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Old 03-14-2011, 03:43 PM
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Default RPM drops while engine is idle.


I have recently run into a problem of major RPM drop while running idle. The RPM is usualy 800-1000 but when staying idle (while in "D" or "P") the RPM is droping to a very low number - sometime low enough to shutdown. When it happening, the step motor kicks in and the RPM get stable again for about 15-20 seconds and then it is happening again. I have replaced the air flow sensor but no improovement. I was thinking of the Oxigen sensor or the coil as the next step. Any other idea?
The engine is a V8 3.9L.

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Old 03-14-2011, 11:13 PM
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Your idle air control valve is either dirty or bad.
Start by cleaning it.
The IACV is what controls the amount of air the engine gets at idle.
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Old 03-15-2011, 08:10 AM
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I think I am having a similar problem with my 95. (read: my head job thread) I have a CEL and code 48 with it.
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Old 03-15-2011, 08:42 AM
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A few good things to do.... RE-check all vacuum lines...Clean the MAF (unless it is BRAND NEW) if it is new to you but previously used. The sensor wires when even slightly dirty will effect its output signal. Next pull that IACV / stepper motor and, as was suggested, clean the cone and lightly lube the spring and shaft with a little spray lube. The mating face (seat) in the plenum needs to be cleaned as well as the butterfly valve and bore on the intake side. All the oil residue in the throttle body needs to be cleaned up.

Is that gasket between the plastic body of the IACV and where it screws into the plenum still good?? If not, make a new one.

The IACV is easily checked for circuit operation... check for ohms across the pins on the backside of the stepper motor for both circuits... you should read about 50 ohms on both circuits (four pins... two pins per circuit)

Are the clamps on the intake hoses snug? is the O-Ring at the downstream end of the MAF there and in good condition?

Is the PCV system all free and clear to breathe? Filter on top of right valve cover is clean / unobstructed? None of the hoses or the "T" are cracked or loose?

Is the Vac. Advance diaphram on the Distributor in good shape... i.e. the diaphram is in tact and not allowing air flow? and is it hooked up to the correct port on the advance housing? some of them have two ports... one for pulling the diaphram out and the other for pushing it in... you want pulling.

After all that if you still have issues you probably have an air leak (ingress) from the lower mating edge of the plenum... if you can spray carb / starter spray anywhere around the plenum and increase RPM's temporarily then you have found a leak....try resealing the plenum.

If this is happening at idle while the car is NOT moving then you can rule out the VSS, TPS, temp sensors... most likely a simple air leak somewhere between MAF and intake manifold.

Last edited by Cosmic88; 03-15-2011 at 09:44 AM.
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Old 03-16-2011, 09:08 AM
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Default be careful screwing around with the MAF

Oferl -

I went down this road last year (or was it the year before) I was ready to junk my disco. Is it worse when it rains or is humid?

I check or replace / cleaned everything the gents above suggeted but my problem turned out to be a small short in the wiring harness right where the transmission meets the engine block.

be careful screwing around with the MAF
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Old 03-16-2011, 04:14 PM
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Many thanks to all of you. Will try everything over the weekend - one by one. I am starting to think it may have something to do with some harness shorting because now that I am thinking about it, there is some correlation of some whistle I've started to here in the radio speakers that its frequency follows the engine RPM...

Anyway - I will update on the results.
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