1986 760 turbo won't run

360beast

Hi all

I have a 1986 760 turbo manual running Motronic 1.1 that's been off the road since 2008. The fuel tank is full of rust and the fuel return line is leaking so I've cut the lines to the tank by the underfloor pump and put them in a jerry can of petrol so it is getting and returning nice new petrol. The spark plugs were varnished and full of black gunk so it has a new set of plugs. It will now start but die after a second or so, it's a smooth engine from what I can tell. I've changed the cambelt and tensioner so it's safe to keep cranking it over, the old cambelt was hard and cracked when I bent it.

I have fitted a "new" ECU and power stage from a car I broke for parts years ago that was a lovely runner and it's made no difference.

I pulled a fuel injector plug off out of curiosity and it still started on all four, so I pulled them all off and it started and ran on all four cylinders three times. I have ordered a set of noid lights and they will be here tomorrow so I can see if the injectors are actually firing, my thinking at the moment is the injectors are gummed up and allowing just enough fuel in to get it to run but they can't keep the flow up so it dies. I will pull the fuel rail off this weekend if the noid light shows it is firing to see if they're actually spraying.

Has anyone got any suggestions as to what it could be other than dead injectors? Motronic isn't my thing so if someone has decent knowledge on this then I'd be very grateful.

Thanks in advance

ramrod

Injectors get blocked on cars that are driven every day. I have seen it happen

What you can do as a test is remove the fuel rail together with the injectors, put the injectors into little cups and crank the engine.

Make sure all the wiring is connected though because the injector ground wires bolt onto the fuel rail where it mounts to the manifold, so you’ll need to make sure they are still grounded.

In any case I would still service (or replace) those injectors because there is a high chance some rust has made it past the fuel filter and the filter in the injector and wrecked it

dmc

I am not familiar with the specifics of Motronic 1.1 though a couple of generic tests may help.

It sounds like fuel is getting into the intake manifold without the injectors firing, two things come to mind.

Is there a cold start injector on that model? If so it may be either leaking or opening.

Second, and most likely, thing is the fuel pressure regulator on or near the fuel rail, which I assume it has. They have a rubber diaphragm in them that can fail and when it does it leaks fuel to the vacuum hose that goes to the intake manifold. Easy to test, pull the rubber hose of with the engine not running and there should be no fuel leaking out. Can also test with engine running, though you may have fuel squirting around a running engine which is not ideal.

360beast

Sorry for the slow reply gents.

I've stuck a noid light in it and it doesn't flash until the engine starts up so I assume that's the cold start injector that gets it running and then the other injectors take over. It dies and then the noid light stops flashing too. The worrying thing is that cylinder 2 doesn't flash the noid light at all but the other three do so I might have a second problem there.

I'll pull the fuel rail off when I get a chance and see if the injectors are even firing, if not then I've got a spare set in a 1998 940 engine I've got in the garage I'll chuck in it and see if they sort it.

I have tried a different ECU, FPR, Power stage module and fuel pump relay already from known running cars so I can rule them out.

ramrod

How does the engine wiring harness look?

Spac

Check the resistance of the injectors (Motronic vs LH2.4) before swapping them.

I absolutely forget all of the details, but I am fairly sure that the impedance changed across the different eras of EFI.
Your worst case scenario is that you replace high impedance Motronic injectors with low impedance LH2.4 injectors and end up burning out the injector drivers in the ECU.

360beast

There is 0.1 ohm difference in resistance.

I've lifted the rail and cranked it, one injector sprays, one injector dribbles and two do nothing! So I've fitted the 940 injectors and it runs for the first time in maybe 16 years! Very smoothly too considering it is on old stale fuel out of a bucket! I've got some 101 octane fuel in a jerry can I'll run it up on and see how it goes.

Thanks for the help everyone