Erling G-P

Some of you may recall I had an engine cutout problem with my 928 last year shortly after getting it.

Shortly, what happened was that frequently when I pressed the clutch to downshift (or stop), engine revs would drop way below idle level; sometimes recovering, other times engine just died (but started right up again).

Original, long thread here:
https://rennlist.com/forums/928-forum/365783-my-engine-cut-outs-a-pattern-idea-emerging-long.html

Suspicions quickly centered around the Idle Speed Valve, and I eventually had it replaced with a new one.

Except for just one time, right after installation of the new ISV, the engine hasn't died this way for months, so I considered the problem solved.

That is, until last Sunday, where I took the car to a track-day for the first time.
Lots of fun, except for the cutout problem resurfacing during the event.

Unfortunately it continues in ordinary driving; today it was back with a vengeance on my way home from work; with the engine dying roughly every 2nd time I wanted to downshift or stop.

Ideas about how the harder track-day use could cause this to rear its very annoying head again are most welcome. I haven't noticed any other ill effects of the track-day use, except for some tyre wear, but that was expected..

I realise of course that it may be a pure coincidence and that it might have shown up again even without the track-day, but I can't help wondering about a connection.

Would be very surprised if the brand new ISV was faulty already after a mere 6 months, but don't know how long they normally last.

Thanks in advance,
Erling

AO

That sounds like my issues. I had very erratic idle issues and it sounded like the ISV was the issue. I replaced it. But it turns out my issue is somewhere in the harness. I know this for sure becasue by moving the harness a few mm, i can make the car surge, idle perfectly, or stall. Unfortunately I cannot predict which will happen. I have a new harness on order and will see if that solves my issues. It had better!

AO

It's also interesting to note thay are both 91 GT's. I also know of a another 91 (S4) that is having erratic electrical issues.

Erling G-P

Thanks for the fast reply Andrew

Very interesting info. Forgive me for what is probably a stupid question; but when you talk about 'harness', I take it it's a fat bunch of cables somehow involved in the engine running/management ?

I guess the much harder cornering and braking I was doing on track compared to public roads, could maybe have shifted the harness and thus giving me the same problem as you ?

Edit: Where do you move the harness around ? Would like to try the same myself...

Cheers,
Erling

AO

Yes. I'm talking about the LH harness that controll the injectors and also carries the knock and crank position sensors to the LH/EZK brains.

The harness comes out the passenger side firewall beneathe the coolant resevoir. It then routes up behind the passenger head. At that point it splits into a few branches. I should point out there are 2 ground points at the rear of the engine below the airbox. Take a look at these first to make sure neither is loose.

After 17 years, and 135k miles my wiring was very brittle. I had to move my harness when i pulled the motors over the winter. I'm quite certain that some of the insulation fell away from some of the wiring or the wiring simply cracked as a result of being moved. If you don't have to move it, don't. It is sometimes better to leave things alone.

IcemanG17

One problem I first noticed at a track day was my slowly dying fuel pump....when I hit a bump or went downhill it would briefly stop pumping causing an odd stutter.....turns out the brushes were worn internally causing the problem...however your does sound like a ISV or vacuum problem

Erling G-P

Just a quick update guys:

Spoke to a Porsche mechanic today, who also happened to be present at the trackday, with his 911. His suggestion was that the ISV probably needed cleaning. I was surprised, since it has barely been in the car for 7 months, but he explained that what has most likely happened, is that the trackday driving has dislodged a lot of the crud typically present in the inlet system, and it has ended up in the ISV.

Sounds plausible to me, since it would explain both that the ISV replacement apparently solved the problem, and that it suddenly reappears after a few stints of track driving.

Bummer of course that you have to take so much apart to get to the ISV. Until I can get that done, I have just been out to try the ol' WD40-up-the-vacuum-tube, to see if it might have any effect.

Have also inspected the readily accessible hoses and vacuum lines, to see if any had jumped off or appear to have leaks in them, but can't find anything suspicious.

Will keep you posted.

Cheers and thanks again,
Erling

AO

Try the WD-40 trick before you take your intake apart. I also thought my new ISV was the issue, so I disconnected it (easy to do on my SCer setup) and it did seem to run better for a little bit, but then I realised that was not the issue when the car stalled under cruise, and I could cause it to die by moving the harness.

mark kibort

I dont know how many times ive posted this, but if you drive a fairly stock ECU managed 928, YOU NEED TO DISCONNECT THE IDLE CUT OFF SWITCH. (not the WOT side, but the idle side) this goes for the S4s and early models , besides CIS
mk

mark kibort

There are two reasons that the circuit has no buisness being used on the track, and is the reason i have a disable switch for use on the track. as you know the circuit is a close loop design and is used to cut off fuel on decel (save gas) and eliminate the little popping decel sounds. however, being closed loop, the fuel is removed, the idles go below target and then fuel is added to bring the rpms up, and then it finds the home rpm level. in racing conditions (or track conditions) this 6000rpm to the find rpm home "mode" when the engine is hot, is extrememly difficult for the system to work correctly in, and often, almost always on the track, a 6000rpm, full load to no throttle lift, will result in a stalled engine.
the other bad result of this circuit is way too much rear wheel engine braking force. (especially if the engine dies, you dont know it ,and then you put it in gear and let out the clutch, as you approach a turn. This can lock up the rear tires for a moment.

its a subtle problem, that i only have issues with at the track. this is why ive installed a cut out switch to leave it disconnnected on the track.

mk

Erling G-P

Thanks for the info Mark. To be honest, I had no idea; being relatively new to 928s and being on track with it for the first time. As it was, the engine didn't stall on track, but while driving slowly in the pit area, after a few stints on track. For future events like this; how would one disconnect the cut off switch, if no disable switch is present ?

Cheers,
Erling

AO

You might be able to snip the line that come back for the idle contact, but I'm not sure if this would adversely affect other operations? One way that I know would work would be to ask someone with a SharkTuner to burn you an LH chip using the stock map, but ask them to uncheck the "Cut off fuel under decel" option on the parameters screen. I would think think this would be easiest way to accomplish this.

mark kibort

you have to find that white wire in the bundle coming from the ECU. ( i forgot the pin number it comes from but its an easy wire to detect by making a small slit in the wiring bundle) pretty easy to just snip it and put an interupt switch on the dash. Its nice to have on the street, as sometimes the car will do some hunting at idle even though this switch cutting the circuit makes the idle about 1-300rpms higher .
It doesnt effect anything else, which is good about the minor mod.

mk