Thread: Is this normal?

The early hotwire 3.5 Disco are prone to erratic idle, made worse in the manual version.

Try adjusting the MAF, you should try it at .9 to 1.0V, I bet it is up around 1.5...This can make for very high rev start ups, especially from cold start.

Back probe the far left and far right MAF wires, at idle warm they need to be .9 to 1.0, clockwise adjustment is higher voltages and richer idle mix. You want to anticlockwise to lean off.

JC

I would be checking the Idle Air Control valve (IAC) at the back of the plenum, note if the the 3 allen bolts are tight, they seem to have a habit of coming loose, remove the stepper motor and clean carbon build-up on the cone shaped plunger and the seat that it seals on, make sure the vacuum hoses connected to it and the Fuel Pressure Regulator are not split, refit and see how it goes. Also clean the teminals on the MAF plug and socket, Regards Frank.

Yes it sounds like a slow IAC stepper to me.

The stepper fully opens on shutdown, so will usually take a few seconds to close on start up. So all of them idle fast until the ECU gets its act together.

It is also normal for the revs to be up on stopping, as the ECU takes a signal from the VSS to idle up if the car is moving. It's more for autos to smooth the 2-1 change on stopping but manuals do it also.
If it stays up when stopped for a short time, that points to a slow stepper.

All cars with Bosch LE Jetronic (14 CUX) do this and my Motronic BMW did also.
Regards Philip A

Originally Posted by PhilipA

Yes it sounds like a slow IAC stepper to me.

The stepper fully opens on shutdown, so will usually take a few seconds to close on start up. So all of them idle fast until the ECU gets its act together.

It is also normal for the revs to be up on stopping, as the ECU takes a signal from the VSS to idle up if the car is moving. It's more for autos to smooth the 2-1 change on stopping but manuals do it also.
If it stays up when stopped for a short time, that points to a slow stepper.

All cars with Bosch LE Jetronic (14 CUX) do this and my Motronic BMW did also.
Regards Philip A

So the 3.9 as well? as mine rev's high on start up before dropping down, gets embarrassing at times especially in supermarket car-parks & servo's with the looks I get