I have a 1984 Porsche 944, I've recently got it running really well except one minor issue. When the car is cold, it runs well. When it gets warm toward the middle tick mark on the dash, it starts to break up underload. Keep in mind it revs fine in neutral. When I give it gas it will break up until I go WOT. When I go WOT it runs fine and pulls like it should. I tried replacing my DME temperature sensor with one from FCP Euro and it was suppose to work on my car and someone else reviewed it and said it work on their 944. So I installed it and tried to start the car, it would start and run for 1-2 seconds then die off, I assume it was going lean. I then unplugged that one and plugged in my old one, which wasn't even installed just chilling on my J-boot, and the car started just fine. I then got 2 other types of sensors from AutoZone that were supposedly suppose to work o the 944, and the car didn't even start on them. But the car started on my old sensor.

Short story: Runs good cold, breaks up underload when warm. Can't find a sensor that works for my car.

Technical Question: What controls mixture. I know the AFM helps control mixture and the DME temperature sensor does too, but are there any other thing that control mixture?

Link to my main thread where everything is documented on the car: https://rennlist.com/forums/924-931-...-no-start.html

TL;DR: If anyone has any advice on other reasons this car could be acting the way it is, the advice will be appreciated. If anyone has experience with a good, working DME temperature sensor they used, please drop a link.

Thanks,
TurrFinn

 

AFM Barn door
AFM temp sensor
http://www.clarks-garage.com/shop-manual/elect-22.htm

re-tracking the AFM wiper arm might help. The AFM temp sensor could be tested as per the link also.

Your DME water temp sensors can be tested also, scroll towards the bottom:
http://www.clarks-garage.com/shop-manual/elect-19.htm

If it starts and runs with a DME water temp sensor plugged in and hanging loose it should run decently under load, that'd be an interesting test. It might stay in open-loop due to the engine appearing to be cool.

Other issues could be an O2 sensor issue, clogged fuel filter, clogged tank "sock" or a failing fuel pump.
I'd probably start by test driving with the DME water temp sensor plugged in but not installed and then try running with the O2 sensor disconnected.

 

In the previous thread, you replaced the DME temp sensor, but you seem to still believe that its the culprit. If you believe the new DME temp sensor is bad, then you should test the new one to be sure. My opinion is that the DME temp sensor is likely not the cause of your issue. Also note that these cars have two temp sensors - DME temp sensor (blue top) tells the computer when the car is in a cold start, and the coolant temp sensor (under the intake), which is the sensor for the gauge ONLY. Verify you are not confusing these.

Secondly, I would recommend you to follow Clarks garage more closely. This is the engine troubleshooting page (Note that DME temp sensor is only listed as a possible cause under issues dealing with cold startup or cold running.):
https://www.clarks-garage.com/shop-manual/ts-01.htm

"Breaks up under load when warm" - Seems to fit with the "Engine Stumbles or hesitates under acceleration", wouldn't you agree?
Possible causes: AFM or Vacuum Leak.

Sounds like you should test your AFM or check for vacuum leaks. In your previous thread you tested the resistance, but it sounds like you did it wrong (ignition off not on), and this only checks the Intake Air Temp sensor inside the AFM and not the AFM itself. You should go through the process of repositioning the pickups and cleaning the contact strip per the above walkthrough. Then test voltage throughout the AFM barn door range - make sure the voltage increases smoothly and incrementally and doesnt jump or drop to zero throughout the range.

Note the test for the IAT sensor inside the AFM:

1. Check temperature sensor I (intake air temperature). Turn the ignition switch off and pull off plug on air flow sensor. Connect and ohmmeter between terminals 6 and 22 on air flow sensor (both outer male terminals on the sensor plug). With air temperature between 15 - 30 °C, the ohmmeter should read 1.45 - 3.3 k-ohms. If the air temperature sensor reads open, it will cause a rich mixture. If it reads a short, it will cause a lean mixture. (I think you said you performed this test with the ignition on in your previous thread)

Now also run through the AFM voltage tests. If it doesnt pass, follow the instructions for repairing the AFM (you may want to just do this anyways). If the IAT (intake air temp sensor) on the AFM still tests bad I think you will need to replace the AFM with the same generation (late and early model AFMs are not compatible, ie: and 84 wont work on an 87 or vice versa). I think your second AFM is likely incompatible.

