auzivision

Simply put it's either lack of or incorrect fuel or spark. No start what so ever I'd hunt spark first. Start run die, start die... I'd hunt fuel first.

That said, fuel can be a lot of things like relays, brains, pumps, valve, filter,damper, etc. and a simple check/monitoring of the fuel pressure will help rule in/out. Also, part store free loaner tools are perfect for the job.

My wild blind guess is a near failing fuel pump that is over heating then cooling off.

fraggle

I like the fuel pump idea. If it's old it's a cheap swap, too.

DanielD

Based on what I've seen I too am leaning towards a fuel problem. It always spins a bit even when it doesn't want to start; it just cranks, spins and then dies.
As I've had these issues for a long, long time. I'm thinking something to do with wiring or maybe a relay. Would a fuel pump take years to completely fail? Also, when its cold outside it runs fine.
I'll see about checking the fuel pressure with a gauge, I haven't done that yet but I expect to see a problem there... That will confirm its on the fuel side of this equation. I'll keep you posted as this moves along.

auzivision

My fuel pumped acted up before SITM a few years back. Got it running by swapping the horn relay and drove down and back no problem. It worked great for a year or two, except one or maybe two other minor fare ups.

Fast forward a couple years, on one my way to SITM, died several times. Roadside repairs (MAF swap, relay swap) appeared to fix, but not really just gave it time. Third time it died we called wrecker. After a two hour tow to 928 Specialists, it fired right up and drove fine. I swapped fuel pump anyways and haven’t had a problem since.

I've heard the pumps have a heat overload kill protection system. So yes, more likely to fail on hot days. Same thing happened to my truck last week on an all-time record hot day here in Indiana. Went to a bar, had a few beers, waiting till it cooled off, drove it to a repair shop. They diagnosed a fault pump the next day and kept five bills of mine.

nexty226

Mine has the same fever. On mornings it starts fine but a few hours later its a pain trying to get it started. The rpm goes up and down ,so low the engine dies and I have to start it one or two more times or even have to push the gas pedal a littler bit so it runs ok. I found the return line strangled. I repair it and change the fuel pump, because it was dead. He stayed about 6 years or so in the garage. The vaccums seems all right, what should I check next?

Rob M Budd

Munk's still has my 928.
They found a vacuum leak. I'm not sure where but Chris Munk said he'll repair that test drive it some more.

Rob M Budd

Finally got the 928 back and it runs fine.
No more hesitation, stalling, surging when restarted while hot.

Chris at Munk's Motors had the 928 for a few weeks and spent a considerable amount of time trying to replicate the issue. Since the symptom appeared only when the engine was restarted after it had already been up to operating temperature, it took a while for it to rear its ugly head.

A vacuum leak was evident and repaired but had no effect on the issue at hand.

Injectors, fuel pump, filter, lines, canister, O2 sensor, relays, distributor cap ,wires, coil were previously replaced or checked out fine.

With a fuel pressure gauge hooked up and visible from the driver's seat while the issue was going on, there was no deviation in pressure.

Chris suggested we send the ECU out for testing. $50 for the test and $400 if it fails and need replacement.
The ECU passed the test but we were told the ECU was not the correct one for the 91 S4.
For $400 they sold me the proper ECU.
Chris installed the new ECU and the problem went away. They performed no less than 10 test drives bringing it up to operating temperature, letting it sit for 15, 30, 45 etc minute intervals.

Since getting it back I've used it for several errands around town and now feel confident the issue is gone.

The original ECU must have been replaced by a previous owner. I never bothered to check the part number on the ECU so that is probably the case.

I've also heard the ECU for 87-91 are all the same. So perhaps the difference in the two could have been naught. However when the old one was replaced, the issue went away.
My thought is that; if they were the same and since the trouble was intermittent, perhaps the old one would have eventually failed when tested over a longer period of time.

In any event, Chis at Munk's Motors took the time and effort necessary to duplicate the issue, verify the fix, and get the 928 back into my good graces.

DanielD

An ECU? I've wondered about that but I haven't checked the part number. Mine is still doing the same thing with all this heat we've had. It's happening every day now. I guess the pn is an easy check. I'll get that done at least.
It's been doing this every summer since I bought it so whatever it is it is a very consistent failure mode.

fraggle

I hjave one from an '89 in my 87. No issues