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Premium Member
2003 Cadillac Seville STS
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60 Posts
Discussion Starter #61
Please be careful when you open the surge tank cap after any sort of driving. If the car has been run hard there's a LOT of heat built up in the reciprocating machinery - the stuff that coolant never touches - and, as it passes off that heat to the block/coolant it's possible that the coolant could warm to a boil point at atmospheric pressure. What cools all the engine guts ? Engine oil - properly operating - it runs at 190 - 220 degrees. Our engines overheat at 270 degrees - and then go into Camel Mode.

View attachment 582049

Note that the recommended 50/50 mix line is out of sequence. Your car uses an 18 psi pressure cap, so do some extrapolation.
Thanks for the graph. It still misfired for a bit even when I bled all the pressure out lol. My best guess now is MAF, Ground, Valve covers or faulty pcm. I doubt it’s the pcm like you said. I did notice what may have looked like some sort of condensation on the outside of some of the spark plug bolts I’m not sure what could cause that or if it would cause the cold start misfire. I am ripping it apart now to change coolant crossover gaskets and valve cover gaskets and the front motor mount. Anything else I should check or change while I’m this deep in? It will be getting fresh coolant again obviously lol as well as I’m gonna put in the new denso boots and acdelco plugs
 

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Premium Member
2003 Cadillac Seville STS
Joined
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60 Posts
Discussion Starter #62
Please be careful when you open the surge tank cap after any sort of driving. If the car has been run hard there's a LOT of heat built up in the reciprocating machinery - the stuff that coolant never touches - and, as it passes off that heat to the block/coolant it's possible that the coolant could warm to a boil point at atmospheric pressure. What cools all the engine guts ? Engine oil - properly operating - it runs at 190 - 220 degrees. Our engines overheat at 270 degrees - and then go into Camel Mode.

View attachment 582049

Note that the recommended 50/50 mix line is out of sequence. Your car uses an 18 psi pressure cap, so do some extrapolation.
If this doesn’t fix the issue I am kinda tempted to start gradually throwing parts at it... I don’t know much more to do other than bring it to a dealer and pay them insane prices not knowing if it will get fixed. What is the likelihood of the computer fixed on adding a ton of fuel to compensate for that big crack I had in the intake plenum boot? I don’t understand why it didn’t throw a code then but it idles rough constantly. I got the check engine light for misfire when I installed the denso plugs. It popped up when I did the front four first Only and drove it like that. Front set were different than the rear would that cause this? Could it be something in the learned memory causing this? The long term fuel trims don’t seem eye opening off
 

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Premium Member
2003 Cadillac Seville STS
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60 Posts
Discussion Starter #64
My two babies I love. Caddy is being a stubborn bi*** though lol
582209
 

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96 FWB
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564 Posts
IDK nuttin' about this motor so just been lurking to learn. This is the point when I'd fall back and punt to either drive over to a friend's for fresh eyes and ideas, or even hunt down a local (insert your make/model here) car club gathering and ask them to take their wildest shot based on symptoms and what you've worked on so far. As mentioned, it's usually, "What have you (or a shop) recently done on the car", or their eyes spy something amiss that I had been blind to and simply overlooked.

I recall some intake gasket attention, but have you replaced (not just looked at) EVERY hose and vacuum fitting? One time I actually caused the problem after snipping the soft rotted end of a vent hose and then a week later getting crappy running and going quite a while before noticing the hose had split at that connection. Another occasion I had several months worsening stumble until I discovered this hidden from view:

 

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Premium Member
2003 Cadillac Seville STS
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60 Posts
Discussion Starter #68 (Edited by Moderator)
IDK nuttin' about this motor so just been lurking to learn. This is the point when I'd fall back and punt to either drive over to a friend's for fresh eyes and ideas, or even hunt down a local (insert your make/model here) car club gathering and ask them to take their wildest shot based on symptoms and what you've worked on so far. As mentioned, it's usually, "What have you (or a shop) recently done on the car", or their eyes spy something amiss that I had been blind to and simply overlooked.

