E30 Zone

I’m hoping these symptoms point to a specific issue and you guys’ experience might save me some time.

M10 b18
L-jet 1983/84 318i
78000 miles

Problem:
Misfires at 0-20% throttle opening.

Background:
Ӣ Car was running perfectly for the last while after valve adjustment and ignition timing was set to spec.
Ӣ Coil, plugs, leads, cap & rotor , air and fuel filter are all less than 12 months old
Ӣ Looked for vacuum leaks just with a can of Easy-Start but found nothing
”¢ Idle is ok, the odd little stumble maybe but it’s steady around 1k, a little higher when hot. This hasn’t changed.
”¢ Misfire problem started suddenly one morning, I thought it may be bad fuel cos I had just filled up, but the issue hasn’t changed even after many more fills.
”¢ After a cold start, it will miss if I rev the engine. But it’s Ok once it warms up a bit. It’s been like that to a certain extent since I’ve had the car.

Known:
Ӣ Car pulls cleanly once throttle is open more than 20% (The issue is really a problem when cruising at very small throttle openings or in town.)
Ӣ Misfire is not affected by load. Happens the same going uphill or downhill, in all gears (but always only at small openings)
”¢ Doesn’t miss under deceleration on a closed throttle.
Ӣ Problem persists even when warmed up BUT it does seem to improve after 30mins or more constant driving (hints at a lack of fuel maybe?)
”¢ Throttle switch clicks as normal but I haven’t yet tested continuity.

Theories
I suspect a throttle switch maybe but I’m unsure exactly what it does.

Could it be an AFM issue? Or does the fact that it only happens at certain throttle positions rule this out?

How about a fuel pump/pressure issue? But wouldn’t this also be apparent at high loads and WOT?

Update:

I'm not getting any voltage at the connector to the throttle switch.

The throttle switch itself is working fine; continuity at closed and WOT positions.

I cleared the coolant passage on the throttle body which was completly blocked.

Next step is to check the L-jet unit. And check for continuity through the harness.

Any good guides for doing this?
Also, I have found a few different part numbers listed for the L-jet units, which is a bit puzzling.
I'm just scoping out ebay incase I need a replacement.

Worn track in the AFM?

DHFiS wrote:Worn track in the AFM?

Thanks.

I haven't opened the AFM but I suspect a worn track would create symptoms also at other throttle openings.

The throttle switch is not getting any voltage (should be 5v).
I will try to resolve that before questioning the AFM.

Update:

I've found my car is different than what is described for L-jet models in the Bentley manual;

Looking for 5V at the throttle switch harness connector with ignition on, I get nothing.

But with the engine running, there is a difference with the switch disconnected...

So I checked voltage at the connector with the car running and find 13.85V from the middle pin to either of the other pins.

Any advice would be much appreciated.

If it is the same as the M42 the three pin connector goes to a variable resistor not a switch. The middle pin is the wiper and the outer 2 pins connect to the ends of the resistive track. The ECU reads the resistance and can tell where the butterfly is between fully shut and WOT.

DHFiS wrote:If it is the same as the M42 the three pin connector goes to a variable resistor not a switch. The middle pin is the wiper and the outer 2 pins connect to the ends of the resistive track. The ECU reads the resistance and can tell where the butterfly is between fully shut and WOT.

Actually it is the connection to the loom I'm checking (for voltage from the ECU).
The throttle switch itself is just 2 switches giving continuity at closed or WOT.
I've already verified the switch is operating correctly.

AFM looked pretty good inside, only slight line in the track, not badly worn.
Despite stuff written online, I dont think you can test this with a standard multimeter.
I adjusted the track position anyway to see, but made no difference so moved it back.

The problem has been getting much worse in the meantime and the car is now undriveable.

Before I buy a new ECU, can someone tell me;

Is the bentley wrong about the 5V at the throttle switch connector with ignition on for L-jet cars???

It doesnt make sense to me that it would be 5V with ignition on then 12v in run.

For the sake of completness, I'll update on this until I can hopefully solve it.

I got the multimeter out and checked the ECU connector as per the bentley.

Firstly, my car has less pins than are referenced in the Bentley and the wiring diagram.
I seem to have no WOT reading from the throttle switch, just one AFM input and no O2 sensor.

The throttle switch itself is switching correctly but there seems to be an issue with the wiring.

With the switch removed, I figure I should have continuity between pin 2 on the switch connector, and pin 2 on the ECU connnector.

Instead, I am getting continuity between pin 18 and pin 2, i.e. it's as if the switch is in the closed position all the time (even with the switch removed).

I'm at a loss to figure out how this could happen.

Diagram here;


*Update: The Bentley diagram of the L-jet unit connector does not match my car

Ok. got back to working on the car today.
Discovered the problem goes away when I disconnect the vac line from the distributor.
Did the suck test on the dizzy and it seems to hold a vacuum ok.

Car drives ok with the line disconnected but I can hear faint pinging under load so need to get it sorted.