high idle on Petrol but not LPG

Hello,

After some advice please.

I've got a '92 3.9 EFI RRC which generally runs fine...

It starts great hot or cold on LPG or petrol, the problem i have and have had for a long time but only really got round to looking at recently is that it has a slightly high idle.

It was particularly bad today @ approx 2000 rpm dropping to about 1200rpm in gear.

So i had a mess around with it all this afternoon, checked for vac leaks etc there was a vac tube that had some off the plenum at the back just under the Idle control valve which i re-attached no idea what it does? It didn't make any difference anyway.

Basically nothing made any difference so i went back to basics, i took it out and thrashed it down some quiet roads accelerating through the gears, full throttle.

That made things much better it was lowered the idle to approx. 1200rpm in park and 900 in drive.

The odd thing is - I'm hoping someone can give me a science lesson on this - that when running on LPG it will idle @ about 850 - 900rpm and in drive about 600-750rpm.

I'm just wondering if anyone has had any similar experience I'm the past that they can help me as i am stumped?

Also before anyone says why don't you just run it on LPG - I don't really live near any LPG filling stations and i can't really be bothered to trek out to fill it up.

I also prefer the way it drives on petrol...

I should probably mention as well:

It has always had this problem since i have had it,
I did have a VAC leak not so long ago which caused all sorts of pain till i figured it out and i went over all the hoses to check etc
I have cleaned the stepper motor etc and around the throttle bodies

Open to any and all suggestions...

Mine runs a slightly different idle speeds from LPG to Petrol. Ultimately, the timing has been set for the higher octane rating of LPG, so when I switch back to petrol the idle drops very slightly.
Over time (and I've not had mine long, but experienced it already) you'll see a small amount of waxy residue build up on stuff like the plugs n ports etc when running it on LPG. As the crap builds up the engine idle begins to drop in theory, though in actual fact because the ECU sees the engine as struggling the idle stepper adjusts up the idle speed.
Mine drives around in town traffic mainly, so doesn't get much chance to stretch its legs....but when it does, it runs MUCH smoother n better afterwards. If it gets really bad, try using some intake cleaner, available in an aerosol.
On the plus side, the LPG residue is much easier to clean out than the sooty Petrol gunge if driving petrol only RRC!!.
good luck

Cheers Joe, thanks for your response but you have both helped and confused me a bit i'm afraid.

So why is mine higher revving on petrol than gas and yours is the other way round?

It is so annoying - surely it can't be just down to timing seeing as it drives so smoothly in every other way on petrol & LPG - better and more power on petrol though for sure.

In summary,
No, its not just down to timing, though optimum engine performance via ignition timing is either set to run with LPG or petrol, not both. The ECU is giving the idle stepper signals that control the idle speed. Crap in the induction system is affecting the idle speed and confusing the system. The ECU gives signals to set the idle stepper assuming everything is clean...which you've already posted, is not.

inscrew and take the stepper out of the plenum. Clean it with intake/brake cleaner, then squirt some through the cavity left in the plenum. screw it back in and give the car another good run

Hello, I know you all like it when there is closure so to update you and anyone else it might help, i think we are there now just about with the idle prob.

I'd already been down the route of cleaning the stepper and it didn't made any difference so i replaced it and gave the whole thing a good clean.

Seemed better first time out but idle was still slightly high - although much improved.

I checked again and just wanted to make sure that i had got everything back on tight and while i was there i've read a few random posts about disconnecting the battery to reset the ECU - i don't even know if this is true but i thought what the hell and gave it a ago.

Second time out seems better again. See attached pics of RPM in Park / Neutral (higher RPM) and Drive.

Does that look about right?!

i would say idle is "better" on gas is cos the lpg needs a tune up..its running poorly and hense has a lower idle...if the idle problem on petrol is sorted i bet you find the idle on gas is crap...

so ignore the gas altogether,sort the idle on petrol AND then get the gas tuned up properly..

Your absolutely right - everything is set for petrol. As i don't really live near and LPG stations and for the low milage i do its just not worth it.

Priority for me is, run as well as possible on petrol and forget about the gas.