Rough idle at Start, please help... - The 1947 - Present Chevrolet & GMC Truck Message Board Network

BigAlVB · 06-07-2003, 02:31 PM

Hi everyone! My '91 Sub w/ 5.7 L FI motor is running rough when I start it up. Only lasts a minute or two and usually, but not always, when its cold and/or damp. I've noticed at times it idles higher than I feel it should--I don't have a tach, just a "seat of the pants type feeling." Also, at times (very intermittently) my Check Engine light comes on. When I shut the truck down, it goes off and doesn't come back on. Seem s to run out fine on the road.
Any help, sugguestions, whatever will be appreciated.

I've been busy! I had a 2.5" suspension lift with 8(!) leaves on the front, and they were going into a negative arch! Yep, it rode like he#$! Replaced it with 4 leaves by Superlift (tapered Softride) and what a difference! Also all of my ball joints were "frozen up"and my tie rod ends were shot so I replaced them with greasable ball joints and new tie rod ends. Rides almost like a new truck now.

BigAlVB · 06-07-2003, 04:35 PM

Thanks Riptide's 78p/u!
With my limited knowledge, I was hoping the TPS is the culprit. How hard is it to replace and approx. how much? Should I order it through gmpartsdirect.com?
Anyone else want to chime in, please?

**old Rusty C10** · 06-07-2003, 05:54 PM

yeah it sounds like the tps but you can gt a cheap tester to see if its storing codes its like $20 to change the tps is pretty easy just two bolts ( small ones ) and ya gotta adjust it so the check engine lites stay off

**swervin ervin** · 06-07-2003, 09:10 PM

I agree.

I just replaced the TPS on my 87 Chevy van. When I first started mine when cold, it would do fine until I gave it some gas. Then as soon as I let off, it would die, or idle real rough. Like you say, it only did this a couple minutes or so. My idle rpm was too high also.

BigAlVB · 06-10-2003, 08:59 AM

OK, gurus, I need help! Replaced the TPS sensor and diconnected the battery for a couple of hours (overkill, I know!) to reset the computer. Checked it this morning and it still "stumbles" when cold. Idle does seem to be a little better, in my opinion.
What else could it be? Make sure to read or re-read my first post to get the gist of the problem I'm trying to describe. I'm willing to try anything before I bring it to a mechanic.
Please help!

BigAlVB · 06-11-2003, 07:11 AM

Anybody?
Bueller...

Bueller...

Bueller...

**swervin ervin** · 06-11-2003, 08:15 AM

I wish I knew the answer, because I'm having the same trouble as you. I replaced my TPS and it seemed to help for a bit, but soon started back. Sorry if I mislead you above, but it did fix mine for awhile, or so I thought anyway.

I'm no TBI guru for sure, just an old carb junkie. The parts for these things are too damn expensive to just start replacing them. Most if not all of them require you to adjust something electrical using all sorts of fancy meters and such. I can't stand this crap either.

I've been reading everything I can find on this and from what I come up with, I've narrowed it down to the idle air control (IAC) valve. Advance sells them for $36.99. but man, I hate to keep thowing money down the drain. There is so much with this setup that can cause things, it's hard to guess. If it was a Q-jet doing this, I would know in a second what to do. I've been searching for a factory sevice manual for my 87 van, with no luck I might add. The Haynes and Chiltons manuals are about useless for this type of thing. I absolutely hate vehicles that are controlled by a computer.

Hey, I got an idea. Why don't you buy a new IAC valve for yours. If it fixes it, then I'll go buy one for my van.

maineiac · 06-11-2003, 09:04 AM

I know it is klike trowing money away but the TPS is a wear item and a good thing to replace anyway as is the oxygen sensor and since no one has brought it up it could also be the culprit by false reports to the computer as to how the engine is running and adjusting the fuel mixture to match. The real problem is all of the sensors interact with each other and even if you have a machine to test them it can point you to the wrong problem. I have seen many garages just basicly do the random sensor guess method so I guess after all this rambling I would say look at it as preventive maintance each one you change is one less that will go bad in the next 10 years. I would look at replacing the oxygen sensor because they are often taken out or degraded by bad gas the egr is also a possibility.

Steve

**swervin ervin** · 06-11-2003, 10:38 AM

Well, those who know me, know I'm not very patient when it comes to stuff like this. After replying to this thread, I went out and removed my TBI. Took the IAC valve out and made a trip to the auto parts store. After a couple ours and $68.00, I'll know in a little while if the IAC valve will fix it or not. I bought a rebuild kit for it too since I messed up some gaskets. It needs cleaning anyway.

I'll be sure to post back here when I get it back on the van.

