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Engine Rough Idle starts about 5 seconds after coming to a stop?

Tiny
NGIMPEL
  • MEMBER
  • 1995 CHEVROLET SILVERADO
  • 225 MILES
I have replaced the plugs, wired(delco factory) cap, roter, Iac valve, idel selonid. I have checked for vacume leaks at the intake manifoul using either at the gaskets. It only happens when engine reaches full temp. When I come to a stop it will idle fine for about 8 to 10 seconds after tack lowers to about 600 rpm's. Help would be appreciated. My mechanic told me to use injector cleaner. I have tried this with no help.
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Monday, August 29th, 2011 AT 10:41 PM

32 Replies

Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Get some choke cleaner and clean the throttle plate on both sides with the engine off. they will gum up and cause this. also clean the hole for the iac. it will start hard after doing this. try that first.then check the fuel pressure

These guides will help us

https://www.2carpros.com/articles/engine-misfires-or-runs-rough

and

https://www.2carpros.com/articles/throttle-actuator-service

and

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

Please run down these guides and report back.
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Monday, August 29th, 2011 AT 10:54 PM
Tiny
NGIMPEL
  • MEMBER
  • 8 POSTS
Recleaned the throttle plates both sides witc cleaner and a brush. Also cleaned the hole for the iac. Still no difference. Could the iac valve be bad. I had replaced it about 6 months ago with the throttle position sensor. Also replaced the EGR I don't know what else to replace except the truck. If you have any suggestions I will try. Thank You for your time.
Nick
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+1
Wednesday, August 31st, 2011 AT 1:38 AM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Oen thing to check is fuel pressure. You havent said what it is in your first question so i'll have to give you a cople of ones here. If it has TBI on it then it's 9-13 psi if lower than that it may give you trouble. If it's a a 7.4L it shoudl be 26-32 pdi. Also replace the fuel filter first if the pressure is low as it may be that and hard telling when that was replaced. Aalso check the evap cannister valve to see if it is open as it shold only work above 30 mph. You will feel vacuum out both lines if it is. If you use an injector cleaner then try sea foam or one made by Chevron. The Chevron one to use is thepro type andis usually over $10 a bottle.
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Wednesday, August 31st, 2011 AT 3:10 PM
Tiny
NGIMPEL
  • MEMBER
  • 8 POSTS
I checked the canister purge valve no vacume at idle. The fuel pump was replaced about 6 months ago along with a new fuel fliter. I'll have to take it back to the shop to have the fuel pressure tested. The truck runs great at any speed. Could my problem be a fuel pressure regulator. If so where is it located and is it complated to replace.
Thank You for all your responses.
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Wednesday, August 31st, 2011 AT 8:49 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Yes it cold be the regulator but you need to do a fuelpressure check to find out or not. You have to let it sit with pressure on it and it depends whether or not you have tbi or port fuel where it's located.
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Thursday, September 1st, 2011 AT 12:56 AM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
The purge valve should have vacuum coming out of one line at least.
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Thursday, September 1st, 2011 AT 12:57 AM
Tiny
NGIMPEL
  • MEMBER
  • 8 POSTS
I have the TBI. I checked the vacume line at the canister when the engine reached normal temp. No vacume at idel. I had replaced the IAC a while back along with the TPS. I have also replaced the EGR. To do a fuel pressure test I do not think I have a test port. So I will have to disconnect one of the fuel lines. I'm not sure which is supply and which is return. Am I correct in saying the pressure tester has to be connected to the supply line and that line must return to the TBI to complete the flow of fuel to return to the tank. I really appreciate you taking your time to help me, Thank You
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Thursday, September 1st, 2011 AT 1:15 AM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Oknow that I know what you have check the gasket under the TBI unit, they have a nasty habit of blowing out and causing vacuum leaks. The hose form the engine should have vacuum and probably doesn't do to eitehr a plugged line or sucked together because it's soft or the gasket is blown out there.
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Thursday, September 1st, 2011 AT 1:26 AM
Tiny
NGIMPEL
  • MEMBER
  • 8 POSTS
Ok I will check both. If I spray some either around the base of the TBI the RPM'S should increase if the gasket is bad or should I just replace the gasket. I know I should have a constant vacume when I disconnect the hose from the purge valve. But am I correct in saying I should not have vacume at the canister at an idle. Again Thank You for trying to solve my problem.
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Thursday, September 1st, 2011 AT 1:38 AM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
