So, this just happened. Damned electricals...

Is there any reason to suspect the XDI system to be faulty?

 

Two thoughts that come to mind, based on the electrical type faults and potentially an over heated circuit are, old engine immobiliser fitted and removed causing an intermittent type fault on the ignition side. The second thought is fuel pump getting hot and faulting, or some switch in the fuel pump circuit faulting and isolating power to the pump...

What are single source failure points, a single wire to the coil, the coil itself, any other simple parts that can be tested or substituted to eliminate the problem???

 

I'm not expecting erratic tachometer from fuel problems. The tach should have no trouble keeping pace with engine speed, a stock engine couldn't accelerate or decelerate fast enough to outpace the needle to any noticable degree.

 

I'd now suspect the XDI box, itself, if you have checked verified the relevant connections/voltages. But since the failure is essentially intermittent, there's probably no way to reliably test it other than replacing it with a known good one, or sending it back to XDI for evaluation...

 

The car i saw with the engine cutting out was traced to split diaphragms in the fuel distributor, it was back firing like no tomorrow and dumping a lot of unburnt fuel past the rings. The sump level increased due so much fuel. U r a bit different but you might want to look to see if the air plate is free, also the accelerator cable is free to move on the engine. If something is sticking the fuel could be either on or off, with nothing in between ?

 

I spent about half an hour combing over the wiring diagrams for both an 82 and an 85 QV, trying to see if there might be something else that heats up and could cause the car to die.

It still feels like the bouncing tach is a critical clue. I'm assuming when you start the car and when you (normally) turn off the car, the tach doesn't bounce. If that's the case, then only the ignition modules can cause the tach to bounce (mechanically, the engine can't do that).

Couple silly questions, what are the purposes of the Engine Temperature Sensor (139) and Thermo-Time Switch (107) in the diagram below?

The Engine Temp Sensor only receives power and has no sense line coming back--so I'm assuming it plays a mechanical role in the warming up of the engine. Have you checked that this device is working correctly? Without knowing the purpose, it's hard to guess if it can kill the engine.

The Thermo-Time Switch feeds the Start Injector, which appears to only be powered when cranking the engine. From the diagram, it seems that while cranking the engine AND the engine is cold AND it hasn't been cranking for very long, the Thermo-Time Switch add's a bit of extra fuel via the Start Injector. Is that right? I can't quite come up with any scenario where this would cause an issue once the vehicle is started, since it's only powered when the ignition switch is in the "Start" position, but wasn't clear if my understanding was incorrect.

Lastly, the Fuel Pump Relay and Start Relay have a strange relationship to each other. The Start Relay serves two purposes: as a "bus bar" to provide the Thermo-Time Switch with power (via the crimped connector at terminal 87) and to determine how the Fuel Pump relay is energized. It appears that when the Air Flow Meter switch (72) is "at rest", the Start Relay energizes the Fuel Pump Relay via the ignition "Start" position. When the Air Flow Meter is "engaged", the Start Relay energizes the Fuel Pump Relay via a direct feed from the ignition "Run" position. I'm guessing this serves the following purpose:
- If the ignition switch is in "Run" position, but not started, the Air Flow Meter is "at rest" (no air flow). This means that the Start Relay is powering the Fuel Pump coil via the ignition "Start" circuit. Since that is not powered, the Fuel Pump is not energized, so it stays off
- If the ignition switch is in the "Start" position (cranking), but not yet started, the Air Flow Meter is still "at rest" (no air flow). Fuel Pump coil is still powered via the ignition "Start", and since it's powered, now the fuel pump starts a-pumping.
- As soon as the Air Flow Meter is "engaged" (air a-flowing), the Start Relay starts powering the Fuel Pump coil via the ignition switch "Run" circuit. Since that circuit is powered either when the switch is in "Run" or "Started", that means the Fuel Pump continues a-pumping.

If that long-winded explanation of the Fuel Pump and Start Relay is accurate, then there would be two potential points of engine failure:
- Air Flow Meter is grounding out when the engine heats up, even tho air is flowing thru it. This would cause the Start Relay to power the Fuel Pump coil via the ignition switch "Start" position...which, of course, is not powered while the car is running. In this case, your fuel pump would just stop pumping.
- The Start Relay is failing when the system heats up, and switches over to the ignition "Start" position. This one is harder to believe, as the Normally Closed position of that relay is to supply power from the "Run" position.

I'd still suspect the ignition modules, but it might be worth taking a quick look at the Engine Temp Sensor, the Air Flow Meter and the Start Relay.

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Oh, and I've got a carb car, so technically have no idea what I'm talking about w.r.t. the injected complications

 

That item 139 looks very much like a WUR.

 

I had a very similar problem with my 308GTSi. The first problem I found was the fuse board failed at the fuel pump lugs and bypassed it. Thought I had it for a couple days. Problem came back. Finally traced it to the multi wire block connector under the /or around the digiplex. I was able to reproduce the problem by wiggling the connector. I sprayed the block with dielectric used some small tools to clean and rough up the contacts used small fine pointed hemostats to tighten the female part of the plug. Haven't had the problem since (20 yrs ago). I did just purchase the SFB (Stinson Fuse Block) and will be replacing it soon. Hope you find the gremlin(s).

