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PostPosted: Mon Oct 26, 2015 8:31 am 
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Re lazy starting (or for some folks, not) in cold weather.

Background = VW T25 flat four fitted with an R90 and standard BLOS unit.
Running just fine and recently tuned by a reputable VW T25 LPG installer ( "Campershack").

At this time of year, over on the T25 owners forum, this cold weather starting thing often comes up, (as per here).

My original open loop mixer ring on the Tin-Top never needed the priming thing that goes "turn the key on until the LPG relay clicks off, then repeat, before starting", but when I had the same open loop mixer ring system fitted to the Hi-Top, I did have to do that (only in cold weather). It was never a bother though. I then went to BLOS on the Hi-Top and it stopped needing it, firing up on gas right away, even on icy days. When I sold the Hi-Top I moved the BLOS onto the Tin-Top (removing the old mixer ring of course) and then I did need to prime it a bit :? (still no bother though).

So all of that ^^^ makes it as clear as mud as to what causes the variation. Some folk over on the T25 forum never need to prime (in cold weather), other always have to (in cold weather), even though we are all using R90s and either mixer rings or BLOS.

Around this time of year, I just do the priming thing as a matter of course, only takes less than 10 seconds, so not at all annoying, I'm just curious. I might try and richen the tick over mixture on the BLOS (dead easy) and see what happens, not sure if doing that may throw out the non tick over mixture a bit though. Don't want to be wasting gas running rich across the range, or having to drive ~80 miles to Campershack to get Andrew to put it all straight again :roll:

Just wondering what you, dear reader, think about this?

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PostPosted: Mon Oct 26, 2015 8:58 pm 
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Even gravity can have an effect, say you park up on a slope with a manifold full of gas/air mix - it can settle out overnight and the heavier-than-air gas can run up one end of the manifold, out the air intake, or whatever - any of which could have a good or bad effect on restart. Could depend on which way the wind blows. The presence of any anti-run-on gubbins, or how any given gas system shuts off, all might affect the end state too.

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PostPosted: Wed Oct 28, 2015 6:06 pm 
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Once converted a 'Bobcat', i.e. shortened and jacked up Rangerover P38, sort of an off road special.. This came to me on the back of a vehicle transporter because nobody had managed to get the (newly built) engine to run. A Rangerover specialists whom I did all the LPG work for had built the engine and the vehicle owner blamed them for the engine not running, the specialist told the owner to come to me not particularly with the intention of converting it to LPG, though that was on the cards, but so I could get the engine running and prove the engine they had built was a good 'un

I found the problem to be due to gummed up petrol injectors, petrol pressure regulator, fuel pump, etc... in turn due to whoever had done the Bobcat conversion getting silicone sealer into the petrol tank (petrol tank on Bobcats is relocated to inside the vehicle). I made a video of the engine running after parking another vehicle with R90 at the side of it and fitting a mixer on the Bobcat, LPG system on other vehicle fueling the Bobcat engine.

So came the questions - What would be involved in fixing the petrol system (answer - sorting all above problems plus probably some yet to be discovered issues - ECUs and wiring all over the place, it was in a bad state) / How much to fix the petrol system and also install an LPG system / How much to make LPG only, removing the petrol tank to make space for a bigger gas tank. In the end the owner chose the LPG only route.

Without the ability to run on petrol it obviously becomes more important that the vehicle should start on gas... What I did on the Bobcat was open the idle bypass slightly on the R90 and install a solenoid override push button. I don't think it ever had a problem starting, the push button not needed, but without any idle bypass there probably would have been situations it wouldn't want to start even if the push button were used... Without idle bypass, very little gas will come out of reducer while pulsing/priming solenoids - and to prevent backfires you're better off starting with a rich mixture in the manifold when cranking. Of course you've got to get this in balance, because if the engine won't start and you keep cranking long enough, if it then does fire it could blow the exhaust off...

Simon

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PostPosted: Wed Oct 28, 2015 6:24 pm 
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I also once made the mistake of using silicon sealer on something in the fuel system and realised that petrol will dissolve it. Although it doesn't dissolve it fully but any excess bits come away and clog up everything further down the line. Not the ideal sealant to use anywhere near petrol.

Thinking about Simon's comments about opening the idle bleed screw a little, it may be that the difference in starting in colder weather may be down to cranking speed. In warm weather when the oil is nice and thin and the battery is in a happy state, the engine will spin over on the starter nice and quickly so easily draw gas in. Once it gets colder, the cranking speed will drop so the amount of 'suck' at the mixer will be lower so make it more difficult to draw the gas through. It could be something as simple as that......

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PostPosted: Sun Nov 01, 2015 1:28 pm 
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Gilbertd wrote:
I also once made the mistake of using silicon sealer on something in the fuel system and realised that petrol will dissolve it. Although it doesn't dissolve it fully but any excess bits come away and clog up everything further down the line. Not the ideal sealant to use anywhere near petrol.
Bobcat was a real bodge, silicone pretty much holding petrol pump in, some had fallen into the tank.. not only that but there was no petrol filter fitted other than the in-tank screen.

Agreed with all points above. Would presume cranking speed not very low or OP would change the battery by default but certainly even a slightly lower cranking speed than usual could make all the difference when starting on gas.. If the engine would start OK at reduced cranking rpm on petrol then mixture on LPG at reduced cranking rpm could be the reason for poor starting.
Maybe before adjusting anything, next cold morning try giving it a shot of ether or propane/butane (I keep a few disposable gas cans with usual aerosol type valve, pushing a length of hard hard vac pipe onto the can valve the other end of the pipe vents gas - mostly useful for finding vac leaks but could also be used for priming) before trying to start on LPG, if that were to make a big difference then maybe opening the idle bypass just a little would work - probably best done with vapour pipe off the reducer so you can see how much gas the bypass is letting through.

Simon

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PostPosted: Tue Jan 19, 2016 3:31 pm 
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Joined: Thu Nov 19, 2009 10:13 am
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Location: John, near Chester, N.W. UK
Sorry for the late response to your input folks, didn't get any email notifications for this thread :?

I reckon you could be correct, whilst my battery and starter are in good nick, the VWT25 has a starter motor cable run of over 8 feet, due to the battery being under the drivers seat and the engine at the back. That, coupled with the effects of cold weather (thick oil and lazier battery) do result in lower cranking speeds on a cold (morning) start.

As said, not a problem, just a discussion, my "priming" with a couple of turns of the key works fine, so no worries.

Ta for the input :)

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PostPosted: Wed Jan 20, 2016 7:13 pm 
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Location: West Sussex
Probably not relevant but early autochoke VWs suffered from waxstat freezing in cold weather.


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