Question about 1991 Porsche 944

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91 944 S2 Cabrio misfiring during acceleration

Have a 91 944 S2 Cabrio and was running great. During heavy rain water got into the back of the engine compartment near the battery. Car started bucking or misfiring with acceleration sounded like it was not hitting on all cylinders and making a tinny sound. Car dried out and everything was fine but now everytime it rains or there is even dew or cold temps starts doing it again and its getting worse. Noticed there is a part behind the battery in the rear firewall that looks like two round plugs (one going in and one going out) with 2 wires between them that make a U shape. Do not know what this part is and cannot find out anywhere? Do not know if this is the problem but the wires are frayed and corroded. The problem does not appear to be related to the distributor cap or the wires but I canot tell if its electrical, air or fuel related. It does not appear that water got into the air flow sensor either only around the battery and perhaps near the wries I mentioned. When it starts to warm up the problem does get a little better except when going uphill and when accelerating hard over 3500-4000 RPMs. Anyone please help.

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  • jmb12166 May 05, 2009

    Thanks for the reply however there is absolutely no moisture or water getting into the distributor cap. Just to make sure I sealed botht the cap and the wires with a sealant. I did find something since my post bnased on a response from somebody elsewhere. There is apparently some sort of fuel condensor in the trunk well behind the gas line and in checking that out I founbd about 8 inches of water had accumulated in the trunk well. I drained it and there did not appear to be any problem with the part but the problem did not get any better and in fact the car stopped firing completely. Mechanic cannot find the cause and says there is no fire. He thinks it may be the hall/cam sensor and we are replacing it but he cannot say that will correct the problem. He says its tough to diagnose because without the car running he cannot perform a computer diagnostic test. Does this make sense or do you have any other suggestions. Thanks for your input.

  • jmb12166 May 05, 2009

    He''s actually a very fine mechanic and I was paraphrasing what he was telling me. He did confirm that there was a short in the cam sensor but from what I can tell he is not sure if that will solve the problem. He specifically said the car is not getting any fire and that is consistent with what you are saying and what it was doing. What would short out the sensor in the first place? . The problem clearly started with water getting into the back rear of the engine compartment and the sensor is located near the distributor cap in the front. Do you know what the wires I previously referred to are? They plug into the firewall and form a "U" and they are located in the rear of the engine compartment behind the battery. Those wires are clearly frayed and it seem like they were getting wet but what do they have to do with the cam sensor, if anything at all? I would like to replace them but I cannot find them in the shop manual and I do not know that they are or what they do. As for the mechanic. he's the best I got and he worked for Jim Ellis Porsche in Atlanta. Im at his mercy since I now live in a podunk town 85 miles away from Atlanta and he's the only guy set up to work on Porsche's. I lived in New Jersey just outside of NY and you couldn't throw a dead cat without hitting a porsche dealer around there. I can and do do some of the work myself but this is beyond me expertese. I will take your advice and change the wires and distributor just for maintenance sake but they so not appear worn in the least. Anyway any input you have is appreciated. The car is red and is beautiful and selling it would be like selling one of my kids so I'll have to decline.

  • jmb12166 May 05, 2009

    Sorry no pic the car is about 85 miles from here. Thanks for the head up on the other lovely issues I' could be facing LOL. I actually thought about the catalytic converter and the O2 sensor as the source of the problem. Someone else mentioned a fuel condensor in the truck and when I checked it out there was about 8-10 inches of water in the trunk well on the side where this fuel condensor is located (clogged drain plug). Frankly when the car orginally started bucking I immediately thought fuel problem and my inital thought was water in the fuel since that is exactly how the car was acting. As I explained from my numerous posts the problem this has been escalating and what started out as a realtively minor hesitation/bucking has now escalated to the current problem. I guess what I'm asking is could a fuel problem cause the hall sensor to fail or could a catalytic converter or O2 sensor failure lead to the failure of the cam sensor. It seems to me that the problem is a fuel/air/fire mixture problem bnut on this car that could be like looking for the holy grail. It could very well be a simple sensor failure but which one?