 

The ETS sensor is not the same as the dash temp gauge sensor. The dash temp gauge sensor is item 12 in the diagram posted above and goes directly to the dash gauges. The ETS sensor is also called a WUR (warm up regulator)...thanks @yelcab for it's more common name. Reading up a bit more, this little guy is part of your fuel flow system and is intended to restrict the fuel flow when the engine is cold. If the ETS has gone bad, it seems like it's possible to affect your fuel delivery.

On the fuel pump, I'm surprised when the engine is dead that the pump is still operating. The fuel relay fuse will still show voltage and there will still be some current draw at the fuse--but I would expect the pump itself to be off (and no current draw at the pump itself). Assuming I'm reading the schematics correctly, it seems that if there is no air flow measured by the Air Flow meter (e.g. the engine is not running), then the Fuel Pump Relay will not be energized. Perhaps the Air Flow meter was removed or disconnected from the system. In that case, you should be able to hear your fuel pump turn on as soon as you turn the keyswitch to the "Run" position (but don't start the car). While carb cars like mine turn on the fuel pump as soon as keyswitch is set to "Run", I'd thought the injection cars had solved this problem by the addition of the Air Flow Meter.

Oh, and by "ignition modules", I'd meant your XDI system (which is an ignition module...just not a Ferrari one )

 

Just a point of reference, the XDI unit cannot send a tach signal directly to the tachometer, there's an MSD tach adapter box used to do that. If you're getting and erratic tach signal, it's either coming from the MSD box or more likely the XDI unit.
Also.....I hate with a passion those magnetic sensors, they are crap.. Worse then crap.. Refuse to use them. Use a hall sensor, cherry, now it's ZF.. it'll need a 5-8v power source, but it's worth the effort. God how I hate those mag sensors, always some issue with them. They need shielding, they are sensitive to heat,etc.. routing the leads on the 308 make them a real PITA... So I don't.

 

I'm sure you've checked this already but my 328 uesed to cut out just as you describe - but not perhaps as frequently. I traced it eventually to the big wiring connector block on the left side of the fuse board where there's a big cable running to earth (?) and the block would regularly get hot, turn brown and burn and therefore created an intermittent circuit that stopped the fuel pump etc. If I let it coll down it would start right up again. I changed the connector block 2-3 times over a 5 year period...! Yours is probably a different arrangement though.

Good luck - very frustrating but great when its fixed - and it will be...!

Mark
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that is the old fuel pump drawing too much current.

 

@thorn what's the latest? BTW, happy to draw out the schematic on the Fuel Pump circuit if that helps you to understand my cryptic words (altho again, I'd suspect your XDI's as most likely culprit).

 

Hi John,

I had a similar problem years ago on my '72 Alfa. After desperately sourcing the problem, sleepless nights, a lot of dirty words and spells, thinking about selling the car I found the problem.
It was the wire from the ignition switch to the coil, which was damaged and caused intermittent failures.
Check the ignition switch too. If the contact of terminal 15 inside the ignition switch is faulty, it would definitely lead to such symptoms. To isolate the problem you can run a hot wire from the battery to the coils. But don't wait too long till you start the car if your ignition system is not protected against open-circuit voltage. For stopping the engine you have to disconnect the hot wire of course.
This is how I found my problem after frustrating weeks. Ran a new wire from the ignition switch to the coils and never had this problem again.

Best from Germany
Martin

 

That's a good point, I have seen that issue with the ignition switch getting faulty and cutting power to the system. Used to be reasonable for those switches... Expensive buggers.. shared with the bmw 2002, but they caught on as well to the price point. Arcing on the contacts is the issue, to bad they are not rebuild able.

 

Thinking here... For a fuel cut test to see if the fuel delivery is being cut... Bottle feed as it gets to the stumbly dying phase, if it'll run on the bottle then you know what direction to go in, if not.. Then it's electrical.
Vacuum line at throttle to syphon fuel from bowl or cup. Or starter fluid at the distributor, though I think the syphon would work well.
Otherwise it's fit the CIS gauges and monitor the control pressures.

All that aside, sounds like an electrical issue though. Inline glow tube for the spark plug to monitor the firing... Or a scope to trace it. Scopes are very handy for electrical diagnosing.

 

And beware of the new reproductions probably coming from China. If they are of the same quality like my repro window switches ( I meanwhile scrapped and rebuilt my old ones) you are in serious trouble.The contact material of the repro window switches is of such mean quality, that arcing destroys the contacts within a few switch cycles.

BTW. Maybe you have heard of the nit-picky Germans Therefore: not only BMW 2002, but the whole 02-series ('Neue Klasse') from 1502 till 2002.

I would love trying to rebuild one. If I find one from the scrapyard I will try it. I think, the only problem is reassembling them with the hollow rivets what should be doable with a self manufactured jig.

Best from Germany
Martin

 

to me it sounds like a bad ground contact somewhere and nothing more, something that loosen with vibrations and heat combined: maybe the problem is to learn "where" it is...

good luck

ciao

 

Or a live wire contact.
On the 308 -late factory electronic ignition- I once experienced a complete cut-out of one cylinder bank each time while left cornering.
After hours of investigation I found one of these poorly made connectors of the distributor pickups on the front bank coil amplifier being faulty.
The tiny movement of the wire while cornering was enough for the connector to lose its contact.

Best
Martin