six35

my 91 c2 runs perfect, unless i hit full throttle. as soon as i push the pedal all the way to the floor, it feels like the car is starving for fuel, the engine just doesn't pull anymore. but if i then let off to just a little before full throttle, it runs fine.
maybe anybody can help me with that?

Reinhard

springer3

The only difference between throttle completely down, and almost completely down, is the WOT (wide open throttle) switch. It could be shorted to something that is somehow putting the engine controls into safe mode. Try isolating the WOT switch, and see if things improve.

If you see the difference when going from 1/2 throttle to full, that could be fuel starvation. There is a procedure for checking fuel delivery and fuel pressure.

Is any part of the problem RPM-related? You could be getting an early rev limit out of the fuel pump relay - and that feels exactly like fuel starvation because it is.

Good luck, and be sure to post the results.

six35

well, the problem is not rpm dependend. i could be at 3000 and if i full throttle it, it feels like it doesn't want to move. i checked at the throttle body' TPS and it clicks at right off idle and at WOT. if i full throttle the car in neutral, i can just hear a few hesitations in the higher rpm, but that is only if there is no load on the engine. as soon as i am driving, at full throttle the car gets really sluggish.

thanks for your help!

Tom W

Does it matter if the car is warm or cold? Does it always happen? I had some similar issues with my 993 that were fixed by replacing the oxygen sensor.

springer3

OK, so it is probably not the rev limiter. That leaves the WOT switch as the culprit. Have you tried to isolate it from the DME, and see if the problem goes away?

If I understand the symptoms, part throttle is fine, 90% throttle is fine, but suddenly there is a problem at 100% throttle. At lower RPM, the power for 90% throttle and 100% throttle are very nearly the same (it is a very large throttle). Therefore, 90% throttle at 3000 RPM should feel about the same as 100% throttle at 3000 RPM. If everything is fine at 90% throttle and 3000 RPM, but terrible at WOT and 3000 RPM, the only real change is the position of the WOT switch contact. I would eliminate that before replacing the rather expensive O2 sensor.

six35

As you said, at 90% throttle, everything is fine, and at 100% the car feels very sluggish. i replaced the oxygen sensor about 5000 miles ago.
The only thing i have done with the TPS is to check if the screws are and if it clicks off 0% and at WOT.
What does DME stand for? and how do i isolate the switch from the DME?

thanks for your help!

DaveK

The WOT does not click off at 0%. There are two switches - the idle microswitch which clicks on as you hit 0%, and the WOT switch which clicks on as you hit 100%.
DME stands for some German word - but basically, it's the main ECU - i.e. the computer which looks at throttle position, air flow, engine speed etc. etc. and decides how much fuel you need, when it should ignite etc.

Someone might suggest something else, but one easy way to rule out WOT switch is to adjust your accelerator pedal so that it never engages. But I guess that is reducing your throttle position, so maybe that doesn't really prove it. I would guess there is an electrical connection to WOT switch so you could try to find that.

Since you thought there was only one switch I suspect you have only found the idle microswitch which is on the LHS of the throttle assembly (as you look at the engine). The WOT is on the RHS - it's not as obvious and takes a while to find.

JasonAndreas

Do you have a tiptronic or 5-speed manual?

Do you have access to a multimeter? To test the WOT switch you want to measure resistance across pins 2 and 3. It should read open circuit and when the switch clicks you should get slightly less than 10 Ohms. If that reads out okay you then take the cable connector off the DME control unit under the seat and measure resistance across pins 53 and 24.

DME stands for Digital Motor Electronics.

springer3

Regardless if whether it is working properly or not, somehow the WOT switch is causing a problem. Perhaps the switch is fine, but the DME reacts badly when it sees the WOT signal. It could be that the wiring from the WOT is picking up noise from the alternator, ignition, or other circuit. In that case, the WOT is just the path to inject the noise into the DME and cause it to go into safe mode.

When having electrical problems. first check power supply (correct voltage, minimal ripple and noise). If that is OK, start checking connections, especially ground connections. It could be a lose ground that is allowing the WOT to have a bad effect on the DME.

Regrettably, these things are not easy to troubleshoot by remote control. The first step is to go to WOT with the wires disconnected from the WOT switch (or possibly move the WOT switch so that it does not engage. I am not sure what the WOT tells the DME to do, but I guess it lets the mixture run a little rich to protect the engine from excess heat. Obviously you will not stay at WOT for long with the switch by-passed, but you need to try long enough to confirm that it is the switch causing the problem.

If it is the WOT switch causing the problem, you still need to determine if the switch is injecting noise into the DME, or if the DME itself is on the fritz. You can jumper the WOT switch at the DME. If that still causes the problem to happen all the time, it is the DME. If it runs fine that way, there is a loose ground or bad connection in the WOT switch wiring. Possibly there is a bad spark plug wire or a bad alternator generating the noise.

Troubleshooting at this level does take patience and a little skill. Go slow, be very careful not to do damage, and understand what you are trying to accomplish as you test different theories on what is happening. Troubleshooting is a process of elimination. Always try the cheap and easy things first - they are usually the problem anyway, and you avoid expensive mistakes if you can find something as easy as a bad alternator, a loose ground wire, or a sparking ignition wire. The DME is expensive, so check everything else first before trying to replace that. It is a really bad feeling to replace something expensive, and then learn it was a loose wire all along. Don't ask how I know. Good luck

DaveK

I don't know if it's the only thing it does, but I know Adrian has said before that it tells the DME to ignore to air flow sensor.

six35

thanks guys for taking the time to explain all the these things to me. i will start trouble shooting everything. as soon as i find out what the problem was, i will post it!

thanks

reinhard