Your next diagnostic challenge! (Fix Posted)

[Jeremy airjer W]

97 GMC Savana full size van

5.7 liter

I was the lucky second guy to try to solve this one. The customer complained of a hesitation/slight lack of power on initial acceleration when the vehicle was cold. He also stated the check engine light was on. A scan for codes revealed lean codes for both banks. The symptoms where not reproduced during the test drive but some interesting info was.

The vehicle was smoked check for vacuum leaks, and the MAF sensor was replaced before I took over. I rechecked for vacuum leaks both with the engine cold and hot and none where found.

The Data I got form the test drive revealed a couple of clues. Long Term fuel trims where at 25% and short terms where well above 10% (+/- 10% is the acceptable threshhold). During acceleration short term fuel trims would spike to over 30%. At idle the upstream O2 sensor would hover at extreme lean, less than 20 millivolts, with an occasional rich spike, over 800 millivolts.

The O2 sensors would quickly respond to a shot of carb spray but I decided to replace one to see if it was just being lazy and got the same result.

Fuel pressure was at 60-62 psi KOEO and over 55psi KOER. Fuel pressure held with the key off. The fuel filter was free of any obstructions.

All other sensor data was fine, including coolant temp, intake air temp, map, TPS, and MAF readings.

So what do you think?

After the repair was made the O2's where back to normal, short term fuel trims where +/- 8% max (the first time I had seen them go negative) and long terms where well under +/- 10%

I'll post the answer in a couple of days.

[Scott K]

Well it sure sounds like a fuel issue. Bad/old gas? Injectors? Wrong fuel pressure regulator?

[deadeye]

Im going with a injector issue. Spiders are the worst injector design ever.

[muddpuppy]

Well I am no auto mechanic but with everything seeming fine per your tests. Along with lean codes I have gotten with newer vehicles. I am going with a bad gas cap restricting the airflow.

[overdalimit]

EGR valve?

[katophil]

fuel filter seen 4 last week

[skolfoppa]

Accelerator pedal position sensor or a squirrel nest in the tb.

[Jeremy airjer W]

fuel filter seen 4 last week

[Jeremy airjer W]

Accelerator pedal position sensor or a squirrel nest in the tb.

All Sensor readings where normal and airway is unrestricted

[croixflats]

Bad smog pump?

[gonefishing70]

Fuel pressure reg. or a small hole in a fuel line.

[shizzy]

did this vehicle have any aftermarket/"performance" parts on the engine?

[skolfoppa]

plug issue.

[Jeremy airjer W]

did this vehicle have any aftermarket/"performance" parts on the engine?

Nope

[ottertailguy]

Clogged catalytic converter

[Jeremy airjer W]

Clogged catalytic converter

Back-pressure was barely measurable.

[Jeremy airjer W]

It ended up being bad injectors/CPI unit. We opted to swap it from a Central Port Injection (which has all eight injectors in one location and they routes fuel through tubes to poppet valves which spray the fuel onto the intake valve.) to a Muti-Port injection (which is essentially the same but locates the injectors where the poppet valves would be eliminating them).

[deadeye]

That's what I was thinking. Those poppets are terrible, they like to stick one way or the other. I wish that setup was available for my blazer (95). I have gotten to replace it 4 times with 3 different brands. Finally got a decent one from borg warner. The conversion unit is a much better setup.

[Big Buck Buster]

I have installed them on a couple motors now. I think that they even increase the performance of the injection system. Great product upgrade.

BBB