Wit's End - Help!

[944sracer]

Hello all,

I am having an extremely aggravating issue with my race car. Here are the details of the car -

1989 951 "S" - Lindsey club gate, stock otherwise

Here are the symptoms -

Cold starts fine
Once it warms up it consistently misses/stumbles
Upon revving (loaded or non) stumble increases
If ignored the car will actually start bucking

Here is what I have done -

New cap, rotor, wires, plugs
Tried 3 different sets of DME/KLR combos - problem still present
New vacuum lines
New fuel pump and filter also looked into fuel tank for collapsing pre-filter
New injector/knock harness
New reference sensors
New reference sensor harnesses

I have hooked up a timing light and I am getting a miss (significant delay in light flashes) on both the coil wire and each cylinder wire.

I am at a loss - I can't figure this out.

My next guess is knock sensor? What else?

TIA

[campbelljj]

Test dme temp sensor, AFM, and more likely have your DME & KLR computers tested since it runs fine cold and stumbles when warm. Cracked solder joints?

[944 Ecology]

You may have a vacuum/boost leak that only a pressure test of the entire intake system will find. You can build a pressure tester from an old fuel filter (with the end cut off) and an air compressor chuck. Pressurize the intake at the J boot (remove the AFM) and use a spray bottle of soapy water to locate any leaks by the bubbles present.

[kdjones2000]

Have you replaced your DME temp sensor?

[tyro]

No AFR gauge?

Tried a different coil yet?

[944sracer]

Thanks for the ideas!

I have not replaced the DME temp sensor or knock sensor yet - I think that is next.

I have tried a different coil

It ran on the Dyno beautifully and was actually running rich so I don't think it is air fuel

I don't think it is the DME/KLR as it does the same thing with different boxes.

[Freaky]

Similar problem here, only mine runs so rich it will foul out plugs in less than a minute. Tried different DME & KLR boxes, 3 different fuel pressure regulators, 2 different fuel pressure dampers, 2 different NTC II (DME temp) sensors, plugs, cap, rotor. Removed MAF and MAF CHIP and installed AFM and stock chip, still barely runs, bellows black smoke and fouls out the plugs immediately.

Anyone with any ideas?

[944sracer]

So I have some good spare Knock and DME sensors coming tomorrow. I will change them and see how it goes..

[guru944]

#1 problem with all 951's any year. Vacuume Leak, Intake manifold at Idle air control feed... very common problem due to the heat soaking from the Turbo exhaust housing & downpipe.

#2 TPS Malfunction. Fuel cut microswitch is bad or sticks.

I see this all the time on these cars... later..

[guru944]

Opps also add to #1 Vacuume leak at other aluminum hose fittings....as well.

Later...

[nize]

+1 on faulty TPS. try replacing it with a known good/new one.

[944sracer]

Ok So this is what I replaced today -

DME Temp sensor - Problem still present

TPS Switch - Problem still present

I did unplug the TPS (after the TPS swap) and the problem is GONE!!!! So I am thinking it is the wiring harness...

Am I right?

Now - next question - is there an aftermarket harness for it?

[nynor]

test the conductivity of the wiring from the TPS to the DME. i can't remember which pin it is at the DME plug, but a search should find it.

[944sracer]

Well I know it is in the harness and I found a kit from Lindsey Racing -

TPS CONNECTOR KIT 944T at LINDSEY RACING - Your Porsche Performance Parts Center

So my question is now - How far back do we think this kit goes? I am hoping it will go All the way to the DME/KLR.

I will call them tomorrow and report back.

Thanks for all the help!

[944sracer]

Update -

I replaced the TPS with a brand new one and the problem is still present. I also tested the AFM per the clarks garage procedure and it tested out fine.

I did make a blink code reader and something strange happened -

I got 0 blinks at first. So I unplugged the TPS to verify operation. It did give me the code of a bad TPS. Once I plugged the TPS back in I still got 0 blinks.

Then I revved the motor up and let the idle drop I got a random series of blinks that did not have any pattern.

Thoughts?

[DWPorsche944]

Does your car still have the stock exhaust? If so, the down pipe is a double wall pipe and the inner pipe collapses over time after many heat cycles. The back pressure makes the car run horrible. Unfortunately, you can't tell if it is collapsed without removing the exhaust and examining with a scope or cutting the pipe at the 1st bend...it collapses where the pipe flattens out.

[944sracer]

Update -

It has aftermarket exhaust. That would not explain why the car runs great without the TPS plugged in.

I dry ran new wires to the DME and KLR from the TPS. Still no change in behavior.

Quick Question - Could it be the cycling valve? Here is my theory -

3 wires run from the TPS to the KLR which also controls the CV. If the CV is faulty when the TPS is plugged in and tells the KLR it is WOT the KLR tries to communicate with the CV to control boost.

When the TPS is unplugged the KLR goes to Default (1.2 bar boost) and the CV maybe behaves more normal maybe because the CV is only wacky when trying to go WOT?

What happens if I run the car with the CV wires unplugged? just to test?

[Ky914Newbie]

Running the car with the CV unplugged will cause it to be fully closed and limit max boost to 1.2 bar.