TB42e Runs ok on LPG but poor on Petrol


After some ideas on where to look next..
Purchased this car a year or so back - runs ok on LPG but hardly even idles on Petrol, I can get it to run but usually ends up stalling and will not idle at the correct rpm and very rough!
The Gas install is a Ventury style - not injected.
I have replaced the O2 Sensor and yet to do tests on that to see if it's working but as sensors go it's the only one that is common to both fuels as the remainder such as motor temp and intake air are not directly fed into the Gas control system..
The only true separation between the two fuels are the Fuel injectors and fuel system such as fuel pump..
I have tested the Fuel pups pressure and it's within spec - about 70psi.. - think that's what it should be so I am guessing both the fuel regulator and pump are ok.
Oh other thing that happens when driving on Petrol is the car backfires a lot - from memory that is due to too much fuel..?
So maybe it is the injectors but I am confident they were replaced by the previous owner..

I have my suspicions the ECU may be at fault but to date I have not been able to borrow one for testing and don't want to buy one only to find it's not the issue.
Other possible issue is poor connection with the EFI electrical plugs - I have unplugged most cleaned and added electrical paste but this will not fix a bad plug..
Again an option is to make up a new harness with all new plugs but that is a heap of work to do if that is not the issue..

Lastly I have replaced the Air Flow sensor with a new one, this did not change a thing.

oh also added electrical earthing from the motor to body but yet to check what other earths are used in the electrical system inside of the car.

Clutching at straws..


Well-Known Member
Re the poor running on petrol. Maybe get your injectors cleaned and checked. They tend to glaze over if not regularly run on petrol. The backfiring could be anything, but make sure your using the rite spark plugs for gas. The gas ones I think are a colder range (lpg plugs).


Well-Known Member
I had a carby model 10yrs ago my mechanic at that time was good with older motors he extended the rotor button and advised me to set the timing at 15 deg btdc it ran great on gas, petrol, never backfired again and got very good econ. Don't know if this will help your issue. Also you have to keep the old girls in good tune they get senitive with age.


Well-Known Member
It could be anything, I have run all my Disco 1 V8's on duel fuel and its a fine art to get both working properly, you always have a compromise with one when tuning.
Your issue is not tuning though, if the LPG installer stuffed the install it will be very hard to find without removing all the wiring and starting again.
First things first, has the car had the issue since you owned it or has it just started?
If its always been there is it possible to ask the previous owner if the issue arose directly after the install and what he knew about it.
Check timing, it should be well advanced for gas, I run about 12 degrees, plugs should be one step down in temp from standard and gaped to .7.
It will not be a plug, lead or coil issue or it would be worse on LPG than petrol.
I had a issue that a crap mechanic spent hours trying to find and couldn't and made me spend big coin on a few bits that didn't need replacing.
Ended up at LPG one in Lillydale they checked the wiring and found the original installer had wired it so when on gas it cut the power to the MAF as well as the injectors so the ECU would go into limp mode and make the car crap on petrol until it was turned off then restarted again. That a a few other small issues fixed and its perfect on petrol and LPG but I do run regularly on petrol to keep the system and seals in good order.
I also need to run minimum 95, most of the time 98 so it doesn't ping its head off when working hard due to the advanced timing


Thanks for the replies,
The Injectors were replaced by the previous owner but think I will look at doing this after my big trip.. being my 2nd car.
When I first purchased the car it ran sort of ok on petrol but could not continue this due to a crack in the after market fuel tank.
I have since had this fixed hence the now attempt at using it.
During this time I found that virtually every join in the intake manifold had no gaskets nor did they seal so I went through and fixed all this up but I made a few errors in how I cleaned things resulting in filling the CAT up with carbie cleaner creating a huge whit smoke problem that took many days of hard work washing and soaking the CAT until it stopped blowing smoke! ... trick for new players..
Last week I found a few issues - firstly the change over relay from Petrol to LPG Created a significant voltage drop to the injectors - partly caused by me as I have now fully rewired the LPG system moving all electrical components to the cabin, mounting them over the motor ECU.
I rectified the voltage drop by adding a relay bypassing the cause of the voltage drop - this improved how it ran but still not enough..
After doing some tests I found the O2 Sensor was not responding correctly - should have a 0 to 0.9V output mine was sitting at 0.2V and never changing so I purchased a new O2 Sensor plus a new ignition module - replaced both and well the car now runs on Petrol and at times will even start so things are improving - I added a monitor to the O2 sensor that shows it's status via a row of LED's this is now flickering about as it should be - ok I love toys :)
Now the fuel consumption both on LPG and Petrol has improved out of sight!!
The car still idles rough on Petrol but seems to be improving, starting to think the ECU water temp sensor may be faulty as on Petrol when cold it runs very rough - but this as with several other ideas will have to wait until later July when I get home..