Why are cars started on petrol and then changed to LPG?

Message

Martec · #1 Post by **Martec** » Sat Mar 19, 2011 5:43 pm

Hi,

I'm new on here,and hope to get some information. I have a 1961 3.8 ltr MkII Jaguar on LPG, but I start it on LPG from cold when ever possible. With an ignition ECU (one map for petrol and one for gas) I have since been starting on gas from +2 deg C (air temperature) and it runs smoothly within a few minutes as the evaporator warms up.

I have a manual petrol tap so the two fuel systems are completely separate.

Am I doing any harm to the system or engine??

Brian

Gilbertd · #2 Post by **Gilbertd** » Sun Mar 20, 2011 8:02 pm

On a multipoint system you have to start and run on petrol for the first mile or so as the mixture will be way out due to the gas temperature, density and the enrichment provided by the petrol ECU (because the gas system is slaved off the petrol system).

On a single point, you should be able to start and run on gas from the word go even in very cold temperatures. You won't harm the gas system and it is unlikely that you will harm the engine.

richard_ha · #3 Post by **richard_ha** » Sun Mar 20, 2011 9:08 pm

The vaporiser requires the heat from the engine to stop it freezing.

Gilbertd · #4 Post by **Gilbertd** » Sun Mar 20, 2011 10:34 pm

But provided you have sufficient anti-freeze in, it won't anyway. It only needs enough heat to prevent the gas temperature dropping below minus 44 C at which point it stops vaporising.

richard_ha · #5 Post by **richard_ha** » Sun Mar 20, 2011 10:43 pm

I've seen a vaporiser that wasn't plumbed in correctly, the car wouldn't run on LPG, on closer inspection there was 1/2" of ice covering the vaporiser.

It's not the coolant that will freeze, it will be the LPG.

Gilbertd · #6 Post by **Gilbertd** » Mon Mar 21, 2011 12:04 am

Ahh but the ice you see on the outside is frozen condensation......

Martec · #7 Post by **Martec** » Mon Mar 21, 2011 2:09 pm

Can I be greedy and ask another question.

Same car (MkII Jaguar) I took to another supposed expert firm to solve a stuttering problem and they lifted my shirt a few times, got embarassed and did nothing again for free that time. This firm sold me a lubricating system to be sucked into the manifold when on gas, I never fitted it, is it useful/necessary on an old Jaguar XK twin cam engine with unleaded valves and seats.

This firm in Grimsby is the reason I contacted TinleyTech, bought a lead and program to sort my own problems, so it is nice to hear advise/opinions from those in the know about LPG.

A while ago an air lock in the jacket water to the evaporator stopped the car, but when bleed out all was fine.

Thanks for the replies Brian

richard_ha · #8 Post by **richard_ha** » Mon Mar 21, 2011 7:39 pm

Which means that it's below -100C, and propane freezes at -40C.

Gilbertd · #9 Post by **Gilbertd** » Mon Mar 21, 2011 8:48 pm

Errm, I always though water froze at 0 degrees C?

Run a gas system from cold on a car with no antifreeze and the coolant passing through the vap will freeze into a solid block. With antifreeze it will be protected down to a lower temperature so even without any heat from the engine it shouldn't freeze. However, assuming the flow is decent and the thermostat is working so all coolant passes through the vap circuit immediately the engine is started, the heat generated by the engine will be enough to stop it freezing.

FatBillSlim · #10 Post by **FatBillSlim** » Fri Mar 25, 2011 9:12 pm

Seriously? nobodys got the answer right!

and i pissed myself at "and propane freezes at -40C" hahaha

billynoband · #11 Post by **billynoband** » Sat Mar 26, 2011 3:44 pm

You can start the aeb based multipoint systems on lpg providing the engine is warm or even in the morning if the day is not too cold especially if the sun has been on your bonnet. It's good to start on petrol a few times a week to keep the petrol system healthy.
I have been doing this for a couple of years now. A tenner of petrol last's me 2 months now.

allante666 · #12 Post by **allante666** » Sat Mar 26, 2011 7:53 pm

Brian, you mention a firm in Grimsby, can you pm me and let me know who and what kind of service you've had? Im considering getting a gy firm to service my 740. Thanks.

