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964 Drivability Issues


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Hi, my name is Chris and I'm trying to solve an issue with a friend's car.

It's a 1990 911 Carrera4 964 3.6 upped to 3.9L, Motronic DME, standard shift 5 speed.

Fairly recent rebuild with maybe 5000 miles on it.

I've spent about 12 hours so far testing and trying to sort out and issue of idling smoothly and the engine suddenly dying.

DME Inspected Repaired as well as a spare that is in working order tried.

Wires all test out as good and no obvious damage to be seen.

Intake rubbers and vacuum lines all intact and clear.

Hall effect switch is in good order.

O2 sensor checked and wiring double checked to be working properly.

Fuel pressure is good 47-55psi.

Air/Idle valve has been cleaned and functions properly.

Fanbelt is factory replacement, in good condition and switch is working properly.

The MAF (Flapper/Barn Door type) had some corrosion and wear but has been repaired to specs but made no difference with the problem afterward.

The engine goes into limp mode when MAF is unplugged and plugged in works well when it doesn't load/starve and die or just die like being shut off.

The issue happens all across the rpm range at any given time as a hiccup when driving intermittently as if the key was switched off and back on again.

Engine can be made to keep running by hold throttle open at about 12-1500rpm but will act as if it is loading up or starving

(fuel pressure increases to 55psi as this happens) and if throttle isn't opened a little more on lower idle or not far enough to begin with and held steady, it dies.

Seems to be after the cold start loop has opened since it idles perfectly when cold but once warm all bets off.

When I left it today it idled perfectly for nearly 5 minutes and suddenly shut off.

No codes stored now.

Previous codes from the Durametric Enthusiast version software at various times were;

O2 + to ground, Throttle potenteometer fault (no TPS on this model) and Hall Sensor.

All these were addressed and codes cleared.

I do have the ignition switch to check out next Saturday as well as install a fresh but used MAF since to other was in such a sad state.

Any directions pointing toward a possible solution would be greatly welcomed.

TIA

Chris

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Just to refresh my memory, where is the hall effect switch located that you tested? Been years since I worked on a 964 and only seem to remember two PMG's(TDC and reference sensors) that read off the flywheel.

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Hall effect switch or sensor is under the shaft driven distributor half.

The crank sensor he recently replaced.

The Hall was my first suspect as it seemed an electrical shutdown but,

after keeping it running by holding it just off idle I could feel/hear the engine load up to almost stalling

and then it would go back to normal idle or whatever rpm I was holding the throttle open to...

..I felt it was more a fuel/air issue but at this point, I'm at a loss.

Edited by Chris1837
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crank sensor replaced , why? is it factory? air gap for sensor checked?

this car does have a throttle position switch, is it adjusted correctly?

The MAF (Flapper/Barn Door type) had some corrosion and wear but has been repaired to specs but made no difference with the problem afterward.

how was the above performed, once the band gets corrosion it is almost impossible to clean it. was the spring taken off and did it get back in the exact position?

also I would replace the DME relay just because

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crank sensor replaced , why? is it factory? air gap for sensor checked?

this car does have a throttle position switch, is it adjusted correctly?

The MAF (Flapper/Barn Door type) had some corrosion and wear but has been repaired to specs but made no difference with the problem afterward.

how was the above performed, once the band gets corrosion it is almost impossible to clean it. was the spring taken off and did it get back in the exact position?

also I would replace the DME relay just because

The Crank sensor was replaced (with a Gennie Porsche Part) because he was guessing at the problem before I ever got involved.

The air gap is good on it and on the Hall switch also.

No TPS on the manual shift. Tiptronic only.

The MAF I wasn't going to attempt to repair as I haven't had very good luck and the PCB looked too far gone.

He cleaned it well and only repositioned the sweeping contacts back about a 1/32 so it travels on a fresh track now.

He did not take the spring off but I have on other meters of the same type in the past to adjust fuel curves and had no problem getting it right where I needed it.

It gives all the proper values now where it did not before.

He still has a replacement due in tomorrow.

He's put 3 new DME relays in, all of which work just fine.

I heard through the grape vine that he has it working properly now since he re-installed a highflow exhaust that it had on when he got the car.

He had not told me the exhaust that was on it was not what was on it when it ran well.

I do suspect there were multiple problems with the O2 sensor and the MAF as well as a restricted exhaust flow.

I'll try to put some closure to this by tomorrow evening.

Thanks Magcuda.

Edited by Chris1837
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you are right on the TPS, sorry a little rusty on these. check the full load and idle switch

is the cat blocked on the other exhaust?

I backed away from the job as even after I showed him it was a matter of a fuel/air problem (with the loading up/starving symtoms) he insisted it was ignition.

It's been my experiance that when a control module goes bad; it just doesn't work.

His was working fine up until the engine loaded so much that the engine just shut down the spark.

Truth is, IMHO, that it is like most drivability issues and a combination of several problems resulting in an issue.

The MAF was in very bad shape with corrosion, his airbox wasn't fitted very well and of course the restrictive exhaust.

I don't believe the cat was blocked but more so the system was designed to work with a 3.6 liter engine

and the extra capacity of the thing being increased to 3.9L was a partial factor in the whole problem.

Anyway, thanks for the responses and when I get to putting the 914 back together I'll be back for more reading and maybe a picture or two.

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