Oh, and Steve, changed the O2 sensor a couple weeks ago. Didn't help anything, but as you say, it didn't hurt anything either. Well, my billfold, but what else is new?

maineiac · 06-11-2003, 11:44 AM

Know all about that wallet thing went tru all the sensors on my wifes minivan after she got some bad gas one more common fail I thought of if it is only when you first start that is a problem there is a cold start valve that acts like a choke on fuel injection that can cause it to run ruff at first but that is only the first few minutes of operation like I said earlier the egr valve can also cause fits.
Good Luck
Steve

**swervin ervin** · 06-11-2003, 01:57 PM

Well, I rebuilt the TBI and installed the new IAC valve. It sure does look purdy.

Good news and bad news. Good news is it fixed my rough cold idle and stumble.

Bad news is now, it's puffing black smoke when sitting still and you give it some gas.

It never done this before the rebuild. Everything on these things is pretty straight forward. Not much way you can do something wrong. Simple as pie to rebuild. But, I know it's something that happened between then and now.

I used carb dip to clean everything. I didn't dip anything that the dip could mess up. I cleaned the injectors with spray carb cleaner.

I'm also starting to think my ECM may be bad. My warm idle is too high and there is no mechanical way to adjust it. It's all done with the ECM.

Ahhhh hell. What did I break this time.

If anyone knows what causes it to puff black smoke (read: fuel smoke), I'm all ears.

So, my answer to can the IAC valve cause it to have a rough cold idle and stumble is yes. I fixed this part of mine. Now I have to figure out what else I fudged up.

06-07-2003, 02:31 PM	#1
BigAlVB Registered User Join Date: Mar 2003 Location: Holland, MI Posts: 74	Rough idle at Start, please help... Hi everyone! My '91 Sub w/ 5.7 L FI motor is running rough when I start it up. Only lasts a minute or two and usually, but not always, when its cold and/or damp. I've noticed at times it idles higher than I feel it should--I don't have a tach, just a "seat of the pants type feeling." Also, at times (very intermittently) my Check Engine light comes on. When I shut the truck down, it goes off and doesn't come back on. Seem s to run out fine on the road. Any help, sugguestions, whatever will be appreciated. I've been busy! I had a 2.5" suspension lift with 8(!) leaves on the front, and they were going into a negative arch! Yep, it rode like he#$! Replaced it with 4 leaves by Superlift (tapered Softride) and what a difference! Also all of my ball joints were "frozen up"and my tie rod ends were shot so I replaced them with greasable ball joints and new tie rod ends. Rides almost like a new truck now. __________________ "When this car hits 88 miles per hour, you're going to see some serious s*&$!" Back To The Future

06-07-2003, 04:35 PM	#2
BigAlVB Registered User Join Date: Mar 2003 Location: Holland, MI Posts: 74	Thanks Riptide's 78p/u! With my limited knowledge, I was hoping the TPS is the culprit. How hard is it to replace and approx. how much? Should I order it through gmpartsdirect.com? Anyone else want to chime in, please? __________________ "When this car hits 88 miles per hour, you're going to see some serious s*&$!" Back To The Future

06-07-2003, 05:54 PM	#3
old Rusty C10 Robert Olson Transport Join Date: Oct 2002 Location: recent transplant to NC USA Posts: 20,310	yeah it sounds like the tps but you can gt a cheap tester to see if its storing codes its like $20 to change the tps is pretty easy just two bolts ( small ones ) and ya gotta adjust it so the check engine lites stay off __________________ Bob 1951 International running on a squarebody chassis "If a man's worth is judged by the people he associates himself with, then i am the richest man in the world knowing some of the fine people of this board" http://67-72chevytrucks.com/vboard/a...t.php?f=25&a=9 (you can review the site rules here!) PM Me for your vehicle/parts hauling needs in the North East US or see my Facebook page Robert Olson Transport Live each day to the fullest.. you never know when fate is going to pull the rug out from under you... I hate cancer!!

06-07-2003, 09:10 PM	#4
swervin ervin You get what you pay for Join Date: Dec 1999 Location: Cherryville, NC Posts: 4,798	I agree. I just replaced the TPS on my 87 Chevy van. When I first started mine when cold, it would do fine until I gave it some gas. Then as soon as I let off, it would die, or idle real rough. Like you say, it only did this a couple minutes or so. My idle rpm was too high also. __________________ Mike 1985 Chevy C-10

06-10-2003, 08:59 AM	#5
BigAlVB Registered User Join Date: Mar 2003 Location: Holland, MI Posts: 74	OK, gurus, I need help! Replaced the TPS sensor and diconnected the battery for a couple of hours (overkill, I know!) to reset the computer. Checked it this morning and it still "stumbles" when cold. Idle does seem to be a little better, in my opinion. What else could it be? Make sure to read or re-read my first post to get the gist of the problem I'm trying to describe. I'm willing to try anything before I bring it to a mechanic. Please help! __________________ "When this car hits 88 miles per hour, you're going to see some serious s*&$!" Back To The Future

