Porsche Enthusiasts Club Forum

On startup i'd imagine the Motronic behaves like a PLC and only initiates combustion when certain criteria are met; i've got an intermittent problem of this criteria not getting met. I've hooked up a bank of status leds to the DME relay and everything seems to be ok there, i think the problem is with whatever has to happen for a spark to be sent. That said, i think i've heard the DME relay chattering once or twice when starting problems have occurred, but that isn't to say the relay is at fault.

It just cranks on for ages sometimes and the starter motor occasionally stalls, however i suspect the starter is generally ok for a 30yr old and always spins up to speed very well. I think the problem might concern whatever is telling the Motronic that the engine is actually, physically spinning; is there some kind of a failsafe to immobilise ignition if no crank speed is detected? I know such failsafes exist in fuelling circuits.

Also, is LE-Jet clever enough to shut off fuelling when you're coasting with your foot off the gas, or was oil too plentiful to be worrying about that in 1986?

I believe the crank sensor / ecu has to see 200rpm before the fuel is turned on permanently, it gets a small burst at ignition on position 2 to pressurise the fuel rail before start

Watch the rev counter needle when cranking. It should move / bounce. If it isn't... suspect the speed / reference sensors on the clutch bell housing. As Waylander says, the ECU needs to see 200rpm (and it uses the same sensor as the rev counter).

Also... see the "Clark's Garage" trouble shooting guide.

http://www.clarks-garage.com/shop-manual/ts-01.htm

That's good intel, cheers. Just as an aside, my tacho went through a phase of reading only half the actual engine RPM at one stage, it hasn't done it for a while now.

I would start by checking all the wiring connections and the cables to the crank sensor

When replacing my crank reference sensor I noticed that the crank speed sensor wire (Between sensor and plug) has been rubbed rather badly through the insulation - I 'fixed' it with a little electrical tape, but it's worth pulling the cable and checking!

My tacho does bounce when i crank it. I'll give the sensors and wiring a coat of looking at.