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 Post subject: Leaky T connector?
PostPosted: Thu Feb 16, 2017 8:02 pm 
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I've been getting very random 'explosions' in my air box for a while when starting in the mornings. The last one even had the neighbours curtains twitching it was quite loud. I've put this down to a leaky injector leaking gas back into the plenum. Anyway, the injectors are 5 years old now so decided to replace them. However, unlike the old ones, the new ones (OMVL reg fast) don't have a take off point for the Map pressure sensor, but come with a T piece to T into the 12mm gas feed. I've fitted this and all seems to be running well but there is a smell of gas at this T piece where the allen screw is. Can someone tell me the best way to seal this up please?


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 Post subject: Re: Leaky T connector?
PostPosted: Thu Feb 16, 2017 9:39 pm 
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identifing what exactly is leaking would be the best start - soapy water will allow you to locate it, or alternatively a gas leak detector spray, if its the hose connection then if its as tight as it will go you might want to renew the 12mm hose rather than risking breaking the pressure sensor as the old hose may be too hard to shape properly round the sensor.

It may not be the injectors that were leaking - if your vapouriser has a vacuum line it may be worth checking that its not leaking gas through that way into the inlet, either put the open end of the vacuum line into some water when its running and check for bubbles (the other end should still be on the vapouriser) or check if you have any pressure coming out of the nipple on the vapouriser (while the engine is running in both cases)


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 Post subject: Re: Leaky T connector?
PostPosted: Sun Feb 19, 2017 3:25 pm 
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Brian_H wrote:
identifing what exactly is leaking would be the best start - soapy water will allow you to locate it, or alternatively a gas leak detector spray, if its the hose connection then if its as tight as it will go you might want to renew the 12mm hose rather than risking breaking the pressure sensor as the old hose may be too hard to shape properly round the sensor.

It may not be the injectors that were leaking - if your vapouriser has a vacuum line it may be worth checking that its not leaking gas through that way into the inlet, either put the open end of the vacuum line into some water when its running and check for bubbles (the other end should still be on the vapouriser) or check if you have any pressure coming out of the nipple on the vapouriser (while the engine is running in both cases)


Hi Brian, the current leak is from the new T piece I've just fitted. The 12mm hose is all new too. It's leaking from the 'take off' grub screw in the T piece. I shall try sealing it with some plumbing sealer I have.

As for the original problem, it was far too random to be leaking from the reducer. If it was, then surely it would happen all the time?

My guess is that just one injector was leaking, and if the engine happened to stop with the inlet valves of the cylinder in a particular position, it would allow the gas to enter the plenum and be ignited by a 'wasted spark' upon starting the engine. This would certainly explain the randomness of the occurrence.

I've now changed both injector rails and time will tell if it's cured the problem :-)


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 Post subject: Re: Leaky T connector?
PostPosted: Sun Feb 19, 2017 9:39 pm 
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colin33 wrote:
Brian_H wrote:
identifing what exactly is leaking would be the best start - soapy water will allow you to locate it, or alternatively a gas leak detector spray, if its the hose connection then if its as tight as it will go you might want to renew the 12mm hose rather than risking breaking the pressure sensor as the old hose may be too hard to shape properly round the sensor.

It may not be the injectors that were leaking - if your vapouriser has a vacuum line it may be worth checking that its not leaking gas through that way into the inlet, either put the open end of the vacuum line into some water when its running and check for bubbles (the other end should still be on the vapouriser) or check if you have any pressure coming out of the nipple on the vapouriser (while the engine is running in both cases)


Hi Brian, the current leak is from the new T piece I've just fitted. The 12mm hose is all new too. It's leaking from the 'take off' grub screw in the T piece. I shall try sealing it with some plumbing sealer I have.

As for the original problem, it was far too random to be leaking from the reducer. If it was, then surely it would happen all the time?

My guess is that just one injector was leaking, and if the engine happened to stop with the inlet valves of the cylinder in a particular position, it would allow the gas to enter the plenum and be ignited by a 'wasted spark' upon starting the engine. This would certainly explain the randomness of the occurrence.

I've now changed both injector rails and time will tell if it's cured the problem :-)


Can't suggest much else on the leak - you know where it is at least. With the injectors it depends, not so much on the position of the engine, but if it is leaking it will fill the manifold/plenum up with gas, this might dissipate with time, also the amount of gas would depend on the pressure present when you stopped the engine etc.

Easy way to tell if its happening is to look at the pressure reading in the software if you have access to it - if when its not running on gas it stays reasonably stable then you should be ok leak wise, if you can see it gradually dropping its still leaking somewhere between the vapouriser solenoid and the injectors.

Fairly random would expect to be injectors though, and given they are the age they are and fairly cheap to purchase probably best to swap those and see what happens as you have done.


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 Post subject: Re: Leaky T connector?
PostPosted: Mon Feb 20, 2017 10:17 am 
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Inlet valve position would have nothing to do with it, since the injector outlet is on the plenum side of the valve anyway.

It is becoming more difficult to buy OMVL rails with a built in pressure take off, so T piece is often included. Some T pieces are not a problem but it seems some suppliers incorrectly assume a standard type temp sensor will seal properly in certain T's. Does the grub screw / temp sensor tighten up OK? If not, or if it continues to leak with the paste you'll do better asking them to replace it with matched parts that are designed to seal against each other.

Reducer leaks, or at least the effects of reducer leaks, can also be intermittent.