06-11-2003, 07:11 AM	#6
BigAlVB Registered User Join Date: Mar 2003 Location: Holland, MI Posts: 74	Anybody? Bueller... Bueller... Bueller... __________________ "When this car hits 88 miles per hour, you're going to see some serious s*&$!" Back To The Future

06-11-2003, 08:15 AM	#7
swervin ervin You get what you pay for Join Date: Dec 1999 Location: Cherryville, NC Posts: 4,798	I wish I knew the answer, because I'm having the same trouble as you. I replaced my TPS and it seemed to help for a bit, but soon started back. Sorry if I mislead you above, but it did fix mine for awhile, or so I thought anyway. I'm no TBI guru for sure, just an old carb junkie. The parts for these things are too damn expensive to just start replacing them. Most if not all of them require you to adjust something electrical using all sorts of fancy meters and such. I can't stand this crap either. I've been reading everything I can find on this and from what I come up with, I've narrowed it down to the idle air control (IAC) valve. Advance sells them for $36.99. but man, I hate to keep thowing money down the drain. There is so much with this setup that can cause things, it's hard to guess. If it was a Q-jet doing this, I would know in a second what to do. I've been searching for a factory sevice manual for my 87 van, with no luck I might add. The Haynes and Chiltons manuals are about useless for this type of thing. I absolutely hate vehicles that are controlled by a computer. Hey, I got an idea. Why don't you buy a new IAC valve for yours. If it fixes it, then I'll go buy one for my van. __________________ Mike 1985 Chevy C-10

06-11-2003, 09:04 AM	#8
maineiac Registered User Join Date: May 2003 Location: South Carolina Posts: 104	I know it is klike trowing money away but the TPS is a wear item and a good thing to replace anyway as is the oxygen sensor and since no one has brought it up it could also be the culprit by false reports to the computer as to how the engine is running and adjusting the fuel mixture to match. The real problem is all of the sensors interact with each other and even if you have a machine to test them it can point you to the wrong problem. I have seen many garages just basicly do the random sensor guess method so I guess after all this rambling I would say look at it as preventive maintance each one you change is one less that will go bad in the next 10 years. I would look at replacing the oxygen sensor because they are often taken out or degraded by bad gas the egr is also a possibility. Steve

06-11-2003, 10:38 AM	#9
swervin ervin You get what you pay for Join Date: Dec 1999 Location: Cherryville, NC Posts: 4,798	Well, those who know me, know I'm not very patient when it comes to stuff like this. After replying to this thread, I went out and removed my TBI. Took the IAC valve out and made a trip to the auto parts store. After a couple ours and $68.00, I'll know in a little while if the IAC valve will fix it or not. I bought a rebuild kit for it too since I messed up some gaskets. It needs cleaning anyway. I'll be sure to post back here when I get it back on the van. Oh, and Steve, changed the O2 sensor a couple weeks ago. Didn't help anything, but as you say, it didn't hurt anything either. Well, my billfold, but what else is new?

06-11-2003, 11:44 AM	#10
maineiac Registered User Join Date: May 2003 Location: South Carolina Posts: 104	Know all about that wallet thing went tru all the sensors on my wifes minivan after she got some bad gas one more common fail I thought of if it is only when you first start that is a problem there is a cold start valve that acts like a choke on fuel injection that can cause it to run ruff at first but that is only the first few minutes of operation like I said earlier the egr valve can also cause fits. Good Luck Steve

06-11-2003, 01:57 PM	#11
swervin ervin You get what you pay for Join Date: Dec 1999 Location: Cherryville, NC Posts: 4,798	Well, I rebuilt the TBI and installed the new IAC valve. It sure does look purdy. Good news and bad news. Good news is it fixed my rough cold idle and stumble. Bad news is now, it's puffing black smoke when sitting still and you give it some gas. It never done this before the rebuild. Everything on these things is pretty straight forward. Not much way you can do something wrong. Simple as pie to rebuild. But, I know it's something that happened between then and now. I used carb dip to clean everything. I didn't dip anything that the dip could mess up. I cleaned the injectors with spray carb cleaner. I'm also starting to think my ECM may be bad. My warm idle is too high and there is no mechanical way to adjust it. It's all done with the ECM. Ahhhh hell. What did I break this time. If anyone knows what causes it to puff black smoke (read: fuel smoke), I'm all ears. So, my answer to can the IAC valve cause it to have a rough cold idle and stumble is yes. I fixed this part of mine. Now I have to figure out what else I fudged up.



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