LPG Forum


All times are UTC



Post new topic Reply to topic  [ 11 posts ] 
Author Message
PostPosted: Sun Oct 16, 2016 3:08 pm 
Junior Member

Joined: Tue Aug 26, 2014 1:48 pm
Posts: 203
Location: South end of North Yorkshire
My wife's Honda CRV with KME Diego ECU, recently has developed an issue. When very low on LPG, rather than switching straight to petrol like it used to, now the engine gradually loses power until it's almost at stalling point and then it switches over. Almost like the switchover minimum pressure is set too low - but no changes have been made to the config. When it's not low on LPG, it appears to run absolutely normally with similar level of power to running on petrol.

Whilst I probably could just raise the 'switching to petrol when pressure less than xxx' value, I'd rather find out why this is happening. Now one thing that might have a bearing on this is that someone ran into the back of it at relatively low speed a few weeks ago. No damage other than the number plate holder in the bumper got pushed back and the spare wheel (mounted on the boot door) got slightly pushed in. Whilst the pipes look undamaged, would it be possible that a squashed pipe or some other shock damage to the tank area might have caused this effect? I guess it's possible that the tank moved on its mounts slightly.

_________________
-
2004 Subaru Outback 3.0Rn auto (LPG)
1991 Mitsubishi Pajero 2.5 LWB (WVO)
2008 Volkswagen Caravelle (Diesel)


Top
 Profile  
Reply with quote  
PostPosted: Sun Oct 16, 2016 10:02 pm 
Intermediate Member

Joined: Mon Jan 23, 2012 12:21 pm
Posts: 881
Location: Milton Keynes
Is the feed pipe copper or plastic? If its plastic I'd expect it to flex, if its copper then its possible that the pipes been distorted enough to restrict the flow slightly, might be just enough at nearly empty tank to cause it to drop off.

Whats the pressure sensor saying in this state and under normal conditions?


Top
 Profile  
Reply with quote  
PostPosted: Mon Oct 17, 2016 8:42 am 
Junior Member

Joined: Tue Aug 26, 2014 1:48 pm
Posts: 203
Location: South end of North Yorkshire
I'll plug the laptop in and find out the pressure when normal, and when it's close to empty and in the reduced power condition. That does of course mean running it until it starts to lose power then switching over manually - so that'll be a few days as she's just filled it up ;)

I will take a look at the pipes and where they run - fairly sure they're plastic.

_________________
-
2004 Subaru Outback 3.0Rn auto (LPG)
1991 Mitsubishi Pajero 2.5 LWB (WVO)
2008 Volkswagen Caravelle (Diesel)


Top
 Profile  
Reply with quote  
PostPosted: Mon Oct 17, 2016 12:39 pm 
Installer

Joined: Wed Sep 29, 2010 5:01 pm
Posts: 2962
Location: Yorkshire
Weather has just got colder so tank pressures will be lower than have been for some time. Low tank pressure, particularly in conjunction with squashed pipe, could cause the reducer to receive part liquid part vapour for a longer period before switching back to petrol than in warm weather when the tank is nearing empty. Same could occur even without any pipe damage and we might expect pipe damage to cause similar symptoms at high engine loads even with a full tank, so in the absence of any other issues (i.e runs OK even at full power unless tank is near empty) might expect squashed pipe not to be involved... Pressure that is low enough to cause lean running should in any case cause switch back to petrol, symptoms do seem due to lean running, so would expect changeover pressure set too high (probably my fault if I didn't adjust it!).

I don't remember the working pressure / switch back pressure settings on your CRV but I believe I changed the setup so reducer pressure isn't referenced to manifold pressure? In cases where reducer pressure is referenced to manifold pressure putting your foot down a bit further on the throttle during normal driving, or putting foot down due to lowered engine power due to lean mixture from 100% gas injector duty cycle is more likely to make for a dramatic drop in metered gas pressure than when reducer pressure isn't referenced to manifold pressure. I did attempt an explanation but it wasn't very clear so I omitted it, anyway I doubt you'll need it to see what I'm getting at. Needless to say I agree with monitoring pressures and looking at the switch back pressure. Where reducer pressure is referenced to manifold pressure switch back pressure is usually around half working pressure and isn't very critical, where reducer pressure isn't referenced to manifold pressure it can be necessary to set switch back pressure closer to working pressure and can be more critical... because particularly in cases of 100% injector dc (where gas pressure isn't referenced to manifold pressure), increasing manifold pressure (putting foot down a bit more) won't necessarily make the system see lower pressure so switch back might not occur.

