2004 LeSabre Stumbles After Warm Start

pcmos · #1 09-18-2010, 02:36 AM

I've been having a strange issue with my 2004 LeSabre (135k miles). If I take the car out for a short trip and leave it off for about 15 minutes it will stumble and run rough after I start it up again. The issue doesn't occur when the car is completely cold and it's also erratic. Morning startups are perfectly normal. I've also never experienced any strange behavior while cruising. By the time I back the car out of a parking space and get it back in drive, the issue has typically cleared up and the rest of the drive is totally normal.

I was hoping it would eventually trip a diagnostic code but so far I've not had an SES light and my diagnostic software doesn't indicate any problem. Tomorrow I'm going to try to catch it in the act with the computer hooked up but I feel like I'm not going to notice anything.

The car drove cross country three weeks ago and took a 500 mile trip for Labor Day with no change in the behavior. I've been waiting forever for the problem to get progressively worse, but it just hasn't. I'm just wondering if anyone has ever experienced a similar issue. I know the UIM is the first thing to suspect, possible coolant contamination, etc, all the usual suspects. I just know these cars so well that I feel like this is something unique. I may go ahead and pull the TB tomorrow, I have a new IAC valve and I can scrub out any carbon buildup.

Blue Bowtie · #2 09-18-2010, 11:13 AM

I wouldn't suspect an upper intake leak first. Instead, I'd suspect injector leakage or a damaged FPR vacuum diaphragm allowing fuel to pool in the intake runners while the engine is shut off. A fairly easy way to confirm this is to monitor standing fuel pressure when the engine is turned off.

pcmos · #3 09-18-2010, 10:14 PM

I felt like the FPR was a good lead so today I purchased an OE Delphi replacement and installed it. I still haven't had a chance to take the car out for a drive. Hopefully the issue has been cleared up. I'll definately report my findings after I take the car out.

imidazol97 · #4 09-19-2010, 06:47 PM

Quote:

Originally Posted by pcmos

I've been having a strange issue with my 2004 LeSabre (135k miles). If I take the car out for a short trip and leave it off for about 15 minutes it will stumble and run rough after I start it up again. The issue doesn't occur when the car is completely cold and it's also erratic. Morning startups are perfectly normal. I've also never experienced any strange behavior while cruising. By the time I back the car out of a parking space and get it back in drive, the issue has typically cleared up and the rest of the drive is totally normal.

Are you watching the coolant level very closely? Measure sitting on the same spot, etc., same temp. I marked my reservoir tank level when hot. Had a slight drop occasionally of maybe a cup over thousands of miles. Then other times no apparent change.

I ran an oil analysis just out of curiosity for wear data and knew it would show coolant getting into engine if there was any. It did show a slight coolant content at 7500 miles on synthetic. I got a phone call from Blackstone before the report came to let me know and suggest how to handle it with short mileage oil changes to avoid buildup of coolant in oil until I could do a replacement.

The seep was at the throttle body, not the coolant passages over and under the lower intake. The GM gasket was too thin to adapt after the upper developed some warp with age and heat (according to the mechanic who I had do the repair). Replacements are much thicker. If I had taken off the throttle body for cleaning at 80K like I thought about, I would have fixed my problem.

I'd recommend pulling the throttle body to see the gasket area for leaks. You mentioned cleaning it...

You might also run an oil analysis--I used Blackstone in Ft. Wayne. Takes a few days to get the free container for shipping from them after phone call, but it was worth it. I also paid for the additive testing and found my synthetic oil still had a good additive level after 7500 miles on the oil this spring and summer.

pcmos · #5 01-15-2011, 06:30 PM

I want to give everyone an update in this post. I know it may be considered an old post now but the problem has been ongoing for quite a while now. Absolutely no change in behavior compared to when I first posted a few months ago. I finally got around to cleaning the MAF and replacing the fuel filter today. I also replaced a number of damaged vacuum line fittings and made sure the vacuum system is completely tight. Next weekend I need to replace the rack and pinion in the car and finish a lot of steering / suspension work. I may get around to pulling the throttle body if my efforts this weekend do not result in any change in this behavior. I'm really stumped, I never get a service light and the car almost never fully stalls.
When the trouble happens I do notice that my voltage drops to about 12V and the headlights dim significantly. Perhaps I'm losing spark due to some electrical problem. I thought it might be an issue with a bad shift solenoid placing an unusually hefty load on the electrical system but all of the shift solenoid resistance values are within spec. I did replace the idle air control valve, fuel pressure regulator and PCV valve with no change in the behavior. Any further suggestions would be appreciated.

pcmos · #6 01-16-2011, 11:49 PM

Cleaned MAF, changed the fuel filter and rebuilt the deteriorated vacuum lines under the hood. No change in the stumbling behavior after a warm start. The car runs better overall but the problem is just the same. I'm fairly certain that the fuel system is losing pressure when I shut the car off. Based on readings I've taken with my fuel pressure gauge it seems like the pressure is bleeding down quite a bit.

I know I need to rebuild the upper and lower intake assembly because I've got oil seeping out all over the place. I'm going to do the intake work and reseal everything, then evaluate the problem once again. If it still exists after the intake work I'll replace the fuel sender unit.

maxwedge · #7 01-17-2011, 01:34 PM

Sounds like a leaky injector or 2.

Jrs3800 · #8 01-17-2011, 03:06 PM

What kind of Plugs and wires are on the car?

pcmos · #9 02-11-2011, 01:13 AM

Plugs and wires were replaced with factory parts. I bought the plugs and wires at the dealer, I think OE is some sort of iridium plug for this car.

pcmos · #10 09-14-2011, 09:52 PM

I finally solved this problem, it turns out that the post-cat oxygen sensor was faulty. After a hot start the system must have been relying on skewed sensor data. Two weeks ago I finally got a code for the post-cat sensor. I replaced the suspect sensor and voila the problem went away immediately. Unless this is just a really weird coincidence I think I finally solved a problem that has been driving me nuts for about 2 years. If I don't post a follow up then it's safe to assume that this was the solution.
After realizing that the oxygen sensor replacement made such a big difference I splurged and replaced the pre-cat sensor as well. Both O2 sensors were purchased at the dealer... car seems to be running better than ever.

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