I recall some intake gasket attention, but have you replaced (not just looked at) EVERY hose and vacuum fitting? One time I actually caused the problem after snipping the soft rotted end of a vent hose and then a week later getting crappy running and going quite a while before noticing the hose had split at that connection. Another occasion I had several months worsening stumble until I discovered this hidden from view:

Nah I haven’t checked every single hose closely no haha I will have to soon. I’ve been ripping this car apart the last few days lol

No bother, but I'm out of ideas.
582463
 

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2002 STS stock
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14 Posts
Wow...you weren't kidding! So that's what they look like from the front! Another 6 inches and I'd be a happy camper to work on 'em!

The '02 STS in my profile is actually my friend's car, and I've been fighting the same P0300 as you apparently are. So I've been researching and reading, reading and researching, doing most of what you've done (also to no avail)... I'm hoping you or I find the solution(s) soon!

I did read about the CKPs failing and causing the PCM to think the engine was missing (maybe worse than it is), and wondering if you were going to do anything with them while you have all that easy access? I was just under mine today, fixing an a/c leak at the compressor (new seals installed) and changing oil, and decided I would not try to access them yet, due to clearance issues.
 

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Premium Member
2003 Cadillac Seville STS
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60 Posts
Discussion Starter #70
Wow...you weren't kidding! So that's what they look like from the front! Another 6 inches and I'd be a happy camper to work on 'em!

The '02 STS in my profile is actually my friend's car, and I've been fighting the same P0300 as you apparently are. So I've been researching and reading, reading and researching, doing most of what you've done (also to no avail)... I'm hoping you or I find the solution(s) soon!

I did read about the CKPs failing and causing the PCM to think the engine was missing (maybe worse than it is), and wondering if you were going to do anything with them while you have all that easy access? I was just under mine today, fixing an a/c leak at the compressor (new seals installed) and changing oil, and decided I would not try to access them yet, due to clearance issues.
You have the same exact issues as me or different? And I might change them since they are right there but I’m not sure yet lol. Idk what could be causing it at this point
 

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2002 STS stock
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Mine misfires a bit all the time, it seems. It started with a semi-hard miss on 7, the cylinder was never totally dead. I've replaced the plugs (with Autolites gapped at .050...have a set of 41-987s going in tomorrow). Cleaned, inspected, and dielectric greased the coil packs, boots, plug wells, etc. The boots were supple and not cracked or tracked, coil towers were nice, ground springs were clean and springy. Some wells had a little moisture (7,8), but hard to tell if it was coolant, oil, or a mix. Cleaned the ICM to PCM connectors, did not check the ICM to coil connectors. Swapped the coil packs across the banks, along with the plug and boot from 7...no change. Still a #7 miss, enough to throw a 0307.

Replaced the rubber intake plenum, both PCV line boots to the cam covers (both dried and cracked), the intake manifold gaskets (only found 1 a bit flattened on #2), cleaned the throttle body manually off the car (the gasket was flattened at the bottom, so I RTV'd it until replacement - also tomorrow), cleaned electrical connectors for IAC, MAP, TPS, EGR, AIR, PCM, etc. Checked all other air/vacuum lines. No change, other than not having a hard cylinder code, just 0300, probably because I didn't let it run long enough.

Cleaned, inspected the MAF, IAT, and zip tube, checked resonator seal, put in new air filter. Cleaned MAF electrical connector. No change. However, just for jollies, I disconnected the MAF while idling and the engine immediately stopped. Left it disconnected, and restarted it. The idle seemed to be smoother, so I left it disconnected. Didn't let it run very long, but the SES hadn't come on yet (I expected it, but ran out of time).

Tomorrow, if I have my way, I'll check fuel rail pressure, install the ACDelco plugs, and run a compression test while I'm in there.

So it's not exactly the same as yours, but similar actions have been taken. I'm trying to get the monitors passed so I can get it smogged, but the misfires are not letting that happen yet. Hopefully, when/if I get that cleared up, there won't be anything else lurking in the shadows.
 

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2002 STS stock
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I was mulling your misfires on cold start thing, and wondering if you had checked your AIR vacuum control valves, thinking if the diaphragm(s) were leaky, they might do weird things to the air/fuel ratio, and therefore the crank sensors/PCM misfire detection.

I have a broken, now tied up, air line, either before or after the pump, at the bottom of the pump. But the AIR valves have not been checked yet. That's also on my to-do list.
 

