Hi,
I need some assistance please.
My 1980 928 with an M28/1 Engine is miss-firing. The culprit is cylinder 8, If you pull the injector plug off, it makes no difference to the way the engine runs.
At Idle, the engine sends vibrations through the body and when you rev it the mis fire is very noticeable.
To solve it I have fitted a new coil, plug leads and plugs, distributor cap and had the fuel injectors cleaned and checked.I've also checked and replaced the auxiliary air valve. I've done a compression test to rule out a bent valve ( all 8 cylinders are equal readings of about 140 psi) and double checked the timing. I have ruled out an airleak by removing the "spider" and replacing all the clips and hoses. And I've fitted 8 new intake manifold gaskets. And to see if it's a vacuum leak I removed and plugged the pipe to the brake servo.
In theory there should be no more mis-firing but it's still there.
Is there anything special about the 928 that I should check?
Any help would be gratefully recieved.
addiotionally my central warning system has a mind of it's own, what happens if I disconect the control unit for it?

Thanks

 

You probably have a bad injector or no signal going to the injector. Put a 12 volt test light on the wires going to the injector and see if iy blinks when the car is running. If it doesn't the wire is bad and if it does the injector is bad.
I'm assuming that you've confirned that the spark plug is firing.

 

try switching the injector wires for 7 and 8 see if the problem moves ... or not ...

 

Try what Jim said. If it does, then you probably have a wiring problem - or the connector is bad. The connectors should be available to you for around US$5 - so you don't have to buy a new harness. If you have lost a wire in the harness, there are other solutions.

If that doesn't yield info, get thee to an o-scope. Nothing fancy, but you need to get an idea of what is really happening electrically at that cylinder and an o-scope is the only way to get a real picture voltage and duration wise.

Even one of the handheld units from Velleman or Fluke will do.

Greg

 

Thanks guys.
I've been through 4 injectors on this cylinder and swapped the 7 and 8 connectors around but to no avail.

I plugged a spare injector into the harness to check the fires and it was clicking away quite happily.

I've even swapped injector around on the fuel rail to see if it made a difference but it didn't.

O-Scope is the next step I think...

 

It's got to be a no signal to the injector problem. Sounds like a wiring issue.

 

If you have already tried 4 injectors and switched #7 and #8 injector wires plus fitted a spare which clicks ....... why are we talking about injectors ?? Is the spark plug wet on that cylinder ? are you sure that the spark plug is sparking ? .........one other thought the two resistors which power the coil are beginning to break down on 78-84 cars and can cause erratic running especially under load as a test try providing 12 volt power directly to the positive terminal small wire the # 15 terminal .. They are located on the inner fender left side next to the P S reservoir .. In addition the green wire from the distributor to the spark box is a shielded wire they break down and can cause erratic timing poor running . But if the # 8 cylinder has compression , has fuel , and has spark it should be running

 

If this were the problem, what would be inolved to fix it? That wire goes inside the distributor housing.

I think I'll check the resistors next.

Thanks

 

It's an ignition issue, no question. You've already figured that out with your switching the injectors out. You've got compresson, so your good mechenicaly.

The green wire, the resisters, and the coil are all for all 8 cylinders. They would not just be specific to the number 8 cylinder.

Question, when did this pop up? After this work on the car, or before?

Are you aboutly sure that that spark plug is sparking? If not get a spark tester, and check it. Cheap, but useful tool.

Have you changed the rotor recently? If not, you ought to when you change the cap, just for good measure. If the rotor's insulation breaks down, it can screw the magnetic pick up all up. That is expensive to fix, because you end up haveing to buy a new destributer.