The missfire came on suddenly, at first only under load over about 3500 RPM, but gradually got worse until the car runs a bit lumpy at idle, and stutters at anything higher. The car still starts first time, no problem, so I'm guessing the timing sensor is ok...I also think the car is running a bit rich because I could hear unburnt fuel popping in the exhaust when I lifted off the gas, and the exhaust smells mildly of unburnt fuel.The plugs are brand new.Here is my shortlist of suspects (in order of what I hope it is!): 1) HT leads arcing or a dirty connection.2) Bad contact at the distributor.3) A bad 02 sensor or dodgy MAF output.4) A dying coil (not likely as it's one cylinder?)5) A dead ECU...6) A broken valve spring... :-(7) A head gasket breach between 2 cylinders?Is there anything I missed? Does anyone know the best way to diagnose a missfire in a 944? Generally?Help!!?By timing sensor, I meant the flywheel sensor... There is a notch in the teeth round the outside and that sends a negative and positive edge pulse to the DME. The sensor that picks this up craps out occasionally, but causes starting trouble. Not getting starting trouble, so I'm pretty sure that isn't the cause.

look through the internet www.google.com

On the one hand you sound like you know what you are talking about. But on the other hand, there's no such thing as "timing sensor." So it is difficult to guess if your problem descriptions are accurate.===You might have multiple problems that is confusion you. I had a dirty MAF and a broken vacuum line and...In general I would concentrate on the ignition portion. If you have a distributor, make sure the timing is not screwed up. My car always stars on the first attempt, yet it's timing was really screwed up - so don't automatically dismiss something.

It pretty much sounds like a firing problem (stutter, lack of power, unburned fuel) so you must start in this order:First "spark plugs" you say they are new but make sure they have the proper gap adjustment; it's also possible that one of them is damaged (even if new). Change them all and try again.Second make sure the spark plus are well connected to the cable, sometimes they might get loos. Also check for the cable order (if they are connected in the wrong order they will miss-fire) Buy new cables, old ones do a bad job.If spark plugs and cables are OK then it's time to check the distributor. make sure all cables are well connected. After that there is little to be done about a missfire in the do-it-yourself department. It's time to go with a certified Porsche mechanic. Since this is an '89 model it's not rare to expect some problems, maybe it's time for a full tune up.

Check these things in that order, i have a 944 Tand the same problem was fixed by #2. When the problem is fixed, you can stop, you do not need to do all these steps1. Spark plugs, you say they are new so move on2. Change the spark plug cables, if you have not done so in 50,000 miles or 5 years, do it anyway, they will go bad soon3. Take out the air filter or open the cap on the throttle bod (mine has it), not sure if yours will or not and empty about 1/2 can of throttle body cleaner in it or in the air intake if don't have the opening in the throttle body4. Change the distributor cap5. Replace the Air filter while you are at it6. Use a GOOD fuel injector cleaner (slick 50 or GumOut) to make sure the fuel lines are clean

My guess would be ignition.take a bottle of spray water.start car and spray down the wires/cap.look for arcing wich is better seen in a very dark room.remove dist. cap and inspect it and the rotor.another easy thing to check is the fuel pressure regulator and dampner wich are the little canisters on the fuel rail.remove the vacuum lines that are attached to them and make sure raw fuel is not pouring out.just a few quick checks! hope this helps

OK bud here we go. i had the EXACT problem as you do with my car. My car always started fine, but would sometimes miss-fire on the road or randomly sputter at an idle, or even at low RPMs. And here's what I did: I bought some expensive ass high performance spark plugs and wires from DENSO and installed them, and my sputter and miss-firing problem magically dissapeared the next time i ran it. I'm not saying that DENSO spark plugs are the way to go for your car (if they manufacture a size for a Porsche...) but you should definately consider buying some new spark plugs and wires.