Can't idle, won't idle

[streaky]

An intermittent problem (which first manifested itself to and from the Hampshire meet last week and then went away) has today become a 'hard' problem. To and from Swindon today, my 4.0 hot-wire has refused to idle at all (both hot and cold).

The symptoms are that, regardless of engine speed beforehand, the revs decline past the usual idle speed of 800rpm, stutter around 400 and then (generally) die.

A bit of throttle keeps them at the 800-1,000 mark with no other trouble, but I didn't have my 'heel and toe' shoes on so slow traffic was a bit of a headache.

Everything else appears OK, she revs freely throughout the band, no misfires at any revs, pulls easily from 1,200 in 5th ...

Grateful for ideas on where to look.

Streaky

[streaky]

Nacnud said:

Mine had a really bad idle which was traced to a pin-hole in the tubing between the airflow meter and the plenum. There's a trick to test for a hole, squirt WD40 (or something, somebody please conform...) over the pipe and listen for a rise in engine revs.

Pipes to/from plenum:

Thin pipe to vacuum advance (from top front of plenum).
3/4" OD pipe to PAB (from outlet at bottom rear of plenum).
3/4" OD pipe - Blanked (from corresponding outlet at bottom front of plenum).
3/4" OD pipe to breather (from area of transition from intake hose to plenum).
2 X 3/4" OD pipe to water (from area of transition from the intake hose to plenum).
3/4" OD 'lazy S' shaped pipe from area of transition of the intake hose to the plenum to a protrusion from at the top rear of plenum with some electrics coming out from the other end of the protrusion.
A thin pipe immediately underneath the above protrusion which goes to a pressure valve (?) in the fuel rail.

There is a Lucas box just after the air filter labelled 'Air flow meter' but there are only electrics coming out, no pipes to/from the plenum.

So I'm a tad puzzled. I don't have the requisite books to hand for technical descriptions of the above, so hope the detail is adequate for your understanding.

Your continuing help appreciated. Many thanks.

Streaky

[streaky]

Ah, ha. After posting the above, I suddenly realised that perhaps the 'tubing' referred to is the air intake duct (concertina hose) after the air flow meter.

Is that correct? If so -- DUH!

Streaky

[streaky]

mungo said:

Just a quick one before I shoot off tonight - I am in no way technical but this happened in my old wedge once.... is the screw on the top of the plenum chamber adjusted right??? [snip]

Humm! There is no screw on mine, so no adjustment possible that way (equaly, no going out of adjustment either) - Streaky

[streaky]

HarryW said:

[snip] BTW the screw that macnud was on about is the one that sets the base idle, if you look to where the hose that goes around to the stepper comes from to the side of the throttle part of the plenum you will see a recess on top(could have a plastic bung in it) inside there is a allen screw which sets the base idle. With the stepper hose fully clamped (i.e. blocked off) the idle should drop to around 500rpm, if not adjust the screw, as a rough guide it should be (but not always) around 2.5 turns away from fully-in. ...

Ah ha! That screw - taht appears large enough to bolt the engine to the floor! I was looking where part of the throttle mechanism bears on the plenum chamber when closed. There is a little 'pip' there. Just about where you might expect the idle adjustment screw for a carburetor to be.

[streaky]

Harry - great explanation, many thanks. Will see if it works later today - Streaky

[streaky]

See also : www.pistonheads.com/gassing/topic.asp?t=51551&f=12&h=0

[streaky]

OK, an update.

Removed and cleaned stepper motor (using carb cleaaner) and inside housing. The Haynes Range Rover manual mentions a washer, but there was no washer there when I unscrewed the motor. I presume this is a fibre washer rather than a metalic one ???

I reassembled (without a washer as I did not have one the right size) and the problems remains.

Tested for a pin-hole between the AFM and plenum - no rise in revs when spraying around with lighter fluid.

It's OK at cold, the problem manifests itself as the temperature rises.

Looking for more ideas ... please?

Streaky

[streaky]

jmorgan said:

Just had a simular problem. It was the bump stop(?) under the throttle linkage. Turned it out a turn and its stopped stalling. Had to re fettle the idle as I thought it was that. Also when mine is serviced he always uses sealant on the stepper motor if its removed.

I used PTFE tape on the threads.

[streaky]

GreenV8S said:

[snip]One thing you might try to keep you going while you track down the problem, is raise the base idle to something the engine will actually idle at. It might be a bit temperamental until it's warmed up, but in principle it works fine with a fixed idle (I've run like this for a few weeks). The base idle is controlled by a screw on top of the throttle housing. If you squash the bypass hose from the throttle housing to the stepper motor assembly, the revs should drop to the base idle speed (somewhere around 400-500 rpm). If the revs don't drop, this confirms the stepper motor is jammed closed but doesn't explain why). If the base idle speed is too low the engine will eventually stop, screw the adjuster out until it keeps idling, you might need to blip the throttle a little to keep the engine running while you do this. [snip]

Is that clockwise (ie. down/tighten) or anti-clockwise (ie. up/loosen)? Recommendation is to tighten fully and back off 2.5 turns. Do I need to back off more or less than this?

Many thx - Streaky

[streaky]

Sorry not to have posted this earlier, but the idling problem went away as spontaneously as it arrived. Obviously that particular gremlin lost its grip under hard acceleration/cornering - Streaky

>> Edited by streaky on Thursday 23 October 17:59