high pressure during cut off

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mgrover
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high pressure during cut off

#1 Post by mgrover » Mon Nov 09, 2020 2:28 pm

bit of a weird one but during cut off the pressure spikes to 3bar still. car is 4.2 SC so thought either the reducer or the solenoid valve is knackered.

but theres quite a bit of a long cable from going the solenoid valve to the reducer, like going across the engine bay. could it be cut off occurs, so injectors stop, then whatever gas left in pipe is pumped so pressure increases?

ac stag ecu if anyones wondering.

the overpressure setting makes no diff

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Re: high pressure during cut off

#2 Post by Brian_H » Mon Nov 09, 2020 2:42 pm

The solenoid just shuts off the feed to the reducer, it has no role in pressure regulation, thats all done by your reducer. When you say cable are you talking about the vacuum hose connection or something else?

There is no pumping going on either, its just controlled by the springs and diaphram in the reducer, all mechanical, nothing electronic doing any of the control side. The pressure sensor is just that - a sensor nothing more. What sort of temperature is the reducer running at when you see the problem? It should be reasonably warm to keep it working correctly, if its too cold it may struggle to regulate pressure properly.

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Re: high pressure during cut off

#3 Post by mgrover » Mon Nov 09, 2020 2:52 pm

nah the gas hose. because before when I had 2 reducers MB1500, the solenoid valve was right next to them. but if the solenoid valve doesnt switch off the supply during cut off then the next question is, is my pretty much brand new reducer damaged.

the reducer is hot enough from what I can see but who knows anymore. since it can easily pump out whats required during high rpms and load. its more when i took my foot off the peddle is has no idea what to do with the extra gas which swell the pipes. at that point i have to stop, release the pressure since the sensor is freaking out.

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Re: high pressure during cut off

#4 Post by LPGC » Mon Nov 09, 2020 6:06 pm

What is reducer pressure set at (this is reducer less manifold pressure)?

The engine is supercharged, so at high load the manifold will be under boost pressure (higher than atmospheric pressure). Let's say manifold pressure at full load is 1.3 bar, if reducer pressure is set to 1.7 bar then pressure relative to atmosphere will be 2.0 bar under boost conditions. If you the suddenly come off the throttle, during over-run manifold pressure may then go down to only 0.2 bar, the gas between reducer and injectors isn't being used (injection is shut off), the measured pressure (reducer minus manifold) is now 2.8 bar.

Especially if the engine bay solenoid isn't directly fitted to the reducer - you don't even have to be running on gas when you boot it for over-run conditions to show high reducer pressure... You could drive around on petrol at part load, the minimum pressure would always be reducer minus manifold pressure, when you back off the throttle the reducer pressure reading increases, run the engine under higher load and the reducer will turn more liquid gas into gas vapour which has nowhere to go. More likely to happen when the solenoid isn't mounted directly to the reducer because when it's mounted directly there's little liquid gas for the reducer to be fed with to turn into vapour.

That said it is still worth looking into potential real issues. If the solenoid valves are open but all cylinders/channels are switched to petrol does pressure continue to rise if manifold pressure stays the same?
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Re: high pressure during cut off

#5 Post by mgrover » Tue Nov 10, 2020 4:53 pm

LPGC wrote:
Mon Nov 09, 2020 6:06 pm
What is reducer pressure set at (this is reducer less manifold pressure)?

The engine is supercharged, so at high load the manifold will be under boost pressure (higher than atmospheric pressure). Let's say manifold pressure at full load is 1.3 bar, if reducer pressure is set to 1.7 bar then pressure relative to atmosphere will be 2.0 bar under boost conditions. If you the suddenly come off the throttle, during over-run manifold pressure may then go down to only 0.2 bar, the gas between reducer and injectors isn't being used (injection is shut off), the measured pressure (reducer minus manifold) is now 2.8 bar.

Especially if the engine bay solenoid isn't directly fitted to the reducer - you don't even have to be running on gas when you boot it for over-run conditions to show high reducer pressure... You could drive around on petrol at part load, the minimum pressure would always be reducer minus manifold pressure, when you back off the throttle the reducer pressure reading increases, run the engine under higher load and the reducer will turn more liquid gas into gas vapour which has nowhere to go. More likely to happen when the solenoid isn't mounted directly to the reducer because when it's mounted directly there's little liquid gas for the reducer to be fed with to turn into vapour.

That said it is still worth looking into potential real issues. If the solenoid valves are open but all cylinders/channels are switched to petrol does pressure continue to rise if manifold pressure stays the same?
1. How do I work that out?

2. Yes it shows increase even am running petrol.

3. I'll move the solenoid back to basically be right next to it.

4. Yes, it just keeps rising to 3ish bar if not more. I've never let it get past that and after that I usually don't activate lpg again so the multi valve is shut.

