Hi Guys,
Im in need of some help as i am a bit stumped on this one. We have a customer with a 1986 944 turbo. He has owned it for the past 2 years. Its not a daily driver but instead used for the occasional supersprint and targa regularity event.

A bit of history with the car, On the road it operates perfectly. On its first track day around 2 years ago, the car started to develop a hesitation at 3000rpm, it happened 4 laps into the session and the car could not be revved past 3000rpm. popping and carrying on. We did not witness this as at the time we were not at this track day.

At this point we decided to do some maintenance on the car as the owner was going to do a targa tasmania regularity and we wanted the car to be perfect. The parts we changes at this time include, Speed reference sensors, TPS, earth straps, NTCII sensor, Rotor, plugs, leads, coil, DME relay. Basically all the inputs that were old. The car did Targa Taz an was good for the first day, but the 2nd and 3rd it started to play up. See video below for hesitation.

On a few occasion the customer drives from his home to our workshop in really heavy Sydney city traffic 35 degree celsius day. When the car gets really hot after 30minutes of bumper to bumper traffic it would start to play up, again not revving past 3000rpm and quite difficult to continue driving. If he let the car set for an hour or so the problem goes away and the car is back to normal operation

So come to this year we decided to remake the entire engine harness and while we were at it install a Vitesse MAF kit to eliminate the original barn door air flow meter. At he same time we installed a VRT8x piggy back so that we could tune it for both 98 octane and E85. I tuned for both fuels and drove the car to and from home for over 2 weeks to test the car. It ran perfectly, boost set to 17psi. Not once during this time did it have any hesitation, but then again i could not push the car hard on the road to really test it.

First track day this february and the car does 3 laps in the first session before the hesitation kicks in, Its still there. This time though its much worse (this is also the first time we have seen the problem first hand) . Customer nurses the car back to the pits, it won't idle and its belching lots of black smoke from the exhaust. running incredibly rich. It won't rev past 3000rpm because we believe there is that much fuel being injected that it can't fire it. I realise at this point that the hesitation is that much worse now as the tune i have on the piggyback is slightly richer then the previous, hence exaggerating the problem. when i say slightly i mean 11.5:1 a full throttle as compared to its previous tune at 12.3:1 at full throttle.

We have a few spare parts we are able to change over in between session. The first is a spare coil and TPS. Its been an hour since last driven so it starts perfectly and out he goes. 3 laps later he nurses back to the pits. Problem still persisting

Next up the DME and KLR computers. When removing the DME computer it was incredibly hot to the touch. Swap the chipboard over onto our spare DME and the customer goes out in the 3rd session. again it had been about an hour, but this time the car started rich, almost got to 2 laps and the hesitation begun again. Customer called it a day as it was raining on the track now.

The next day he drove to our shop again, this time the problem was there the entire time, from when he stated to the shop. This was great because now we could really try and diagnose this. The problem previously had only shown up when it was driven very hard and the car got very hot. On our Air/fuel meter it was reading extreme rich below 9:1 AFR. It was at this point that i swapped the DME computer back over. The idle was perfect agin and the AFR came back to 14.5:1. Thinking that we may have found the culprit being that his original DME was actually faulty and only worked properly when hot. But it was not to be as of this weekend the car again played up. Driving from his home to work, about a 15minute highway drive it begun to hesitate and become undrivable. He pulled over turned it off then turned it back on and it all seemed fine for the rest of his drive to work.

Something on this car is switching over, causing rich running conditions and for the moment I'm a bit stumped. In between these events we have also tried a replacement ignition switch, new battery, NTCII, coil, TPS, DME relay and a few other components i can't think of right now. Anyone ever had an issue like this or have any ideas? I'm going to get the car back and take it to the dyno, hopefully we can get the problem to arise and i can log some detains with the Vitesse MoMonitor
Thanks for any help
Sean

 

Did you try changing the fuel pressure regulator ?

 

"When removing the DME computer it was incredibly hot to the touch."

I had an issue related to an aftermarket coil a couple years ago that caused my DME to overheat. The symptoms were quite similar to what you describe. When the DME gets too hot it will reset. When it resets, the car will misfire and when the DME starts back up (a few milliseconds later) its initial readings are messed up causing the car to run open loop and rich in my case. I wonder if something is causing the DME to overheat. You can check it with an infrared temperature gun. The case should get only a few degrees above ambient. Mine was about 25F over ambient. My problem got progressively worse. Before figuring it out I toasted a DME and a coil.

 

Yes new FPR Blade,

Thanks interesting Shawn, The DME in this car gets really hot, i would hazard a guess at around 50-60 degrees Celsius. seems very excessive to me, but i had never checked any of our cars DME's after a track session. I also noticed that when he drove here to our shop it was quite hot just from street driving. possibly 40-50 degrees.