 

From your othe thread it also sounds to me like your AFM door may be seizing or getting hung up. I had this issue on my car as well, and required a good deal of sanding/filing the inner wall/barn door of the AFM to stop the barn door from catching on the inner wall. Not sure what caused this, but it seems like the barn door actuator may have gotten slightly bent or something. The barn door must have free, unimpeded motion (besides the spring) for the AFM to work correctly.

 

The issue started as a cold start issue, then when it warmed up it got better and the issue was no longer there even at operating temperature. The next day or so it starts the hesitates when warm issue. I presume the DME coolant temperature sensor was bad since I tested it, I don't recall how I tested it but I did it with ignition on and off. It didn't test within spec. Now I did have assumptions of the AFM air temperature sensor being bad, I feel like it might be bad however the issue doesn't seem to come from IAT's but from the temperature of the engine. I had the car running yesterday with the cap off my AFM, when I opened the throttle the arm would swing in an orderly fashion. The potentiometer strip is very lightly worn, its in better condition than the one shown on Clarks Garage. I do need to test my AFM air temperature sensor again, but before I do that I'm going to try driving the car with the original DME temperature sensor plugged in outside of the block. So it doesn't get warm, if it runs well at operating temp, I presume the sensor will be considered bad. I will get back to the thread within the next 2 hours to update how the test goes.

TL;DR: Issue seems to be related to coolant temp, I have yet to test it out.

Edit: I did read through the engine diagnose on Clarks Garage and tired to pinpoint my issue, but none of them seem very viable. A vacuum leak seems unlikley because when I go WOT it opens up and goes like its not a problem, likes it switched fuel maps. Maybe I'm missing something but I'll be sure to test my AFM again.

 

I just re-tested my AFM, which I previously confirmed gad a bad air temp sensor, it once again read 0 ohms. However it doesn't seem to present my issue, my issue seems to be reliant on the temperature of the engine.

Just to fix it without buying a new air temp sensor, could I pull the air temp sensor out of out 87' AFM and put it in my 84'?

Edit: My bad, I was wrong. My meter wasnt set properly. I believe it's set correctly now. I'm still learning how electrics work in cars considering its my first car.

Approximate OAT 67F or 19.4C

 

What about the voltage tests for the AFM?

Please go test it, reposition the pickup arms if needed (will require opening the afm). The resistance check you just took measures the IAT sensor function, but thats only half of what the AFM does. Testing the voltage is the other half of the equation. It will tell you if there are dead spots on the pickup strips (which is solved by cleaning the pickups and repositioning them to a less worn part of the pickup strip). Confirm the barn door moves without any obstruction as well, this is critical. If it gets hung up or sticks, you may need to sand away some of the inner wall or barn door to eliminate any areas where its hanging up. Lastly, make sure the AFM you are using is the correct one for the DME installed in the car.

The AFM meters air coming into the engine (quantity and temperature). If not working properly it will cause hesitation or stalling under load because the DME doesn't know how much fuel to add to balance against the amount of air coming into the engine in order to maintain an acceptable air fuel ratio. This is why clarks says this is a possible cause of hesitation or warm running issues under load, which I believe is exactly what you said your issue is.

 

I tested the voltage when you open the barn door previously. I just put in the sensor that the car doesn't like running on in the sensor location and plugged in its favorite sensor outside of the block. The car ran fine. I do notice the car sometimes having a hard time recovering from a rev, its subtle most of the times but I can tell its happening. The car is also making a faint metal sound, not sure if it should be worrying or not.

I'll attach some photos and videos in a minute.

Edit: I will recheck the swing of on the barn door tomorrow and check voltage just to be sure.

 

Video link:

You Tube

Current temperature sensor arrangement

 

Update: Just drove the car for a 10-15 minutes with the original DME temperature sensor plugged in outside of the block and it drove great. It drove good when it was warm then when it was at operating temperature. The only thing I noticed is my issue flipped, when I go WOT it breaks up but at any other point in the throttles range it feels like it should. I guess that's a better issue to have but the car doesn't have a working DME sensor in the block. I think I'm going to order the DME temperature sensor from pelican parts (Bosch sensor not Genuine Porsche sensor) and try that one out.