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Re: high pressure during cut off

#6 Post by LPGC » Wed Nov 11, 2020 10:33 am

1. Some systems software allows you to control gas solenoids manually through software (i.e. at any time you can click in software to open/close solenoids at any time regardless of the operating condition) in which case you can click to open gas solenoids even when driving around on petrol. In some other systems you can effectively do the same by using settings to e.g. set the system to idle on petrol when the switch shows the engine is running on gas (during which the system will keep gas solenoids open). If no such option is available in software it is always possible to run a wire from battery to solenoids to open them.

But you don't need to do any of the above... Because 2 and 4 imply that there is a problem with the reducer. Gas pressure should not keep rising.

I wouldn't bother doing (3) either... there may be some misunderstanding regards (3). What I meant was that if the reducer solenoid is directly attached to the reducer, then even if the reducer has a fault where gas pressure slowly rises in the vapour lines when no gas is being drawn from the reducer, we may not see the fault occur because there will be very little liquid gas between the solenoid and reducer for the reducer to draw on to turn into vapour - even if the reducer had a massive fault where liquid gas feed connected directly to rubber gas vapour hoses it wouldn't see rubber hoses get over-pressurised if there was no liquid gas for the reducer to send to the rubber hoses. In which case to determine whether the reducer had such fault we may have to resort to (1).

Other than the slight differences when checking for a faulty reducer it doesn't make much difference to most installs whether the solenoid is directly mounted to the reducer or if there's a length of pipe between the solenoid and reducer, directly mounting the solenoid to the reducer is only really advantageous if the ECU is capable of pre-heating gas injectors. To preheat gas injectors during the engine warmup stage (when the engine is still running on petrol before switching to LPG) the ECU keeps gas solenoids closed while slowly pulsing gas injectors until gas pressure reaches zero, it then quickly pulses gas injectors. This all happens before switching to gas - the pulses of gas injectors starts slowly when there is still gas pressure so that gas flow from injectors only has a negligible effect on overall engine fuelling but as gas pressure falls to zero the gas injectors can be pulsed faster because with no gas pressure it doesn't matter how long injectors are pulsed there will be no effect on overall fuelling. Just before the gas solenoids open the injector warm up phase ends, gas injectors stop pulsing, solenoids open, the system then switches from petrol to gas as normal. If gas injectors heating is used it will take longer before gas pressure falls to the point that the system can pulse gas injectors quickly without effecting overall fuelling because while injectors are being pulsed slowly the reducer will be turning any liquid gas from between the solenoid and reducer into vapour, so to maximise the effect of gas injector heating and/or minimise changeover to LPG from petrol time it's better to have the solenoid directly attached to the reducer.
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Re: high pressure during cut off

#7 Post by mgrover » Wed Nov 11, 2020 11:08 am

I think this ECU does control it via software since I can select in software to bypass the valve.

Ah I've got the preheat injectors enabled, not that its really required currently.

Ill order a replacement reducer then. If it fixes the issue I can confidently send the other one back rather than having the car unusable for a few days. I would normally just get the reducer repair kit but lpg shop is out of stock.

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Re: high pressure during cut off

#8 Post by LPGC » Wed Nov 11, 2020 1:43 pm

Sounds like a good plan.

One thing that causes premature reducer failures is operating too cold, the most likely conditions this occurs under are either insufficient flow of hot water or the driver putting his foot down shortly after the system has switched to run on LPG. Failures due to the latter can be avoided with some systems because they can be set up to momentarily (while the driver has his foot down) switch back to petrol if Tgas and or Tred fall below a certain temperature, such facility allows a comparatively cool switchover temp to be set while avoiding problems if the driver puts his foot down during the warm up stage.

My car switches at 20C, which means in this (cool) weather if I start the engine and immediately start driving I only get around 200yards before it switches to gas.. Which means temps could potentially fall very low if I booted it shortly after changeover, which could cause problems... except I've prevented such conditions causing problems by setting the system up to momentarily switch back to petrol if engine load rises above a certain level while either Tgas or Tred are below certain levels. If I hadn't set these conditions it would still be fine for me to drive it shortly after switching to gas because I'd know not to boot it (or to manually switch back to petrol if I wanted to boot it) but another driver might boot it just after it had switched to LPG from a cold start and have problems. But because I did make use of these settings no driver could cause the system to have any problems regardless of how they drove the car at any time, not even if they booted it just after it had switched to LPG. I employ these settings (and injector pre-heating) on all my installs (although to a lesser extent on vehicles where the engine runs say a very rich open loop mixture for a long time during warm-up), thus they all switch to gas at relatively cool temperatures, so the vehicle runs on gas for more of the time / uses less petrol / save the owners more money while at the same time being driver-proof. It is possible to set some systems up to switch back to petrol above a certain engine load momentarily (automatic switch back to gas after booting it) with temperature readings higher than the low load changeover temperature.
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Re: high pressure during cut off

