Rough idle and dies

FREMONT

MEMBER
1998 CHEVROLET LUMINA
180,000 MILES

Only on start up and if car is shut off for 5 min. Or longer the engine runs really rough and loopes when started. But once it quites going though its fit's it seems to straighten out and idles just fine and runs good, but once you shut it off, it starts all over again? Checked coils, IgnitionCM, PCV, MAF, fixed all vacumn leaks, good fuel preasure, wires and plugs are good, cleaned idle control module and port. Checked all connections they are all clean and tight. I am not getting no codes? If I did things would be more simple and it would be fixed. :) I have looked at almost everything, I am stumped! I did take the car to the car wash and washed the engine, but that has been two weeks ago, but it did kinda start acting up at the same time? But I am not getting any codes if something electrical got wet and went south?

6 Replies

HMAC300

MECHANIC
48,601 POSTS

Check fuel pressure if it's not 41-47 psi then its 'not good. Clean throttle plate with choke cleaner on both sides and iac hole.

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Wednesday, May 8th, 2013 AT 8:17 AM

FREMONT

MEMBER
8 POSTS

I already told you I did take throttle housing apart and cleaned the hole unit inside and out and my fuel pressure is good right at 45 and stays there when the car starts acting up. So please read my question again? The only two things I have not looked at are the cam and crank sensors. Like I said I am not getting any codes! I would think if one of those were bad I would get a code? I am leaning towards the crank sensor? I did watch my timing deg. On my scanner and my timing was jumping all over and when it was acking up and was idling rough my timing was at 24 deg and idle at about 700. I dont think that is right? So what do you think? Thanks, Fremont

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Thursday, May 9th, 2013 AT 7:18 AM

HMAC300

MECHANIC
48,601 POSTS

When we get a lot of questions on here people will say fuel pressure is good and it'sonly at 35 psi which for a gm product it won't run. Sorry I misread abot the throttle plate. Crank and cam sensors can/will go bad and not give codes. The only way to test them is by resistance which of course we don't have available here so about the only thing I can suggest is check against a new one. If it is high cold then it will get higher when hot hence a starting problem but it may also be the connection problem. But with how hard it is to get the cranks sensor on this i'd just suggest replacing it as it a pain to get out with out breaking it. Cam sensor is much easier to check on this also if there is some interference on the cam sensor wires it can cause a problem as well

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Thursday, May 9th, 2013 AT 9:35 AM

FREMONT

MEMBER
8 POSTS

Well I tested the resistance on the crank sensor and compared it to a new one and the old one was much higher, so I replaced it with a new one and still got the same problem? Cam Sensor? I didn't check but I don't think it is? This problem that I have with this car is only on start up(cold) and when I shut it off for 10 min. Or longer, it will start running really rough again and straighten out and run really good? So to me it is something bleeding off or sonsor cools down? My fuel PSI stay's UP and I even changed the fuel filter, all my connections are clean and tight? The owner told me yesterday that the car has sat for about two months, so I am going to put some high octane gas in along with some GOOD fuel inj. Cleaner and drive it down the highway. The Injectors were replaced some time ago, so maybe this will help or not? Maybe this will give the computer a chance to run threw it's steps and get back in its perimeters? Because I did change the water pump and disconnected the battery. I should have not have washed the motor! LOL ( I know it's not good, but I have washed many of them and I tend to be reall carefull and never had this kind of problem) NO codes make it even harder to diagnose! HELP! Thanks Fremont !

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Friday, May 10th, 2013 AT 7:23 AM

HMAC300

MECHANIC
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Check the cam sensor for resistance as well. Washing the engine may have got some waterin the connector. The parameters on the computer don't worry about it will learn as you drive. Just make sure on the cam sensor you put back on the conduit around the wires if it doen'st come with it. As the spark plug wires will effect it sometimes as well.I know you said you checked vacuum hoses and that if there are any soft ones replace them because cold the engine doesn't run with computer in closed loop so it runs on the richer side also if this conitues to happen take a screwdriver and bang the maf with the handle if the car quits replace the maf or clean it with choke cleaner. Hard to remember tricks form 20 yrs ago lol.