944 Engine Woes

JonVaccaro · 06-02-2003, 04:22 PM

I have an '86 944 and I've been having engine problems since I got home from college two weeks ago. At low RPM's I have very very low horsepower. At high RPM's, however, the horsepower is normal. I can feel the switch from lousy to normal power as I accelerate- the car jolts a bit as it makes the cross over at about 2700 RPM. On the highway the car sometimes lightly shudders at about 2500 RPM, but not lower and not higher. The car has a brand new coil, sparkplugs, sparkplug wires, and distributor cap, and a few months ago the clutch was replaced along with the two magnetic sensor things associated with ignition timing (I'm still confused about that part). The problems first began back when my clutch and sensors were replaced but I'm hesitant to draw any conclusions. It's an old car so things go wrong all the time.
Does anyone have any idea on how I can diagnose my car? I'm a college student on a low budget so I'd like to do it myself. I'd really appreciate any help you can give me- I'd really like to get this beautiful car up and running again. Thanks,

Jon Vaccaro

Roger Hall · 06-02-2003, 05:35 PM

When was the last time you replaced the fuel filter?

carsontc · 06-02-2003, 07:04 PM

Quote:

Originally posted by JonVaccaro
The problems first began back when my clutch and sensors were replaced but I'm hesitant to draw any conclusions.

Todwic may not be as hesitant

todwic · todwic's Garage

I'll draw that conclusion and sketch an assertion or two.

todwic · todwic's Garage

I'll do this semi quick and easy There are two sensors that sit on a bracet right above your flywheel. TDC and RPM sensors. The RPM sensor counts the teeth on your flywheel and this how you DME knows how many RPMs the engine is going. It uses this 411 to keep fuel pump running, keep the injectors injecting, etc. The TDC sensor counts a 5 mm high pin screwed into your flywheel. everytime the pin goes by, the sensor sends a signal to let the DME that the engine is at TDC. This lets the DME decide when to send the ign. pulse to fire your secondary ign. circuit. Your spark plugs, man. Now the clearance of the teeth and pin (the distance between the sensors and pin/teeth is .8mm. If whoever installed your sensors moved the bracket, broke a sensor, didn't tighten the sensors into the bracket, or moved the bracket too close and sheared the pin/broke sensor, then DME isn't getting proper 411. I'd try swaping the sensors (while you have them out, inspect them) and switching plugs (making sure you keep the TDC sensor [closest to motor] on the TDC plug)

carsontc · 06-03-2003, 04:44 AM

of course, I agree with Todwic...they somehow messed up the installation of the new sensors

FLA944 · 06-03-2003, 06:01 AM

I concur w/ todwic and tom. Note: todwic has the most experience with those sensors than any individual dead or alive. It has been rumored renaming the speed sensors as todwic 1 and 2.

Rick

JonVaccaro · 06-04-2003, 06:02 PM

I gotta thank you guys; you saved my butt. Thanks for the expert advice. Damn, I love porsches.

todwic · todwic's Garage

and now it's time to pay for tom's advice.
Fee: Tell us a story. car fixed? what wrong?

JonVaccaro · 06-17-2003, 06:46 PM

Well, I finally got a chance to talk to the mechanic who replaced my TDC and RPM sensors today. He said that if either sensor were to be bad or improperly installed that I would have a constant horsepower problem not low horsepower at low rpms and normal horsepower at high rpms. He seemed pretty convinced that the sensors weren't my problem, but I told him the same things that I posted in this forum. I could really use some advice, I'm afraid of getting cheated by this mechanic but I need evidence of what the problem is. Any help you can give would be much appreciated

carsontc · 06-20-2003, 03:39 PM

could you give a little more detail...

did this problem begin immediately after the clutch was replaced?

or immediately after the plugs, coil etc were replaced?

did the mechanic install a new sensor bracket? ...or

a new reference sensor? ...or

a new speed sensor? ...or all? ...or some?...what exactly?

or did he just simply remove the parts during the clutch job and then reinstall these original parts (which is what is normally done during a clutch replacement)

yeah...it could any of several other different things, but if the erratic running engine problem began immediately after the clutch work...it would seem most likely something he had just worked on

Mark_944S2 · 06-20-2003, 03:55 PM

Jon,

Logic says that its those sensors. This trouble started when they were fitted, right? So they could either be faulty or improperly fitted.

