Carpokes Porsche Forum

First I want to apologize for writing a novel here, but I wanted to make sure I gave all the info.

TLDR: Idle, hard cold start and intermittent miss issue, with a ton of stuff already checked and replaced.

I’m chasing down a lingering drivability issue on my 1994 Porsche 968 (Tiptronic). The car runs well overall, but I’m dealing with an intermittent hesitation at low RPM and light throttle. It shows up most when pulling away from a stop or just easing into the throttle. Once you’re into moderate or heavy throttle, the car pulls strong and feels completely normal (usually).

At idle, when the engine is fully warm, there’s occasionally a slight fluctuation—maybe 25-50 ­RPM (the work I have done so far has improved this, most of the time is is steady now)—but it is missing, it is random, not a hard miss that happens every time the engine rotates. It’s subtle, but it’s there. The issue tends to be more noticeable after heat soak, like in traffic or after the car has been sitting hot, but it’s not 100% consistent. The car starts quickly when it is warm, doesn’t stall, and doesn’t have any misfire behavior. It’s just this persistent low-RPM hesitation that hasn’t completely gone away. And the random miss.
One additional symptom: on a cold start (especially in colder weather), the engine initially feels like it’s running on two cylinders. If I shut it off, wait 10–15 seconds, and restart, it clears up and runs normally.

A lot of work has already been done on the car. The fuel system has been refreshed with a new pump, filter, and high-pressure line, and the injectors were recently cleaned and balanced. Ignition components including plugs, wires, cap, and rotor have all been replaced. The engine also had a full front service with timing belt, balance belt, rollers, and water pump. The coolant temp sensor has been replaced, and the alternator and battery are both newer.

More recently, I installed an aftermarket MAF. It was slightly oversized and needed to be sanded down a bit to fit the airbox properly. (purchased from 944online) After installing it, the car improved noticeably—idle smoothed out and the hesitation got about 85% better—but it didn’t completely resolve the issue.

I’ve done a smoke test and found several small vacuum leaks, which I fixed. That helped, and I think that I’ve found all the vacuum leaks as today’s smoke test didn’t reveal anymore of them. I’ve also checked ECU grounds and connector seating, and everything there looks solid.

The throttle position switch has been tested and is behaving correctly, with proper switching at idle and full throttle. Fuel pressure has been checked at the rail and looks good, and leakdown holds as expected.

The idle control valve has already been removed and replaced, so that’s no longer a suspect. Doing this also improved the cars idle behavior. Sometimes it is even running perfect. On Friday after installing the new idle control valve it ran perfect all day, smooth with no misses or hesitation. then began to miss again today.

what’s left generally falls into vacuum leaks, airflow measurement, lambda/O2 feedback, or fuel control issues. Today I went back over most of the things I’ve done, checking the new temp. sensor, distributor, rotor, plugs, wires, grounds, and more smoke testing, which revealed one last leak. (I Hope)

At this point, I haven’t replaced the O2 sensor, and I haven’t gone as far as pulling the intake manifold to inspect every vacuum line underneath it. The EVAP system also hasn’t been fully inspected yet.

What I’m trying to figure out is where to focus next. The car is clearly much better than it was, but something is still slightly off. I’m wondering if this pattern points more toward a small remaining vacuum leak, an aging O2 sensor that’s slow to respond, or possibly inconsistency from the aftermarket MAF. Or something that I’ve failed to even consider. I'm starting to feel like I'm chasing my tail and getting nowhere.

If anyone has run into a similar “mostly fixed but not quite” hesitation on a 968, I’d be interested to hear what finally solved it. I’m especially curious about common vacuum leak points that are easy to miss on these engines before I commit to pulling the intake apart.
Also one last thing: there are no currently stored fault codes.

Had the same issue on my 94 when purchased last year. I noticed a bag in the boot with odds & ends in it and two other DME relays. Before purchasing this transaxle car I saw a lot of DME diagnostics posts.

I purchased the f9 solid state DME w/ diagnostic & pump prime since it’s not a drop top. Also got the diagnostic fan relay while I was there. POOF! Cured. Issue gone, haven’t looked back.

https://www.ftech9.com/new-products/993 ... BsnMH-GL2h

I'll give that a try. I may have replaced that relay when I first got the car, but I can't remember. we'll try it and see what happens.

Intermittent problems are always a challenge.

Sounds like you have been looking at all the right things... so just a few additional thoughts.

If the age/condition of the O2 sensor is unknown, I'd probably replace it regardless. I don't normally recommend throwing parts at a problem, but O2 sensors are often neglected and changed less frequently then they should be anyway (i.e., every 60k miles).

After chasing similar issues in another car (450SL) I finally installed a wideband oxygen sensor in the car so I could actually 'see' the AFR when it was stumbling, etc. They are invaluable for issues like this -- if for no other reason than to confirm it's an AFR problem. I don't keep it installed -- I just added the port to the exhaust and capped it, so I can check the AFR with a wideband whenever helpful. (Comes in very handy for smog testing/tuning too.)

Since you seem to be all over the usual suspects (vacuum leaks, temp sensors, weak ignition, fuel pressure, etc.) I'll throw out a few other ideas. Engine/chassis grounds can cause all sorts of issues, so if you haven't cleaned yours, that's a no-regrets and free thing to do -- will do the car good even if not the cause of this issue.

Harness problems also come to mind, including in particular the speed and reference sensor wires and the injector wires. They both suffer from frayed wires under the boots. A free and easy shade-tree trick is to just shake those wires while the engine is running and see if you get stumbles or changes. In a similar vein, cracked solder joints in the DME are fairly common and can create infuriating intermittent issues. That's another free/easy check with a magnifying glass and bright light.

Not sure what to make of the aftermarket MAF that you had to sand to fit? That sounds a little wonky. I'd be tempted to swap back in the original and/or monitor the signal coming from the MAF to see if anything weird is happening when it stumbles.

Not sure if any of that helps, but it sounds like you have a good handle on what to check, so just throwing out more possibilities if helpful....

Great minds think alike. I have an O2 sensor on the way! I will try the wiggling the wires trick and see what happens. The only reason I haven't cleaned all the grounds on this car is because I'm not really sure where all of them are. I had a stereo install last year and I need to get in there and see if they stacked a ground with the DME ground. if they did then that could be part of the problem. maybe it's giving some kind of weird feedback. thanks for the info. I'll keep plugging away at it.

Tom briefly mentioned it but if you get nowhere with the other stuff, seriously consider pulling the ECU and touching a hot solder-tipped gun to all the joints (or any with little lines on them)

I've done this twice in 18 years and each time it has solved little annoying issues, some similar to what you describe