"mis-Firing" On Petrol But Not On Gas

Mike Floutier · January 13, 2011

Hi Guys,

Well I haven't had to ask a question for a while which must be a good thing BUT this morning I had a problem and I'd appreciate some help with the diagnosis. Ok, bear in mind that I've had a gas conversion so it runs on petrol or gas. 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, and 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 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 gas - very soon (1 - 2 seconds) 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 - I didn't stop the engine or erase the codes btw.

8. I tried switching between petrol and 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 it appears that the problem "mis-firing" is occurring only when running on petrol and NOT happening when I'm on gas - 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 gas.

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

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

Many thanks.

Mike

Ps Which cylinder is which btw. eg. is No.1 front left etc. also bear in mind I don't know if bank 1 is left or right - lol!

Edited January 13, 2011 by Mike Floutier

steve2006 · January 13, 2011

Hello Mike,

From what I understand the LPG system has an "Injector Emulator" which basically tricks the petrol fuel injection system into thinking it is working as normal when in fact the engine is running on LPG.

Seeing as the car runs fine on LPG this would suggest the ignition system is all OK (coils,plugs,leads) or it would also misfire on gas as well.

It may be worth getting the LPG system checked out or have a look yourself for wiring damage to the LPG system.

Mike Floutier · January 13, 2011

Hello Mike,

From what I understand the LPG system has an "Injector Emulator" which basically tricks the petrol fuel injection system into thinking it is working as normal when in fact the engine is running on LPG.

Seeing as the car runs fine on LPG this would suggest the ignition system is all OK (coils,plugs,leads) or it would also misfire on gas as well.

It may be worth getting the LPG system checked out or have a look yourself for wiring damage to the LPG system.

Hi Steve,

Thanks for responding.

I hadn't thought of the gas system somehow causing the petrol system to mis-fire.

I guess the best thing to try, in this respect, is to completely disable the gas system, by pulling it's fuses. Then if the problem persists I will know it's not the gas system causing it. On the other hand, if the problem goes away, I will know where to look.

I'll give it a try in the morning.

Regards,

Mike

steve2006 · January 14, 2011

Hi Mike you're welcome,

If you disconnect the LPG system it should default to running on petrol only but dependent on how the switch over is done the car may not start at all on petrol.

Just another thought with the strong smell of petrol it could be the car is running on both LPG and petrol at the same time i.e. both sets of injectors are active together which would cause serious running problems due to over fueling.

LPG fitting centres should be able to check the system and perform a diagnostic check.

Mike Floutier · January 14, 2011

Hi Mike you're welcome,

If you disconnect the LPG system it should default to running on petrol only but dependent on how the switch over is done the car may not start at all on petrol.

Just another thought with the strong smell of petrol it could be the car is running on both LPG and petrol at the same time i.e. both sets of injectors are active together which would cause serious running problems due to over fueling.

LPG fitting centres should be able to check the system and perform a diagnostic check.

Thanks Steve,

You were exactly right!

I did try pulling the LPG fuses "to test whether the lpg system was causing the problem" but the problem remained.

At this stage I felt the best thing to do was visit my installer in the hope that he could reprogram the lpg system to start at a lower temperature or even start on gas - to save the cats from damage. Unfortunately the latest Prins software doesn't allow that.

They then followed your suggestion of testing the emulators. This was done by removing the emulators and "shorting" their multiplugs to effect a "normal" petrol environment using a custom plug.

Unfortunately this seemed to make no difference. Then one of the lads suggested that because there was an outstanding OBDll fault code the petrol ECU would still think there was a problem and rightly keep the relevant petrol injector shut down.

He cleared the fault code and, aaaah, smooth running was restored.

So as I said, you were totally on the nail, it was a faulty emulator in the gas system causing the petrol injectors to misbehave.

Fortunately they had a new emulator handy so I could drive away smoothly on petrol or gas.

Thank you Steve!

Kind regards,

Mike

steve2006 · January 14, 2011

Good result Mike, I'm wasting my life repairing TVs and all the other domestic electronic rubbish produced these days!!! lol

Mike Floutier · January 16, 2011

Good result Mike, I'm wasting my life repairing TVs and all the other domestic electronic rubbish produced these days!!! lol

Unfortunately, the exact same problem has recurred after a couple of days working fine with the replacement emulator; same cylinders affected.

I checked the date on the "new" emulator and it's only a few weeks newer than the broken one it replaced; ie. Sept 2008.

I wonder what's happening here? The replacement emulator temporarily fixing the problem proved that the petrol injection system works ok.

Presumably it's an electrical/electronic fault.

But is it likely that this second, albeit old, emulator would fail so soon? Is it possible that some other problem is causing the emulator to fail?

Any ideas?

Regards,

Mike

Mike Floutier · January 30, 2011

Just to update you on this, 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!!

Many thanks.

Mike

SteveLS400 · January 30, 2011

Just to update you on this, 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!!

Many thanks.

Mike