Have you ever seen a lifetime/failure graph? The most likely time for a failure of a new component is immediately after new.

I can easily think of a few ways that the sensors could cause trouble at low revs but not at high. I think they are magnetic or inductive, (can someone tell me how they work?) either way the pulse HEIGHT gets higher with RPM. So if the sensors are too far away and the pulse height is just on the edge of being big enough for the computer you could easily get the effect you're seeing.

Mark

JonVaccaro · 06-20-2003, 04:27 PM

I brought my car into the mechanic yesterday; he's a good guy and extremely knowledgable. He ruled out the sensors pretty quickly, saying that reference sensors and speed sensors either work or they don't- there's no in between. So, since the car runs he felt that the problem was probably not sensor related. We took the car for a test drive and he felt the problem immediately. The car idles perfectly but as soon as you try to accelerate the car feels like it looses 30% of its power. The engine continues to drag like that until about 2700 rpm where the power seems to return. The power problem at low rpm holds true for all gears and if I'm cruising on the highway I can feel the car lightly shuddering when I leave it at 2700 rpm.
After driving for a while my mechanic felt that it might simply be a squirly fuel problem. We replaced the fuel filter on the spot and put some BG Injector Cleaner in the fuel tank to see if it wasn't simply an injector problem. He suggested getting an injectors and head cleaning service performed. His father (it's a family owned business) thought that my problem may come from soft carbon build up- as fuel is squirted into the engine, the soft carbon initially absorbs a portion of the fuel, hindering horsepower. Within seconds, however, the soft carbon becomes saturated and the engine experiences a boost of power.
Anyway, the fact of the matter is my car is still broken. I'm going to have my injectors and heads cleaned and if that doesn't work I'll replace my fuel pump. I like my mechanic- he only works on porsches and he's an important member of the PCA in the area- but I'm still not convinced that this isn't a sensor problem. Does anyone have any advice on how to diagnose the car myself? I'd appreciate any help anyone could give.

Roger Hall · 05:15 PM

I have no experince with 944's so consider this advise as suspect.

On my 81 924 I had similar issues. It was a fuel delivery problem. On your car I would suspect the fuel pressure reulator, TPS, and any temp sensors that areused by the fuel injection system.

Also the DME relay. Relay's usually are not intermitent though. Relays usually work or they dont.

Just my .02 hope it helps.

carsontc · 06-20-2003, 06:05 PM

dirty fuel filter or weak fuel pump would not cause the engine to run better at higher RPMs

and

Quote:

Originally posted by JonVaccaro
He ruled out the sensors pretty quickly, saying that reference sensors and speed sensors either work or they don't- there's no in between.

I'm surprised he said that

check out this thead:

Sensor stupidity

Todwic had entered the "no in between" zone

that's why I was asking those questions (btw...none answered yet)

if they are new...I would recheck the .8mm adjustment per:

http://www.clarks-garage.com/shop-manual/ign-02.htm

but still good to try the BG Injector Cleaner in the fuel tank

just don't start changing big ticket items like the Throttle Position Switch or Air Flow Sensor or Fuel Pump just yet

Roger Hall · 06-21-2003, 08:27 AM

Quote:

Originally posted by carsontc
dirty fuel filter or weak fuel pump would not cause the engine to run better at higher RPMs

and

I'm surprised he said that

check out this thead:

Sensor stupidity

Todwic had entered the "no in between" zone

that's why I was asking those questions (btw...none answered yet)

if they are new...I would recheck the .8mm adjustment per:

http://www.clarks-garage.com/shop-manual/ign-02.htm

but still good to try the BG Injector Cleaner in the fuel tank

just don't start changing big ticket items like the Throttle Position Switch or Air Flow Sensor or Fuel Pump just yet

I was thinking that the fuel filter could cause hesitation.

I think it is possible that the sensors are intermitent, but I would doubt it. I would think that if the sensors are intermitent it would not be confined to a certian RPM range. However electrical sensors can act up.

I agree that you should not start replacing parts without testing them. Throwing parts at a car will get expensive and frustrating.