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Premium Member
2003 Cadillac Seville STS
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60 Posts
Discussion Starter #73
Mine misfires a bit all the time, it seems. It started with a semi-hard miss on 7, the cylinder was never totally dead. I've replaced the plugs (with Autolites gapped at .050...have a set of 41-987s going in tomorrow). Cleaned, inspected, and dielectric greased the coil packs, boots, plug wells, etc. The boots were supple and not cracked or tracked, coil towers were nice, ground springs were clean and springy. Some wells had a little moisture (7,8), but hard to tell if it was coolant, oil, or a mix. Cleaned the ICM to PCM connectors, did not check the ICM to coil connectors. Swapped the coil packs across the banks, along with the plug and boot from 7...no change. Still a #7 miss, enough to throw a 0307.

Replaced the rubber intake plenum, both PCV line boots to the cam covers (both dried and cracked), the intake manifold gaskets (only found 1 a bit flattened on #2), cleaned the throttle body manually off the car (the gasket was flattened at the bottom, so I RTV'd it until replacement - also tomorrow), cleaned electrical connectors for IAC, MAP, TPS, EGR, AIR, PCM, etc. Checked all other air/vacuum lines. No change, other than not having a hard cylinder code, just 0300, probably because I didn't let it run long enough.

Cleaned, inspected the MAF, IAT, and zip tube, checked resonator seal, put in new air filter. Cleaned MAF electrical connector. No change. However, just for jollies, I disconnected the MAF while idling and the engine immediately stopped. Left it disconnected, and restarted it. The idle seemed to be smoother, so I left it disconnected. Didn't let it run very long, but the SES hadn't come on yet (I expected it, but ran out of time).

Tomorrow, if I have my way, I'll check fuel rail pressure, install the ACDelco plugs, and run a compression test while I'm in there.

So it's not exactly the same as yours, but similar actions have been taken. I'm trying to get the monitors passed so I can get it smogged, but the misfires are not letting that happen yet. Hopefully, when/if I get that cleared up, there won't be anything else lurking in the shadows.
I was mulling your misfires on cold start thing, and wondering if you had checked your AIR vacuum control valves, thinking if the diaphragm(s) were leaky, they might do weird things to the air/fuel ratio, and therefore the crank sensors/PCM misfire detection.

I have a broken, now tied up, air line, either before or after the pump, at the bottom of the pump. But the AIR valves have not been checked yet. That's also on my to-do list.
Never have checked them no. You’re talking about the bulky valve on top of the rear valve cover and the one next to the water pump area right? I never really thought about that. I know it still misses and nothing changes after the pump quits running. It’s only pushing in air for like 30 sec right then it’s done? Or am I wrong
 

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2003 Cadillac Seville STS
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60 Posts
Discussion Starter #74
Mine misfires a bit all the time, it seems. It started with a semi-hard miss on 7, the cylinder was never totally dead. I've replaced the plugs (with Autolites gapped at .050...have a set of 41-987s going in tomorrow). Cleaned, inspected, and dielectric greased the coil packs, boots, plug wells, etc. The boots were supple and not cracked or tracked, coil towers were nice, ground springs were clean and springy. Some wells had a little moisture (7,8), but hard to tell if it was coolant, oil, or a mix. Cleaned the ICM to PCM connectors, did not check the ICM to coil connectors. Swapped the coil packs across the banks, along with the plug and boot from 7...no change. Still a #7 miss, enough to throw a 0307.

Replaced the rubber intake plenum, both PCV line boots to the cam covers (both dried and cracked), the intake manifold gaskets (only found 1 a bit flattened on #2), cleaned the throttle body manually off the car (the gasket was flattened at the bottom, so I RTV'd it until replacement - also tomorrow), cleaned electrical connectors for IAC, MAP, TPS, EGR, AIR, PCM, etc. Checked all other air/vacuum lines. No change, other than not having a hard cylinder code, just 0300, probably because I didn't let it run long enough.

Cleaned, inspected the MAF, IAT, and zip tube, checked resonator seal, put in new air filter. Cleaned MAF electrical connector. No change. However, just for jollies, I disconnected the MAF while idling and the engine immediately stopped. Left it disconnected, and restarted it. The idle seemed to be smoother, so I left it disconnected. Didn't let it run very long, but the SES hadn't come on yet (I expected it, but ran out of time).

