2000 Cadillac STS Intermittantly runs terribly

BPHODGESUS

MEMBER
2000 CADILLAC STS
V8
FWD
AUTOMATIC
90,000 MILES

Hi I bought my wife a 2000 Seville STS about a year ago. It has had some minor problems, but other than a starter, nothing too costly. Lately it has had an intermittant problem of running very rough, with low power as if there were water, or other contamination in the fuel. The problem then clears up, then returns. I pulled the plugs and they were clean, the exhaust smells rich when it is running poorly. I had a guy check the fuel injectors, and he said they were fine, but that the voltage reading to the injectors was higher than expected. We also have some electrical anomalies in which the CEL, Brake, and other lights will come on and go off, and the occasional "Stability System Needs Servicing" warning light and gong will come and go as well. It seems that there should be some unifying electrical issue that is causing all of this. The car is essentially undrivable today as it stands now, but I may get home from work and have it drive fine. Any suggestions other than trading it in when it is running okay? I am concerned that it may not run good again as it seems to have fewer good days. Any suggestions would be appreciated. Here are the codes I was able to get from the OBC.
ABS: C1287, U1000
PCM: P0300, P0420, P0741, P1106, P1860
SDM: B1001
TTM: BC1983, U1016, B1009, B2739
Thank you.

5 Replies

FACTORYJACK

MECHANIC
4,159 POSTS

Does this problem occur after a period of cruising, and typically when warm. It is possible that the EGR is sticking open, and causing a large vacuum leak. The increase in intake pressure might be causing the p1106, which is intermittant high MAP sensor voltage. When it is running poorly, try tapping at the base of the valve with something metallic, and see if it clears up. If the rough run occurs out of the blue, this would not apply, as the egr valve has to open to stick open. The P0741 and P1860 are torque converter clutch related faults, there are some issues with them as well, but they would not cause a rough run. The ABS and service stability message may be relevant to the C1287, which is a fault related to the steering position sensor.

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Monday, December 7th, 2009 AT 9:40 PM

BPHODGESUS

MEMBER
3 POSTS

Thank you. I hope that is it. I will check it out when I get home from work tomorrow. I think it does get worse after warming up, and after a restart (going into store, etc.) Would this cause it to run very rich? It uses alot of fuel when it is running bad, and the exhaust smells of raw fuel. Is it something that can be cleaned, or is it necessary to replace it? Thank you again.

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Tuesday, December 8th, 2009 AT 1:59 AM

FACTORYJACK

MECHANIC
4,159 POSTS

It could, in theory, cause it to run rich. It is essentially a large vacuum leak after the mass airflow sensor, and O2 sensors are going to reflect the extra air as a lean condition, causing it to add fuel. If the exhaust smells of raw fuel, you could have an injector that is sticking open. I suppose to look into that, you would need to observe fuel pressure, and when it is running bad, shut it down and see if the fuel pressure holds or bleeds off. It should hold for a bit. Also, I have not seen one intermittant, but a fuel pressure regulator can leak past the diaphragm, and put raw fuel into the intake system through the vacuum line. Typically, this can cause a hard start, and rough run, but can be cleared up by throttling it a bit.

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Tuesday, December 8th, 2009 AT 10:39 PM

BPHODGESUS

MEMBER
3 POSTS

So if I try tapping the EGR, it may improve if it is just a bit sticky, and if I check the vacuum I should notice a difference between the readings when running good and when running bad. If this doesn't solve it I will try to get a fuel pressure check. I think you are right about the fuel pressure regulator being less likely, given the symptoms that you described as it does not have a hard start problem, and does not clear up with throttling up. In fact if I try to rev the engine it cuts out and the rpms drop rapidly when I hit about 4400 rpms. It seems that it is shut down (to idle) by the brain. As far as the EGR is it cleanable with carb cleaneror similar solvent, or does it need to be replaced if it is sticking? Thank you very much for your help.

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Wednesday, December 9th, 2009 AT 11:33 AM

FACTORYJACK

MECHANIC
4,159 POSTS

The RPM cut out is probably the rev limiter. As far as cleaning, I have not had great success. It may work for a while, but I have found it temporary at best. Checking the vacuum may not rule the egr as the smoking gun, when it is running bad, you would probably notice vacuum faulting anyway.