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Hi everyone!
I have a Porsche 944 -85B and it has had some strange problems the last year. I bought the car without oil pressure which turned out to be nothing more than a clogged up oil filter and after putting in a new one there was good oil pressure again with a steady needle. After this problem it would start and then stall immediately but could be held running with the foot on the pedal. After some head scratching I smoke tested the intake and sure it was a vacuum leak. A huge one! After this the car started fine and ran properly for some minutes till it warmed up. Now the car had a idle speed issue. This is pretty much where I left it since last February, until last week. I did some research and most likely it was the Idle Speed Valve that was the culprit. It was! I ordered a new one and put it in the car. Now when bridging the terminals in the diagnostic connector the idle speed sunk and i was able to set the correct base idle at the screw on the throttle body. Before I installed the new one I compared the specs on both of them and the old one seemed fine. It has the correct ohms and everything but still did not work properly. Now to my final issue. I have a random misfire occurring at idle and it doesn’t seem to be a specific cylinder. It seems to me that it happens randomly at all cylinders. Before changing the ISV I had a slight misfire feeling when driving at partial throttle however changing the ISV took care of that problem. Probably because the ISV was behaving erratically before. When I had the intake off I checked all the sensors and everything. The lot! The only sensors I have’t checked are the Speed and Reference sensors. The reason for that is that I don’t have a lab scope, yet! There is one other thing I haven’t checked and that is fuel pressure. I have ordered a fuel pressure gauge so I will be able to test in a few days. I will keep you posted. I have done a compression test which resulted in 185-200 psi on all cylinders. That seems fine to me. When driving the car on high loads, for example during acceleration the engine runs fine. No misfire or anything. My thoughts are that I now am left with what appears to be a bad spark situation which only shows up at idle speeds. I have changed the plugs and the old ones, which only had 700 miles on them, were fine apart from a light blackness to them. That would indicate either a slightly rich condition or a weak spark, right? This car has had a new AFM sensor put in it a few years ago according to receipts I got with the car. I am thinking that it also could be a mixture problem at idle as the CO correction screw is on the AFM. I have tried to increase the mixture by screwing it in and the misfire goes away. Could this be the cause. Last year at the MOT, I had a CO value of 0.2% which is low so I increased it a bit. Now the thing is that Porsche says the CO should be within 0.5-1.5% at idle so I don’t want to increase it too much. It is better to find out if something else is the cause. Could more fuel help combustion if the spark is weak? I haven’t changed cap and rotor or spark plug wires yet because they are quite expensive. I also want to be sure of what it is first. Sure, the car could probably need a cap, rotor and wires cause they are old and show signs of wear. I have also made breath test through a vacuum line and the engine can pull about 9.5 psi vacuum at idle which is good to me. I would be very grateful for any thoughts on this problem! Oh, I almost forgot. I am from Sweden and our cars are mostly imported from Germany and this being a -85B it has not got a catalytic converter so there is no O2 sensor to worry about. The car has got L-Jetronic fuel injection and the timing is controlled by the DME.
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What you describe sounds like a vacuum leak. Vacuum leaks will show up more at idle and go away toward WOT. If it was missing under load I’d be looking at the ignition system but since it isn’t I’d be looking for vacuum leaks. I would stay away from messing with the fuel mixture as you could really set things out of balance. You can also check to see if it’s a specific cylinder by doing a power balance test. Lastly, don’t rule out a mechanical issue. That’s an older engine in a sports car. You can’t rule out that it’s a bit worn out. Do a compression test or better yet a leak down test to determine the engine’s health.
Vacuum leaks
Voltage leaks
Power Balance
Compression testing
Leak down testing
Finding TDC
Solving Performance Problems 1
Solving Performance Problems 2
Re Performance Problems
Thank you Eric!
I think I have seen all your videos and they are great!
After some furhter investigation I have found spark leaks from the spark plug wires thanks to your method Eric. I sprayed water on them and with the lights out I could see the spark jump. The leaks are everywhere, to the fuel rail, the head and a lot of other places! Even if that is not the cause there is for sure something to look in to. In addition to my first post I have done a fuel pressure test and it checked out with 2 bar att idle and 2.5 with vacuum removed. This car with L-Jetronic has a vane-type MAF sensor so i cannot clean it. However i have tested it on the bench with a regulated 5V source and it had a consistent voltage drop from 210mV at closed position and nearly 5V in the fully open position.
I have not yet done a Dynamic Compression Test as I have not had time. I haven’t done that for some time and I have to refresh my mathematics but if I’m not wrong I work out how much the engine is breathing and then I can calculate an estimate DCR value by multiplying it with the CR?I’ll keep you posted! Thanks again!
If you found voltage leaks I would start there. Go for the simple stuff first always when dealing with performance issues. As for dynamic compression they actually have special tools to do that. I’d replace the wires however and recheck for your performance issue.
Thank you again Eric for replying! And everyone else too ofcourse! I will start with replacing the wires and go from there. I haven’t had time to do much else with the car so i’ll make another post when I know something new. You all take care in the meantime!
Hi again everyone,
I have now had some time over to spend on my car. And money! I took a break from the engine issue and spent some time fixing my stuck door locks. They were corroded in place so to speak. Back to the engine. I have now replace to cap, rotor and the spark plug wires. It did much difference. It now sounds much smoother but there were still some misses now and then. Now I didn’t really know what to do so I called a guy that has worked on a lot of 944s and at the moment has my neighbors car to do some suspension work. I had to take the whole story about the engine and some history what had been done to it before. After some thinking and talking about fuel system and God knows what we ended up in a mixture problem again. Here is the thing.. What my small brain didn’t understand before is that the mixture screw on the AFM only corrects CO on IDLE. Not when the throttle is off the TPS idle position! In other words you can lean the mixture out so much that it starts to misfire at times. I thought the mixture was good before because when I buried the pedal in floor it ran good. Obviously Bosch designed this system this way for some reason I don’t know but perhaps the engine needs a bit more fuel at idle to run smoothly? Anyway I turned the screw in a couple of turns and the misfire went away completely and also the engine rpm rose a lot. I corrected the base idle on the throttle body and removed the jumper cable and now it works like charm. The question is how it could have become this way.. I have a receipt from a Porsche dealer where they adjusted the CO after the faulty AFM was replaced so it should have been good! The things is this I believe: Because of the faulty Idle Speed Valve someone had tinkered with the throttle body and therefore the base idle was way to high. Even after I put things right it used to idle at about 1100 rpm. Because the idle was high the dealer had not got the mixture right for the idle. So now that I fixed the ISV and the idle now is back to normal 800 rpm and the base idle set to 840 rpm the mixture is too lean at that engine speed. I am not an expert on this but I believe it must have been something like this. Later on when I’m going to take the car out of the garage and start driving it I will have to take it to a shop with a gas analyzer and correctly set the CO.
For those of you who wonder how to do that:
1. Warm engine up
2. Defeat idle stabilizer (ISV) at diagnostics port
3. Connect a rpm meter to the ignition coil
4. Adjust CO with a gas analyzer in the tailpipe and make sure the base idle is at the right interval of 840 +/-40 rpm ALL the time.
5. When CO is correct and idle is correct disconnect the rpm meter and remove the jumper cable
6. Mark base idle screw so that you know if it moves and get a new drill plug for the mixture screw on the AFMThanks again everyone for the help and a special thanks to Eric!
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