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Mike Floutier

Mis-Firing On Petrol But Ok On Propane

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Well I haven't had to ask a question for a while which must be a good thing BUT yesterday I had a problem and I'd appreciate some help with the diagnosis. Ok, bear in mind that I've had a propane (liquified petroleum gas) conversion so it runs on petrol or propane (we call propane gas over here which of course is confusing). I'll refer to gas(oline) as petrol.

Anyway, this is the sequence of events:-

1. Start up from cold - immediately I notice it's running slightly rough (just like 100,000 miles ago when one of the coils needed seeing to) so I suspect a coil.

2. OBD11 codes registered are: 1) P0300 - Random/Multiple cylinder mis-fire, 2) P0301 - cylinder 1 mis-fire, 3) P0307 - cylinder 7 mis-fire, and 4) P0301 - cylinder 1 mis-fire (pending).

3. After a little while the engine warning light begins to flash - indicates that mis-firing may cause over-heating which may damage the calalytic converters.

4. A strong smell of exhaust fumes and possibly petrol is evident even inside the car and especially outside - I'm still feeling slightly sick as I write.

5. NOW, as the car is warmed up it switches automatically to propane gas - immediately the mis-firing stops completely

6. Another few seconds and the engine warning light stops blinking.

7. An OBD11 re-scan now shows NO CODES.

8. I tried switching between petrol and propane gas and found that this sequence would happily repeat itself.

As I say, at first I suspected one of the coils and that possibly a poor connection or something was curing itself as the car warmed up.

However the problem "mis-firing" is occurring only when running on petrol and NOT happening when I'm on propane - which is good as this should help with the diagnosis. eg. I think it rules out the coil/plugs part of ignition as the problem would be evident on both petrol AND propane.

However, the car has to start on petrol and won't change to propane until the coolant is sufficiently hot. The problems with leaving it like this are: 1) Damage to catalytic converters, and 2) the clouds of white smoke whilst warming up.

Does anyone have any clue as to what's happening here?

Kind regards,

Mike

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Well I'm not that familiar with the conversion you added but it occurs to me the problem is still with the base petrol functionality.

How is the propane fed to the intake manifold? Could there be some vacuum leaks which are causing the erratic idling? Why don't you swap the coils and see if the mis fire moves to the other cylinders? Maybe you just got another bad coil again.

Not sure how the ECU deals with the bait and switch fuel swap (petrol to propane). I suspect there are some pretty complicated algorithms and fuel maps in its programs and maybe when you switch to propane those algorithms get confused. Why not shut off the propane system completely for a few days and see if the system self corrects (ie will a petrol cold start after a petrol warm shut down make a difference)?

Could it be the dreaded coolant temp sensor. Causing ECU to think engine is perpetually cold and sending too rich a petrol mixture?

Not concrete but a couple ideas

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..."However the problem "mis-firing" is occurring only when running on petrol and NOT happening when I'm on propane - which is good as this should help with the diagnosis. eg. I think it rules out the coil/plugs part of ignition as the problem would be evident on both petrol AND propane."...

Not so sure. By your own logic, cold start=Petrol, warm engine=propane. It could be an intermittent in coil and at low temps it shows itself, ie on petrol, and as engine warm it goes away and simultaneously you're on propane. I guess seeing how it runs on petrol on a fully warmed engine might be revealing. Also ignition voltage requirements of petrol and propane may be quite different (does propane have lower flash over point?) thereby a marginal coil might work on propane but not so well on petrol.

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So, I am intrigued by this system, Mike. If it initially runs on petrol and switches over to gas when warmed, how long does a tank of petrol last for you? I would be somewhat concerned that the petrol could turn putrid before all used. In any case, would not a "bad" tank of petrol take a long time to get used up?

Besides coils, you might also consider spark plugs. I would imagine the petrol is not as easily ignited as the gas as CuriousB mentioned.

Also, possibly clogged injector(s)?

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Hi Guys,

Sorry to be a while getting back but I had another problem arise in the meantime and I wasn't sure if it was related or not.

Also, I thought it was solved by a replacement fuel injector "emulator" in the propane system but that turned out to be a red herring and the problem returned a couple of days later.

Thanks CuriousB for all your input. To answer some of the issues you raised:

1. The system is a Prins VSI (vapor sequential injection). The liquid propane is fed via a vaporizer directly in the same way as the gasoline.

2. Regarding it being a warming up problem, there is a facility to switch manually between propane and gasoline so I was able to confirm it was not this.

3. You mentioned different ignition requirements for propane versus gasoline. Unfortunately it's the other way around and the propane mixture actually requires a higher voltage to get a spark to cross the gap - hence iridium plugs are essential to avoid mis-firing (having lower voltage requirements) - This factor ruled out the coils as I mentioned.

Yes Landar the gasoline is used fairly slowly, I typically fill the tank about 1/4 and this lasts a couple of months.

My propane converter encouraged me to change my spark plugs suggesting that they may be causing problems but why they would go away when switching to propane......

Anyway, we had all given up trying to understand and diagnose the problem so I agreed that it was probably time for action.

What I did was:

1. Change all the spark plugs,

2. Put in a bottle of injector cleaner, and

3. Clean the Air Flow Sensor.

The problem went away, which was good news.

The next morning I had a mis-fire again which was a bit strange especially as this time it was present on propane and gasoline. However, this time the obd codes were different. No mis-fires, instead I got P1305 which was the coil circuit I mentioned in my original post.

Firstly I tried cleaning the contacts on the coil, spraying them with switch cleaner and refitting as this is what cured it before. No good!

I then swapped the coil with a good one to see if the problem moved - no good. Then tested the circuits for power, continuity and shorting - all fine.

I assumed that the new plug must be ok, especially as this problem was intermittent/semi-heat related. I ruled out the ECU on account of cost - lol!

That just left the coil connector. I realised that although I'd cleaned the coil's connection terminals, I hadn't cleaned the plug/female sockets. I did this with a sliver of folded wet/dry glass paper.

It now all works fine!!

Thanks so much for your continued support guys, I know I always say this but I really wouldn't be able to persevere with these problems by myself.

Kind regards,

Mike

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That just left the coil connector. I realised that although I'd cleaned the coil's connection terminals, I hadn't cleaned the plug/female sockets. I did this with a sliver of folded wet/dry glass paper.

Are the contact pins gold plated or silver? Tinned connector contacts are the evil scourge of the electronics industry. They oxidize and over time can build up a insulating layer. Since these connectors are seldom mated/unmated you don’t get a "wiping action" to clean the oxide away. This was a big problem with electronics in the 70's as many semiconductor (IC's) were plugged into sockets on circuit boards. Random mysterious failures.

Gold eliminates the oxidizing problem as it is a stable element. It can get dirty and contact springs can get bent so it isn’t foolproof though.

Glad to see you're back on the road (even if you're driving on the wrong side :lol: ).

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check your timing. If it's off it will run like *BLEEP* on a cold start. Take the timing covers off and see if the lines are matching up with the cams and the crank.

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