Can You Solve This Mechanical Challenge?

[911cedric]

Hello everybody,

here i come again with a persistant engine problem;
maybe you guys could have interestig suggestions.
I have an american 1996 Carrera 2 cab, running terrible, and i can't find the reason.

This car runs rough when cold or hot as well. the engine shakes A LOT and runs heavily. It shakes so bad that you can ear the exhaust and the clutch release bearing shaking too. Idle is stable, oil pressure is great, no overheat. shows no leak.
The mixture is not balanced, at cold especially you can smell it.
I couldn't for sure talk about MISFIRE, because ignition seems not faulty and engine performances are normal.

The motors "cough" really bad, and i can't find the reason.
There is no smoke, the motor has very good driving performances, but the low regime is really poor; so you can feel the car shaking during deceleration at low speed. I mean when you give 3000 rpm you couldn't say that there is a problem but at idle it's dramatic. It's sometime very difficult to start the car.

I don't have any diagnostic device for now as my durametric just died; so i have to go oldschool with a multimeter.

I did a lot of maintenance on that car as i bought it in poor mechanical condition.
Here are all the tests and engine repairs i did:

- spark plugs change
- rotors + ignition caps change
- changed the intake distributor seals because of a leak
- changed the fuel pump
- Fuel filter replacement
- cleaned the complete fuel system
- Oxygen sensors change
- DME relay replacement
- heat/ temperature sensor
- measured all cylinder compressions with 11.5 (perfect)
- checked the distributor belt; all rotors work
- checked the injectors one by one
- checked the heat sensor wiring harness

i checked with somebody in town the car with a KTS301 Hammer,
and it appears that my new temperature sensor is dead.
this was all what he had on his KTS.

I measured today the Sensor wiring harness and finally the sensor itself
that showed no resistance value(ohm); so, it's dead.
Anyway, this sensor should only have influence on fuel injection at cold but not when hot. So i don't think it's the key of the problem. I will change it anyway monday.

what i did not yet and should do:

- checking exhaust system components if they are clogged especially the cat and the mufflers (but difficult to evaluate)
- testing the spark plugs wires one by one with an ohm meter
- testing O2 sensors alimentation/communication with a multimeter
- testing fuel pressure or changing the Fuel pressure regulator.
- checking the DME unit itself for potential defects (in a Porsche center)
- testing the engine wiring harness + the complete DME wiring harness
(no recall seems to have occured on that car anyway)

I 'm also interested in "the ignition switch unit", but i don't really know its function in the ignition process. could somebody explain?

i have an appointment at my Porsche dealer on 7th of juilly for a full electronical
checkup in case it's not solved by the time.

I hope i will find by the time.

Please, Let me know if you have an idea, it could be really helpfull.

cheers

[budge96]

911cedric ,all those tech hammers and stuff are great but you need to get back to basics w/
how your car operates..
The sounds and running problems and hard starting are clear indicators as Don mentioned
likely your dual mass flywheel is seperating giving false readings and subsequent codes.
If the reference and crank sensors are not getting consistent signals off the flywheel toothed
chopper the thing will fire eratically...period.
Your really fortuneate it starts at all, likely as the engine warms and the rubber loosens it
may get worse..allthey need do is change position slightly and you get poor running ..period.
Sounds like time for a new or replacement flywheel,I would try and check allthe sensors too!
You'll get it it just needs diagnosing, Bert

[911cedric]

thank you for your diagnostic,

this could be the reason...

i just wonder how to diagnose this.
it's a pretty serious repair, and i have to make sure it's the real cause.
i would have to remove the engine.
i should maybe first have a look on the flywheel position sender?

there should be one or two flywheel sensors that send signals to the dme.
if there would be a real problem with the dual mass flywheel, i should probably have a fault code from the dme?

please let me know if there is a reliable way to diagnose this.

Cedric