Surging engine idle at low speeds / deceleration

November 26, 2006

I've got a 1992 GMC Yukon 2-dr 4WD, 350 TBI.

This "surging" issue has been going on for quite awhile, but I recently had work done to it including a new exhaust (cat including) and was hoping the problem was related to the converter being plugged. It wasn't...

Here's what happens: If I'm driving at very, very low speeds (think grocery store or mall parking lot- 5 mph or so) the engine has a "surging" idle to it. Feels like someone is on/off/on/off/on the gas pedal. Only happens at these low speeds which is right around 800-1000 RPM, just barely above idle. The other time it happens is when I'm decelerating (coming to a stop sign or light) and the engine rpm passes "through" this zone; only this time it sounds like the engine is about to die just for a split second then it corrects itself.

I can accelerate through this zone and not feel it (although 2nd and 3rd gear seem very underpowered) and otherwise the engine runs and idles very smooth.

It almost feels like a vacuum advance leak, only this engine is electronically advanced. Timing is right on because the mech at the exhaust shop adjusted that. They also resealed the throttle body because they said there was a leak in the idle control something-or-other. Since I've had them do the repairs the truck does run alot better and the surging is not nearly as bad as it was but it is still there and the mechs are pretty unsympathetic, I.E. "Come back by and we'll take another look at it".

Really hoping someone could lend me a clue!

Thanks in advance

GM-Tech · November 26, 2006

Sounds lean, like a possible vacuum leak somewhere. Did they physically pull the throttle body off the vehicle? The gasket between the TBI and the manifold are known to leak. Usually, they cause a higher than normal idle speed, but I suppose it could depend on the size of the leak.

November 26, 2006

Sounds lean, like a possible vacuum leak somewhere. Did they physically pull the throttle body off the vehicle? The gasket between the TBI and the manifold are known to leak. Usually, they cause a higher than normal idle speed, but I suppose it could depend on the size of the leak.

<{POST_SNAPBACK}>

Here's what part of the Labor Description says on the invoice,

"...Diagnosed very hot exhaust and lack of power. We found that the timing was off by 8 degrees. Corrected the timing. We also found the throttle body leaking, we will need to reseal the throttle body."
"Throttle body assembly - remove, Install and Overhaul -[includes: Install repair kit and adjust idle speed.]"

So based on that I would assume they physically removed it.

Could it be distributor related by chance? I had to replace the pickup coil a while back, perhaps I didn't put it back together correctly? That's the only other thing I can think of...

Otherwise should I do the 'ol carb cleaner vaccum leak detection trick?