Engine Performance problem
1998 Chevy Truck V8 Two Wheel Drive Manual 53,000 miles
This problem really started a year or so ago with my 1998 C3500 dually. I was going to sell this truck and gave a potential buyer a test ride. Everything was fine. I always drove the vehicle at least once a month to keep it in shape. Well, the next time I went to drive it, battery dead. No problem, I jump started it. And this is when the problem started. The engine was running rough and when driving, I could tell there was a loss of power. My immediate suspicion was stale gas so I put in fresh gas. No help. I checked ignition next, and the only thing I found was a bit of oxidation on the contacts in the rotor cap. I replaced it and the rotor at the same time. I put in a new $70 set of platinum plugs. No help. I borrowed my neighbor's IR thermometer and brought the engine up to temp and checked exhaust port temps and found a couple of them to be low. Started suspecting clogged injection for those cylinders.
I gave up for awhile but now I am back at it. After putting in a fresh battery.. it barely ran, then straightened back to where it was before. This time around, armed with better knowledge of fuel trim and having a scan tool (btw the CEL is NOT on, and no codes are pending), I've started checking some other things.
Fuel pressure is good, just over 60 psi during the two second run, falling back to about 58 and holding with KOEO. I replaced the inline filter anyway since it's easy and only $6. Fuel pressure did not improve, but it's already in spec anyway.
I've ohmed out all the injector coils, they are at 12.5 to 13.2 which is a good reading.
I've done a compression test to eliminate a bent push rod or something worse. When I pulled the plugs, I kept them in order and wrote down compression readings for the corresponding cylinders. The picture I have is very interesting. Remember, these plugs have seen very few miles. The three cylinders having the lowest readings, have plugs that look like they've never seen the inside of an engine. But even the lower numbers are not very low... just 'lower'. 195-215 is the range. The plugs with black on them were dry, not wet.
The fuel trims are essentially saying bank one is lean on long term, but around 0 percent on short term. Bank two is LT 25 and ST 50. These are percent values reported by this scan tool. Positive percentages indicate the attempt to correct for a lean condition. But the plugs in bank two look like they are running too rich.
I'm still thinking a good injector clean with high pressure air and cleaning solution (dealer procedure) is a good idea, but I wanted others to have a look at this picture and give their impression. There is no abnormal noise, just the increased vehicle vibration one would expect from a rough running engine.
Thanks very much in advance,
Thursday, November 20th, 2008 AT 5:27 AM