#9 Post by mgrover » Fri Nov 13, 2020 8:32 am

ive been super conservative and switching at like 40-50c because of the high pressure required to power the car at all rpms.

but ill keep that in mind, i do live very close to a ring road and honestly can't remember when i first used the reducer its been so long. but the problem only seems to of gotten progressively worse. so would this kind of thing normally mean a rip in the diaphragm? since now if i do floor it when at temp it feels like its struggling/choking to get the car going but if i gently get to certain speeds it much happier, albeit still breaks during cut off.

i was wondering is there anything else that can be going wrong when a cars trims lean out during cut off on petrol? is it just a vacuum leak? because i've smoke tested and smoke tested with no luck.

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Re: high pressure during cut off

#10 Post by Brian_H » Fri Nov 13, 2020 11:40 am

Just a thought here, and it could be completely wrong, but is it possible its manifold pressure your seeing leaking back through the injectors being a SC engine? Smoke test might see that if you disconnected the gas feed to the injectors under the right settings, but might be hard to test for.

Might also be worth saying what injectors your using, as it doesn't seem visible in your post?

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Re: high pressure during cut off

#11 Post by mgrover » Fri Nov 13, 2020 11:50 am

AC STAG WF02 injectors. am not quite sure what you mean tbh. but new reducer is on the way, so ill installed that and see how it goes.

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Re: high pressure during cut off

#12 Post by LPGC » Fri Nov 13, 2020 5:43 pm

I know what Bri means... If you try to blow through an injector from the gas inlet side you won't manage to blow through it unless the injector's solenoid is open (injector is open, injector is being electronically pulsed). But if you try to blow through it from the outlet side toward the inlet (on some types of injectors) you can blow through it that way. Inside the injector there's a plunger held against the outlet hole by a tiny spring, blow against the plunger/spring and you can cause the plunger to open and therefore blow through it - some one way valves have the same type of design.

The supercharger would likely provide enough pressure to open the injector plungers and cause a boost leak if the injectors were not connected to the pressure reducer (if the inlet side of injectors were not effectively sealed). When the inlet side is effectively sealed any pressure that makes it's way from the outlet side to the inlet side will remain in the inlet side - there wouldn't be a boost leak in practice because the injectors are connected to the pressure reducer so effectively sealed, and once pressure in the inlet side equalled pressure on the outlet (manifold) side the 'one way valves' (injectors) would close trapping the pressure on the inlet side.

But the supercharger wouldn't be capable of pressurising the gas pipe between reducer and injectors to 3 bar because it isn't capable of pressurising the manifold to anything like 3 bar (we might expect a 4L V8 running 3bar boost to make closer to 1200bhp than 400bhp), I forget how much boost sc's on 4.2sc rangerovers make but it'll only be maybe 0.5bar > 1bar over atmospheric pressure.
Last edited by LPGC on Fri Nov 13, 2020 5:56 pm, edited 1 time in total.
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Re: high pressure during cut off

#13 Post by Brian_H » Fri Nov 13, 2020 5:56 pm

Had a suspicion it was unlikely. Hopefully you get somewhere with the replacement reducer and relocating the shutoff valve.

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Re: high pressure during cut off

#14 Post by mgrover » Mon Nov 16, 2020 4:48 pm

so some updates. ive removed old magic reducer. one thing I noticed was the coolant was all crusted inside the pipes? Is that normal? Also I know for magic you need to put in an inline filter in the coolant. its just a little mesh, it says to the enter side of the coolant, to me thats the right side where it says on the label the amount of BHP for 2 outlets.

also I hooked up the temp sensor, but no idea which way around the wires go, I assume it doesn't really matter.

also I need to extend the wires for the solenoid valve since I've moved the valve to where the reducer is and now it connects directly. am guessing the wires also don't matter which connects to which?

I used this to connect the gas pipes together where I removed the safety solenoid

https://www.lpgshop.co.uk/8mm-copper-pi ... bow-90deg/

and used this to connect the solenoid to the reducer

https://www.lpgshop.co.uk/nipple-m10-to-m10-equal/

hopefully nothing blows :P

edit found a small leak but i need to fix that. also still need to get some cable.

but weird thing i was driving around with obviously like a bit of a twat, the solenoid valve isn't connected. so valve should be completely shut no? well when i floor it i notice the gas pressure can rise to like almost 0.7 bar. can someone explain how that happens? the pipes past the reducer should be empty.

