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PostPosted: Wed Jul 05, 2017 10:00 pm 
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Location: Milton Keynes
4eyes wrote:
LPGC wrote:

Has it been mentioned yet that a lean mixture on petrol can cause knock... Lean mixture on petrol could be due to LPG system calibrated too rich.

Simon

Thanks Simon. I have read on some American Subaru forums that my engine is tuned a little lean in places, Alot of people seem to have this issue with no fix apparent. Would you be able to expand on the point above about the possible rich set up of the lpg ecu effecting the petrol ecu? Thanks.


I think Simon missed this bit of your reply - The petrol ecu is still active when used on lpg, and will still adjust its trims according to the lambda sensors. So if your lpg side is set too rich it will lean it back off - causing it to be corrected. Doesn't sound like a problem until you switch back to petrol and see it takes a while to relearn the trims for petrol (if they are very far off it may not relearn on some cars until the trims are reset). effectively if the lpg isn't setup with the correct calibration the petrol ecu will be trying to correct it, and the lpg will then be adjusting it as well.

The real question is does it keep doing it after a few miles on petrol, or does it gradually disappear?


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PostPosted: Thu Jul 06, 2017 7:41 am 
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Thanks Brian. I have tried running on petrol, including super unleaded, for a few hundred miles and it doesn't seem to improve. It's only a slight ping only at around 1200 revs and is short lived. It is predictable and repeatable. The symptoms are identical to many other owners reports who have no lpg. I don't think it's set up too rich on the lpg side as it was recently serviced and calibration checked. If it was set up too rich to an extent that the petrol ecu couldn't correct it when back on petrol, shouldn't I expect a check engine light? Thanks again to everyone who has helped, this issue isn't a major issue for me, but I'm getting quite anal about it !


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PostPosted: Thu Jul 06, 2017 8:29 am 
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Pre-ignition would only normally occur when you put your foot down and are making the engine work harder. If that is the case, what are you doing trying to accelerate from 1200 rpm? Surely, you'd want to drop it down a gear (or two) to get the engine into the power band? If it is a common problem and the dealers state that it is normal it does seem likely that it has been mapped with a very lean mixture around that rev range as it would normally only be expected to be used when cruising at a constant speed. One of the driving courses I have done was a drive to survive course taught by ex-police instructors and we were told that unless the rev counter is showing over 3,000 rpm, you're in too high a gear.....

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PostPosted: Thu Jul 06, 2017 10:54 am 
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Joined: Wed Sep 29, 2010 5:01 pm
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Location: Yorkshire
I wouldn't accept it as normal... Could check the plugs for signs of detonation, if the signs are bad you might want to continue looking into it?

Edit - Hibernated my laptop while browser was open on this page last night. The last post I read before this reply was from 4eyes.

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PostPosted: Thu Jul 06, 2017 2:36 pm 
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Joined: Fri Nov 15, 2013 4:09 pm
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Thanks Gilbert. My car has a CVT so I have no real control over rpm apart from the throttle. It only occurs with a warm engine when feathering the throttle going up hill at slower speeds. It will also do it in "manual" mode when I can select a lower gear. It does not last long and doesn't happen with more aggressive throttle input, and does not happen at all on lpg. I did change the spark plugs anyway about 6 months ago and it made no difference. Old spark plugs were fine.


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