Simon

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 Post subject: Re: Leaky T connector?
PostPosted: Mon Feb 20, 2017 3:52 pm 
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LPGC wrote:
Inlet valve position would have nothing to do with it, since the injector outlet is on the plenum side of the valve anyway.



But surely the position of the valves would come in to play here, because there must be a spark to ignite the gas which has leaked into the plenum? If not, how is the leaked gas being ignited, if it's not the 'wasted spark' on the exhaust stroke?


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 Post subject: Re: Leaky T connector?
PostPosted: Mon Feb 20, 2017 4:31 pm 
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I hadn't noticed before but Brian told you before me 'not particularly the position of the engine'.

Indeed a spark must get to the gas in the plenum to ignite it and cause a backfire, but this isn't really relevant to position of the engine valves when the engine stops spinning, and in fact you don't know which cylinder (if any in-particular) are igniting the gas in the plenum. At some point, when the engine is spinning again, a spark is introduced to the mixture in the plenum but you don't know the exact point at which it occurs and any/all cylinders could do that.

A leak from any LPG injector, regardless of engine stood position (inlet valve position) will fill the plenum with gas - with inlet valve closed the gas can only go into the plenum (not into the cylinder), or with inlet valve open gas will fill the cylinder and plenum at about the same rate.

The problem is the gas leaking into the manifold in the first place, no point looking any deeper into it. But if you were curious as to how backfire could occur (given that carb engines have explosive mixture in the plenum all the time) it is due to combination of valve timing and wasted spark system designed for an engine where plenum only contains air.

Take it this isn't on your other problem vehicle, the Bongo (which hasn't been fitted long enough for leaky injectors / reducer to be expected)?

Simon

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 Post subject: Re: Leaky T connector?
PostPosted: Mon Feb 20, 2017 6:09 pm 
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Don't forget that a weak mixture will cause a 'spit back' through the inlet irrespective of valve timing or wasted spark system. Anyone who has ever tried to balance multiple carbs will know that the one with the flames shooting out of it is the one that is running lean. If there is a fuel/air mixture at even remotely close to the optimum mix will ignite in the intake manifold or plenum.

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 Post subject: Re: Leaky T connector?
PostPosted: Mon Feb 20, 2017 6:30 pm 
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That's another good point Gilbert.

Also implies that a plenum flooded with gas mixture that is too rich to ignite (so no backfires) to start with will get leaner as engine is cranked as gas is pumped out and air is pumped in, until it reaches the point where gas/air mixture in plenum is correct to ignite and then engine aspects (valve timing, burn speed in cylinders, wasted spark) can do their bit to cause backfire.

Worth saying that for a petrol injected engine that is being cranked on petrol, the petrol system should provide rich enough mixture (on it's own, regardless of extra fuel in terms of LPG) to prevent slow burn in the cylinder from causing flame presence at wrong side of inlet valve IF cam timing isn't also a contributing factor.

Slightly off topic but remembering when I had the fuel injected BMW 750 V12's, one oldish bloke told me 'Its alright having 12 cylinders but it is getting them all balanced that is the problem'. He was harking back to the days of multiple carbs on a V12, so I told him 'Not a problem with fuel injection'.

Simon

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 Post subject: Re: Leaky T connector?
PostPosted: Mon Feb 20, 2017 10:09 pm 
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Thank you for your posts, it does help me get my head around what's actually happening.
It does make me wonder why this scenario isn't happening more often, given that any leaky injector can fill the plenum with gas. Or maybe the design of my engine makes it more susceptible for some reason.
Anyway, it's only been 3 days but so far so good, no sign yet of any more unwanted explosions, I'm not sure how much more my air-box could take!

No Simon, no such problem with the Bongo yet, although I have used the same set up and injectors on this one too.



[Backfire: The term derives from parallel experiences with early unreliable firearms or ammunition, in which the explosive force was directed out at the breech instead of the muzzle. From this came the use of the word "backfire" as a verb to indicate something that produces an unintended, unexpected, and undesired result.]


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 Post subject: Re: Leaky T connector?
PostPosted: Mon Feb 20, 2017 10:20 pm 
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colin33 wrote:
Thank you for your posts, it does help me get my head around what's actually happening.
It does make me wonder why this scenario isn't happening more often, given that any leaky injector can fill the plenum with gas. Or maybe the design of my engine makes it more susceptible for some reason.
Anyway, it's only been 3 days but so far so good, no sign yet of any more unwanted explosions, I'm not sure how much more my air-box could take!

No Simon, no such problem with the Bongo yet, although I have used the same set up and injectors on this one too.



[Backfire: The term derives from parallel experiences with early unreliable firearms or ammunition, in which the explosive force was directed out at the breech instead of the muzzle. From this came the use of the word "backfire" as a verb to indicate something that produces an unintended, unexpected, and undesired result.]


Trouble is something leaking won't always leak - think how taps go, sometimes they will drip, other times they will be fine.

If the injectors (or for that matter the reducer) is gummed up with heavy ends then the chances of something sticking are much higher - if you have any damaged seals then bits that have been pulled off may sit down and seal sometimes, but not always hence sometimes its fine, others not. Other variables like the temperature of the injectors can play a part, hotter injectors less likely to be problematic in this respect, if they were the problem. similar if theres a small hole in the reducer it may not always leak for reasons you may not know.

3 goes does sound like you might have some luck fixing it though. Simon did a better job of explaining what I was trying to say originally!


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