 

Still sounds like an AFM issue. These tend to be very hit and miss as the pickups sometimes connect and sometimes not which can vary at different throttle positions and rpm. One day might be running great, the next day its hesitating and stumbling (at least that was my experience when mine went bad). Luckily repositioning the arms, cleaning the contacts and fixing the barn door sticking issue worked for me

Glad its running better.

 

I'm going to check the swing on the potentiometer as soon as I get home. When I was driving the car it felt much better with the dme temp sensor out of the block, but one thing I have noticed about driving it the clunk between gears. I'm new to manual I really learned to drive manual in this car, so I'm not sure if I'm casuing this. When I go from 1st to 2nd and let the clutch out it starts lurching and lugging. I get on the gas a little and it will still do it for a second. The revs seem fine, I guess I'd describe it as a sloshing feeling. The car will lurch then die then lurch then die, it's not actually dying but it's being clunky. If it's not me causing it my guess would it's from a slosh or the swing of something like the afm. If I don't give it gas to get out of the clunk it will just continue doing it.

I don't know if that made sense or if it was just a ramble. I'm also typing on a phone so my formatting isn't good.

Sorry about the rant and formatting,
TurrFinn

 

Probably your trans mount or rubber centered clutch needs to be replaced. The rubber center clutches have deteriorated over time and crack, separating itself from the disc itself - these are an extremely common issue on these cars if you have the original clutch. Once this happens, there are small "ears" on the disc which act as a fail safe to keep the clutch functioning. If you search you will find a near endless amount of information on this. The solution is to install an updated Sachs clutch that doesnt have the rubber center. Continuing to drive the car with this situation isnt recommended, so this might be the next thing youll want to look into diagnosing and fixing.

There is a pretty low cost DIY for a urethane trans mount mod which is pretty easy to do. The clutch job, however, is one of the more difficult jobs on these cars, requires lowering the rear suspension and moving back the torque tube, removing the bellhousing and clutch fork to access the flywheel and clutch. Its also a good time to address other while you are in there things such as the pilot bearing, rear main seal and clutch fork bearings. Lots of guides and walkthroughs on this on youtube and online.

I think you should focus on one thing at a time and continue testing and investigating the AFM. I am curious if slightly moving the pickup points will solve your issue. "Checking the swing" needs to be done with a voltmeter, and you will probably just want to reposition the pickups regardless of what the voltmeter tells you as these are usually pretty worn anyways and need it.

Once thats finished, then look at the clutch.

Unfortunately (or fortunately) any 944 you purchase is going to need many many issues addressed. Most of us on these forums have gone through this journey, but once you get through it, youll have a really nice car that you can be proud of driving because you fixed everything yourself. You will also have intimate knowledge of your vehicle so you can come on the forums and help others with their "journey".

 

AFM test: I just tested the AFM according to Clarks Garage. The swing seemed to be a normal smooth operation. voltage would slowly and steadily increase to a max of 8.50 volts give or take .50 volts. Input DME voltage was about 10.25-10.50 volts.

Clutch: I was driving the car previously, a few weeks ago when it was running much worse compared to now. I heard a sound in the rear end at a stop with the clutch out and it was clanky. I checked Clarks Garage and it sounded like what you mention where the rubber was torn or worn out from age. I don't know if I'm not able to do a clutch as I don't think I posses the knowledge on the 944 to perform one.

Questions: Is the clutch on the flywheel? Any good videos your prefer to reference when inspecting the clutch/replacing the clutch. Does a clutch replacement require any specialty tools, minus the thing you stick in the center? If you have replaced your clutch where di you buy yours? I'm local to Lindsey Racing and they have so many clutches, I do see they have a Sachs clutch.

Edit: Just drove the car and it went well, still breaks up at WOT. Might try bending the arm on the AFM tomorrow.

 

Going WOT should ignore all inputs from the AFM, temp sensors, and oxygen sensor, and go into open loop. I would NOT bend the arm on the AFM. You could cause even more problems. Did you ever test the throttle position switch? Maybe your WOT isn’t being sensed. Or someone misaligned the throttle switch. See Clarks on how to test.