it constantly but I assume that’s normal. You can actually remove cats and fool the computer as I’m sure you already know
 

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Premium Member
2003 Cadillac Seville STS
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60 Posts
Discussion Starter #15
Whatever the problem is, it isn't the "computer". The PCM (Powertrain Control Module) in the air filter box is basically bulletproof - and no one has hacked into the 2000 - 2005 FWD Northstar PCM yet.

Remove the cat and you'll get all sorts of emissions and powertrain codes. It is a honeycomb high flow cat.

Way back in this thread - oil in the plug wells and a statement about "condensation" on/in plug boots. If it's water/moisture condensation, that's most likely your problem. Oil is an insulator, so oil-wetted boots should not cause a miss.

Rent a fuel pressure tester from a parts store. The test port is on the corner of the fuel rail, passenger firewall corner - #2, 3. Spec pressure at Key: ON and at idle is 41 - 47 psi.

View attachment 581297
So I’ve done the following
-New Denso double platinums pre gapped
  • New coil cassette and module
  • Swapped injectors around
  • Compression tested all 8
  • Ohmed out injectors and control module connector
  • Ran a few tanks of Techron fuel system cleaner
-Changed Intake plenum boot and intake manifold gaskets
-New air filter and cleaned TB and MAF
-Sprayed around for vacuum leaks while engine is cold and hot
-Just did multiple WOT pulls and engine braking
-Swapped my new spark plugs around
- Plug boots look good
So if my fuel pressure checks out with key on and at idle what would you suggest I do next? Thank you for all the advice I really appreciate it. I did put a couple tanks of 92 octane in just to see what happens but nothing like I assumed would happen haha
 

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White Diamond '03 DHS (with DTS floor shift)
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86,969 Posts
I'm not surprised that Techron and/or 92 octane didn't help. It never does.

Since you did a WOT run with no problems, I doubt you'll find a fuel pressure problem, but check it anyway.

When was the last time you cleaned the TB? Odd as it may seem, we have seen that cause misfire before.
 

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Premium Member
2003 Cadillac Seville STS
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60 Posts
Discussion Starter #17
I'm not surprised that Techron and/or 92 octane didn't help. It never does.

Since you did a WOT run with no problems, I doubt you'll find a fuel pressure problem, but check it anyway.

When was the last time you cleaned the TB? Odd as it may seem, we have seen that cause misfire before.
Yea I recently did the throttle body a couple months ago. Just saw it yesterday and it looks brand new. I will do the fuel pressure but it ran very strong 24:7 during my WOT. Maybe I’ll have to check with a vacuum gauge because I’m not sure what else it could be. I sprayed everywhere for a vacuum leak but the idle never once moved haha
 

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White Diamond '03 DHS (with DTS floor shift)
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Oops, I missed post #15 where you said you cleaned the TB. Sorry for the redundant question.


Is a 2003 northstar with 130k miles likely to run into the headgasket problem?
Not "likely", but anything is possible. Sounds like you've changed everything there is to change and checked everything else. Are you having to top off the coolant?
 

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Premium Member
2003 Cadillac Seville STS
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60 Posts
Discussion Starter #19
Oops, I missed post #15 where you said you cleaned the TB. Sorry for the redundant question.




Not "likely", but anything is possible. Sounds like you've changed everything there is to change and checked everything else. Are you having to top off the coolant?
I’ve noticed that after a long time the coolant will go down like half an inch or less. It takes forever. Not sure how I could find something that small of a leak. I did do that liquid combustion gas test on the fill tank for multiple minutes and it never changed color. I’m hoping it’s not the very beginning of the headgasket leak but I beat the crap out of it yesterday and it never moved past 12 o’clock lol. I really like the car the transmission shifts very smooth. I have Amsoil engine oil and AMSOIL tranny fluid. Literally all my fluids are brand new. Would you suggest to keep on using dex cool or would it be beneficial to completely flush the coolant and refill using new HOAT or OAT coolant? I’m not sure what kind dex is
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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DEX-COOL is perfectly good coolant. No sense changing horses in mid-stream. As with any other coolant, exchange it every 3 or 4 years - "long life" recommendations notwithstanding.

"Plug boots look good...." did you install the boots on the same plugs they came off of ? Or did you mix them up ? Those boots DO track occasionally - and use a small smear of silicone ignition dielectric grease in each boot top and tip. Post #3.
 
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