The CPS [Crank Position Sensor or "pulse sender" (in PET) or "RPM sensor" (in wiring diagram)] plug often fractures on the sensor side and it can come unplugged easily. I had to tiewrap mine together until I got around to replacing it. That worked fine for several years. Anyway, if you find the plug fractured or the shielding broken, you can replace this USUALLY pretty easily. It's located on the top of the bellhousing under the air filter housing. Remove that and the MAF (Mass Air Flow sensor). Locate the sensor as I mentioned, then trace its wire back to a bracket that sits on the block under/behind the MAF. Some of the critical grounds others mentioned are on the two ends of this bracket (lots of brown wires fastened down by two bolts). Check/snug those too. There is an Allen screw that holds the sensor in the block. After you remove that, rotate the sensor a little and pull up on it with a Vice Grip/pliers. If it comes out easily, great. If not, slather it with some penetrating oil, try again. On rare occasion it has to be drilled and a screw put into it to pull it out. You MUST get it out if you wreck it. If it's stuck despite these efforts, the flywheel must be removed to push the sensor remnant out from underneath. I don't want to go into that in detail. Usually you pull the motor, but the S4 automatic flywheel CAN be removed with the motor in the car. I've done it. But it shouldn't come to that. My CPS came out easily.

If you suspect the ignition switch as Doc suggests, try wiggling the key when the mishap occurs. My headlights started to go off at inopportune times and it took me a while until I discovered the ignition switch was the culprit when I wiggled the key.

 

I have a much better way of removing the CPS. Remove the allen screw that holds it in place, hit it with penetrating oil of choice, and then wrap it up in safety wire so that the wire rests under both sides of the sensor. Wrap the other end of the wire around a long piece of metal, rest a wooden block on the intake manifold (if you care about the paint, also makes more travel distance), lever up. I didn't think this would work, but I tried it after Roger Tyson suggested it to me and it was 100% easy. Best tip I've had in a while.

 

Excellent, Thomas.

 

Also, CPS sensors are cheap. There may not be a reason to replace it, but if you are going to remove it, I would just put a new one in anyway. It can only eliminate a future problem.

 

Steve, welcome to Rennlist.

First, if you haven't yet, go sign up for the 928uk mailing list - found here:

http://www.928.org.uk/mailman/listinfo/928uk

Next, how far are you from Cambridge? Get on the phone to John Speake of JDSPorsche and if feasible, book in to go see him and get your car plugged into a diagnostic computer (John will be able to upgrade your ECU chips to MY88 ones which support the diagnostics tool.. your 87 has the wiring just not the ECU support that later cars have).

Your symptoms sound to me like LH failure, or possible Crank Sensor - however getting the car properly diagnosed, checked for ECU fault codes, and having all the actuators tested, will help you a lot in eliminating any issues. Even if you can't physically get to John, odds are he'll know where there's a good 928 mechanic local to you who has the right tools for the job.

John will also be able to organise a replacement LH (fuel) ECU and MAF (Mass AirFlow sensor), as well as test your ones.

The reason I say LH is I've seen another car with similar symptoms, and also the part of the LH that commonly fails also controls the idle air valve (along with the O2 loop - but you don't have to worry as your car doesn't have an O2 loop.. UK cars didn't get them until 1990).

Welcome to the tank - and post a note on 928uk saying hi to the members if you're not on there yet.

 

Great advice from Hilton as usual.

 

+1 ^^^

 

I have an '82, so I don't know if your year would have the exact same component. I had the same problems as you and it was the Ignition Control Unit. Would run well when cold, when warm would hit 3200 rpm or so and cut out. Engine would not shut down, but would burp and fart to the point that there was no power at all, rpm needle would fluctuate back and forth or register 0. When I lifted off the gas and she slowed down, the problem would stop. Very frustrating, could never go over 70 mph or so. Finally was fortunate enough to get a used unit from 928 Intl when the had their December 50% off sale one year, replaced the unit and never had a problem since. But, like I say, not sure if your model would be the same as mine.

Please be sure to let us know what the problem was when you get this fixed!

 

Spoke to Steve on the 'phone today. I agree that it sounds like CPS or its connector. The clue that the rev counter drop like a stone when the engine cuts out strongly confirms a CPS diagnosis.

I aslo suggested as a remote possibility to remove the kick down rerlay, as this is also fed from the same rpm signal as the rev counter, and the feed to pin 1 LH ECU... it's just p[ossible the kick down relay has a problem. Easier than removing the CPS :-)

 

Thanks for all the replies, I'm really surprised at the number of people giving their 2 cents, it's really great here.

I took Hilton's advice and spoke to John Speake today, as John himself has mentioned. I tried unplugging the kickdown relay, but it had no effect.

I'm going to find time tomorrow to clean the earths in the V (already done the others I could find) and check out the crank sensor, I'll post the outcome here.

erwalker, thanks for the info on your similar problem, I've been trawling the forums for a few days but haven't found this 'maximum revs' problem mentioned, so it's good to hear from you. Once I've eliminated the CPS, I'll move onto your suggestion.

thanks again, everyone. By the way, here's a pic (I haven't got the D90s, the owner sold them separately and I considered them, but I actually really like the stock ones) The eagle-eyed amongst you will notice that it's got the hinged spoiler. The x-ray eyed will notice the RMB...

[this image was taken by the previous owner- if he's not happy with me using it I'll take it off...]

 

Nice!

You should find the problem in short order. BTW, you don't have the same Ignition Control Unit as in an 82. You have the EZK and a final stage control on the left side under the front edge of the hood. These rarely fail and there are separate final stage units for each half of the later ignition system. CPS is first on the list.

 

Hmm, not sure about solving it in short order- my local Porsche dealer had a crank sensor on the shelf, it was about 50% more expensive than other sources, but I got him to let me have 20% off, so I went for it, rather than wait for one in the mail.

Allen bolt was rounded, ended up drilling it out and using a stud extractor. Hoped the sensor would come out easily, used the wire-and-bar technique mentioned above. The entire plastic bit came away from the bit inside the bellhousing, I'm now staring blankly at about 3mm of protruding metal that is so hard I can't drill it (I think it's a magnet). And so smooth I can't get anything to grip it. Please don't tell me I have to take the flywheel off! Is it too risky to heat it with a pocket size gas torch? Help!

 

Did you soak it overnight with Plus Gas or similar ?

You need to drill around the metal central core with a small drill to remoce all the windings. You should then be able to extract the iron core, collapse the outer shell (aluminium) inwards and withdraw upwards.

Use the new sensor you judge how far to drill down without going through the base of the sensor.

 

Tap-out from below may be the most straightforward, least frustrating approach.


I just had that happen to a manual car about 6 months ago.
Had to take out the clutch.
But, to get access, had to remove aftermarket headers.
But, to get access to them, had to remove the crossmember.

Wished the whole time it was an automatic.

The aluminum bellhousing had corroded, powdery white, all along the length of the stainless steel section of the CPS. I worked for several days soaking it and drilling and trying to get a bite on it from above. Tap out was the sure way.

Then again, others have solved it as John Speake suggests. I just didn't succeed at it.

 

Thanks John, I'll try the drilling idea today. I was intending to get the sensor from you, but when the dealer had one on the shelf, the idea of having a proper drive that evening made it impossible to wait! I'll make a list of other bits I might need, and see what you can sort me