High idle ...what am I missing ???

[gmw9666]

Right then chaps, last few jobs done ready for the fest but I'm struggling with a high idle

All ideas are welcome

Here are the symptoms......car starts ok in 3-5 seconds and coughs into life. Needs a bit of throttle so doesn't stall then idles at about 1050 ish.........runs at that level for a bit then goes up to 1200

By this point on the road and fully warm when slowing down to a stop the revs drop and the car stalls. She will re start straight away no issues.

The AFM screw thing is out 3 full turns and still smells a bit rich

I've been over all air pipes for leaks with wd40 and nothing there

I had a new temp sender so fitted that and still the same

Checked all plugs and gaps and leads with known parts and no difference

I've currently tweaked the idle screw so now she won't stall but there is clearly something amiss

What do we think chaps?

[ElvisWedgely]

To me it still sounds like an air leak, though not nessesarily in the pipes. Inlet manifold? Plenum? Etc.

Tony.TCB.

[adam quantrill]

Yes air leaks.... try sucking on the vacuum hose to the brake reservoir, should hold some vacuum.

Also blow down the breather hose to the plenum - this should give resistance like blowing up a baloon. (Or else the rocker gaskets are suspect).

Split in the air intake hose? THe big one to the plenum.

[gmw9666]

Forgot to mention this is just an idle issue........driving and accelerating is frankly excellent (even compared to the 420!) there is no hesitation or lack of power or misfires

Yes, have been sucking / blowing hose as well.......mmmm tasty not!

One other thing, the plenum butterfly when closed appears to be fully closed, can't get a feeler gauge in

[KKson]

adam quantrill said:

Yes air leaks.... try sucking on the vacuum hose to the brake reservoir, should hold some vacuum.

Also blow down the breather hose to the plenum - this should give resistance like blowing up a baloon. (Or else the rocker gaskets are suspect).

Split in the air intake hose? THe big one to the plenum.

I was going to suggest vacuum pipe to brake servo. Mine is looking a little perished and I've noticed the tickover increasing a little. Spoke to Bradderztvr earlier this evening and he used washing machine water hose to replace his old hose and it works well and doesn't get deformed by the vacuum.

[mrzigazaga]

Mine started doing that a week ago and so i went over all the jubilee clips as i do periodically and the AAV hose clamps were a little loose..Not much..Maybe 1/4" turn on them, And now its gone.

Also check to make sure the idle control valve housing is sealed properly as this can cause this as well..Also is the air filter clean....

[adam quantrill]

gmw9666 said:

One other thing, the plenum butterfly when closed appears to be fully closed, can't get a feeler gauge in

That isn't right - there should be a gap - can't remember the figure now but I'm sure you can google it. Found it - Check that there is about (manual says no more than) 0.05mm (0.002inch) gap at the bottom of the throttle butterfly disc.

1. Set the gap.
2. Set the throttle pot voltage.
3. Set the mixture.

[Wedg1e]

Frayed, kinked or badly routed throttle cable holding the throttle open a tad?

[gmw9666]

Wedg1e said:

Frayed, kinked or badly routed throttle cable holding the throttle open a tad?

Cable routes fine and moves without sticking, nice and smooth

There does appear to be a little bit of play on the linkage pivot, not a lot but a little

Will tweak the butterfly and report back also

[adam quantrill]

Clean all the black gunge away from it too. Also blow down the breather pipe into the plenum in case that is blocked and needs a good clean.

[Sireatalot]

Morning Glen.

I had symptoms similar to this a while back where the wedge would start / run / accelerate all fine but when warmed up and coming to stop at lights etc.. it would want to stall and the revs drop into the 100 / 200 range..

It turned out to be my vacuum unit's ( on the dizzy) diaphragm had perished and not controlling the revs like it should be..

Replaced the unit and the pipe going to the plenum and all is back to normal and it hasn't stalled since..

[gmw9666]

Sireatalot said:

Morning Glen.

I had symptoms similar to this a while back where the wedge would start / run / accelerate all fine but when warmed up and coming to stop at lights etc.. it would want to stall and the revs drop into the 100 / 200 range..

It turned out to be my vacuum unit's ( on the dizzy) diaphragm had perished and not controlling the revs like it should be..

Replaced the unit and the pipe going to the plenum and all is back to normal and it hasn't stalled since..

Taking the hose off the plenum to the dizzy.........its is clear with no splits........how hard should it be when sucking to hear the dizzy diaphragm flap move..........I'm giving it all my might and only then can I hear a small flap noise........should it be easier therefore is it stuck a bit? Would this cause the high idle?

[Sireatalot]

gmw9666 said:

Taking the hose off the plenum to the dizzy.........its is clear with no splits........how hard should it be when sucking to hear the dizzy diaphragm flap move..........I'm giving it all my might and only then can I hear a small flap noise........should it be easier therefore is it stuck a bit? Would this cause the high idle?

I'm not an expert but remember it didn't take that much suck to hear a change in the revs...
I would say less force required than blowing a party ballon up.. in terms of sucking..

Mine would regularly idle at 1200 / 1300 before the vacuum unit change.. now it idles at a happy 900-1000 and when stopping, doesn't go low enough to want to stall.

you may not need to replace the pipe - I just did as an extra.

This is the one I bought.
http://www.simonbbc.com/ignition-spares/lucas-35d-...

[adam quantrill]

When checking the dizzy vacuum advance best to put a timing strobe on there and you will see the timing move. If the diaphragm is holed you will be able to suck through the pipe continuously.

[bradderztvr]

KKson said:

I was going to suggest vacuum pipe to brake servo. Mine is looking a little perished and I've noticed the tickover increasing a little. Spoke to Bradderztvr earlier this evening and he used washing machine water hose to replace his old hose and it works well and doesn't get deformed by the vacuum.

A piece of washing machine hose is great in the winter, but realised today in the heat it goes very soft and looses form. Not such a good bodge after all.
Also check the idle screw on the plenum for tightness, I kept adjusting mine every day, as it kept winding in or out when driving. Keith (Kkson) put a bit of thread tight on it when we set the FPR, smooth tick over now, and doesn't move.

[mrzigazaga]

bradderztvr said:

check the idle screw on the plenum for tightness, I kept adjusting mine every day, as it kept winding in or out when driving. Keith (Kkson) put a bit of thread tight on it when we set the FPR, smooth tick over now, and doesn't move.

You can also use a rubber grommet...

[jindle]

Also check the little hose going from the top of the plenum to the fuel pressure regulator, if that's buggered then you'll overfuel when hot

[AM400]

Overrun valve sticking?

This was the cause of my high idle.

[adam quantrill]

Good suggestion also if the EAV is stuck open this would let in too much air.

You can block off/plug the pipes to each of these easily to eliminate without spannering.

[gmw9666]

Right, I just adjusted my flap a thou and reset TPS and now running at 2k idle with the idle screwed all the way in

Got me thinking this is all to do with the TPS or the linkage

So lets look inside my TPS, mmmm, one side almost all the carbon has come off and the copper thing has worn its balls off

Gave it a clean and popped it back on and the car runs back to high idle 1200

Frankly I think I'll worry about it after the fest when you guys can have a look for me :-)