740 engine died

Brad · 2023-11-20T00:18:12+00:00

Yesterday I was driving my mum's 1990 740 and the engine quit twice. The engine was at operating temp and there was no warning or indication that anything w...

Brad

I just got it started again. Engine has cooled a fair bit though. Thinking this is a heat related issue. Something is getting hot and failing.

Tow truck should be here shortly so I'm not going to drive it.

ramrod

Fuel pump relay can do that

Ex850R

Test everything at home, the tow costs and time lost add up. Swap the modules out for known good ones?

nugget_940

..power stage module?

Brad

nugget_940 I didn't put any new thermal paste on the replacement one but I'll do that this afternoon. Thanks for the reminder.

I've considered going to wasted spark on my 940s. Just haven't got around to it yet.

Brad

I did some digging around after getting towed home. Of course it started and ran fine while sitting in the driveway.

On a whim I replaced the ignition power stage with a used spare one from my stash. So far I've taken it for 2 roughly 10 minute drives with about a 5 minute break after each. Now I've just got back from about a 20 minute drive. On all 3 drives she ran perfectly and no hint of any issue. I am hopeful this is problem solved but not yet confident enough to say that with certainty. I'll see how she goes over the next couple of days.

Thanks everyone for the input so far. Really appreciate it. I will provide an update in a day or two if all goes well.

Major Ledfoot

Brad Nice catch. Guess the things are getting to the age now where the old igniters are starting to fail.

Wisdom suggests a spare igniter ought to be carried in the tool kit from now on, just in case.

Rock Auto has them, at a not indecent price.

nugget_940

Brad good to hear,

i think i replaced mine early on in ownership but cant recall.

Did you put thermal paste on the back of it?

FWIW if you convert it to wasted spark you bypass the factory power stage module 😛

Brad

Major Ledfoot

The Bosch part number on the old power stage is 0 227 100 124. It's still available from a number of places in Oz too surprisingly. It appears to be used by many different makes including Peugeot, Porsche, Citroen among others.

If this problem is indeed fixed I'll get a new one for it so I'll still have a spare in the stash.

dmc

Glad you had a quick resolution time.

The thermal paste going off is what kills them, they lose the ability to transfer the heat out of the module into the heat sink and overheat. Then something in them fails. Re-applying fresh paste every few years is a good preventative measure.

Ex850R

dmc
I imagine wiring and connections need attention too?

ramrod

Ex850R

Only on the radio suppression relay

The connectors tend to come out of the relay often on those

Brad

Looks like the 740 is fixed. It's been driven a few times since replacing the power stage and there have been no breakdowns. Happy Days.

I also replaced the thermal paste on the back for good measure. Don't really need another breakdown.

Is it possible to mark this as solved?

jamesinc

@Brad yup, I highlighted the solution already, you should see a ✅ and the answer in green at the top of the thread.

Brad

Thanks James.

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