Engine Performance problem
1995 Toyota Camry 4 cyl Front Wheel Drive Automatic 137000 miles
1995 Toyota Camry, 137K miles, automatic, 4 cyl. About 6 weeks ago the car did not want to start, and when it did, it was idling incredibly rough. Took it to a local repair place, they diagnosed a tuneup, so plugs, wires, and the cap were replaced. After doing the tuneup they said the car had developed a stumble when it was at a stoplight, and they said the car was losing fuel pressure. Fuel pump replaced. They found no other issues with it, driving home I noticed it was still stumbling a bit when idling after driving for a few minutes and the engine was getting up to temp. Took it back, they cleaned the battery terminals and said it was working again. I started it up, let it idle for about 10 minutes with no issues, then starting driving. Less than half a mile later, the car did the same thing. Drove it back, they eventually replaced the distributor and it worked a lot better for 2 weeks, although it would occassionally do the miss/stumble when idling again. Then yesterday, driving back and forth less than a mile, the car idled rough after starting it up and stalled out. It continued to run rough at idle and I had to drive with one foot on the gas, the other on the breaks to keep the RPMs up. After coming home, I tested a few things I saw online. I unplugged the ECT sensor when the car was idling and that caused it to idle worse, so the sensor appears to be working. I used some canned air to blow out the EGR vacuum modulator filters but no change. I also made sure the throttle body was clean and the air cleaner hose going to it didn't have any cracks. I can't see any vacuum hose cracks but I don't have the tools to really get down into the engine to check all of them. It's a rather intermittent issue; after all the work done on it it would still drop out sometimes at a stop light, but it was only for a brief moment and the car would not stall. It seems to only be happening after the car has been run for a bit, and I don't notice any issues while driving at highway or street speeds. The only other thing I have noticed that is different is that when the car is cold, immediately after starting up it will make a " fart" type sound for a second, but this does not persist on subsequent starts or happen any other time other than right after a cold start. It normally idles at around 700rpm when it has been driving for a bit, when it starts to stumble it doesn't lose RPMs noticeably until it gets worse. When it does stumble, you can see a very brief interuption in the exhaust. Also, when they replaced the fuel pump, you could hear it hum from inside the car, and when it stumbled, you could hear the hum of the fuel pump kind of drop off for a second.
From the symptom description, it seems there is a misfiring and the most likely cause are the spark plugs, its wires or the distributor cap. Since those parts were newly replaced, I would suggest rechecking them.
Quite often replaced parts that are faulty are overlooked when you start looking beyond them.
Check the spark plug wire installations and the spark plugs as well.