Manager Forum

Hi everyone. Got my own head-scratcher today.

This one came from the used car lot down the street. Engine surges real bad most times when accelerating from a stop. OK at cruise. No codes. Fuel trims are all normal when symptom is happening. Customer already replaced plugs, coils, MAF, throttle body and who knows what else. Looking at scan data, all appears ok except engine goes into open loop when symptom occurs. AF sensor is new with no change. Scoped pedal position sensor because it really felt like a TPS problem of days gone by. That checked out fine too.

WTF? I'm missing something here and brain quickly becoming mush. I need a spark to get me in the right direction.

Scope all 4 injectors at once and see what there doing during the surge event. Testing volumetric efficiency might eliminate some possibilities.

Put your low amps probe on the fuel pump and watch the waveform when it happens. Check your air boots real good. Had one doing that once. Ended up having a crack in the accordion boot that only opened up during accel. Scope your crank and cam sensors. I know its alot of work, but...

Great ideas. Thanks.

Checked fuel pressure and volume during symptom...good. Checked the boot for cracks, but I'll check again cuz it's easy to miss. Haven't scoped cam/crank yet. Tomorrows another day and the owner knows this is a weird one.

Thanks for the input.

Just thought of something, does it have VVT?

Yes. VVT is present.

does trans pick up right away on accel surge? maybe low on fluid?

It picks up initially, then falls on it's face, then picks up, then falls. If you hold the throttle steady it will continue to do this off and on thing all the way up to cruising speed. Really whacked; feels like fuel is being taken away or not available, but pressure is good and trims are spot on.

Well folks, this one's fixed. Turned out to be in the electronic throttle system. There was a short in the harness that only occured when the engine moved slightly under load. The short was on one of the two throttle position sensors in the throttle body. Didn't catch it the first time through and nearly dismissed the throttle system since the other shop had done a swap out. It took two of us on a road test to catch it. I'll be getting paid for this one today.

great find. do we ever get fully paid on jobs like this one? thanks for posting.........