Mart993

Hi Everyone,

I thought I would ask for your advice one more time before dropping a lighted match into the fuel tank of my 993! In June I posted about a shake after startup which cleared after five to sixty seconds. Since then I have taken the car to several "specialists" and a main dealer but no one can give me an idea of the real cause of the problem or sort it. Items changed so far are:

Plugs (twice, was originally supplied turbo type) leads, rotor arms, distributor caps, DME relay, all earths cleaned, all DME plugs cleaned, alternator rebuilt.
Head temperature sensor, intake air temperature sensor, throttle position sensor, oxygen sensor, idle control valve cleaned, new clutch and DM flywheel
Air filter, fan belts (genuine Porsche) fan pulley halves, cam cover gaskets and various checks for air leaks. Injectors cleaned and new seals fitted, new fuel filter.
DME control unit independently checked, standard chip installed.

Once the shaking stops (it smells very rich), the car is fantastic, with brilliant acceleration, and when driven sensibly (not often!!!) 28 to the gallon. Car uses a bit of oil but no more than you would expect for a 993 (about 1000 mile/litre)

When it is shaking, the change over valve clicks open and shut, disabling the valve doesnt make any difference. Only fault code is "check engine light" but mine is a rest of the world car and doesnt have one.

Apart from the match, or taking the car to be exorcised, has anyone got any other idea as to what this fault can be. Any help gratefully received before the money runs out

Many thanks

Mart

crg53

Was the problem there before you had the injectors serviced?? Reason I ask , I had very much the same issue after my injectors had been serviced, except mine would do it on and off while I was driving, CEL would come on and the engine would run on only 5 cylinders for a while, then back to 6.
I fixed the problem by isolating the bad injector and replaced it with a new one.

Mart993

Hi there and thanks for the reply

Unfortunately the problem has been there before and after the injectors were cleaned. Thats not to say that there isnt an injector faulty either way. What is strange is that the shake starts about the same time every time, and just isnt random at all. Once clear the car runs fantastic! It's never missed a beat under load or cruising on the motorway.

Thanks once again

Mart

RS man

Sounds like you are in UK? Who have you spoken to so far?

techman1

Are there any other dash lights flickering on?
No other electrical issues?

Sounds very much like a flooded cylinder, or 2.
Does it do this on every startup, or only cold starts? The longer it sits, is it worse?

Wiring harness in the states has been covered by PCNA, are you aware of that issue? Possible one or more of the injector wires have been compromised?

Mart993

Hi there,

Yes I am in the UK, I have spoken to my local Porsche main dealer in Newcastle, Ninemeister, an independent in Leeds and even Adrian who wrote the book.

I have checked and the car has had the replacement loom. It does the shaky bit most times when its started from cold or hot, summer weather or the -18 that we had a few weeks ago. Revving the car doesnt make it go away, neither does switching on all the electrics to add load. However i turned on the ignition and left it on for a couple of minutes before starting. It still did it after the usual time. I also turned off the engine while moving, and bump started it (ie without using the starter motor) again it did the shaky thing. So it would appear to be something that maybe the ECU is checking - maybe

Very grateful for all the help and suggestions

Mart

Mart993

Oh sorry,

No other dash lights go on, the tachometer doesnt do anything dodgy. I have fitted a bulb into the check engine light fitting and it works as it should, and doesnt illuminate during running. I can even check the fault codes with it (so clearly the wiring is there even on a UK car!)

Wild isnt it!!!!

Mart

RS man

Did you actually take the car in to these shops or talk over the phone?
If you haven't taken the car in to them perhaps you should. Colin at Ninemeister has a 993 (albeit a highly modified RSCS) so if you haven't taken the car into him you may want to try that.

RS man

Just out of interest, would something like a bad engine mount do this (question for someone who may know)?

Mart993

Hi there,

I am in the process of taking a day off to go to ninemeister, they do have quite a good reputation. I did have the car at the main dealer, and three other independents. None of the test equipment showed up anything wrong apart from the main dealer which showed an earth fault to the DME relay which was due to a corroded earth lead/bolt. What really confuses me is why it should run so well apart from the shaky bit. Is there any posibility that the tracker/alarm or some other system is causing the problem?

Mart993

Thanks for the thought RS Man, not sure of the answer to this, however the engine seems quite stable in position, and it feels like a fuel or ignition type of fault. I will check it though - just to make sure. Frankly, I will try anything now!!!

Thanks once again

Mart

RS man

Looks like you didn't do the one thing I would have done in your situation - go and see Colin.
Just looking at the net on bad motor mounts for information, it may be the motor mounts if they haven't been changed. I've never had problem with the mounts so I don't know for certain.

RS man

Another thread - not motor mounts in his case:
https://rennlist.com/forums/993-foru...d+motor+mounts

Rinty

Good luck with the diagnosis, and let us know what they find out.

1pcarnut

Have you had a sniffer up the tail pipe while it's doing this to see what the AFR (air/fuel ratio) is doing? Might help isolate things a bit. Looking through your list of items changed I didn't see that you've tried swapping out the MAF sensor. This is an easy to do swap if you could find a friendly owner to loan you his for a few minutes.