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Old 10-04-2017, 05:00 PM   #1
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Default Weird rough idle condition

Long time listener, first time caller....

1999 Oldsmobile 88 LS 3.8L engine. I just bought this car last week and have an issue I wanted to ask you all about. The car runs and drives great. Been well taken care of. No check engine light since I have owned it. I did put a thermostat in it the other day because it was never getting much past 170 deg and if it did, it didn't stay there long. Now it runs a consistent 200-205 deg. It does seem like it takes a little long to warm up all the way though.
All that said, the problem is, that there is a small window of time when the car starts idling and running rough and then stops. When I first take off in the morning it runs fine, but then 5-10 minutes later when it gets almost warmed up it starts running and idling rough. That will last for about 5 minutes and then it is fine again and usually has been fine the rest of the day.

Any thoughts? I wonder if it is a temp sensor, or it is taking too long too warm up and messing up the closed loop system. Or, an O2 sensor. Thanks

Branden
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Old 10-04-2017, 09:56 PM   #2
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What do you know about the service history of the car? How many miles are on it?

I would just start with the basics, and work up from there.
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Old 10-05-2017, 05:01 AM   #3
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Quote:
Originally Posted by rjolly87 View Post
What do you know about the service history of the car? How many miles are on it?

I would just start with the basics, and work up from there.
Hi, sorry. There'* 186k on the car. From what I can tell it has a pretty good service history. I got a stack of receipts with the car. I found one that says that the MAF sensor was replaced a few years ago but with parts these days, that doesn't mean much to me. All of the other sensors look original as far as intake, throttle and idle go. Plug wires look "newer". I would even almost bet that the thermostat that I replaced was original.

I agree, I have started with the basics pretty much. I replaced a dirty air filter and pcv valve, which is what I first thought of. But at this point, it'* clear to me the way it acts is that the computer goes into some sort of sequence and then comes back out of it and is fine. I drove over to Autozone the other day (5 min away) and it started doing it. Shut the car off, went inside and when I left it didn't do it anymore. Then other times it just quits on its own while the car is running and then is fine. It just seems like it'* only when the car is almost, or recently up to temp.

Thanks
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Old 10-07-2017, 01:28 PM   #4
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Are there any thoughts on this issue? Anyone ever seen this happen? Pretty much it just has about a 5 minute period at the end of it'* warming all the way up that it runs like this. It misses at low speed cruising and idles rough and then it quits and runs fine the rest of the day. No codes or light.
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Old 10-08-2017, 11:22 AM   #5
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Couple things to check....

Are you losing coolant? Check on a cold engine, a couple days in a row...

Also, pull vac line off the fuel pressure regulator, and see if there is fuel in the vac line....

Also spray carb cleaner around engine joints, like upper plenum, lower intake, throttle body, valve covers ETC., and see if there is a change in rpms....
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Old 10-08-2017, 11:48 AM   #6
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Forgot to mention, that the throttle plate and bore should be cleaned, along with removing the IAC valve and cleaning it'* pintle and also the passageway in the throttle body......best way to do this, is remove the throttle body from the upper plenum...will need new Throttle Body gasket.....
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Old 10-09-2017, 06:33 AM   #7
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Quote:
Originally Posted by Tech II View Post
Couple things to check....

Are you losing coolant? Check on a cold engine, a couple days in a row...

Also, pull vac line off the fuel pressure regulator, and see if there is fuel in the vac line....

Also spray carb cleaner around engine joints, like upper plenum, lower intake, throttle body, valve covers ETC., and see if there is a change in rpms....
Awesome! This is great info. I appreciate it. I understand all of what is happening, just not good with computer controlled engines. I still run points ignition in my old Ford truck hahaha. I will do these things and report back. Thanks again

Branden
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Old 10-12-2017, 05:52 AM   #8
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UPDATE: Well, I hadn't had a chance to work on the car yet the other day when it finally gave me an engine light for this issue. I was able to read the code and clear it and it was a P1404 EGR valve stuck open, code. This was great, although I don't understand why it took so long to give the code. I'm assuming it has to have the condition for a certain amount of time before registering a fault code. Oh well.

At any rate, I took the EGR valve off and cleaned it out good and worked the pintle (plunger) up and down until it was smooth. It was in fact sticking open when I pushed it down. Sure enough, that took care of it! It also took care of the hard shifting that it was doing when it warmed up, which also makes sense. Anyway, I wanted to give a closure to this thread and report the remedy to the problem. Thanks for the help.

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Old 10-13-2017, 05:45 AM   #9
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Thanks for letting us know what it ended up being.
Glad ya got it working well again!
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