    If you have not read some of my earlier posts this problem first started after a heavy rain. The car immediately started hesitating and it sounded like it was missing a cylinder (like a hollow tinny sound). Once the car dried everything was fine until the next rain. Than the problem progressed to dew or cold but again once it dried out it appeared fine. If I covereded the side of the engine compartment where the battery was and there was no problem after a rain (this is why I thought about the wires on that side). Than it progressed and even with dew or cold it appeared relatively fine but hesitiated at 4000 RPM or above. Than it progressed to its current problem in that after rain or dew it hesitated or bucked until the car warmed up (about 15-20 at idle). On the last day it started and after idling it would hesitate/buck/miss until it seemed to catch and than it ran smoothly up to about 4000-4500 rpm and started with a minor hesititation. I had actually called the mechanic and set up an appointment for the next day. Well as fate would have it I started it the next day and everything seemed okay. It occasionally missed over 4000 RPM but would catch and be fine. Well I was 80 miles from home and we had a brief shower. I started it and right away it bucked/missed and it did not clear for about 30 min. I tried to drive it home and it cleared somewhat but would start once the car reached 4000 RPM. Obviously I tried to nurse it below 4000 but it gradually got worse and it eventually started bucking and clearing bucking and clearing until it sounded like a cylinder went out. This was about a mile from home . The car would not accelerate over 60MPH. When I pulled into my driveway it spitted and sputtered for about 30 seconds and cut off completely. It would not start again and was towed to my guy. Unfortunately while I think the cam sensor may be shot and replacing it may get the car to start I do not think this will solve the ultimate problem. I certainly don't want to start replacing all the things we mentioned. Now that you have the whole painful story can you add anything to your prior suggestions. Any experience with these cars after they get wet or have you heard about anything similar from anyone else? The car has 120K miles and between 80-120 it ran like a dream. Just replaced the clutch, water pump, air filter, oil filter, plugs and the timing belt within the last 5K miles. Lastly, why would a broken inner valve spring not be evident at lower RPMs. I ask because since the valve cover will be off to replace the hall sensor I may have him check these as well. Anyway sorry to be so long winded but I thought if you had the whole recent history it may help with any sugestions. Dont worry about the tone of your responses either. Porsche owners cannot afford to have thin skin. Thanks again.

  • Dave  C
    Dave C May 11, 2010

    I have a better solution, sell the car to me....but only if it's red or black :-)

  • Dave  C
    Dave C May 11, 2010

    Hall effect cam sensors are very easy to test, they have a power lead, a ground lead and a 5 volt square wave output. If he doesn't know this then get the car away from him. "He says it's tough to diagnose without the car running"???? What? This is the opposite of every mechanics nightmare, which a complaint that you can't get to happen at the shop. A "no start" is the easiest thing to diagnose, spark, fuel, compression. It's a piece of cake!
    Replace the distributor cap, rotor and wire set. It's maintenance!


  • Dave  C
    Dave C May 11, 2010

    The problem does appear to be related to the distributor cap and the wires. Replace them.

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  • 1,050 Answers

I'll go out and have a look at my car to see if I can figure out what these wires are, post a picture of them if you can. You have to re-size down to 40 kb so get a close-up. If you can't do this, you need to repair the frayed wires regardless, so as they say in the Nike ad, "just do it", but I don't think this is related to your problem.
Sorry about being so adamant about the cap, rotor and wires, I'm not trying to be rude, text talk does not come easily to me. It's just that you really can't see cracked or tracked ignition problems. High voltage has a way of jumping to all the wrong places if it has established a path in the past. Also, when you have an ignition problem, too much is put at risk. If the engine is missing, the computer will see oxygen in the exhaust stream and think it's running lean and then add fuel, this melts the catalytic converter and fouls the O2 sensor as well. If the spark plugs haven't been done for awhile, I would do those as part of the process as well.
This has nothing to do with your "no start" problem, but just something to keep in mind, a friend of mine has a '90 S2 and he had a problem with a "miss" at higher rpm, we scratched are heads over this for about 2 years. The problem was finally traced to broken inner valve springs! Turns out it is a very common problem, especially if you drive the car hard. He was a weekend warrior at our local auto cross. We changed the springs without removing the head, you can do this by bringing each piston to TDC while you plumb air or rope into the plug hole to hold the valves up. It is rather difficult to see the broken inner springs, you have to remove the valve cover obviously, but it is very hard to tell that they're broken because you have to peak through the outer spring. Some people have reported finding pieces of valve seals in the oil as it drains out during an oil change, this is a sure sign the inner spring has come apart. If you have a lot of miles on the car, this may be an issue.
I'd come and give you a hand, but I can throw a rock into the Pacific where I'm at, so I guess this will have to do. :-)

Posted on May 05, 2009

  • Dave  C
    Dave C May 06, 2009

    I'll blurt out a couple of things quickly, I'm on the run.
    I noticed that you didn't change the fuel filter last service, probably a good idea now. I don't think it's water in the fuel because you say sometimes (when dry out) it ran OK, the high rpm cut out may be a restricted filter though. Kind of surprised this wasn't done as soon as you noticed the problem- cheap maintenance. I would put a fuel pressure gauge on it and drive it to see if the pressure stays up under load as well.
    Typically, wet weather problems are ignition related, can't stress, cap, rotor, wires enough.
    Engines with dual valve springs are designed such that each spring on it's own, is quite weak, together they are strong and dampen each other as the frequency of compression changes. If one spring breaks, you are left with one weak spring with no dampening effect as icing on the cake. A dual spring valve will float (actually bounce between open and closed position, when it is supossed to be closed) without one of it's springs.


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