Now how to find out what may be causing the DME to overheat?

 

The coil harness is separate from the engine harness. May want to check that wiring.

Not sure if you have access to an oscilloscope but Joshua found my issue by looking at the ignition driver output with an oscilloscope. My issue was a resonance that was occurring between the DME and the HVC II I was using. We fixed by adding a capacitor to the DME. Obviously your situation is different but I would say something is causing the DME to get too hot. I measured it on several cars.

 

Hi Sean,

Being able to reproduce the problem is a big part of the solution. So far we can conclude that the problem is not related to the DME & KLR boxes, as the problem occurs with different boxes.

Let's figure out if the problem is caused by an input to the DME or on the output side.
On the input side (I doubt it, but should be eliminated), a simple data log with the MoMonitor and you can catch any irregularity.
On the output side, you have the Ignition and the injectors. Both generate heat!
A few things to help you narrow it down (when the car is acting up):
- If we can determine if the problem is related to a single cylinder or across the board, we'll have more to work with. With an infrared temp gun, check the temps on each of the exhaust runners (about 1" from the head). If all runners have similar temps, then the problem is across all cylinders. Any runner with a big temp delta between it and the other runners will be your problem area.

- If the signal between DME and injectors is shorting to ground, then the injectors will activate at 100% duty cycle and will cause serious black smoke out of the exhaust. And will probably kill the engine at idle. Testing this may be a bit more involved. A continuity test helps, if you can get it done fast enough before it cools down. You can always run a dedicated test harness to the injectors, bypassing the existing injectors harness.

- As mentioned earlier, the ignition side can be the issue (high probability). I'm sure you already tested or replaced the distributor cap & rotor; so assuming that's not the problem, let's concentrate on the ignition circuit and path to the coil.

If for some reason the coil is causing the DME to overheat. For testing, you can bypass the DME ignition circuit and use one of the external ignition modules I sent you way back (email me for instructions on how to perform this test). If the DME stays cool and the problem goes away, then you isolated the problem and we'll have to figure out the details.

I would start with the MoMonitor log, then the temp test. Both are simple and quick to perform. The coil test will be next on my list, then the injectors.

 

This is sounding very promising guys, i really appreciate both your help on this one.
We have an oscilloscope at work Shawn and i can check that test.
John, i think that external ignition module sounds like a good place to start. Seeing as i can't get the car to play up right now. We do know the DME ecu gets really hot with just 20-30 minutes of driving. If we install the external module at least we can then isolate the factory module from the equation. If it removes the heat from the DME then we may be onto a winner.
Its very frustrating when we can't get the car to play up when we drive it. hence the option to run on the dyno with the MoMonitor and sees if it picks anything up.
Regards
Sean

 

Just wondering about injector impedance if the injectors are not stock... not knowing the history of the car, I know that some install aftermarket injectors which have a different impedance to stock without also using ballast resistors. This could cause more current to run through the injector drivers than they were designed for and could cause the DME to heat up.

Just a thought guys... good luck getting it all sorted out!

 

Good point. However, it does not apply here as the problem occurred with stock setup.

 

Sean, check your email.

 

another good point Scott,
the problem did occur before we installed the larger injectors but to a lesser degree such as in the video above. When it plays up now its much more violent and very difficult to drive afterwards. The owner has said that once this issue arrises on his way to work it can stay in that state for quite a few minutes possible up to 20.
The injectors we have now i think run the same impedance as the stock ones.

Got it John

 

Alternator output ?

 

From memory, stock injectors are supposed to be around 5 Ohms... I never found any larger after market injectors with that impedance but I haven't looked recently.

Interesting that the problem started with stock injectors... I wonder if they were still within the specified range for impedance? From what you wrote earlier it seems that the problem is not only more aggressive with the new injectors but the onset is also earlier.

 

Check the DME temperature sensor.

 

I once used a Nology coil "amplifier" that made the DME smoking hot. I burned the bajesus out of my finger trying to find what was causing the DME to be so hot. Spoiler alert, it was the ignition transistor... Removing that reduced the DME temp dramatically. If you have stock everything, I'm not sure what would cause two different DME's to heat up like that.

Just some random thoughts that seem unlikely to help... The ignition pulse is first produced in the DME. That signal then goes to KLR9 AND to the tachometer. It seems possible --in theory anyway-- that a faulty tachometer (or dash harness) could screw up the initial ignition pulse somehow. Also, you mentioned rebuilding the harness, but it might be worth noting that the final ignition pulse runs from DME1 to that 14 pin connector on the firewall, and then from that connector to the coil -- i.e. not in the engine harness you rebuilt...