BLUE C2

Have now removed and cleaned the ISV as best I could. Degreaser, hot water wash and de-watering fluid. Checked it with a 9V battery and it shut when power was applied which I assumed to be OK. Re-assembled with new clips and reset idle switch to go open circuit just as the slack in the throttle linkage is taken up (ie as the butterfly starts to open). Engine started from cold instantly which was a first. Cold running also perfect. So I thought I had a complete cure. Next morning it needed two goes to get it to run and there was a hint of the very light throttle juddering when cold. Do I have an ISV problem?

johnfm

Blue

have you got a friendly independent or porsche wrecker who could 'lend' you an ISV from another car? Maybe SImon at Porsche Apart would be able to help out...


..you seem slike a VERY competent DIY'er, do you work in the motor trade??

Ade - C4 91

Hi,

Water is never a good thing to clean these kind of electrical/mechanical parts, the ISV has a sensitive little motor inside it which is easily damaged.

It certainly does sound like you have a problem with the ISV area, but it could also be the idle switch still not seating correctly - try moving the switch a little closer to the throttle stop and try again.

Good luck

Ade.

Scrooge

Where is the idle switch located?

Ade - C4 91

On the throttle housing itself, just in front the throttle spindle stop (where the cable connects to the spindle). Its actual positioning varies a little between the 89-91 and 91-on models, but it's thereabouts. Look for a little black box (1cm sq) with black wires running from it - it has a micro switch on its surface which clicks in when the throttle stop rests fully closed.


Ade.

BLUE C2

Last night the car took 3 attempts to start and ran like a pig ‘till warm so it it ain’t fixed. I pulled out from a side street only a few hundred yards after starting, into a reasonable traffic gap but the engine had no power. More throttle just made it worse. Very gently going on the power eventually got me going but by then the Audi which had been a dot in the distance was right on my tail and none too happy.
I was expecting the worse this morning but the car started first time and ran perfectly. Looks like the 2-3 attempts required to start and the juddery partial throttle running when cold issue are linked but other than this I’m struggling to see a pattern.
I’m a design / development engineer working for a racing engine manufacturer supplying engines for F3000, GT and LeMans. We also do our own engine management systems and I’ve done a lot of track support work where problem diagnosis formed a big chunk of the job. My only Porsche / Bosch engine management experience has been since acquiring my C2 Targa a couple of months ago so I’m a keen amateur on this front and hence all the questions. I’ll take “VERY competent DIY’er” as a compliment, thanks Johnfm.

johnfm

Blue


Have you already stripped down the airflow meter?? I think it may be worth stripping that area down (air flow/throttle assembly), checking all electrical connections and reassembling. It is possible that an electrical connections is intermittent and is 'made' when things heat up.

you have a private message BTW

Adrian

I haven´t had time to check the whole thread so you might have already been told some or all of this.
1/. Reset DME.
2/. Cylinder head temperature sensor. Has this been checked. It is a negative temp co-efficient resistor so when cold should be around 3.6 to 4.4 K ohms. You can purchase a small resistor in this range from your local electronics shop and use it to substitute the resistor. This will help you in troubleshooting. Do not leave it in though the engine will run badly when warm.
3/. Reasonance valve solenoid. This is located on the air intake assembly. This valve should open on test when you turn on the ignition and then close. It will not open again until you reach 5000 rpm or there abouts. If this valve is sticking open or leaking the symptoms you describe will occur. I recently found this problem on another 964 with similar problems.
4/. Sticking fuel injector. Either sticking open or not opening. These are pulsed injectors individually operated.
5/. Sticking or contaminated airflow sensor vane.
6/. Incorrect gap on flyhweel reference and speed sensor, bad connectors on sensor or damaged sensor.
Ciao,
Adrian
911C4

Ade - C4 91

Adrian, if you have time, can you describe the location of the Resonance valve solenoid and how to diagnose it?

Ade.

Ade - C4 91

Blue,

Have you checked your battery condition and earth cables (both to the battery itself and the earth straps on the transmission)?
What's the condition of the coils, caps, rotors, HT leads?

Ade.

johnfm

I think he's checked the caps, rotors, leads, head temp sensors etc....read his first post.

Ade - C4 91

Yes - sorry - so he did.

Ade.

Adrian

Dear Ade,
The reasonance valve and solenoid assembly are mounted inbetween the two air intake halves. You can see the sections connecting the two havles. You will see an actuating arm and connected to this actuating arm is the vacuum solenoid valve. You will easily find the connector. Quite often this assembly is in somewhat bad condition. To check the operation is quite easy. Locate the actuating arm, whilst watching it and keeping fingers out of the way of spinning things, get somebody to turn the ignition on and start the engine. You should see the actuating arm move to the opposite position and then return. What this arm does is open a butterfly valve and effectively extend the length of the air intake. This is done at high rpm to allow the airflow to stabilise, remove some of the turbulence.
Ciao,
Adrian
911C4

Ade - C4 91

Thank Adrian, I'll check it out.

BLUE C2

I'm out of the office all next week and so will not have Internet access but I may get time later in the week to try some more of the ideas suggested on this board. The air flow meter strip is probably something to do on the bench back at work. I have made a variable resistance box / lead to sub’ for the head temp sensor and may still have a play with it but as the sensor values check out hot and cold and the connections are all OK I’ve assumed the head temp sensor side of things is not the problem. Battery and earths all OK.
I had found that one of the plug leads hadn’t been pushed fully on by the people who had done the last service 6 months ago. The engine didn’t run on the middle cylinder RH bank if the coil pack that runs the upper spark of each cylinder was disconnected. A new plug and correctly fitted lead sorted this out and the engine runs on all 6 on either coil pack. I’ve heard of strange problems occurring after disconnected plug leads find their own ground for the HT. Any thoughts as to whether this may have some relevance to my current problems?