96 SVT stumble, no chk eng light?

[thecoop]

Sup yall

question,
my buddy's 96 svt stumbles under hard acceleration, no check engine light like i said.

hooked a scanner up to it today and monitored data as we drove, when it stumbled:

TPS 93.3%
Load 98%
all 02's at about .850 mv
s/t fuel trims at about 11-14
fuel system 1 goes into open loop

then when you let off,
TPS 20%
Load 23%
all 02's go to 0.000 for about 5 sec
s/t fuel trims -3
goes back into closed loop

weird thing is TPS never goes below 20% even when fully off throttle and the load calculation never goes below 23% and when rolling into it goes right to 80%

Are these things known for bad throttle position sensors?

Any ideas?

 

[thecoop]

also forgot to mention, it accelerates pretty good under light and moderate throttle input without stumbling.

 

[wmburns]

CLean the MAF

MAF is dirty. Classic sign. Fuel trim neg at idle. Big positive at load. AF will go lean at application of power. There is a delay for the O2 sensors to detect the lean condition and add fuel.

Use MAF cleaner.

It is normal for the load numbers to never go below 20%. Not as sure about the TPS. There are posts regrading how to index/check the TPS. Might be a good place to start.

Somewhat concerned about the "Open Loop" at WOT. Normally, the PCM would not go into open loop unless a sensor failed/no signal. The O2's going to zero may be the failed sensor and the reason for open loop operation.

Is this a custom tune?

 

[thecoop]

no custom tune. car is compeltely stock. I think the 02's going to 0.000 are a result of the problem since there are no 02 codes and all 4 go out at the same time. Never have I seen 4 02's go bad at once, not even on a German car! Lol.

Will try cleaning the MAF and see what happens.

Need to research more on the TPS signal and why its range is 20% to 93%, it never even gets to 100% under WOT...weird.

Thx.

 

[patman0911]

I thought the computer swithching to open loop at WOT was normal? A/F ratio goes rich at WOT and makes the O2 sensors useless to the computer.

I think your TPS readings are fairly typical. 10~20 degrees maybe when closed mid 90s when fully open. I think people used to try to fine tune it to equal 100 degrees when opened but I don't think it matters as the computer calibrates itself to scale to the range of values it sees. I noticed mine sitting around 18 degrees closed when I had my scanner hooked up the other day, didn't think to look at WOT though.

 

[Twista]

change the spark plugs and wires =]