Can be a problem with setting switch back pressure close to working pressure with reducers that aren't great in terms of pressure stability - Set switch back pressure too close to working pressure and the system might switch back to petrol randomly or, say, when the reducer is at a certain temperature.

Simon

_________________
Full time LPG installer
Servicing / Diagnostics / Repairs to all systems / DIY conversion kits supplied with thorough tech support
Mid Yorkshire
2 miles A1, 8 miles M62,
http://www.Lpgc.co.uk
Twitter https://twitter.com/AutogasSimon
07816237240


Top
 Profile  
Reply with quote  
PostPosted: Mon Oct 17, 2016 3:45 pm 
Junior Member

Joined: Tue Aug 26, 2014 1:48 pm
Posts: 203
Location: South end of North Yorkshire
Cheers Simon. That does make sense, and yes you left it not referenced to manifold pressure. So I'll go and find out what the pressure is during full tank and just as it's starting to run a lower power (have let her know to switch to petrol manually so it doesn't use any more if this happens when she's on her way to or from work, than I'll plug the laptop in and go for a test drive when she gets home).

I'm happy to try raising the switchover pressure slightly to see if that helps. Maybe it is simply the cold weather and the bump has nothing to do with it - she normally fills up before it runs out anyway, so it's only been on a couple of long journeys recently where it's happened.

_________________
-
2004 Subaru Outback 3.0Rn auto (LPG)
1991 Mitsubishi Pajero 2.5 LWB (WVO)
2008 Volkswagen Caravelle (Diesel)


Top
 Profile  
Reply with quote  
PostPosted: Mon Oct 17, 2016 6:37 pm 
Installer

Joined: Wed Sep 29, 2010 5:01 pm
Posts: 2962
Location: Yorkshire
No probs mate.

For the benefit of other readers: I set it up like that because it seemed petrol pressure wasn't referenced to manifold pressure on the CRV. For any given Pinj and multiplier, fuelling on gas is only the same as on petrol if the difference between petrol pressure and manifold pressure and the difference between LPG pressure and manifold pressure are consistent (i.e. this doesn't mean both pressures are the same, it means they are both unchanging in relation to manifold pressure)... So if petrol pressure is 3.2bar relative to atmosphere (not manifold pressure) then on a normally aspirated engine petrol pressure relative to manifold pressure may fluctuate between, say, 3 bar, which is 3.2 minus 0.2 (where 0.2 is probably the lowest manifold pressure the engine will see while fuel is actually being injected) and 3.2 minus 1 bar = 2.2 bar (where 1 bar is manifold at atmospheric pressure at full throttle), the 2.2 bar to 3 bar range represents around a 33% shift in relative petrol pressure. Meanwhile if reducer pressure is referenced to manifold pressure it's pressure relative to manifold pressure doesn't fluctuate at all (with perfect reducer pressure stability) so represents a 0% shift in relative LPG pressure. There are pros and cons to connecting the vac pipe on such installs and with most ECU's a perfect compromise isn't possible... whether it it best to connect vac pipes, or not, depends on spec of the engine, reducer and injectors. On this car connecting the vac pipe made for a map with too high a multiplier at idle and too low a multiplier at high loads, multiplier had too steep a downward curve. Disconnecting vap pipes made for a flatter multiplier but for this to be workable (let alone improvement over connecting vac pipes) it is more critical to check minimum Ginj is above injectors minimum pulse length (for given injectors) and Ginj doesn't run out of duty cycle window at higher loads. On some vehicles with none manifold referenced petrol pressure, particularly those with turbos where the % range of petrol pressure fluctuates by an even greater amount, it is necessary to reference reducer pressure to manifold pressure... like the C65AMG V12 BiTurbo I converted this year but on that install I fitted an ECU that allows compensation of Ginj for manifold pressure (compensation is seemless between vacuum and boost and adjustable at many points a bit like having another map besides the multiplier map). On none turbo applications Pinj can be plotted fairly consistently against manifold pressure (except on Valvetronic types) with only small margin for error (which is mostly due to charge trapping ability of the cam at different rpms), after all a given engine load implies a given manifold pressure so the multiplier map can compensate for the difference in pressures. But on turbo applications it is more complicated because the range of manifold pressures may be double (or more) that of a normally aspirated engine and you couldn't not reference reducer to manifold pressure because if you did there'd never be enough gas pressure under boost conditions and would be way too much pressure under idle conditions. I was going to do a thread on my manifold pressure compensation calculations etc for the pressure compensation setup on the C65AMG 6L V12 BiTurbo 800bhp conversion I did this year, haven't got around to it yet. I'll just mention that without compensation fuelling was incorrect around the point boost kicks in, because when boost kicks in Pinj rockets due to both increased demand for petrol and lower petrol/manifold pressure difference, if Ginj followed the Pinj rocket fuelling became overly rich causing misfires even though off boost at the same Pinj fuelling on LPG was correct.

