Hello,

My engine occasionally goes into some kind of safe mode when hot in traffic. I can drive on the highway and as soon as I pull up to a stop light it shuts down and the engine won't rev over 1500 RPM. If I pull over and raise the hood for 5 minutes or so, it will let it rev free again for awhile.

Specs:

87 NA
Replaced the thermostat with dealer supplied OEM
Replaced coolant temp sensor(the one tucked below the alternator)

Any ideas?

 

Disconnect the TPS plug. The car will on the whole run normal. Might idle a little less than ideal. See if the problem goes away.

The other thing might be a fuel problem. But you'd need to tee into the fuel pressure line with a gauge to do that. At idle you'd see approx 28psi. With key ON, engine off, you'd expect to see 37-39 psi (fuel pump check connector has to be jumpered to do that).

To check the fuel pumps full output you'd just deadhead the gauge to the pressure line out of the fuel filter and jumper the fuel pump check connector with key ON. Pressure should be what the FSM says. Memory says 64-80 psi ???? for a non turbo. Higher for a turbo. See FSM for exact figures.

On thing you might do is idle the car and pull the vacuum line off the FPR to see if the line has vacuum on it. Should.
Those figures would represent fuel rail pressure as seen by the fuel injectors.

Driving would vary because the FPR is vacuum controlled, but would be in the 30's on the whole. Like if you let off the throttle while driving it'd drop to the 28 figure, but if you stomp the pedal it'd go to the 39 figure. Normal driving somewhere inbetween.

I think your problem is a TPS plunger sticking problem. I don't KNOW that. Just guessing. If the TPS is disconnected and the problelm goes south, then I was right.

 

In addition....the S4's don't have a safe mode, if I remember correctly.

 

Is there a way to fix a sticking TPS, or is it an item that needs to be replaced?

What would be the best way to diagnose the issue...drive it to the point where it is stuck at 1500 RPM and start unplugging things until the engine goes normal?

Which of these parts is most sensitive to heat? I suspect that it is heat related as when I raise the hood for awhile it seems to go away after 5 minutes or so. Occasionally when I am in an area that I can't pull over it will go away after driving in fifth gear(I have determined that you can go about 55km/h in fifth gear without going over 1500RPM)

I didn't get to drive it a great deal yesterday, but it seemed more pronounced after replacing the temp sensor. Probably not enough driving to say for sure, but it was odd that it occurred right after getting off the highway. It was like clockwork. I was driving fine, at about 40k, and as soon as I slowed down and restarted for a light it happened.

 

Nah. Just unplug the TPS and leave it disconnected. The car is very driveable without a TPS in the system. That's true for series four cars.

If the problem recurrs, then it was not the TPS. End of story.

If not the TPS, then try the fuel suggestions I mentioned.

 

Thanks for the help. I'll try disconnecting the TPS and the thermocouple and isolate where the problem is coming from the next time I'm stranded at the side of the road.