I don't know what else to try except a straight jacket!!

[mhpd31]

Ok guys, I have been working on my 1988 944S for a few months now, and I am at the end of my rope with no clue where to go from here. I will start from the beggining and add as much detail as possible.

When I bought the car, it had a rough idle at startup, and a slight miss once it was warmed up, but nothing drastic. It also had a shutter from a dead stop. I thought theese were all problems that could be fixed with a tune up and routine maintenance. As I planned to modify the car anyway, I figured I would do it all at once. instead the modifications just exaggerated the issues.

Modifications were as follows: (all from Lindsey) Billet replacement non diaphragm BOV , Performance tune up pack (black magwires, plugs), 944T 300rwhp package, 944T Quad M mass air flow meter kit, 4 inch exhaust no cat pre muffler 02 bung, polished tip, Black vaccum hose kit, hoses gaskets, etc etc. I got everything put together and all the earlier problems were GREATLY exaggerated. Rough idle, intermittent backfire, car ran out pretty good with boost, but if I did not come out of the hole above 2000 rpm from a stop the "shutter" would kill the car.

YouTube - mhpd31.MPG

So I called Dave at Lindsey and we tried everything, over the next few weeks, from new chips, to a new mass air flow, we tried things until he ran out of things fo me to try. So I took the car apart (for the third time) checked every vaccum hose, replaced the TPS, the ICV, rebuilt and cleaned the injectors (thanks to the great post on here), rebuilt the throttle body and..... same thing! Idle was a little smoother, backfired a little less, but, same problems, and when you take off, you either have to feather the gas (like its powerless under 2 grand) or come out over 2. If I would disconnect the tps (unplug the harness) the idle smooths right out, and it runs great. I am assuming that this puts it into a default mode, and have not tried to actually drive it this way. I set the TPS where it clicks immediatly as soon as soon as you roll the throttle. Also the fuel pressure is a steady 38 psi.

Last time I worked on the car was 5 days ago, and I found a guy in town who supposedly has a plug for ANY car ever made to do diagnostics, so I was going to drive it down to him. Today was new, I got in started the car, and it died, did this 5 times, runs for 5 seconds and then just dies, if I really play with the pedal and feather the gas I can make it get to about 1800 rpms and run for about a minute then it dies!!!! I don't know what else to do, or try, I AM SO CONFUSED !!!!

[chally72]

If you can unplug your TPS and it runs decent, you have found the source of your problem. Either the TPS itself, or the wiring connecting the TPS to the computer, is funky here. Since you say the TPS is new, the wiring has to be the culprit.

There is no damage done by running without the TPS, except you will run very rich. It defaults rich so as not to damage the car when it does not know where the throttle is positioned. I did this for a few days when I needed a new TPS and this car was still my only form of transportation. Good luck getting it to idle though, you'll have to be fast with your feet at stops, especially on hills. Try unplugging the TPS, and go for an extended drive. See if the car still has any problems (other than problems with being rich).

My TPS problem originally presented as misfiring at RPM, along with very rough idle, and moved quickly to the entire car dying in gear, bucking/stuttering so hard I could hardly get it above 1500-2000 RPMs, and not idling at all.

[minho78]

Was your car cold when you started it? It looks like your car is not enriching the Mix at start up (if it was cold). Coolant temp sensor maybe the culprit for that. The TPS should be next Unplug it?

[mhpd31]

Ok so if the TPS is new, and I have it set right (clicks almost immediatly when you roll the throttle). That would only leave bad wiring (which really sucks because electrical is where I am at my weakest). So does this mean I have to replace the entire wiring harness for the car??? I did clean the contacts on the plug for the tps. Is there a way to test this, and if I have it towed to the guy that says he can scan the car will it do any good for my situation?

[mhpd31]

Minho it acts the same at all temperature ranges.

[minho78]

OK, from what I saw with your AFR gauge your car is really lean. O2 Sensor?

[chally72]

Aha, two-three possibles. I forgot about the O2 sensor. Don't have experience with coolant temp sensor problems.

The guy with all the diagnostics BS will not be able to help. These are pre-OBD cars. No diagnostics port other than one that can be read with a simple blinking LED light.
Unplug the O2 sensor (under the intake in the rear) and see what happens. If this doesn't work, unplug the TPS like I mentioned. Run it for a while and see if your problems still exist.

There are tests you can perform on the TPS with a multimeter. They are fairly quick. If you unplug the TPS and your shuddering problems disappear, and if the TPS checks out, you have to look at wiring.

[Grandad#3]

Hello, Check out the link below for the TPS.

Throttle Position Switch - Information, Troubleshooting, Replacement, and Adjustment

Cheers,
Larry

[minho78]

Originally Posted by mhpd31

Ok so if the TPS is new, and I have it set right (clicks almost immediatly when you roll the throttle). That would only leave bad wiring (which really sucks because electrical is where I am at my weakest). So does this mean I have to replace the entire wiring harness for the car??? I did clean the contacts on the plug for the tps. Is there a way to test this, and if I have it towed to the guy that says he can scan the car will it do any good for my situation?

Sometimes new items might be bad from factory. Remenber electronics don't like to be tossed arround inside a box when shipped. Test the tps and the O2 Sensor.

[Techno Duck]

Pull back the boot on the TPS connector and look at the wiring. Cracked insulation and shorted wires under the boot are not uncommon and are relatively easy to fix.

Have you reset the idle speed with the ICV disabled? There are instructions on how to do this on ClarksGarage.

[nize]

Originally Posted by minho78

Sometimes new items might be bad from factory. Remenber electronics don't like to be tossed arround inside a box when shipped. Test the tps and the O2 Sensor.

+1

it is possible you have a faulty new TPS. the best way to check this is to use a known good TPS from another 951 that has no problems.

also keep in mind that sometimes the TPS can check fine when tested while cold, and be faulty when warm/hot. that's what happened to mine.

[KEV951]

have you checked the basics? look at your spark plug gap, cap, rotor, have you checked fuel pressure or delivery? your idle vac is very low, seems like fuel starvation, check for pinched send lines under the car on the passenger side where the metal line ends a and a small bend of rubber hose connects to the metal line ahead (8mm tube) . cut again, check fuel pressure, delivery, and check the gap on the plugs, its sounds to me like checking the basics will solve this for you
good luck
]

[wizard13]

Just out of curiosity did you tune the MAF controller? If not i would start with that because im pretty sure with MAF conversions you cant just drop it in and go without setting your air flow scaling.