Simon

_________________
Full time LPG installer
Servicing / Diagnostics / Repairs to all systems / DIY conversion kits supplied with thorough tech support
Mid Yorkshire
2 miles A1, 8 miles M62,
http://www.Lpgc.co.uk
Twitter https://twitter.com/AutogasSimon
07816237240


Top
 Profile  
Reply with quote  
PostPosted: Sat Oct 22, 2016 9:44 am 
Junior Member

Joined: Tue Aug 26, 2014 1:48 pm
Posts: 203
Location: South end of North Yorkshire
OK - some information. At idle (760rpm) with about half a tank of LPG the pressure is 1.89 bar. Changeover pressure is set a less than 0.5 bar by 0.4 seconds. There are two error codes stored, LPG pressure too low (although you get this every time you run out and it switches over, so it's not a lot of help) and LPG injectors open 100%.
That second one to my mind would tend to agree with Simon's assessment that the pressure is too low and it's unable to open the injectors long enough to get enough fuel into the manifold, hence it runs lean and power drops off as the petrol ECU starts retarding the valve timing and ignition timing to compensate.

I don't have the pressure when it's doing this, but hopefully later this weekend I will.

_________________
-
2004 Subaru Outback 3.0Rn auto (LPG)
1991 Mitsubishi Pajero 2.5 LWB (WVO)
2008 Volkswagen Caravelle (Diesel)


Top
 Profile  
Reply with quote  
PostPosted: Tue Oct 25, 2016 11:46 am 
Installer

Joined: Wed Sep 29, 2010 5:01 pm
Posts: 2962
Location: Yorkshire
Without reducer vacuum reference, if pressure is 1.9 at idle (where manifold pressure will be around .3 bar) then flat out (when manifold pressure is 1 bar) pressure will be 1.2 bar less a bit for reducer pressure fall-off when it is flowing a lot of gas.. Could probably set switchback pressure to somewhere around .9 bar.

Simon

_________________
Full time LPG installer
Servicing / Diagnostics / Repairs to all systems / DIY conversion kits supplied with thorough tech support
Mid Yorkshire
2 miles A1, 8 miles M62,
http://www.Lpgc.co.uk
Twitter https://twitter.com/AutogasSimon
07816237240


Top
 Profile  
Reply with quote  
PostPosted: Tue Oct 25, 2016 4:45 pm 
Junior Member

Joined: Tue Aug 26, 2014 1:48 pm
Posts: 203
Location: South end of North Yorkshire
Not a bad estimate Simon!

With the power dropping the pressure is now only 1.6 bar at idle, dropping to 1.3 bar at light throttle and 0.65 bar at full throttle. So yes, it wouldn't have switched over on the existing setting on 0.5 bar. I've upped it to 0.9 bar and can confirm that although it'll idle on LPG, it switches back to petrol as soon as you try to accelerate much. Tomorrow will refill the tank and we'll see what it does when it gets low next time, and what the range is like.

_________________
-
2004 Subaru Outback 3.0Rn auto (LPG)
1991 Mitsubishi Pajero 2.5 LWB (WVO)
2008 Volkswagen Caravelle (Diesel)


Top
 Profile  
Reply with quote  
PostPosted: Tue Oct 25, 2016 8:28 pm 
Intermediate Member

Joined: Mon Jan 23, 2012 12:21 pm
Posts: 881
Location: Milton Keynes
Just a thought - if this suddenly came on after the accident or any inspection work resulting from it has the manual shutoff been moved from fully open if its fitted? (Assuming its a single hole tank not 4 hole) Would cause the same sort of issue as a squashed pipe?


Top
 Profile  
Reply with quote  
PostPosted: Wed Oct 26, 2016 7:34 am 
Junior Member

Joined: Tue Aug 26, 2014 1:48 pm
Posts: 203
Location: South end of North Yorkshire
Good idea Brian - I looked a the pipes and the security of the connections but didn't look at the valve. I'll take a look at the weekend (when it's light enough!)

_________________
-
2004 Subaru Outback 3.0Rn auto (LPG)
1991 Mitsubishi Pajero 2.5 LWB (WVO)
2008 Volkswagen Caravelle (Diesel)


Top
 Profile  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 11 posts ] 

All times are UTC


Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB® Forum Software © phpBB Group