[p0isonra1n]

Hey guys,

My dad has a TB45 that has a perisitant miss. It is on LPG but it misfires on both Petrol and Gas.

We have replaced the following and it has made no difference:

• New Spark Plugs
• New Ignition Leads
• New Dizzy Cap
• New Rotar Button
• New Coil
• New MAF

We have also checked compression, 120psi across all. We have also tested the whole distributer in my TB45 and it runs no problem so it is not the distributer itself.

We are at a loss of what else it could be as it does it on Gas and Petrol so it can not be fuel system related.

Any things we have missed to check would be great. Thanks for the help.

 

[nigel3780]

Does it miss at any rpm?

 

[p0isonra1n]

Does it miss at any rpm?

It's worse at idle/low rpm

 

[nigel3780]

Have you tried disconnecting leads While running to see which cylinder it is?

 

[dkril]

How about by throttle position? Is it better or worse at WOT?

 

[Trollbasher]

probably a vacuum leak in the inlet manifold or gummed up PCV valve messing with the fuel mixtures.
other causes could be worn cam lobes on one cylinder - but the only engine I know that regularly did this was the holden 253 v8.

 

[Daves GQ]

Hey guys,

My dad has a TB45 that has a perisitant miss. It is on LPG but it misfires on both Petrol and Gas.

We have replaced the following and it has made no difference:

• New Spark Plugs
• New Ignition Leads
• New Dizzy Cap
• New Rotar Button
• New Coil
• New MAF

We have also checked compression, 120psi across all. We have also tested the whole distributer in my TB45 and it runs no problem so it is not the distributer itself.

We are at a loss of what else it could be as it does it on Gas and Petrol so it can not be fuel system related.

Any things we have missed to check would be great. Thanks for the help.

It surely has to be something on the ignition side in spite of the things you have changed. I had an issue a while back where my TB42E ran horribly on petrol but fine on gas, which was eventually traced to a fuel pump failing to deliver sufficient pressure. Now replaced it again runs superbly on either fuel. Setting the distributor advance so it suits both fuels should be straight forward.

The thing with LPG is it is usually a highly reliable and very clean burning fuel and uses very little of the EFI system when in use. The vehicle is tuned for LPG with a sensor placed in the exhaust pipe, and once the system is set it basically just breathes LPG and burns it, so things like your MAF and oxygen sensors etc make very little difference. Provided your ignition is sound it should run OK on gas. The only issue I have with gas is the engine likes to be warm to switch to it, if you start using it prior to the thermostat opening you can get a miss when accelerating hard, but if running gently it is ok. Petrol runs fine at any temperature, and I start on it before switching.

There are other sensors in the throttle body, but they largely dictate things such as idle settings. The suggestion to try and trace which cylinder might be at fault could be a clue. If it consistently one cylinder then your task to trace it becomes a lot easier. The fact that you have replaced the dizzy cap rules out a hairline crack, and as long as you have used good quality spark plug leads and the correct temperature range spark plugs, changed the coil etc, then you have done all the obvious things.

Is it possible something is shorting out occasionally, such as a spark plug lead? Hope this helps.