This is a new one for me. About a week ago the first symptoms appeared, the idle was hunting like crazy, 2k then 3k then back to 1k. The car would cruse fine, but every time I came to a stop it would be revving the engine. Then it started to buck at low RPMs if I gave it too much throttle.

Now the only way to put it in gear is to gently press the throttle until its 3-4k and put it in gear gently. It cruises fine, it idles fine, but does hunt a little.

What I've checked/done:
NO CELs
No misfires detected
Replaced Coils/plugs/wires
Cleaned MAF
Reset NV RAM
Attempted to run without MAF, same symptoms.

IAT/ECT/MAF all indicate normal in logs.

Right now the only ideas I have are to replace the ECT sensor and clean/replace the ESS sensor.

Any suggestions?

 

A faulty neutral switch can cause hunting , but not to the degree you are seeing .

 

They symptoms are just damn odd... If I jab the pedal it dies, if I rev it very slowly it's fine. Its almost like the change in RPM is the problem, but no ESS codes being thrown.

It's definitely something that feeds the computer, but I'm pulling my hair out trying to figure out what.

 

Throttle body??

 

I think you already explained the most likely candidate. (In my opinion)

If the MAF was stuck reading extremely low then this issue would get progressively worse as you opened up the throttle. The fact that the car acts identically without a MAF at all tells me that the one you have is worthless. Which should be anything but the case.

On a side note, our cars run without the MAF?

 

Yes, but in fail-safe mode

First thought I had was the neutral switch too. It usually will make quite a fuss at idle surging up and down. I only know because when I had a trans rebuilt they installed a sealing washer between the neutral switch and the case, which it doesn't have one from the factory. It was keeping the switch end from engaging the shifting shaft properly, which the result is the same as if it had failed.

A bit of a pita to get too, but easy to check once you do; on/off, continuity or not.


.

 

Replaced the ESS and cleaned the TB, no dice. After reving it a few times I'm starting to think it's a fuel starvation issue. If I put my foot halfway down while reving it <4k it will start to respond for half an instant then turn off like a light switch, above 5k it might break up and then rev or it might die.

Guess I'm gonna throw a spare fuel pump at it and see what happens.

 

So you're still doing all this zoom-zoom pseudo-science ignition and engine control stuff, yet you don't have the equipment to do a proper fuel pressure check before just swapping parts hillbilly-garage style?? This is why I always felt your turbo thread should be titled "highly improbable' rather than 'impossible'.

 

I actually have a fuel pressure sensor on my other 8, but it only gives a 0-5v out and it's tied into the CAN bus. I could move it over and use a data logger, but that's a pain.

Fuel pump flows fine... The tank level was above the fuel pump so I had to pump it down, worked like a champ. Also tried bypassing the fuel pump resistor, still same symptoms.

At this point a throttle position sensor failure, or a throttle pedal position sensor failure both seem likely.

As for throwing parts at it hillbilly garage style, well I learned it from watching the dealership. and they have all been parts I had on hand which vaguely matched the symptoms.

I'm really at a loss at this point though, it's really like something is dropping out when I open the throttle and the ECU is telling me nothing.

 

Swap over the pressure sensor beside the air pump ! Very good chance that's your issue .

 

Disconnected the BARO sensor and symptoms disappeared. Haven't driven it around the block yet, but looking good so far.

 

Confirmed, no relapses after disconnecting the Baro sensor. Thanks Brettus!

I have a working theory for what was happening and why. It seems to me the sensor thought I was driving at the top of Mt Everest and the Max load table was causing me to lean out past a certain throttle position/load. Before this I never thought that much about the Baro sensor, but now I know it can cripple the car.

 

Glad I could help . Remembered seeing similar symptoms on a race car a few months back . Surprising how dramatic the effect is when that sensor goes bad !