No the one line coming frm the engine will have vacuum. The valve opens to suck the fule mix out of ht ecannister after 30 mph. And yes if you sspray ether around the base tehre will be a rpm change up or down. I wouldn't reccomend using ether as it drys the wasll of the cylinder and is not a good idea. Choke cleaner is much better.
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Thursday, September 1st, 2011 AT 5:45 PM
Tiny
NGIMPEL
  • MEMBER
  • 8 POSTS
Ok, Thank You again. I checked with carb cleaner around the base of the TBI and all vacume lines. No change. I pulled the vacume line from the MAP sensor and and had good vacume. I pulled the vacume line for the purge valve and had no vacume from the line from the basr of the TBI to the purge, which I think should be constant. I don't think that is my problem. Hod do I know if the plunger from the iac valve is set correctly? Thank You.
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Thursday, September 1st, 2011 AT 7:39 PM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Clean out the line going to your prge valve as it may be soft and closing or plugged, tbi units will do that as well. Also check yoru pcv line for the same thing. The iac valve should set to proper distance when you drive over 30 mph. That is assuming that you matched it up with teh other valve for length when you installed it.
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Thursday, September 1st, 2011 AT 8:19 PM
Tiny
NGIMPEL
  • MEMBER
  • 8 POSTS
I have checked all the lines. All seem to be ok. I was concerned why I did not have constant vacume in the line from the TBI baseplate to the purge valve, I did set the new iac plunger the same as the one I removed. Do you think it could be play inside the distributer shaft. Or a modgle breaking down when it gets hot. I'm leaning away from a fuel npressure regulatio because ti starts with no problem every time. I will try the seafoam if I can figure out what it is. I will look it up. Thank You very much for your time helping me. If I keep replacing things I will eventually find the problem.
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Friday, September 2nd, 2011 AT 1:12 AM
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Why don'tyou go to where the purge valve hookup uis an seeif you can clean that out so you do get vacuum. If it idles ok while in gear then just leaveit alone. It could be the things you are talking about but if thereis no problem other than dropping down a little bit then don't worry about it. As long as it doesn't stall and you aren't complaining about that. Just clean the pipe out asit may have carbon in it. Yo don'tneed totake the tbi off and if any drops in the intake it won't hurt anything if it gets sucked through the engine.
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Friday, September 2nd, 2011 AT 3:18 PM
Tiny
JEFFIEB
  • MEMBER
  • 1 POST
  • 1994 CHEVROLET SILVERADO
  • 180,000 MILES
It is having a lot of trouble idling and in some cases dies. It runs fine it just has trouble when idling
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Monday, November 16th, 2020 AT 11:37 AM (Merged)
Tiny
DOCFIXIT
  • MECHANIC
  • 18,828 POSTS
Maybe injectors leaking hook up timing light remove air cleaner start point light at TBI injector spray should be 360 degree mist no drips no gap in spray pattern
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Monday, November 16th, 2020 AT 11:37 AM (Merged)
Tiny
ROBSMITHTN
  • MEMBER
  • 1 POST
  • 1993 CHEVROLET SILVERADO
  • 295,000 MILES
My 1993 chevrolet z71 4x4 truck cranks, then runs like crap for 25-30sec then idles correctly.I have checked plug wires to make sure they have good connection and not shorting out on block or manifold.
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Monday, November 16th, 2020 AT 11:37 AM (Merged)
Tiny
ASEMASTER6371
  • MECHANIC
  • 52,797 POSTS
Make sure the injectors are not dripping after you turn it off. It may be flooding the motor and needs to clear out when first started.

Check for vacuum leaks as well.

Check fuel pressure as well.

Roy
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Monday, November 16th, 2020 AT 11:37 AM (Merged)
Tiny
BIGJOHN6292
  • MEMBER
  • 1 POST
  • 1992 CHEVROLET SILVERADO
  • V8
  • 4WD
  • MANUAL
  • 205,000 MILES
I have a 92 chevy silverado when it idles the rpm is at 1000 and every few seconds it drops about 500 rpms. I have have replaced plugs, wires, ignition coil, distributor, spark control, tps sensor which made it worse, fuel injectors, o2 sensor, and idle air control valve. I am am at my witts end please help.
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Monday, November 16th, 2020 AT 11:37 AM (Merged)
Tiny
FISHER4X4
  • MEMBER
  • 1 POST
I would check that truck for a vacuum leak. Especially if it's worse when cold. Sometimes a vacuum leak will cause the iac to act erratic. First check all vacuum hoses and connections. Then check the intake gaskets. To do this you can either use intake air cleaner spray and spray the intake gaskets along each side and front and back and listen for rpm change. Or use torch gas along intake gaskets and listen for rpm change.
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Monday, November 16th, 2020 AT 11:37 AM (Merged)

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