968 rough idle, need help!
 

I really need help, my 968 has a rough idle. Hunts up and down and at the bottom the whole car shakes then rises again and then repeats. I bought a new idle control valve but no luck. Above 1000 tpms the car runs perfect.
Just spent 400 dollars at a reputable experienced Porsche shop to fix it but they were unable to find the problem and they had my car almost 3 weeks so I got tired of it and picked it up today.
Here is what they checked/tested......
1) checked ignition, new plugs, checked cap and rotor and coil and wires with good units.......checked ok

2) checked fuel pressure.....ok

3) smoke checked intake....no leaks .... ok

4) checked CO......OK

5) checked compression.....all cylinders 135 psi+-

6) checked basic cam timing......ok

7) checked exhaust temps......1 and 3 slightly cooler than 1 and 2....inconclusive

8) checked power supply to maf.....ok

9) checked throttle position switch.....ok

10) checked oscilloscope pattern.....ok

11). Used thermal imaging camera to find heat difference.....no problems noted

12) they said fuel injectors seemed to be firing

13) checked grounds


Let's say for a minute that their tests were ok, what else can I look for?
Do good CO levels mean the o2 sensor is in fact good?

Car has new fuel filter and new engine temperature sensor as well

Check engine vacum

Possibly head gasket leaking. Do you lose coolant with no noticeable signs

Vacuum leaks were checked with smoke machine and no coolant leaks.

The CO test is just an adjustment for places that do idle-speed emissions tests.
CO is an indicator of incomplete combustion...but fuel ratio issue would show up as a HC reading.

Try swapping the O2 sensor as I mentioned on RL...they are 3-5 year parts, and if you don't know the history of the one you have it may be time to do it anyways.

Maybe the shop will split the cost with you if it fixes it?

I have been chasing the same problem for last two weeks. I have just installed a new Bosch O2 and it hasn't done it for two days now. Fingers crossed

135 psi is very low for a 968, I wonder if that is what they really saw?

It's easy to replace the injectors. I run more modern EV6 injectors in my 1992, Bosch 0280155831, 0-280-155-831, listed as 32 lb/hr. If yours have not been cleaned it might be worth replacing them with EV6 or if you must, clean the ones you have.

I always replace an old O2 sensor also rather than troubleshoot with an old one. You should be able to disconnect it for troubleshooting also.

An unlit propane torch can be helpful, see if propane smooths out the idle.

The MAF could be suspect, they can be rebuilt or exchanged with a Volvo MAF that flows a tiny tiny bit worse (you would not notice) Bosch 0 280 213 012, 0280213012, probably $40 used on ebay or local scrapyard. Good to have a spare in the event of. Volvo 960 V6 or Saab 9000 Turbo.

If need be, Injection Labs in Boulder can rebuild your stock MAF. You could try cleaning yours first, and I'll assume there is no goofy hot air intake on the car. MAF problems should throw a code. Should.

-Joel.

That's great to know. Thanks.

Thanks Joel, this shop is very experienced and the tech working on my car has been working there on Porsches since 1984. They told me the compression was good and never mentioned any problem with that. I would think they would have said "hey your compression is really low". I may call them and inquire.
I have cleaned my MAF, and I will probably order a new o2 sensor. Thanks for all of the info I'll look into spare maf and injectors.

With engine warmed up and hunting idle, disconnect electrical connector to idle-valve. What does it do!

With steady idle, use assistsant to hold throttle if necessary, what is vacuum in intake manifold?

With steady idle, what is MAF output voltage?

Yea the 135 psi seems low my 172,000 mile 968 runs 195psi with recent head gasket.

I called them about it and they said that they only cranked the engine over enough times to see if all cylinders were roughly the same so I guess it wasn't really a "full"compression test.

I will have another one done in the future though.

I did disconnect it a while back but I don't remember now exactly what happened. if I recall correctly the engine quit.

Sounds like a vacuum leak between the MAF and intake-valves. Extra air is getting in there that's not measured and the DME doesn't inject enough fuel to match. With the idle-valve connected, it's trying its best to cut off idle-air to compensate. When disconnected, it stops in the middle position and lets through some air, which is too much for the already-lean mixture.

Measuring actual vacuum in the intake will tell us a lot. And the MAF output-voltage. It's like knowing how much gas is in the tank, you need measurements and a display to know for sure.

So this is a really experienced tech (over 30 years) but he halfassed the compression test so now that needs to be done again. Got it.

An experienced tech would be aware of the bad idle you get with poor static compression, like in all those BMW V8s with bore erosion or all these newer DI cars with carboned-up intake valves. The 968 does not have DI nor nikasil bores but it can get low static compression due to scuffing caused by detonation (cheap-or-fake gas, hot weather). It can also be a half-broken valve spring. Honestly I cannot imagine getting most of the way to a decent compression test and then quitting when trying to sort out a problem like this. Having had the car for three weeks. What the heck?

Injectors can be sticky in such a way as to be more noticeable at idle and unless they have been serviced pretty recently the car would benefit from that, but you could diagnose this without pulling injectors if you wanted to.

-Joel.