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Author Topic: missfire on LPG  (Read 1200 times)

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Bojan

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missfire on LPG
« on: 03 April 2010, 11:35:42 »

today my 1996 MV6 started developing missfire while driving on LPG. it seems to be happening only while accelerating after the gas pedal was released (egg. accelerating after braking).
it runs fine when switched on gasoline.
LPG system is with injectors.

what could be causing this problem?

the lpg filter wasn't renewed recently, so it is one possibility.

camshaft cover gaskets are leaking a bit. could this cause spark leads to deteriorate so much, that it would cause a missfire, but only on lpg??

any other possibility? bad lpg in tank?
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Darth Loo-knee

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Re: missfire on LPG
« Reply #1 on: 03 April 2010, 11:39:20 »

I have being told several times by a Very Good Gasman I know, that LPG needs a very good spark.
If you have no codes I would be checking the HT side of things.
Plugs, Leads, Dispack
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Bojan

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Re: missfire on LPG
« Reply #2 on: 03 April 2010, 11:48:43 »

now, when i think of it, the word 'missfire' isn't the proper one.

i had a dispack failure on previous mig, it run on 5cyls, and it wasn't near like this. you could feel it while driving.

in this case, the car is twitching and caufing, like when runing out of gas.
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tunnie

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Re: missfire on LPG
« Reply #3 on: 03 April 2010, 12:20:22 »

Quote
now, when i think of it, the word 'missfire' isn't the proper one.

i had a dispack failure on previous mig, it run on 5cyls, and it wasn't near like this. you could feel it while driving.

in this case, the car is twitching and caufing, like when runing out of gas.

Filters need to be changed every 10,000 km's, thats whats stamped on mine. But apparently the LPG you get is a lot dirtier than what we get over here, so the 10k limit is stamped on filters to cover the manufactures.

I'd start by putting a replacement filter on, then go from there  :y
« Last Edit: 03 April 2010, 12:20:39 by tunnie »
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Bojan

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Re: missfire on LPG
« Reply #4 on: 03 April 2010, 19:34:14 »

i've read couple of posts here, and it seems that the correct word for the problematic behaviour is 'jerking'.

it is as if the engine is struggling to accelerate. there is no pattern in this struggle, and it only happens in low revs while accelerating. above 2500 rpm there are no problems.

can the lpg filter be cleaned? can't buy new one until tuesday (easter and stuff -> stores closed).

i hope it isn't the dispac or crank sensor.  :-/
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Bojan

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Re: missfire on LPG
« Reply #5 on: 04 April 2010, 18:15:22 »

drove it today, and here is the feedback.

when cold, it runs perfect on petrol and lpg. only after being driven for an half an hour or more, it starts jerking, and only on lpg. on petrol runs fine all the time.

i pulled out 2 leads (uk passanger side). there was some oil, but just a bit. i've got it out, and gonig for a test drive now.
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tunnie

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Re: missfire on LPG
« Reply #6 on: 04 April 2010, 18:24:12 »

how old are the plugs? It runs fine on petrol yes? Any faults are magnified by LPG, it needs a stronger spark to ignite it. I'd stick some fresh plugs in  :y
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Bojan

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Re: missfire on LPG
« Reply #7 on: 04 April 2010, 19:04:42 »

plugs changed 3 weeks ago.
genuine GM/bosch.

on petrol, it runs quite fine.
it used to run very nice on petrol, apsoluetly no signs of problem, until yesterday. and then, all of the sudden, this happens. 
i filled it up with lpg yesterday. could the bad, dirty lpg cause this problems?
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Lazydocker

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Re: missfire on LPG
« Reply #8 on: 04 April 2010, 19:59:57 »

Quote
plugs changed 3 weeks ago.
genuine GM/bosch.

on petrol, it runs quite fine.
it used to run very nice on petrol, apsoluetly no signs of problem, until yesterday. and then, all of the sudden, this happens. 
i filled it up with lpg yesterday. could the bad, dirty lpg cause this problems?

Did the problems start immediately (soon) after filling up?

If so it is possible that you've got some dirty gas... Only way to tell is to use the tankful and try again ;)
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Bojan

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Re: missfire on LPG
« Reply #9 on: 04 April 2010, 20:13:34 »

soon after filling up,maybe 50km after.
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Lazydocker

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Re: missfire on LPG
« Reply #10 on: 04 April 2010, 20:45:49 »

Quote
soon after filling up,maybe 50km after.

Could well be bad gas then... You're (unfortunately) in a country which does have some problems sometimes. See what happens on the next fill up :y
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Bojan

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Re: missfire on LPG
« Reply #11 on: 05 April 2010, 10:48:41 »

crap...it started to behave the same when running on petrol.

so, first thing i can do is to change the leads. this i can do by myself. the rocker cover gasket is leaking, but not much. i can't do that by myself, and the mechanic guy can do it at friday or saturday, at best.

so, i was thinking to change the plug leads tomorrow, and the rocker cover at the end of the week, or at the begining of the next week.

will some oil in the plug wells do any noticeable damage to new plug leads in weeks time ?
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Bojan

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Re: missfire on LPG
« Reply #12 on: 05 April 2010, 12:41:12 »

update: just did the paperclip test.

i'm getting code 19. that's a crank sensor, right?

could that cause the problem.
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Kevin Wood

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Re: missfire on LPG
« Reply #13 on: 05 April 2010, 13:05:25 »

Yep, sounds like crank sensor needs changing.

In addition, the cam covers need sorting before they do destroy the leads. As a temporary measure I would drain away the oil, clean up the existing leads as best as possible and perhaps use an ignition system sealant spray such as "damp start" on them before inserting them onto the plugs. Might keep the moisture at bay until you get the cam cover gaskets done.

Kevin
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Bojan

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Re: missfire on LPG
« Reply #14 on: 11 April 2010, 17:27:07 »

car was fixed few days ago.

first the crank sensor was changed, which made the 'code 19' go away.
but the missfire remained.
so the plug leads and cam cover gaskets were changed, and now car runs fine.
it is noticeably smoother and somewhat quieter.

thanks for help to everyone :y :y
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