Mini backfire, just a pop really

[Clifford Pope]

The car is a 1992 240 with open loop system and mixer.

It had been running well for several years, and I've done about 80,000 miles.
The idling on LPG has always been a little bit wavery, but over recent months this has gradually got worse, with the engine hunting rather than ticking over evenly.

I gave it a thorough service - plugs, leads, distributor, throttle body, idle air valve, all inlet and breather pipework, air filter, and the hunting has vanished. It now idles just as smoothly on LPG as it does on petrol.

However, on two occasions recently it has made this quiet pop, I'm pretty certain from under the bonnet.
The circumstances have both been the same:
engine just warmed up - perhaps 10 minutes into the journey.
moving very slowly in 2nd gear, and then popping just as I open the throttle a little to full away. Nothing violent, I wasn't trying to drag race or anything, simply creep forwards at walking speed.

No damage has resulted, everything is tight, the engine hesitates but then continues to run perfectly. No more recurrences, until once a week later.

I suspected an air leak somewhere, but the usual test with WD40 does nothing. Is there anything else I could look at? It seems an odd coincidence after a service that otherwise was such an improvement.
Does this indicate a weak idling mixture perhaps?

[volvobaggen]

The weather has been cold, and an open loop system cannot adjust or autocompensate for external factors such as temperature. I would think its a air/fuel mixture issue.

There is some interesting reading here:
http://www.lpgforum.co.uk/viewtopic.php?f=28&t=11843

[Clifford Pope]

That's interesting. I hadn't considered the temperature effect, other than observing that both incidents happened at the same state of engine warm-up : in fact at the same place to within 50 yards, 3 1/2 miles from home.

I had some time ago noticed that I needed to be cautious not to switch over to gas too soon, because the vapouriser had not warmed sufficiently. This effect is made much more noticeable if I open the heater valve too soon, as obviously it draws hot water that is needed by the vapouriser.

What I do now in cold weather is delay switching to gas until the thermostat has opened - ie the temperature gauge has gone up to just over horizontal, then suddenly dipped as the thermostat opens, then after a few minutes stabilises again at half way. Before I adopted this procedure I had been switching earlier to gas, but clearly it seems the dip as cold water circulated after the thermostat opened was cooling the vapouriser.

Anyway, the thread on the LPG forum concludes that the effect of cold air plus colder gas can tilt the mixture either richer or weaker depending on circumstances. In my case I make sure the gas is warm before switching over, so presumably the cold air effect is dominant, which will weaken the mixture?

It occurs to me that the air filter when I changed it was quite dirty - I had let it run too long through forgetfulness. That would have camouflagued the weakening before the service by making the mixture richer?


So is the correct conclusion that the mixture, at least in the early warm-up period, is weaker? And does this predispose it to backfiring?

If that is correct perhaps I can just be aware of it and use a few more revs, dropping into 1st in the early part of a journey rather than trying to use a lazy low gas flow pull in 2nd.

[volvobaggen]

Yes, in a open loop system the mixture will get gradually richer as the filter gets dirtier. Changing to a new filter will make the engine run leaner.

From what i have read, both hunting and backfiring are typical symptoms of a lean-running engine.

It could also be that you have heavy ends clogging up your vaporizer. They are mostly paraffin waxes, so they could also cause restriction and changes to the delivery of propane.

I don´t know how much difference the density of air means to combustion in engines running on lpg, maybe we can get some more qualified people to answer?

[Clifford Pope]

Many thanks Volvobaggen.
I'll certainly check the vaporiser drain.
If weather has anything to do with it, it seems to be associated with clammy very damp air rather than cold. Curiously, I have noticed the same with my carburettor-engined landrover - prone to mild icing in wet chilly weather, but absolutely fine in freezing dry air.

Perhaps I should just turn the mixture up 1/4 turn for a winter setting, and back down again for summer. I'd got it spot-on for several years so am reluctant to fiddle again, but with marking and careful record it would be all right.

[volvobaggen]

I remember reading a somewhere on this forum that you had the best mileage I have ever seen on a old red block Volvo on lpg. Maybe your engine has been running right on the edge of being to lean for a very long time?

For me as a lpg novice, tuning the open-loop for optimum performance and fuel economy takes a lot of trying and failing.

My 245 from 1977 with 4-speed gearbox use about 1.15 litres lpg pr 10km on country roads, but I don´t want to experiment making it run any leaner. Running on the leaner side can burn valves, blow up air intakes, and for cars with a catalytic converter, its probably not good making the exhaust gases to hot.

Making the mixture slightly richer in the wintertime is probably a good idea, and will most likely eliminate the hunting and backfiring.

[classicswede]

When cold the LPG is denser and that is why you do not need any choke with cold running. If you use much throttle when cold the vapouriser can fail to vaporise the fuel resulting is some liquide going through or it can freeze reducing the gas flow.

240's are sensative to ignition components and can struggle if not fitted with genuine volvo parts.

You can check what your fuelmixture is doing by hooking upto the exhaust sensor.