Tomorrow, if I have my way, I'll check fuel rail pressure, install the ACDelco plugs, and run a compression test while I'm in there.

So it's not exactly the same as yours, but similar actions have been taken. I'm trying to get the monitors passed so I can get it smogged, but the misfires are not letting that happen yet. Hopefully, when/if I get that cleared up, there won't be anything else lurking in the shadows.
Damn let me know how you do and keep me updated on what you do and find. Let me know your compression numbers. I did my test cold when usually you do it warm lol. I have so much torn apart now that I’m about to say f it and drop the engine....
 

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2002 STS stock
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Yes, the Bank 2 control valve by the pump, and also the two valves by the left (rear) bank, mounted on the bracket over the coil pack, that must be unbolted to allow the coil pack to be removed. The small one with the electrical connector controls the vacuum to the other two injector control valves, the ones with the metal flex tube going to the exhaust manifolds.

Yes, the pump runs for a minute or so, then stops. It's purpose is to clean up emissions on start-up, helping to burn unburnt fuel until the engine gets hot enough to get the fuel system into closed-loop.

So your miss continues until the engine reaches operating temp, even after the AIR pump shuts down? Ok, I ass-u-me-d your misfire stopped around the time the pump would turn off...dohhh!

Still, while searching for inexpensive straws to fix my misses, I was thinking that could be having an effect on either the intake air through the vacuum lines, or the upstream O2 sensors.
 

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2002 STS stock
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I'm very familiar with Ford's modular 4.6 engines, and I've always run compression checks cold on them. They have aluminum heads, with not very many (3-3.5) threads holding the sparkplugs into the earlier heads. Many of these have blown out the plugs due primarily to over-torquing, easier to do on a hot engine. So I'd only mess with plugs on a dead cold engine...

I'll definitely keep you updated on all I find. Like I said, inexpensive fixes are my goal...I can't drop an engine anymore, especially a transverse mounted one! I might still be able to do a Crown Vic engine, but what used to take me a few hours would now take me a week!
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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69,052 Posts
Emissions loop change occurs at about 160 degrees coolant temp, depending on ambient temperature. Before loop change the O2 sensors are out of the system. AIR operates for X to XX seconds depending on ambient temperature, coolant temperature, time since last run event, time since last start, other inputs. Sometimes it won't run at all.

2002 STS temp gauge - loop change at about 11 o'clock.

Temp gauge - my numbers.jpg
 

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Premium Member
2003 Cadillac Seville STS
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60 Posts
Discussion Starter #78
Yes, the Bank 2 control valve by the pump, and also the two valves by the left (rear) bank, mounted on the bracket over the coil pack, that must be unbolted to allow the coil pack to be removed. The small one with the electrical connector controls the vacuum to the other two injector control valves, the ones with the metal flex tube going to the exhaust manifolds.

Yes, the pump runs for a minute or so, then stops. It's purpose is to clean up emissions on start-up, helping to burn unburnt fuel until the engine gets hot enough to get the fuel system into closed-loop.

So your miss continues until the engine reaches operating temp, even after the AIR pump shuts down? Ok, I ass-u-me-d your misfire stopped around the time the pump would turn off...dohhh!

Still, while searching for inexpensive straws to fix my misses, I was thinking that could be having an effect on either the intake air through the vacuum lines, or the upstream O2 sensors.
that could definitely be a possibility. Yes it does misfire past then for a while but they slowly get better watching the miss counter on 7. It gets better and better then it will never miss again until it cools down
 

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Premium Member
2003 Cadillac Seville STS
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60 Posts
Discussion Starter #79
Emissions loop change occurs at about 160 degrees coolant temp, depending on ambient temperature. Before loop change the O2 sensors are out of the system. AIR operates for X to XX seconds depending on ambient temperature, coolant temperature, time since last run event, time since last start, other inputs. Sometimes it won't run at all.

2002 STS temp gauge - loop change at about 11 o'clock.

View attachment 582516
Yep my misses get a lot better around 11 o’clock then they go away
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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Cylinder 7 is right at the location of the Fuel Pressure Regulator - driver's corner of the fuel rail.

Fuel rail - metal.gif
 
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