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Re: high pressure during cut off

#15 Post by Gilbertd » Mon Nov 16, 2020 10:02 pm

Some solenoid coils have a built in diode across the coil to stop any back emf from damaging the driver transistors in the controller, they will be marked + and - and must be connected the right way round (get it wrong and you'll blow the fuse). To check for leaks, a squirty bottle with a mix of washing up liquid and water will show them up immediately.
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Re: high pressure during cut off

#16 Post by mgrover » Tue Nov 17, 2020 8:27 am

Gilbertd wrote:
Mon Nov 16, 2020 10:02 pm
Some solenoid coils have a built in diode across the coil to stop any back emf from damaging the driver transistors in the controller, they will be marked + and - and must be connected the right way round (get it wrong and you'll blow the fuse). To check for leaks, a squirty bottle with a mix of washing up liquid and water will show them up immediately.
ah ffs i was hoping it wouldnt matter, since the other cables are like purple and grey or someshit. so not sure which is positive and which is negative.

i found the leak pretty quickly tbh. the L joint I put in, the connection screwing into it wasn't really in straight.

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Re: high pressure during cut off

#17 Post by LPGC » Tue Nov 17, 2020 10:50 am

Most modern solenoid coils don't have the diode or any polarity markings and most are not polarity sensitive. If there are no polarity markings one pole may still be marked 12v, in which case I still wire positive to that pole just in case of a diode / polarity sensitivity.

The nipple you linked to is M10 to M10. There are 2 types of MJ reducer - the Standard and the Extra. The Standard has M10 gas inlet and is supposedly rated 250bhp to 350bhp, the Extra has M12 gas inlet, is a larger unit than the Standard and is supposedly rated up to 450bhp. Did you fit a Standard or Extra?

MJ reducers are sometimes supplied with gauze strainers but you don't need to use them and they'll flow more if you don't use them. Crud can build up anywhere in the cooling system but a lot of build up in a short time could point to the cooling channel drying out sometimes, you don't want the reducer to be above the minimum height of water when the engine isn't running.

The vapour lines pressure rise is possible because there will still be a little liquid gas between the solenoid and the reducer when the solenoid valve is closed.
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Re: high pressure during cut off

#18 Post by mgrover » Tue Nov 17, 2020 4:58 pm

LPGC wrote:
Tue Nov 17, 2020 10:50 am
Most modern solenoid coils don't have the diode or any polarity markings and most are not polarity sensitive. If there are no polarity markings one pole may still be marked 12v, in which case I still wire positive to that pole just in case of a diode / polarity sensitivity.

The nipple you linked to is M10 to M10. There are 2 types of MJ reducer - the Standard and the Extra. The Standard has M10 gas inlet and is supposedly rated 250bhp to 350bhp, the Extra has M12 gas inlet, is a larger unit than the Standard and is supposedly rated up to 450bhp. Did you fit a Standard or Extra?

MJ reducers are sometimes supplied with gauze strainers but you don't need to use them and they'll flow more if you don't use them. Crud can build up anywhere in the cooling system but a lot of build up in a short time could point to the cooling channel drying out sometimes, you don't want the reducer to be above the minimum height of water when the engine isn't running.

The vapour lines pressure rise is possible because there will still be a little liquid gas between the solenoid and the reducer when the solenoid valve is closed.
ive hooked up the solenoid and it clicks on turning switch so win win. i havent had a chance yet to setup the adaption or even see if it works yet. work got in the way. i mean the cabling right now is disgusting, i found 2 random cables i had and made my own super seal and some wire splicers haha. but it works.

do you mean the diff between Compact and POWER? Because I has DA POWAAAAA. Fair enough as long as its not a massive deal.

The reducer is connected directly to the solenoid. So should the pressure rise at all? Or is it just vacuum pressure? Ie the reducer pumps, but theres nothing to pump so its just vacuum?
Last edited by mgrover on Tue Nov 17, 2020 8:42 pm, edited 1 time in total.

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Re: high pressure during cut off

#19 Post by Gilbertd » Tue Nov 17, 2020 8:17 pm

Reducer doesn't pump, it regulates the pressure down from the 10 bar or so that the liquid entering it is at. What Simon is saying is that with the valve closed there will still be some residual gas, in liquid state, in there which will vaporise and increase the pressure. A litre of liquid will become 270 litres of vapour so you don't need a lot of liquid.
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Re: high pressure during cut off

#20 Post by mgrover » Tue Nov 17, 2020 8:20 pm

not to spam :P

so this is where my leak is.

https://photos.app.goo.gl/uELt3ADBmK4on3c49

am i using the wrong connector? it seems to come out of the bit before the screw

or is this better?

https://www.lpgshop.co.uk/8mm-orange-ho ... connector/

also i went on a drive and it seems fixed. but that could just be the gas leak :P

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