svo-ed

The car is a 1984 Euro 928. It starts right up on the first try. It runs smooth and is very responsive, however, once the car is running if you rev the engine to say 3000 RPMs the engine will drop back to idle, even if you keep the gas peddle at the same position. If you try to give it gas as it drops down in RPMs, the engine stumbles and/or slightly misfires or has almost no effect.

This happens at any RPM 2000, 2500, 3000, 4000. The throttle responds instantly, then it is impossible to hold a steady RPM. Needless to say, the car is not drivable.

The car did not run when it was purchased, so just running is a huge step forward.

I have installed new Bosche plugs, installed a new fuel filter, polished all the grounds to mirror finish, installed a new air filter, new large diameter hose and "O" ring around the MAS, new temp 2 sensor, and have replaced a number of questionable vacuum lines.

Any ideas? I am thinking Fuel Pump? I was told by a mechanic friend who is by no means a 928 expert that it may be the TPS, but I am not sure there is a TPS on an 84? (seems like that started in 85-86?

Thanks,
Ed

The Forgotten On

1984 Euro cars have LH so they have TPS just like the 85-86 cars do. So you can replace it, if it doesn't help then you have removed one of the causes and can keep searching. A bad engine ground can cause problems with the fuel injection so you should clean those as well. Hope you can get it working right.

svo-ed

So would a faulty TPS (which I have read is not like a typical modern day TPS) cause the car to not hold a fixed RPM?

I have also read that the TPS is quite a pain to change. Most say to do a top end refresh while your at it.

What exactly is involved with the "Top End Refresh"?

Thanks again.

Opelotus

I once held the idle switch closed on my engine (US '84) while opening the throttle to see what would happen and it performed much like you describe... engine revved to XXXX RPM then returned to idle, then immediately revved up again, etc.

If you can hear the injectors shut off (light ticking should stop) when the RPMs drop then I would guess TPS as mentioned. When the idle switch (or TPS is your case) is closed, the ECU will cut the injectors if engine RPM is above ~1200 IIRC. This is supposed to be for coasting, however if the idle switch is stuck closed you would get an effect as you describe any time the engine revved >1200 RPM.

You could try unplugging the TPS and retest for that condition before you replace it?

svo-ed

Very interesting....so if I unplug the sensor, the sensor would be "open" rather than stuck closed?

My understanding is that the TPS has two switches. one for WOT and one for idle.

Could the car be driven with the sensor unplugged?

And how would it effect the idle?

Thanks for the input.

Mrmerlin

follow the testing procedure in the WSM to test the TPS at the computer connectors in the passenger footwell

svo-ed

I installed a new TPS but this did not correct the problem.

Mrmerlin

yeahbut, did you test it first?

It almost sounds like you should invest in a WSM before you buy any more parts.

davek9

About two weeks ago seen this on an '87 seen similar issues; first it would not idle (RPM would go up and down), swapped the MAF with a known good spare (mine) and that smoothed out the idle.

However when we held the throttle at ~2k or 3k RPM it would die like the fuel was being cut off, checked fuel pressure and it was good, replace EZF relay, no change, replaced LH relay and it improved but not totally.
Swapped the LH w/ known good (thanks to AO) and that fixed the problem, the MAF and LH were sent to JDS Porsche (North American agent) for a rebuild. I also noted the MAF and LH both had stickers that someone else had attempted an unsuccessful repair in the past.

Dave