Cold start misfire

[Harry Hindmarch]

Right I've had this problem for a few months now and I've tried a number of things and not had any luck. Hoping someone in here might be able to shine a light on the issue. 

So my sc430 is an 03 with 160k on the clock. Its had lpg fitted by the previous owner a few years ago. 

When I start the car it starts on petrol and automatically changes over to lpg once the cars warmed up a little bit. 

The problem is when I start the car it runs lumpy like a misfire. When the car is put into drive it struggles to run but as you accelerate the car powers through the misfire. But if I come to a stand still again it will start to misfire again. Sometimes the misfire is bad enough that the engine light comes on and flashes also bringing on the traction control light switching the fraction control off. 

Once the car switches over to lpg after a couple minutes of running it runs smoothly without a misfire. 

I have replaced all the spark plugs and all the ignition coils with high quality replacements. I've check for vacum leaks and anything that looks suspicious but without luck. Also replaced the temperature sensor but no change.

Also since buying the car it's always had the engine light on showing a p0430 code for the lambda sensor on the right side of the car. I've replaced the pre cat sensor but the code comes straight back after the car goes from cold to warm again after the codes cleared. I know this could be a dying cat but I can't see how that would cause a misfire? 

I'm unsure what to try next and don't want to throw money on parts that don't make a difference. 

If anyone can shine some light on the problem I'd very much appreciate it as I'm lost on what to do next. 

Thanks everyone

[Britprius]

I think a compression check would help sort this problem. It maybe that through valve seat regression the valve clearances are tight holding the valves open slightly. If this is the problem it does mean the cylinder heads will need refurbishing. This is not unusual with some LPG conversions.

John.  

[messi]

Hi there

I had this issue on an sc430 I had in the past, I replaced all the parts you have, and still had the issue, it turned out to be the variable valve timing solinoid, they are located at the front of the engine, the drivers side one controls the left side, you could either clean it or replace it, I replaced it and all was good. There's a video on you tube showing where it is and how to replace, it's very easy. I bought the part from rockauto, make sure you take the Battery terminal off before replacing the sensor

 

Remember bank 1 is the passenger side and back 2 is drivers side

[messi]