Martec · #13 Post by **Martec** » Sun Mar 27, 2011 7:58 pm

Thanks for your replys so far, some interesting comments there.

Allente666 - as a new member I am not allowed to PM - yet.

I was very pleased with the firm that commisioned my system after I had fitted the kit (10 years ago), they also gave me a certificate to satisfy insurance, which it still is, but they shut up shop and went commercial in Ireland.

I returned from sea to find a flat battery on the Jaguar, on recharge it had a flat spot which in hindsight was from the fuel cut out at set revs (approx 1100). I went to the then nearest LPG agent in Grimsby as they had the progam for my Leonardo ECU.

I was never allowed to see the screen of the laptop, I went to see them at least 3 times but each time the fault reappeared as I got off the Motorway at Scunthorpe. I was shown an expensive multipoint LPG injection self-learning system, but they couldn't fix my basic gas carb system.

I finally contacted my original supplier who put me onto Tinley Tech and I bought a program and lead, using an old desktop sytem in the garage I just disabled the the cut out at set revs and solved the problem!!!!!!!!!!

TT has been updating the software as I have moved to later microsoft operating systems, but until I read Simon Hibson's articles I didn't know how it is supposed to work.

When I can PM you I'll give you the firms name

I later found a guy called Peter Aldridge in Scunthorpe who seemed to know what he was talking about and immediately knew what I had done.

Brian

SimonHobson · #14 Post by **SimonHobson** » Sat Apr 02, 2011 8:33 pm

Just for reference, I've been running Rover V8s for nearly 12 years now (first a Disco, now a 110). With the Disco on carbs, and the 110 on carbs before I dropped a Disco EFI engine in, I've started on gas as a norm whatever the weather (and we've had some pretty cold weather during those 12 years) - and in fact for significant periods I've had gas-only as the petrol pump has been broken.

I'm starting the 110 on gas at the moment as I still haven't got round to swapping the petrol pump. But normally, with an injection system it's started on petrol simply because "it's easier that way". Petrol burns over such a wide range of mixtures that you can just "lob some in and fire" to start the engine. LPG is more finicky and getting it just right can need a bit of practice. Petrol also lights up with a weaker spark which can be an issue in cold weather when the oil is thick, the battery is cold and down on power, ...

And many people run gas-only vehicles - no option to start on petrol at all.

So don't worry about it - you're not doing any hard starting on gas, and arguably a lot less harm that starting on petrol which can wash the oil off the bores with a "choke out, bung loads of petrol in" start.

Gremlin · #15 Post by **Gremlin** » Mon Apr 04, 2011 2:00 pm

I guess by the age of your Jaguar that it might have manual choke/s?

If so..... no choke for starting but might be able to use it to increase tickover in cold weather.....if needed...

Martec · #16 Post by **Martec** » Mon Apr 04, 2011 10:15 pm

With the 3.4 and 3.8 ltr engines the twin HD6 carbs were fitted with an automatic cold start carb. An absolute disaster, so most are now controlled by a remote switch under the dash. When switched on it throws petrol in with a bucket.

It can at a pinch be used when the engine is hot but the engine can easily stall.

Brian

Gremlin · #17 Post by **Gremlin** » Mon Apr 04, 2011 10:38 pm

I posted somewhere else with a pic showing how I overcame the autochoke on my Bentley fitted with SU's I use an elastic band to hold the choke open...It will then start on gas OK.... Was
thinking about fitting an overide push button to I could get petrol when I switched the gas off, as I was having to start on petrol because it wouldnt start on gas because of the choke. Wont bother now tho. Will replace band with a spring one day.............. one day......

LPG Discussion Forum