740 engine died

Brad

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 was wrong. As I turned the corner into my street the engine quit. I walked the last 300 metres home and grabbed my spare fuel pump relay and radio suppression relay. Swapped the fuel pump relay and the engine fired up. Beauty! Or so I thought.

Got about 100 metres down the road and it quit again but this time I saw all the warning lights randomly flashing a few times until they finally stayed lit. I swapped in the spare radio suppression relay but no go. I jumped the fuel pump relay pins in the board and got the fuel pump running but it still wouldn't start.

I checked for fault codes and the Fuel Injection side displayed code 311 which is something to do with the speedo signal being absent. My research indicates that this would lead to an idling problem but wouldn't cause the engine to quit. And i hadn't noticed whether or not the speedo was working at the time.

I tried cranking it again and the engine started so I got it home and left it idling for a bit but it didn't quit again.

I am wondering if this might be the crank position sensor. Can they go randomly faulty without causing a code to be logged? I know they can become heat soaked and fail. As far as I know the sensor is original so, being a 1990 model, it's 34 years old. I checked the plug and the wiring for it but didn't find any obvious problems but I didn't remove it so at the moment I can't be sure there is nothing wrong with it. I am going to try swapping it with a new spare one tonight and see how it goes.

Is there anything else I should be looking for? Thanks guys, I appreciate any help.

Major Ledfoot

Speedo signal isn't from the RPM sensor - the speedo signal is from the diff.

The RPM sensors at the rear of the engine often fail when the plastic wiring insulation ages, becomes brittle, and eventually shorts out the RPM sensor. Before shorting out completely, they will fail intermittently. If you've had coolant hose leaks from the feed to the heater core in the past, it will happen eventually. It's worth removing the sensor and inspecting the condition of the wiring, particularly where it goes into the top of the sensor.

ramrod

Yes change the crank sensor

It needs changing anyway even if not faulty

Brad

Thanks Major Ledfoot and ramrod. I'll try swapping the sensor tonight. I've swapped cars with Mum for a few days as she won't drive the 740 until it's fixed so I'll be able to test drive it a few times and, hopefully, confirm a fix.

Ex850R

Also fuel tank filter sock on pump clogged? Fuel filter blocked? Fuel lines old and sucking in sealing off flow?

jamesinc

Cluster lights flashing and no speed signal at the ECU may indicate a problem with the cluster, given that the ECU speed signal comes from the cluster. The signal from the diff is processed by the cluster and the output signal is fed to the ECU.

However given what you've said I would just replace the crank position sensor anyway. If and when it goes dead it'll properly strand you, why wait for it to happen?

Brad

Thanks Ex850R and Jamesinc. Both very good suggestions too. I'll keep the post updated as things progress.

Inski

I would check the intank fuel pump and check the hose hasn't cracked or broken off.

dmc

I would suggest determine the cause of the no-run first. I had a similar situation in a 740 a while back, it was the ignition module/igniter. When it is failing is there spark? Is there a fuel pump under failing conditions (sounds like this will be fine)?

There is not much in these old engines so not much to check. Considering it just flat died and did not slowly die it sounds electrical rather than fuel pressure related.

Brad

@dmc, yeah I get what your saying. I didn't have the presence of mind to check for spark when it died but realised later that I should have. I'll keep a spare spark plug in the car with me so I can do a quick check if it happens again.

Brad

Okay I replaced the crank sensor last night. Decided to drive it to work this morning. I am now sitting on the side of the road waiting for a tow truck. Same symptoms as before. Just suddenly cut out with warning. Tachometer dropped to 0.

I managed to get it started a couple of times but it dies after a few seconds later.

I have used a timing light on each of the ignition leads in turn and determined there is no ignition. There are no fault codes. I had been driving for about 20 minutes so engine was at operating temp.

What to check next? Ignition coil? Ignition amplifier?

dmc

Check power to the coil, 12V at the + side with the key on - this will be fine though best to check. There is a slim chance it will be power to the ignition system (EZK, coil etc, from a fuse). You said the tach died straight away so it sounds like no signal to coil (as that is what drives the tacho). Next is to check if there is a signal getting to the ignition module/igniter/amplifier. Not sure how to do this on the side of the road. It sounds a lot like the ignition module, or maybe (super rare) the EZK failing when hot.

Getting closer though. Also check the spark output from the coil lead, not sure this is easy on a 740 though unplug from the dist cap and rest 1/2 to 1cm from something metal, like a strut top bolt or intake manifold and then crank it over. You will know if there is spark.

ramrod

Have you done anything with the radio suppression relay and it’s wiring?

740’s suffer from this failure.

Do you have an injector test lamp?

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.