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Golden Wrench
Golden Wrench, Shop Foreman
Category: Car
Satisfied Customers: 348
Experience:  ASE 30 Years of fixing severly broken cars and light trucks
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1989 GMC: idles..rough..a very low RPM to a Higher speed..injector

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A 1989 GMC Jimmy idles very rough, but will run well on the road. The idle is like an oscellation from a very low RPM to a Higher speed. I got a fuel cleaning/injector service but so far it still idles roughly although it seems to be improved somewhat. Do you have a probable cause?
I'm GW. Is the check engine lite on?
Customer: replied 8 years ago.
No there is no check engine light on. The truck has not been driven much. It is in the garage most of the time, but has run well before.
You may want to check for a vacuum leak at the rear of the throttle body gasket.
Customer: replied 8 years ago.
I went out this afternoon and checked every vacuum hose that I could find that comes out of the housing where the fuel is being induced and I could not find any that appeared to be loosing vacuum. When I removed the hoses and covered the port they were connected to with my finger, I could feel the suction. I am not sure where the location is that you say to check for vacuum loss. It may be somewhere other that the connections I see that pull a vacuum, I don 't know. I really didn't see anything I thought was a problem. Maybe I'll need a mechanic with instruments and know-how to find it if it is a vacuum leak. I'll agree that a vacuum leak seems to be a likely cause because when the engine is ideling tha vacuum is considerably less than when the engine is at the running RPM. That could account for the fact that it runs OK on the road because while the engine is running faster it overcomes any possible vacuum leak.
The vacuum leak usually occurs when the base gasket pulls through. You can hear it. Or spray wd-40 at the back of the throttle body, and if the engine smoothes, it's leaking.
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Customer: replied 8 years ago.

I think I see the gasket you are talking about now. I sprayed WD-40 around it and after a short delay noticed the engine did run a bit smoother. I think it may be the gasket although the engine will sometimes run quite smooth on its own for a short time so I'm still not certain that is the only problem. As an expert, do you think the replacement of this gasket is a job that should be undertaken only by a trained mechanic or could it be done by a somewhat mechanically inclined person who can do some minor repairs and upkeep. The throttle assembly seems to be held in by 3 bolts into the engine block. If these are removed can you lift it to replace the gasket after disconnecting the attached hoses and linkages. My concern is that these holdown bolts have been in there for 20 years and may be very difficult to deal with.


I do value your help and will not skip out on payment!


Yes, you can, just be sure to clean the old gasket off the manifold anf tbi unit. Easy repair. GW
Let me know how it goes. GW
Customer: replied 8 years ago.
Thanks a lot. I don't think I'll be able to do anything until after New Years. I want to allow plenty of time.