'92 Chevy Problems


I own a '92 Chevy Lumina (3.1 L v6 engine) with 96,000 original miles. It ran well until recently.

Question 1: At complete stops, the automatic transmission appears to ‘down shift’ hard. Earlier checks showed fluid level ok. Any idea what’s causing this?

Question 2: After recent fill-up, engine developed loud knock behind firewall. Could it be as simple as ‘bad’ fuel and is it possible that it’ll disappear with the next fill-up? Higher octane? Counterperson at auto parts store recommended changing the oil with a heavier grade oil that may take care of it. Any chance that it could? Does it sound as though the engine is beginning to fail? I do not want to sink a lot of money into this car.

Any thoughts would be greatly appreciated.



On older cars that had transmission work done, the bolts would unscrew and the transmission would be loosely attached to the engine, causing knocking (around). The harsh downshift may be caused by some sort of detent cable being stuck, but I don’t know if this still happens these days. The old kickdown rod would rust in the wide open throttle position and cause the same problem. There was an elongated slot in the linkage that would allow the throttle to close and leave the rod behind.


There was no transmission work done (I’m original owner). What are the next logical steps?

By the way, on Question 2, the knocking only appears upon acceleration, not starting or idling.


Is there any white smoke associated with the knocking?

I’m wondering about the vacuum modulator for the tranny. It’s basically a diaphragmatic control that uses the engine’s vacuum as a variable to tell the tranny when to shift/downshift. It the diaphragm is leaking, not only will the trannny misbehave, but the fluid can get drawn into the engine intake and it knocks like crazy on acceleration. In the old days it would also product white smoke, but that may be captured by today’s cat converters, I really don’t know.


This sounds like the classic sticky lock-up converter. There is a mechanism that locks the torque converter above 40 or so miles per hour. This helps save gas. It’s supposed to unlock as you slow down, but if it gets stuck it kind of “breaks loose” as you come to a full stop. The classic cheap fix is to unplug the locking solenoid wire.


A knocking on acceleration is generally a problem of octane too low, compression too high, or timing being off. Try a higher octane fuel for one tank and see if that helps. This may be pointing to another problem, but will tell you if it’s a pre-ignition type of knock or something more serious. If higher octane helps I’d get a mechanic to check the timing and compression.


No white smoke. Where is the vacuum modulator and is it a large effort to repair/replace?


Where would the locking solenoid wire be? Thanks!


I’m going to check with the higher octane and see where that leads us. Thanks!


Somewhere on the transmission. A transmission shop should be able to disconnect it in a couple minutes. Even a general mechanical shop should be able to do this.


www.autozone.com has a good explanation of the modulator, but I was unable to find the location…probably due to the tiny size of the graphics on my CRT. A manual may show it.

In a transverse mounted engine it may be hard to get at. I can only hope it isn’t on this car.