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3.9 V8, random hi idle and bouncing needle


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Wanted to ask a quick question.

About once a week when I start my 96 disco it doesn't behave properly.

After ignition it stays on 1000 rpm instead of falling down to 500ish.

When driving I can get it lower by slowing down on hi gear, but on idle its always over 1000.

Then after a short while it starts reving up and down, up and down. From maybe 1000 to 2000 and back. Over and over for ever.

Restarting the engine works sometimes, but not always. Last time I just ignored it and drove on the hi-way for 1 hr, it then went back to normal.

Ive disconnected the MAF, and that didn't help. The v8s are not my strong side.

Any tips?


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Check the simplest things first, crank case breather and air filter is clean, disconnect and reconnect the vehicles battery to reset the EFI ECU, and for any leaks in the hoses connected to the plenum chamber.

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check the air intake hoses too some of them when they get older, compress as the engine speeds up then reinflate when it slows, it causes exactly the symptoms you are describing. if it is that you can put a spring coil of wire inside to hold the pipe open and keep you on the rouad while you get a new hose.

Just make absolutely certain it can't move around inside the hose.

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Was Gona say MAF but you said you checked that. Have you got a replacement MAF to try or borrow?

Sounds like it's hunting, (rev counter dancing to it's fav tune)

Once every now again sounds to me like there's a sensor on its way out?

Wonder if it could be the MAF or a leak (air).... faulty penometer? Or idler at the back of the plenum chamber?

If you can borrow them from someone to see if it makes a difference.

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