I have a 94 Oldsmobile Cutlass Ciera, (3.1L V6 GM3100 engine). When I start the engine it idles very rough and usually stalls if I put it in gear. Eventually once it warms up I can drive it, but when I do drive it’s extremely sluggish on accelerations and acts like its about to die. At constant speeds it runs mostly ok. We figured it was a vacuum leak, but we’ve tried every test we can think of to find it and talked to a couple mechanics and tried their tests and it passed all of them. Supposing it isn’t a vacuum leak, anyone have any ideas what it might be?
Have you checked the fuel pressure? How about ECM codes?
Fuel pressure was the first thing that came to my mind too. Dying fuel pumps imitate vacuum leaks. They both result in too little fuel available for the air getting drawn in.
MStoobs Sr. here. We have not yet tested the fuel pressure, however I wonder if the change in performance with engine temp is consistent with low fuel pressure. In addition, when cold, the engine stalls with what sounds like a light backfire and the exhaust smells of unburned fuel. We cleaned the MAF sensor and the intake air temperature sensor and checked the air filter (clean as a whistle). Since it is pre-OBDII I need to figure out the incantation to get any error codes, however it does not even have a lit check engine light. Regarding possible leaks: we tested by spraying starting fluid around the most likely leak spots with no reaction. I even clamped off the vacuum hose to the brake booster to rule out a leak there.
Maybe it would be a good idea to run a compression test and make double-sure you’re dealing with an engine that’s not losing a cylinder or two due to low compression.
A precursor to a compression test could be a vacuum check with a vacuum gauge. Personally I’m not a big fan of using aerosol ether to find a vacuum leak but that could just be my dislike of the stuff.