Thread: Disco Problems, Maybe MAF?

Is there any way of testing the MAF besides buying another unit?

Well ther eis a rough way.
Get a multimeter on say 2 volts or 12 v setting.
Peel back the boot on the connector, then find the signal wire. I think it is the third from the engine but it will be the only one that varies with revs.
One will be 12volts ( feed to the MAF)
One will be about 1-1.5 volts constant ( reference voltage -nearest to motor)
And the third should vary with revs.( the wiring diagrams are not clear but it should be blue green wire)
If you do not get a wire where the value goes up as the revs rise then you have a dead MAF.

I would also check the water temp sensor, a 2 wire sensor usually located on the RH front of the inlet manifold.Value should be 400 Ohms or less at normal operating temp.
Regards Philip A
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Originally Posted by Fade

Hey all,
I seem to be having a few troubles with my 94 Disco . I believe it may be the MAF though am not entirely sure. Engine is running seriously rich, after 1 week new spark plugs are black. She is completely un-drivable at the moment. Have tried twisting pins of MAF and cleaning it the best I can with no improvement to driving. With MAF plug connected and disconnected there is no real difference in running, which makes me think MAF for sure. Though are there any other items I should be looking at? Is there any way of testing the MAF besides buying another unit?

Thanks in advance
Matt

Edit: I have already changed dist cap, rotor, and new spark plugs (now fouled). Symptoms started over a week ago when driving to work, the car was running fine but struggled and backfired over 2000rpms, over couple of days the engine would do this at lower rpms. However now it will do it through the whole rev range. Idle is unstable most of the time as well.

Matt, Try the Valve on the back of the plenum box, it is called the By-Pass Air Valve (Stepper Motor), it has an electrical plug going to it and a rubber hose, it is held to the back of the plenum by 3 bolts (Allen head), check to see if they are tight, everyone I have seen has been loose. Remove and clean valve body and plunger (carefully) and clean the electrical pins with contact cleaner and reassemble, may solve your problem, Regards Frank.

Thanks for the advice so far. Working late tonight so will have to tackle these tomorrow night. Hopefully the will point to the problem or at least rule out components.

GOt around to do my MAF readings today.

On the pin closest to the engine (reverence pin) I am getting around 3.3v, does this indicate a dead MAF? The third pin was varying with the revs, between 1.5v and 2.5v (at approx 2500rpm).

Is this all normal or faulty?

Matt

If your MAF is giving a reading of 3.3v when you are measuring the two outside pins then it will run seriously rich.

Back it off to between 1.0 and 1.5v. Probably closer to 1.0v if it is a stock ECU. The higher the voltage the richer it runs.

Thanks for that, didnt realise how to adjust it at first, then i remembered the magic screw on the maf. Reduced it down to 1.2v. Problem still there, but least I have ruled out the MAF. Have cleaned out the stepper valve and MAF, is idling better but still not running properly.

I know the Vacuum Advance unit needs replacing, could this cause my symptons?

Other than the blackened plugs what are the symptoms you are having?

I thought mine was running rich due to sooting of plugs and spitting black stuff on the garage door but having posted here and reading into it I found that it was normal. It didn't cause any driving/runnning problems though.

Perhaps now you have adjusted your MAF if you clean the spark plugs it will run okay?

I remember when I adjusted my MAF if I turned it up to 1.5v it was spitting sooty crap onto the driveway constantly whereas it didn't at the lower voltage. I carried some white A4 paper around with me for a little bit and held it behind the exhaust pipe at different engine temps to see what came out. I found it spat more black stuff when cold. I think it didn't spit any at normal operating temp.

If the vacuum advance isn't working it would use more fuel but I don't know if it would cause whatever problems you are having or not. I have only ever had a faulty vacuum advance on an old BLue 202 in a VH Commodore. It used bucket loads more fuel than it should have but it ran okay.

Yeah thats what I though about the Vacuum advance as it should only advance the timing for idle and while cruising.

Basic problem I am having is when Im trying to drive it there is a complete loss of power. Sometimes the power would return to normal for a short period of a few seconds, but most of the time would just splatter along. Can only reach 20kmph and thats over a fair distance. That is after cleaning plugs and adjusting maf.