NSW928

My 1987 S4 has developed a 'surge' when it gets to about 2400RPM. If I try and hold it at those RPM's it intermittently surges back to 1000RPM and then back up again, it feels like the engine is cutting out. If I run it below 2400RPM she runs fine, which means I can hold her right on the default Aussie speed limit of 100kph with no issues.

The problem started after I was out on a drive in the country and I got a split hose near the heater valve. I was able to nurse her the 100km or so home by driving at around 80kph and topping up with water every 15 minutes or so. Next day I removed the airbox, found the leak and replaced the faulty hose. Then this issue started happening. From what I have read here I am inclined to think that the leak has wet/ruined the CPS which is obviously in the vicinity of the heater valve. Would someone with mechanical knowledge of the symptoms of this issue be able to confirm that I am probably on the right track here, before I set to work on removing and replacing it? The annoying thing is that I had the fuel rails out about 6 months ago.

Shark2626

The CPS is just a fancy magnet with a few wires, doubtful that it is your problem in this case. Carry on...

On a side note, I have replaced my CPS and it was a bitch! But I was cautious and mine didn’t break, unlike the vast majority that you will find written about here on Rennlist. You really don’t want to attempt it unless your car absolutely will not start and you’ve done literally everything else.

NSW928

OK thanks Shark, I will certainly explore all other options before attacking the CPS then! This now leads me to whether it is something related to the initial overheating - I watch the gauge diligently whenever I am driving, obviously overheating alerted me to the water leak so I pulled over. At that stage it wasn't in the red (probably about 2/3 up), but I left her idling and when I got back in the warning light was on. At this point I turned her off while I investigated the leak and let her cool down. I then added the 5L of water I always carry and drove slowly back to a service station (about 5km) in the town I had just been through. This is when I commenced the 'nursing it home' procedure.

When I changed the coolant after fixing the hose, for peace of mind I also changed the oil and there was neither oil in the coolant nor vice versa, and the car runs well below 2400RPM so I am sure I haven't done any major damage to the engine. What electronically may have been damaged that would cause these symptoms? Should I get the computer reset maybe?

Mrmerlin

has the LH computer been rebuilt?

NSW928

Not in my ownership and no evidence of it from POs' receipts.

Kevin in Atlanta

Make sure you did not disturb the MAF and create an air leak. It should be fully seated in the elbow.

NSW928

Ok thank you, I will check that when I take out the airbox again tomorrow

Mrmerlin

post a picture of the engine has an intake refresh ever been done

GregBBRD

Not likely the CPS.
However, if it has the plastic male connector crumbling, cracked, and falling apart (like the vast majority of 1987 CPS), it probably needs to be replaced, anyway.
"Catch" that rear knock sensor, while you are there, if the plastic male connector is the same.

"Surges" are generally super rich conditions....
Pull a spark plug (or two) and see if they are black.
Pull the vacuum lines off of the fuel pressure regulator and the dampers to see if they are leaking fuel....super common. (We now make replacement regulators and dampers, which are less than 1/2 the price of the Bosch pieces, from Porsche.)
Clean the MAF connection and make sure the wires going to the MAF are not deteriorated and toughing each other, right at the plug.

You are "messing" around in an area that will likely have very brittle fuel hoses. Be careful and replace these hoses, if they are original and deteriorated. (We make the best replacement hoses available....don't be tricked into buying low pressure rubber hoses with clamps!)

NSW928

Thanks everyone for your help. I took out the airbox and checked and tightened all the connections. I think the issue was the plug in the foreground which I think is the other end of the CPS lead? It's at the back of the right hand fuel rail, one of the 3 wires was out, plus they were touching. Took her for a drive and she is back to her best.

NSW928

PS I know the 'plug' isn't meant to look like that! Most of the ones on my vehicle are like that, I aim to replace them all eventually.

GregBBRD

That's a knock sensor attachment, not the CPS.

You've got some "deferred maintenance" to do, before your car will be reliable....

NSW928

Thanks Greg. Yes, the car needs a bit of work however she runs surprisingly well. BTW the spark plugs and leads, and the front fuel hose, and the notorious 'fire hazard' one at the back of the engine have all been replaced recently.

The main issue I have is the vacuum system. It doesn't work. I have vacuum but nothing that is controlled by vacuum works - no climate control, no cruise control, rough changes in the transmission (which has been rebuilt) etc, etc. I have no idea where to start with this!

StratfordShark

Yes indeed rear knock sensor connector. If that is not connected the ignition will retard 15 degrees, and you’ll be leaving a lot of power on the table!

Mrmerlin

you need an intake refresh