930 hesitating on low RPM

[himizoli]

HI

We have a 930 turbo with the following problem
- On low RPM 1000 to 2500 hesitating on little throttle. (like misfire the engine)
After 2500 RPM the car pulls really nice

I have checked the follows
- Ignition system Ok (cap, rotor, cabels , plugs new)
- Fuel system WCP 3.6bar system Pressure 5.2bar(this a bit low???)

Any idea?

Thanks

Zoltan

[flightlead404]

Originally Posted by himizoli

HI

We have a 930 turbo with the following problem
- On low RPM 1000 to 2500 hesitating on little throttle. (like misfire the engine)
After 2500 RPM the car pulls really nice

I have checked the follows
- Ignition system Ok (cap, rotor, cabels , plugs new)
- Fuel system WCP 3.6bar system Pressure 5.2bar(this a bit low???)

Any idea?

Thanks

Zoltan

You don't mention the coil? Also, when you say cables does that include the green wire and also the two leads to the coil (mine were separating at the crimp)?

What about individual injector flow rates and spray patterns?

Idle mixture?

[himizoli]

Yes, the green cable is also new.
The Fuel distributor is recently rebuilt, spray pattern are ok.

The Idle mix is AFR 12,6

If I rev to 1500RPM the car the AFR goes 15 around and making the misfire.

[RarlyL8]

Is this a Euro or USA (Lambda) engine?
Have you checked the air meter plate is free and correct height?

[himizoli]

This is a EURO 3.0 year 1975.
The WUR is 054.
The air meter place is moving freely. ( I pushed down and comes back easily)
What you mean about the correct height?

[flightlead404]

Originally Posted by himizoli

This is a EURO 3.0 year 1975.
The WUR is 054.
The air meter place is moving freely. ( I pushed down and comes back easily)
What you mean about the correct height?

Euro 75 so no Lambda sensor or valve, correct?

[himizoli]

No lambda sensor, no extra injector on Fuel distributor

[flightlead404]

Originally Posted by himizoli

No lambda sensor, no extra injector on Fuel distributor

So you're getting a lean stumble when you come off idle.

There are (if the '75 is the same) some different ports in the throttle body that provide vacuum sensing for various things such as advance/retard in the distrubutor.

Perhaps you have one of those ports not connected, or a leak somewhere in a line such that when the port is covered/uncovered you're getting an unmetered air leak and going lean?

Or some other air leak and the idle mixture has been turned up rich to compensate, and you're loosing that band aid coming off idle?

[RarlyL8]

That's the wrong WUR for a '75 but shouldn't have anything to do with the stumble.
Does this stumble happen when the engine is cold or warm or all the time?
Can you get AFR and fuel pressure readings during the stumble?
Checked for air leaks?

[himizoli]

Checked for air leakage, find one but the stumble still comes.
Installed a AFR gauge and on idle everything fine AFR 12,6
Start to drive and stumble comes 1500 to 2500 Rpm , the AFR goes up to 15,5...
After 3000rpm the carb pulles very hard and AFR is 12-13
I think still I have air leaks....

[flightlead404]

I think a warm idle mixture of 12.6 is a bit rich. I found mine to idle best around 13.8.

Others may have different experience and ofc I'm running a 3.3 1986 car

[himizoli]

I was set the WCP to 3.0Bar and it makes better, but still comes the stumble 1500 to 2500Rpm. The AFR abound 14 in this RPM range....

[flightlead404]

Originally Posted by himizoli

I was set the WCP to 3.0Bar and it makes better, but still comes the stumble 1500 to 2500Rpm. The AFR abound 14 in this RPM range....

You mentioned there's a cdi box under the seat, is there a narrowband sensor in the exhaust? Is there a lambda valve on the fuel head? If so, is it buzzing?

I don't remember the details off the top of my head but I think if there's a lambda valve but its not connected at all it is open, and if its powered but not controlled its around 50% duty cycle. I could be way off here perhaps someone can correct me if so.

So, normally it would be running around 50% duty cycle then controlled by the cdi box below (I think 3,000 rpm) to get an idle mix meeting the emissions requirements (call it close to stoich).

If the lambda valve is then disconnected and fails open we then reduce the control pressure, thereby richening mixture. If the idle mix is then adjusted lean to get it back to a reasonable idle maybe that would cause a lean stumble coming off idle?

Just spitballing here....