1998 Chevrolet Truck



November, 20, 2008 AT 5:27 AM

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,



2 Answers



November, 20, 2008 AT 7:28 AM

Very thourough diagnosis, I have to admit, good job.

Fuel trim and injector pulse is controlled mostly by O2 sensors. Although engine temp. Must be at closed loop operation.
I would check the engine temp with the scanner, make sure it at operating temp.
I would scan O2 sensor feedback with scanner, they read in millivolts.
Probably time to change the O2 sensors even though it is low mileage, they are 10 years old.
I would remove and clean the TB and IAC.
I also think you are right about cleaning the injectors.

Finally I would check backpressure on the CAT, to make sure it is not clogged or partially clogged.

Whenever you store a vehicle, or do not drive much, use a fuel stabilizer, to prevent varnish build up in the fuel system.

Just for grins, I would have the charging system and battery tested at most autoparts stores for free, they provide a printout (assuming everything is normal) and this will impress potential buyers.



November, 20, 2008 AT 9:16 AM

Yeah, the battery is brand new (the old one wouldn't even charge after sitting for so long which I kind of expected to happen) and alternator output is good.

Something I forgot to mention in my first post was that fuel pressure during engine run at idle has a fluctuating needle in the 50's range, not steady. I have resisted the notion of pulling the upper intake up to this point, but looks like I might need to. That way, I can clear any clogged injector/poppet issues and check the FPR at the same time.

And yes, I do dump fuel stabilizer in the boat/lawn mower, etc. But thought I'd be driving this vehicle often enough to keep fresh fuel in it. : )

The fuel system is operating closed loop (both banks) and the two upstream O2 sensors are fluctuating as they should. I don't have graphing capability or I'd include a picture.

Also forgot to mention that I carefully cleaned the MAF sensor and screen with the appropriate MAF cleaning aerosol. I'll pull the IAC and clean it too.

Thanks for your comments,


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