Not idling on LPG

hiswitsend · Post by **hiswitsend** » Thu Dec 14, 2006 11:41 pm

Hi

I wonder if anyone can help me.

I have a 1988 range rover classic with a 3.5 EFI V8 with the flapper vavle air flow meter. It has a single point LPG system fitted.

Had an lpg backfire the other night and blew a hole in the pipe between the plenum and the AFM. Since then it would run roughly on gas and rev, but not idle on petrol.

Found a small pipe that had been blown off from the back of the plenum, as well as a pipe that could be the PCV(?) Now it runs sweetly on petrol, but refuses to idle on gas; it revs on gas ok, but stops stone dead as soon as pressure is taken of the accelerator pedal.

Could it be that there is no air getting into the plenum when running on gas, but ok with petrol? Or are there other components that could have been taken out, plus if its running ok on petrol, not tried it extensively yet, does that mean that the AFM survived?

Lots of questions, I know, but this looks like the place to find the answers...

Dave

LazyDocker · Post by **LazyDocker** » Fri Dec 15, 2006 8:46 pm

Sounds to me like it's sucking too much air and running lean.

Check very thoroughly all around the plenum... There are quite a few pipes which could have been blown off in the backfire!!

Also, have you replaced the pipe from the AFM to the plenum? It's no good just taping it up as it will still suck air... Trust me, I know!!!

As for the AFM being damaged... You may be lucky as it found a weak point further back in the system. However, you wont really know until MOT time.

As an aside, if you run predominantly on LPG then it's well worth advancing the ignition until it just pinks on petrol and then retarding it a gnats willy... You'll find the petrol runs a little lumpier but the LPG is much, much better and you reduce the risk of getting valve overlap, which causes the backfire! I'm not saying it will prevent it, as that would be tempting fate, but since I've done this to mine....... Touch wood!!

hiswitsend · Post by **hiswitsend** » Fri Dec 15, 2006 11:39 pm

Was thinking the opposite in that no gas is getting into the plenum.

Just connected two pipes back on the plenum to get it to idle on petrol, which it now does very sweetly. With the pipes off it was running and idling on gas, a bit rough though, but not idling on petrol, but did rev om petrol.

No idea what else has gone wrong now, unless the idle bypass(?) hose is blocked or not used when running on LPG.

ChrisJC · Post by **ChrisJC** » Mon Dec 18, 2006 1:37 pm

Can you hook it up to a gas analyser and see what the mixture is doing when you reduce the revs slowly down to idle speed?

Maybe the vapouriser is knackered?

Chris.

LazyDocker · Post by **LazyDocker** » Mon Dec 18, 2006 10:10 pm

Hmm, confusing!!!

Generally, FWIU, LPG backfire is generally caused by either the mixture being too lean, or valve overlap (timing!)

Is the spark VERY strong when idling on petrol? LPG needs a much stronger spark.

Other than that, a few quid in the right direction in your local MOT station and they should let you use the gas analyser and try

hiswitsend · Post by **hiswitsend** » Wed Dec 20, 2006 10:00 pm

LazyDocker wrote:Hmm, confusing!!!

You're telling me...though would ahve to admit that that tends to me usual state of mind

Looked at the throttle butterfly; according to haynes(...) there should be 0.05mm of a gap between the plenum wall and the butterfly. This is most certainly not the case, no matter how much I adjust the idle screw.

So, maybe it has been knocked out of adjustment and petrol works because it reads the AMF, whereas I think the gas just goes only by the TPS...or am I underlining my initial statement...

Dave

hiswitsend · Post by **hiswitsend** » Sun Dec 24, 2006 3:59 pm

Its fixed

There is a door opener, that is attached to the AFM to allow air when running on LPG, that had jammed; I was able to free up the mechanism by operating iy by hand and now it idles and runs on gas agian