Behr 964

I haven't posted much on the forum apart from an initial introduction along with some photographs of my car. I have however used the forum extensively and it's been of tremendous assistance with researching the various concerns/issues/broken things/sleepless nights that I have experienced in 964 ownership to date. I have also followed some truly entertaining build threads.

The one thing I have to ask about is a recent issue with my car that resulted in a happy ending, the only problem being I'm not sure what fixed it!

To set the scene, in January I had a complete top and bottom end rebuild along with a whole host of other work done. This was all performed by a local specialist shortly after I bought the car. 6 weeks ago I started to experience idling problems. Mainly that the idle would undershoot then try to recover dramatically. Most of the time it would recover, other times it wouldn't catch it in time and the engine would stall. This got gradually worse to the point that I was suffering from hesitation during acceleration. It felt like I was running out of fuel. This became increasingly less intermittent and more problematic to live with. After a couple of days I decided enough was enough.

Following research on Rennlist, I removed and cleaned the ISV then reassembled it. This didn't help at all (the ISV wasn't really dirty). I then read about the effects of disconnecting the battery to perform a part-idle adaptation. To my surprise the car quickly started and settled down into a nice idle. As soon as I started driving it again the problem reappeared. The same as before.

I didn't have much time available so bit the bullet and went back to the specialist. They ran a diagnostic and found a knock sensor fault but that was about it. They tried a new ISV, took the car out for a drive and deemed it fine. I picked it up that night and it stalled within 2 miles. Time for me to get my hands dirty.

I then did some deeper research on the Rennlist forums and made a list of things I needed to do. I should say at this point that I deem myself a relatively competent amateur mechanic, albeit one with very little free time, hence the reliance on a specialist.

So with the help of my research I made a list of everything I needed to attack and also put some parts on order:

Check for vacuum leaks
Check leads and wiring
Check distributors (rebuilt in January)
Oxygen sensor?
Replace the DME relay (nothing in history to say it's been changed)
Replace the 25 year old fuel pump (as above)
Replace the fuel filter (was changed in February, but hey - why not)
Check the AFM wiring (well known bulletin for early cars)
Replace the gas cap (rubber seal perished so needed doing)

I started by replacing the fuel pump. This was simple (I have a lift) and the threads here were really helpful (mainly in finding the pump!). Fired her up and no difference, straight into poor idle even after 30 minutes of running. I then changed the gas cap and installed the DME relay (I wasn't expecting gas cap to fix it, merely maintenance). Fired her up and again, no difference.

Not wishing to give up I removed the airbox cover and set to changing the fuel filter (stamped late 2014 which corroborates that it was done recently). This is when I noticed the bolts securing the airbox to the AFM (4 x M8 bolts in a square flange pattern) two of them were very loose, one was barely tight. I took them all out and checked for a gasket in between. This was present and looked ok. I put a small dab of grease back on the gasket to help tack it in place and gently teased it back in between the airbox and AFM. I replaced the bolts, tightened them and then changed the fuel filter. At this point I fired her up and hey presto, great idle. After a few days of driving the fault has still gone she is running like a champ.

The question is, was it the fuel filter partially blocked or the air leak between the airbox and AFM causing varying resistances in airflow at the AFM? Bearing in mind that the fuel filter was changed 4 months/1000 miles ago and has very little resistance to being blown through now that it's on the bench. Could it be that I disturbed some wiring - perhaps the AFM? Even so, the fault is still gone. Any input or opinions from you are all welcome.

Here are the threads and articles that were useful in my fault finding efforts just in case they help someone else:

Good thread with troubleshooting info from Steve W.

https://rennlist.com/forums/964-foru...low-rpm-5.html

Fuel Adaptation (disconnect battery)

https://rennlist.com/forums/964-foru...e-battery.html

Info on DME and an annotated picture of engine components

http://www.964uk.com/html/FAQs%20-%20Engine.htm

Technical bulletin regarding MAF wiring from Porsche manual

http://www.pcarworkshop.com/images/b...0590geolab.pdf

Other search keywords I used:

Idle bounce
Idle undershoot
AFM (reposition wiper)

Tore-B website

http://www.bergvillfx.com/index.php/...4-and-993.html

But finally, thanks to all here who contribute to make this a great friendly resource.

Behr.

RallyeChris

Any air leak before the AFM is un-metered, so in theory should not cause any poor running issue. The engine isn't aware if it, so no issue, with only the possibility of un-filtered air entering the engine. It is possible something got into the new fuel filter, restricting flow. Replacement of the fuel filter may have eliminated the blockage, if there was one. If the connector for the AFM was corroded, you could have had a low voltage issue at the AFM. This could certainly cause your described issue. By "messing with" the connector, the connection could have been improved and eliminated any low voltage issue at the connector.

jevvy964

As chris says pre afm air leaks shouldnt cause a problem but maybe as the engine was shaking during idle it opened up the gap between the afm and the airbox allowing more free flowing un filtered air in which confused the afm a bit making it flap around a little. Might be worth a small dab of threadlock on those bolts to stop them coming loose again, be a world of hurt if one gets sucked into the engine.

Kappi2907

Disconnect the battery for at least 30min and try again. Resetting the motronic can solve the problem.

boxsey911

A friend had a stalling problem with his C2. It was solved by replacing the airflow meter.

18T_BT

Is there a good place to get a new AFM? These buggers are hard to find and are a bit pricey.

crg53

You can start HERE , they always carry a lot of Porsche parts.

No affiliation............

18T_BT

Thanks! I saw that already. It's used so doesn't necessarily guarantee that it will work long term and new they are twice that. I also see that it's the same part number as the 930's. Wondering if it's worth switching to MAF system and remap the DME.

Behr 964

Hello everyone

I'm now really glad that I started this thread. The symptoms I reported in the first post have gradually crept back in the last few months and 500 miles of driving. This time the part-throttle hesitation and stalling is even worse than before, whilst the idle issue is there but less pronounced than it was. I just have a sneaking suspicion that my fuel filter is suffering.

The questions are therefore - would it be unprecedented to have to clean the fuel tank on a 964? I just can't help but feel that there's a contamination issue. If so, is the tank relatively straightforward to remove? I'm in Hazmat so have no issues with the dangers of working with fuel.

Potentially a breather issue?

Am I missing something else and is this a coincidence - is there something else that could be causing this stalling/hesitation other than what I have already done and researched? I appreciate that's wide reaching question.

Any and all help sincerely appreciated.

Behr.

LPMM

Fuel Pressure Regulator? Fuel Injectors?

speedbagger

did you check all the plug wires for damage/broken boots/arcing (assuming your plugs are good)?

what about the coils?

good luck.

dhc905

Try this (Solved my idle problem):

http://www.durametric.com/porsche964faultcodes.aspx

Or get a true diagnosis.