Daxmanian

Hi Rennlisters,

I've been reading here for quite a few years now but never posted.
So here goes :

I've been plagued by a minor engine problem on my '94 Porsche 993 for more than a year now and have not found a solution yet.
When I start the engine it starts normal, but after about one or two minutes the engine tone changes and it loses a bit of power. Engine sounds like one of the cilinders is not getting fuel or not getting a spark. Engine makes a "put-put-put" sound, like an old VW beetle.
Duration of this phenomenon varies, sometimes a few seconds and sometimes up to two minutes. When the engine behaves normal again it doesn't come back the entire ride until I shut down the engine and restart it a few minutes later.
It only does this in the first minutes the engine runs. Never later than the first minutes. It idles solid and you only notice the problem on accellerating.
When the engine returns to normal again it's instant, like an on/off switch.
Lots has been done on the engine the last years but the problem persists.
Dme relais swapped, swapped dme from other 993, still same problem. Changed plugs and ignition wires, new ignition caps and rotors, belt in dual ignition changed, final drive under driver seat changed, dual coils changed, compression on cilinders measured, all within tolerances. Ground (earth) contacts checked and cleaned, braided ground wire from battery changed, oxygen sensor changed, fuel pressure valve changed, all connectors on engine checked for corrosion, MAF sensor cleaned, ISV checked and cleaned. Throttle position sensor checked and measured. Hooked up Durametric diagnostic equipment, no faults. Checked all sensor readings with Durametric while driving, all normal values.
So you can imagine I'm running out of ideas now.
9 out of 10 drives the engine behaves this way. Any help would be highly appreciated.

NC TRACKRAT

A few suggestions: Have you changed the fuel filter? If not, change it and cut it open with an exhaust pipe cutter to check for debris. It could be water or trash in the gas tank.

Daxmanian

Yes, this has been changed with the maintenance about a year ago.

Daxmanian

I forgot to mention in my initial post that the injectors were also swapped with no results.

Daxmanian

No more suggestions?
Would it help if I showed a picture of the car


Attachment 841539

NP993

Can you post a Youtube clip of your engine doing this?

You could try shutting it down while it's making this noise and pulling a spark plug from each cylinder. If you've got a dead hole you may be able to see it on the plug (wet with fuel, etc).

Are you sure you don't have a vacuum leak? Clamps on the intake boots are all snug?

LexVan

New MAF sensor. Not a cleaned one.

NC TRACKRAT

Doubt that this could be the cause but worth checking: I don't know if it affected ROW vehicles but, here in the U.S., there was an engine harness recall for all '95 Carreras. You might check to see if your car was included.

Daxmanian

Hey guys, thanks for the suggestions.

Car has got the new wiring harness.
I'm going to try to make a youtube clip when the engine is acting funny in the next few days.
MAF sensor replacement is a possibility.
Vacuum leaks not checked yet. The vacuum system is holding its vacuum. After switching the engine of and cycling the resonance flap 30mins later it cycles normally so I think its holding its vacuum.

The engine only acts up when its in the warming up fase, never when it's stone cold and neither when it's fully warmed up.
Does anybody know what changes in engine parameters when the engine is going through the various fases of warming up? I thought I read somewhere on this forum that the DME goes through several stages before reaching the fully warmed up stage.

Anyway, thanks for trying to help me out

SwayBar

It sounds like a bad lifter.

After you shut the car down the oil bleeds out, and when you restart it, it takes the lifter a while to pump-up again with oil.

NP993

Vacuum for the Varioram system is different than what's commonly called a "vacuum leak" in the intake. This is when air is introduced upstream of the combustion chamber, causing a cylinder to either run lean or not at all. Only place on a 993 to check is the rubber intake boot at the base of the varioram manifold, and the injector block (the plastic part the bottom of the boot sits on). Easy to check, but I doubt this is your problem.

Ed Hughes

I'd clean MAF before replacing. Mine was dirty and resulted in a nasty flatspot, so you never know what behavior could come of it. CHT sensor is new too?

CRC MAF cleaner is what you use. Pull it, and spray the wire. Don't touch the wire with a rag or anything.

Daxmanian

Bad lifter,mmm didn't cross my mind yet. Is this something I could check without taking the engine out? Engine only behaves strange about 1 or 2 minutes after start, before that no problem and after a brief period problem stops again.

I cleaned the MAF already and swiped the sensor carefully with a swab leaving some grey deposit on the swab.

CHT sensor was checked with durametric displaying actual values. Normal behaviour, engine temp rises normal during warmup, no funny spikes.

Ed Hughes

Bad lifter would make a racket.

Daxmanian

No strange mechanical noises so I can rule out the bad lifter?