M48 equipped 911 - won't idle, stalls

[Jwasbury]

The subject is an '83 911 race car with individual throttle bodies and a Motec M48 ECU. The engine was dyno tuned and runs fantastic at high rpms. It won't idle, however, and you have to keep it alive with the throttle. It stumbles even with the light throttle until rpms get above 1800-2000. Since this is a race car, I have been living with the issue because it goes like stink on the track, but it is a pain when pulling into the pits and I hope to fix it in the off season.

Was able to get my laptop interfaced with the ECU and found that while running, it's throwing a sync error diagnostic code. The crank sensor is a mag type...36 tooth if memory serves. sync sensor is hall effect. The air gap btw the crank sensor and toothed wheel looks proper...haven't laid eyes on the sync sensor yet.

I'd appreciate any tips for where to start on troubleshooting this issue.

Thanks,
Jacob

[stevieturbo]

So it's always been a problem ?

The sync error could be misleading, or it could be the problem.

But sounds like it could be an installation or tuning issue.

Are all the ITB's balanced for even airflow ? What is your current lowest idle speed ?

You basically need to start from this, and then adjust the blade position smaller incrementally and adjusting tuning as required to maintain idle until idle speed is at a sensible level.

[Jwasbury]

I just bought this car, driven it about 5 trackdays, and it's been like this since I got it.

The ITB and Motec conversion was done by Rothsport in Oregon for the previous owner. I think it's tuned very well for high rpm, but it doesn't like to idle, especially when hot. I think it does a bit better when cold, although my impression is that the idle problem has been getting progressively worse. Hard to say for sure since I have only used the car 5 times.

I don't know if the ITBs are balanced, and don't exactly how to check this. Have done balancing on multi carb motorcycles and assume its similar, but haven't got the correct tools or the know how as to specifics on the ITB set up on this car. The ITBs are Jenvey units and I have looked at their website for info on set up, but I haven't fully dived into them yet. I did try adjusting the primary rod of the throttle linkage (which doesn't change balance) to see if I could get the idle speed up, but it did not seem to have any effect. At this point, even if I use the gas pedal to keep it alive, it stumbles and wants to die until rpms get up to 2k or so. The thing screams at high rpm, seems to run perfectly on the track without slightest hesitation, stumbling, etc. The gearbox is close ratio, and rpms are probably never below 3500 while lapping. Not scientific, but the tailpipe soot is a nice golden brown, which I always thought was a sign of good tune (at WOT at least )

[David Ferguson]

Well, I would work with Rothspeed on the idle issue. May just need a different fuel or ignition map for the current fuel you are using, or perhaps a sensors isn't correct, and it's compensating for an invalid barometric pressure for example.

What idle speed are you trying to achieve? What is the ignition advance when it's near that speed? Sometimes you need a bit more ignition advance below your target idle speed and a bit less above, so that the ignition can try and help hold the idle steady. From your description it sounds like more advance might be needed, or the fueling is just wrong - what are the Lambda readings?

One reason you might have less advance at low RPMs, it to make cranking and starting easier, so there is a tradeoff.

[pftuning]

If you are seeing RefSync error while running, it certainly warrants a further look by your dealer.

[Andrew C]

I too run an M48 on a 911 race engine. We had the same issue. Check air flow to the ITBs!!!! then reset your idle speed to 900rpms See if it will hold 900. Good luck..