Vy new engine cutting out when on gas but fine on petrol

[muso]

I've just had a changeover 160,000 km V6 ecotec engine fitted to my Vy which was/is on gas (dual fuel) It previously (with the old engine) never missed a beat on gas and I drove it around for about a year. But ever since I had another engine fitted I found that it stalled all the time on gas but ran fine on petrol, the engine cuts out whenever the revs drop such as when slowing to brake before a corner. There has been a few loud back fires, fortunately I undid a few clips to give the air filter cover some movement. I've since taken it to get a Gas tune and there hasn't been anymore back fires but it still cuts out, gas tuner says I need new spark plug leads but it never missed a beat with the same leads with the previous engine.

The new engine has NGK plugs which I've tried re-gapping to a smaller gap .9mm for lpg....it runs a bit better but still cuts out in traffic causing me to say bugger it and switch over to petrol a lot of the time.

So as far as I know all that's been changed is the engine so why doesn't this new engine like LPG? I can go out and buy a good set of Eagle plug leads but I really don't think that will stop it from cutting out.....and having just forked out for a gas tune (with no success) I feel that getting new plug leads will be a waste considering the current Bosch leads were never a problem with the previous engine. Could it be that the new engine also has a new computer which has not "learned" to run on gas?

I honestly thought a gas tune would fix this but it didn't really help much at all, I'm sick of it stopping in traffic, I'm also sick of driving around in low gears to keep the revs up so it won't stop......any tips would be much appreciated, but I'm not removing the gas system.....when it is running right it is really good and very cheap to run!

 

[brae123]

You need to make sure you are using the correct spark plugs for lpg, standard plugs wont work properly with lpg. Have the tuner check the gas pressure again. The leads may need to be replaced if the are the originals and depending on the kms they have done. May even need to have the computer reset and the gas tune re-done.

 

[EYY]

Replace the coils, spark plugs and leads before you do anything. You can use standard gapped plugs, or plugs with smaller gaps. It won't make a difference in reality, but make sure your plugs are one step colder than standard.

If it's not running properly on gas, make sure that the mixer is sealed against the throttle body properly with a good quality silicon (assuming its a positive style gas system here). LPG is much more sensitive to vacuum leaks than petrol.

Have you made sure that the cooling system is bled properly? If you have any air pockets, the gas converter may not be getting hot enough and freezing. This will restrict the supply of gas.

 

[muso]

You need to make sure you are using the correct spark plugs for lpg, standard plugs wont work properly with lpg. Have the tuner check the gas pressure again. The leads may need to be replaced if the are the originals and depending on the kms they have done. May even need to have the computer reset and the gas tune re-done.

Reset the car's engine computer or the actual gas ecu? Will disconnecting the car's battery for 30 mins do this?

 

[sleepa]

LPG is very fussy compared to petrol. I have had a few different cars with LPG (mixer and Injected) and have had a couple of minor issues.

Things that look fine could be no good. Check/replace the air filter as a slightly dirty one can make the engine run rough, and as above, change the Spark plugs to LPG ones and get some good 10mm LPG Leads (or similar).

Have you made sure that the cooling system is bled properly? If you have any air pockets, the gas converter may not be getting hot enough and freezing. This will restrict the supply of gas.

That's a very good point, he just had the engine replaced, maybe it hasn't been bled properly. Do it again to make sure.
And if the converter is freezing it will limit gas flow at low idle, like pulling up at the lights causing it to stall.

If any of this is the issue then it will run great on petrol and not so good on LPG.
If it's injected gas then the LPG ECU piggybacks of the main ECU so basically copies all the fuel info that has been fed into it (don't think this will be an issue as you have had it tuned, all it does when I get mine tuned is slightly more power or economy, runs the same otherwise)

If it is a mixer system and all the plugs/leads/etc are good, then it's all in the physical tuning of the converter, and they are a pain in the ass sometimes. I'm guessing you need more gas flow when its idling.
Maybe the mixer could have been damaged when they replaced the engine?

 

[muso]

Surely the gas tuner would have picked up a damaged mixer
I will check the cooling system. How can you test the cooling system is bled#properly?

 

[muso]

Well you won't believe it........I put my old spark plugs in from the old motor and it runs without a missing a beat on gas just like it used to, the old plugs were holden ones (GM 92141875) very happy

 

[sleepa]

Glad you got it sorted :thumbsup:

Just remember this for the future.

 

[muso]

It's gone all of today without issues, it's threatened to stall only a couple of times when cold, the ignition lights flashed but it didn't stall, previously it wouldn't make it to the corner shop without stopping several times. It could very well be the spark plug gap in the old plugs, i think they were 1.2 or 1.3 mm as opposed to 1.5 standard correct me if I'm wrong.

 

[muso]

Sorry I hate to say that it only lasted a day or two before it went back to doing it again, once you back off to slow down the tacho needle drops and hits 1000 rpm then starts bouncing and then usually stops. The car is hard to start on LPG after that so I just switch over to petrol and it starts and runs fine. But it idles fine on both fuels, only stalls when slowing down in drive, keeping it in lower gears 1 and 2 when slowing down keeps the revs up so it doesn't stop.

So the gas tuner who tuned it said the next step is a good set of Eagle 8 mm LPG leads........done that but it still does it.

Gas Tune done = Check
New plug gapped to 1.4mm = Check
New lpg leads 8 mm Eagle = Check
Problem still present

Where from here? This is getting expensive and the car still stops in traffic, never did it once with the old (buggered) engine, went for a year without missing a beat until the engine failed. Since having the new engine installed it is nothing but trouble on gas, but I'm not going to run it on petrol it's so much more expensive......no way