S2Cabby

I am in the need for some help. I recently purchased a 1990 944 S2 Cab with 92,000 miles.

I love the car I used to own a 951 and I have to admit I love this car more. The problem I have encountered after only 1900 miles of ownership is a confusing one. I have spent days reviewing similiar posts on this and other boards with no fix.

7:00 am I start the car and it starts right up no problem. I allow the car to idle for 5 - 8 min while i say good buy to family and I take off. I live in an area that has a lot of people traveling to work so the car doesn't get much of a work out, typical shifts are at the 3700 rpm range. this is were the story gets confusing, when I do push the car after driving 10-15 min the car will stumble and buck and the tach will move up and down wildly and if I am approaching a red light the car will just come to a stop when applying the clutch. I have also taken the car out on the highway and after a period of driving the car will exhibit the same tendencies but due to the speed I am carring the car maintains it's course and does not die. Upon occasions when i do stall out, the car willnot start until I have allowed it to cool down. I can limp home but if I were to try andf push it, it will start to buck and as if it was starving for fuel, this usually occurs at rpm's above 3800 rpm's.

I have replaced the following items; DME relay, Plugs, Plug wires, Fuel filter, TPS, fuel pressure regulater, air filter,one bottle of Chevron techron feul cleaner, had the Fuel Pump checked and all was okay along with the speed sensor attached to the bell housng, and fuel pressure.

I am left with checking the Airflow meter and the DME itself. What else should I look for. I think SoCal has been the closest of the people I have seen as far as coming close to diagnosing this kind of issue.

Please help.

Thanks in advance
Maurizio Vozza

Jim @ EuroWerks

It sounds like it may be the DME. But first ck the grounds going to the bell housing. I've seen this same symptom from a bad ground here. It will eventually kill the DME if it is the problem. So do not try someone elses control unit in your car but instead your controller in their car. What about the cap and rotor. I've also seen a rotor button do this. Remove the rotor and look for a burnt spot on the back. Usually if the tack is going crazy it's something ignition related. Ck the ground 1st since it can be the most detrimental. One last thing, ck to see what your voltage output is. If the voltage regulator is malfunctioning it could kill anything electrical in the car. Hows that for a bucket load?Good luck! Oh, sorry I've seen a bad connection at the positive post of the altenator cause overvoltage also. Boy it just keeps going doesn't it.

SoCal Driver

I'm with Jim on the electrical part. I too strongly suggest that you loosen and clean all of the grounds especially the ones on the back of the engine. The key here is that the problem is temp related. Nothing will torment a DME more than low voltage.

There are other electrical components to the ignition on the 944S's. There is a modual between the DME coil trigger signal and the coil. These have been known to flake out. So has the balasts for the injectors.

Danno

Isn't the tach driving off a cam-sensor on the S2???

Might be a good idea to hook up a voltmeter to the AFM signal and see if it does anything bizarre when the stumbling & bucking occurs.

S2Cabby

I think you guys may be on to somnething with the electrical checks ecspecially the rotor and cap. I might not have mentioned it earlier but this did start to happen the very next day after changing the Plug wires and plugs. The clutch was just replaced and I am pretty sure that he ground on the bell housing was clean.

Thanks again you guys are awesome.

Maurizio
1990 944 S2 Cab

S2Cabby

I just thought of one other thing I would appreciate an opinion on. The other item that may be playing a role is this. When i purchased the car it was low on freon. I raised the car to get to the fitting and found that the cooling tube running to the alternator was undone from the front side port. I went ahead and put a clamp on it and reafixxed the hose. Perhaps the revious owner new something I did and he removed the hose because it was not getting enough air to the alternater. Possible???

SoCal Driver

Alternator vent hoses just come loose. It happens.

Tach connects to the hall sensor? Interesting. How does the hall sensor signal get back to the DME so it can figure out the timing for the knock sensors?

Justin Browning

Try tapping the DME itself fairly hard and see if that fixes the problem. If so, bad DME.

SoCal Driver

Found it. This means that if the tach is bad or the connection to the instruments is bad then the knock sensing system doesn't work.

S2Cabby

When you say "Found It", does that mean you may have figured out the specific solution to my problem, or you found out the answer concerning the relation of the Hall Sensor to the tach. When I said it effec ts the tach, what I was trying to say was that when the car goes throught he bucking and sputterring as if the fuel was getting cuttoff the tach goes from let's say 5500 rpms to 0 back up to 4000-5000 rpms and back to 0 and then the car will be okay again once I reduce my fuel draw and I have enough speed to carry me through.

Thanks by the way for all this help. How often does a loose or dirty ground usually take care of ignition/fuel problems?

SoCal Driver

The 944S models (including the S2) have a slightly different approach to engine control. More of an add on really. Your problems with the tach bouncing around and the car sputtering is part of this difference.

The basic electrical connections on these cars are critical for all models. If you don't have decent connections between the battery, chassis and engine the low/intermintent voltage to the DME can play hell with the DME's "thinking".

So one starts at the most obvious and proceeds to the least obvious. Clean the connections to the battery and from the battery to the chassis and engine. Check that the battery is getting about 14 VDC when running. Have the battery checked for a dead or dying cell.

At the starter check that the wires are tight and not corroded.

In the engine bay check that the ignition modual is getting 12 VDC with the key on.

Check that the connection to the sensor (there is only one) is tight. Might want to check it's clearence too. 0.8 mm.

Check that the connection to the Hall sensor at the front of the cam housing is tight and the wires not cut or exposed. The tach (as Danno relates) is driven off of the Hall sensor.

Check that all the other electrical connections are tight and corrosion free.

S2Cabby

So If I read this right, if all grounds and electrical conections to starter and alternater are correct I would be safe to assume that my Hall sensor is the culprit. Correct? The tach bounces up and down as the sputtering is occuring.

Jim @ EuroWerks

What have you ck'ed since we last talked?

S2Cabby

Unfortunately, I do not have access to the car. I brought the car in to the shop for some suspension work and the replacement of the TPS, and Fuel pressure regualtor. I did not feel confident that I would be able to make it home without it bucking and then stalling out on me. It was pretty warm here on Friday.

I will keep you all informed as to what we find tomorow.

S2Cabby

I just came back from my mechanic who says that while the speed refrence sensor looks okay initally from the osciliscope when cool it starts to fluctuate when warmed up. The band increases and decreases and he feels that it is the speed reference sensor that is bad. I am expected to pick up the car tomorow once the new part comes in. I will update you when I drive the car and see if the problem has truly been solved.

I know alot of you said it could be this sensor but initially the wave on the ocsiliscope checked okay.