Can a bad distributor cause the engine to stumble especially on acceleration? I ask because about six weeks ago I replaced the distributor and last week the car started running like crap. At first I assumed it was dirty fuel injectors but the cleaner from Autozone did NOTHING. Here is what I have done: dropped the gas tank to change the fuel filter. Old filter was only a few years old. Checked fuel pressure. It runs at about 52 psi regardless of vehicle speed or load. Removed the fuel rail. Visually looked at each injector. They appear clean from the outside. No carbon. Did a compression check on the engine. Compression is equal on all cylinders @ about 180 psi/four compression strokes. Car has 155,000 miles on it. Checked for vacuum leaks. Gauge held steady at 15 lbs at idle. I am at 5000 ft. above sea level. Can't hear any leaks either. Wires and plugs are new a year ago but not Bosch or dealer wires. They are Duralast brand. The only code I got when running my Auterra scanner on my laptop is misfires. P0300, P0301, P0302. I cannot see any burn marks or signs of shorting out on the wires. Plugs look excellent. Champion brand.
The car runs better cold than hot. In other words it doesn't stumble as much. When I push on the gas from a dead stop when hot, it almost dies, recovers a little bit but runs......just like it is missing.

 

Could you have crossed spark plug wires when changing the distributor?

 

no. It ran great up until about a week ago. Here is additional tests I did today (day #5):
tested the EGR valve for leaks....holds a vacuum just fine. Isolated the EGR tube from the intake manifold, capped the opening and test drove the car. Still misses just as before other than knocking from no exhaust gas. Hooked that system back up.
Isolated the evap vacuum hose from the intake manifold. No difference.
Removed upper intake manifold, disconnected coil and fuel pump, had someone crank the engine. I visually verified EACH injector is working.
Drained the gas tank and replaced fuel with 2 Gallons of Chevron (come to find out my brother in law has been getting gas at Sam's Club). It did run better at first, but once the engine temp was up to 192 or so, stumbling and cutting out on acceleration or at idle continued.
Tested the PCV valve. It functions fine. No vacuum leak there either.
Disconnected the battery and waited 15 minutes, then reconnected. Car was warm at the time. Instantly still runs lousy.
I am almost out of ideas here. Honestly, I don't think my problem is ignition related. It acts just like rotton gas or a vacuum leak (of which I can find none). Now something that I don't understand: Over the past five days of working on this car, I'd go out in the morning when it's cold (40 degrees or so) and try to start it. It would crank but never turn over. ACTING as if it was getting no spark. (I knew the fuel pump is working cause I can hear it and have verified it's pressure, etc etc). So with the key still ON, I removed a spark plug and it is drenched in gasoline. I dried it off, connected the spark plug wire to it, grounded the plug and cranked the engine again. It IS getting spark. Every time I've done this test, the plugs are getting spark but still wet with gasoline. So I put everything together and eventually the engine would start but would take several cranks. So that is where I am at right now.

 

BTW: when "warm", manifold pressure = 9.2 at idle, engine coolant temperature sensor = 192-199, IAT = 100 degrees or so

 

Problem solved. It was a bad distributor. Even though the plugs were getting spark, it wasn't enough spark. A bad coil can mimic symptoms of a vacuum leak or lousy fuel supply. Interesting.

 

Yes, improper timing will mimic a vacuum leak symptom. Watch those aftermarket rebuilt 2.5 distributors. They are bad right off the shelf it's very common.