Mechanics

BAD IDLE SPEED

1991 GMC Sonoma • 6 cylinder 2WD Automatic •

I asked a question about a problem I have with my truck: GMC Sonoma 1991 4.3l 4x2, low idle speed, I received a quick response from you, thank you very much but it was a very short answer indeed: clean throttle body and check idle air system. I can't find help in the repair manual about these particular tasks. Is there any possibility that you can help me a little further with some clues or small details on how to perform these tasks?
Thank you very much.
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JoseMLopez
December 28, 2010.



Yes, that is a very basic resposne. What I would suggest is this. First, check around the engine for damaged or disconnected vacuum hoses. They are about 1/2" in diameter and provide a route for engine vacuum to travel. If there is a leak, it can cause a low and rough idle. Next, as far as the throttle body, I think what the original post ment was to try a cleaner that you mix with the fuel in the tank. If you stop at a parts store and ask for one, I'm sure they will have a few to recommend. The idle air control valve is a different issue. It is mounted on the throttle body (on top of intake). It's purpose is to maintain a steady idle. When they go bad, usually you will experience a looping idle (up and down).
Let me know what you find with the hoses and if the injector helps.

Joe

Thank you Joe for your help. This is the thing: my GMC Sonoma 91 starts ok, runs fine for a few minutes and several miles, when warm starts the problem: at traffic stops I have to put it in neutral or park and press gas pedal because very low idle speed, sometimes even turns off. I have changed myself lots of parts: tps, pcv and egr valves, put fuel cleaner in tank already, went to the mechanics they charge me almost $300, checked compression, fuel pump, all ok but couldn't fix it, just yesterday to be sure I changed myself injectors, no luck, I am getting desperate don't know what else to do, I am going to check the hoses like you say, more carefully because already did that too, maybe I missed something. The thing is I have taken good care of this truck even changed eng and trans, new paint job plus a lot of parts, but this particular problem is going to make me throw away the truck that otherwise is running fine and still looking good, it's a shame.

Tiny
JoseMLopez
Jan 3, 2011.
Have you checked the EGR? Also, have you checked the intake gasket. In 91, there were problems with the intake gaskets.

I changed EGR myself, I don't know if it's possible the new one is faulty, and yesterday I changed one gasket when I replaced injectors, when I removed fuel press meter/fuel regulator, do you think I have to change all gaskets in tbi unit? Is it possible fuel press meter/regulator make this problem if faulty?
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Tiny
JoseMLopez
Jan 4, 2011.
No, the injectors should be fine. When you replaced the EGR, was the intake clean where it mounts?

That is a good point, it's a fact that mechanic work can't be done in a hurry, you have to dedicate time and patience, I have to admit when I installed EGR valve I didn't apply this rule as well as I should, that combined with the fact the egr valve is a little difficult to access, it's possible something is wrong, I will have to remove it again and check if it's clean, but to dedicate the time for this I will have to do it on weekend. I will let you know, thank you very much for your help and your patience with me, unfortunately I can't find this kind of help with the mechanics around this town, and I have tried several shops already it's not uncommon for me to have to finish the work they do to my car after they charge me a lot of money, for example I let them do once transmission maintenance, after that the car was leaking fluid, I went down to check, ALL the screws in the trans. Pan were lose, if I didn't check myself I would have ended with a broken trans.

Tiny
JoseMLopez
Jan 4, 2011.
Hay, you can ask anything you want. That is why we are here. I will keep my fingers crossed for you. I know they are a pain to get to and I wouldn't want to have to do it twice. However, it isn't uncommon for the intake to carbon up where they mount. Please let me know what you find. If that isn't the problem, we'll figure it out together.

As far as the shops where you are, they are everywhere. I wouldn't trust many of the ones I work with to change my oil.

Good luck and let me know what you find.

Joe

Hi, Joe it's me again with my old GMC Sonoma, low idle speed problem.I am going to check EGR valve like you told me but I need to ask you a question about something I noticed and is bothering me big time: every time this low idle speed start, at the very same time I see oil pressure going down too, sometimes dangerously close to the red line, I wonder if it's normal that oil press go down because of low idle or God forbid, it's the other way around: idle speed happens because oil pressure problem. Thank you again for your help.

Miguel

Tiny
JoseMLopez
Jan 8, 2011.
THe idle speed will play a big role in the oil pressure. If the idle gets low enough, the pressure will drop. Just make sure the oil is full. I feel confident what you are seeing is expected because of the low idle.

Hi, Joe, I removed EGR valve to check as you told me, it's clean, unless the new EGR valve is defective, the cause for the low idle speed is not this valve. I checked all hoses again, the only thing I found is that months ago while changing some of these old hoses, there is one, is not a flexible hose but a plastic tube L shaped that goes from the manifold to the TBI unit, it broke, couldn't find a replacement, they don't have at the autoparts store, so I had to replace it with a flexible rubber hose, don't know if this can cause a problem, I don't see a difference but again, I am an auto technician, I mention this to give you all the information I can even the small details in hopes that it can be useful to you to help me solve this problem. Thank you very much for your attention. If I was closer to your town I would have driven there and pay you what it's worth to fix the car, I am sure that you would be able to fix it right, because the mechanics around this town, can't trust any of them.

Tiny
JoseMLopez
Jan 11, 2011.
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