Question80000 miles. I bought it a month ago and have put 2000 miles since. New plugs, wires, and oil filter. Shortly after getting the truck it began to idle a little rough. When you would let your foot off the brake from a stop and let it roll it would sort of lurch forward with a pulsing action. I took it to my mechanic and he said that the computer said that there was a random miss fire. So he pulled the plugs out and said that they were a little fouled and that he had cleaned them up and it had improved the situation. Sort of. He said run premium fuel with good injector cleaner. Drove 2 tanks like this on a road trip with a 5500lb trailer for 600 miles. Reached my destination strong, even through the mountains. Next day I let vehicle idle for about 20 minutes and check engine light came on. Refueled with premium and injector cleaner for unloaded ride home. Light still on. Drove half of the way home and decided to run regular. The light then goes out. Get home and the situation remains the same, still shaky idle, but at speed it runs smooth. A week later the light comes back on. Shortly after that the temp gauge goes up real high. Bad water pump, fixed that. The mechanic said that the rough idle could be due to clogged injectors and that the system should be cleaned. He said that the code he was getting was a miss fire on the number 4 and number 6 cylinder. An express service place down the road had a special on fuel injection cleaning, $47 I think. Told him what the problem was and he said that he wanted to check to see what was going on before he started. He gets a code saying miss fire in #3. Pulls plug out and there is a decent amount of oil on it. Cleans plug and replaces cracked boot. Still idling rough. Puts compression gauge and says that the compression was low and that the engine was bad. He said it could be a bad valve or a burned piston. Obviously I am getting a lot of mixed signals here. What do you think?
AnswerGee Eric,
I'd sure hate to muddy the waters with more signals...without even touching the truck.
I like the idea of the compression gauge, but it needs to be used on every cylinder, and then compare the readings against each other.
Now, if #2 mechanic tested all cylinders, and found that one cylinder to be considerably lower than the others, and adding oil before a second test didn't improve the pressure, what he said sounds very possible.
There is another test, similar to a compression test, where you can isolate where it is leaking.
Use the spark plug adapter, and compressed air.
On each cylinder in turn, at tdc compression stroke,for that cylinder, apply shop air pressure. Listen for leaking air in the throttle body(intake valve leak), exhaust pipe(exhaust valve leak), and oil fill cap(piston or ring leak into the crank case).
If you have excessive air sounds at the throttle body or exhaust pipe, you might get by with a valve job only.
A valve seating on carbon intermittently could cause the intermittent problem.
A leaking piston or ring, or a burnt valve, should never clear up and run good, so you may have a spark problem, or a dirty EGR valve, causing a large vacuum leak.
Which leads to another test. Connect a vacuum gauge, and see if the needle jumps all over.
Steady is what you would like.
Good luck,
Van
