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    Posted: 12 Nov 10 at 20:39
I've tried digging around for the answer to this, but haven't come up with anything so I'm hoping a fellow brickie can enlighten me. I thought that the Digifant engines control timing via the ecu... but do they? And if so, how? Or rather, what does the ecu base its ignition timing on? Does the lambda probe play any part?

Or have I just got it all completely wrong?

Thanks in advance Beer


Edited by SebRogers - 12 Nov 10 at 21:10
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ..lee.. Quote  Post ReplyReply Direct Link To This Post Posted: 12 Nov 10 at 21:23
ecu does controll the timing. it has a 3d map. check the timing at 3500 rpm should be 35 degrees +/- 5 degrees if its out you will need to follow the timing adjustment method which is a pain in the arse.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ..lee.. Quote  Post ReplyReply Direct Link To This Post Posted: 12 Nov 10 at 21:25
oh. lambda does not play a part in the timing map but the engine temp sensor does
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Aidan Quote  Post ReplyReply Direct Link To This Post Posted: 12 Nov 10 at 22:33
Digifant ECU will do whatever it can to run right AND produce as clean an exhaust as it can so it will adjust fuelling and timing, which is partly why when you swap over from gas to petrol it's all a bit whooooa as the ECU has probably been leaning off the fuelling (it thinks) and running more advanced timing to get a clean burn on lpg

It would be nice to be able to interogate the ECU when it's running on gas just to know what it thinks it's doingGeek
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SebRogers Quote  Post ReplyReply Direct Link To This Post Posted: 13 Nov 10 at 11:25
Originally posted by ..lee.. ..lee.. wrote:

ecu does controll the timing. it has a 3d map. check the timing at 3500 rpm should be 35 degrees +/- 5 degrees if its out you will need to follow the timing adjustment method which is a pain in the arse.

Thanks Lee. That's not quite what Haynes says though (?)  Confused
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SebRogers Quote  Post ReplyReply Direct Link To This Post Posted: 13 Nov 10 at 11:34
Originally posted by ..lee.. ..lee.. wrote:

oh. lambda does not play a part in the timing map but the engine temp sensor does

Hmm. Interesting. Bang goes that theory then (see next post...)
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SebRogers Quote  Post ReplyReply Direct Link To This Post Posted: 13 Nov 10 at 11:49
Originally posted by Beaker Beaker wrote:

Digifant ECU will do whatever it can to run right AND produce as clean an exhaust as it can so it will adjust fuelling and timing, which is partly why when you swap over from gas to petrol it's all a bit whooooa as the ECU has probably been leaning off the fuelling (it thinks) and running more advanced timing to get a clean burn on lpg

It would be nice to be able to interogate the ECU when it's running on gas just to know what it thinks it's doingGeek

Yep, it'd certainly be interesting.

The problem I'm having is that switching to petrol isn't really whooooa, it's more spluttersplutter followed by misfiring and no power until I reset the ecu by switching off and back on again (at which point it's all fine). Unplugging the lambda solves the problem, which is what made me wonder whether it had anything to do with injection timing. 

The only thing that seems to clear it is holding the revs at c.3000rpm with no load for (quite) a few seconds when I switch to petrol, and even then it's a bit hit and miss. It's also hard to do that in traffic Confused

I'm getting the feeling that I can either chase my tail around trying to get it all to play nicely with the lambda plugged in, or unplug it and live with running petrol on the default map for an easier (albeit slightly less economical Wink) life.

The other issue is that in some ways the engine seems happier without the lambda. The idle is less lumpy and the emissions are still well within reasonable limits (currently c.2.1% CO, but I think that can come down a bit).

Seems to me part of the problem is trying to persuade a basic open loop setup (lpg) with different timing requirements to play nicely with a basic closed loop setup (digifant petrol) which runs in emulation mode on lpg. I can see why it gets confused switching back to petrol - what I can't understand is why simply removing the lambda from the equation sets everything straight again if timing feedback is coming from the temp sender Confused
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Aidan Quote  Post ReplyReply Direct Link To This Post Posted: 13 Nov 10 at 14:41
removing lambda means the ecu isn't looking to adjust anything based on lambda it just runs default map which is a bit rich anyway, I'd say that your gas is either rich or lean to quite an extent from optimum; as described when I come over to petrol from gas it's a bit down on power and won't accelerate if you try giving it some pedal but about 5-10 seconds later, it understands where it is at and then is completely driveable. If switching from gas to petrol or vice versa at idle it won't always change over without stalling, but above 1500 rpm it will change over happily; the lpg is a compromise setting , not too rich at idle but mustn't be too lean at high revs. Remember to disconnect lambda before restarting when setting idle mixture on gas, generally set 1 - 1.5% lambda off but check with Steve
If lambda is connected it must rank very highly in the ecu's logic, especially at idle it's all about emmisions, MV can make 0.3% just like a modern engine, your 2.1% is acceptable for a DJ which is allowed 3%, generally the MOT doesn't know the difference between MV and DJ so you are tested with 3% limit - first time my garage tested mine they got 0.3% reading and told me the engine was running very lean until pointed out that it was lambda/cat engine; similarly if lambda is not connected then it will just run as a sort of DJ by map alone
If the lambda is working well and the ecu is happy with what it's hearing lpg open loop works fine - remember I told you about the Thompson lambdas that even though the test meter shows it scaling correctly at idle as soon as you got properly hot on a run it would be as if it had failed lean. If you restarted immediately and tested the output it was fine, but the ECU didn't like it and would struggle to get the mixture and timing right - hence the subsequent use of the Bosch ones with no problems at all in last 5 years
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SebRogers Quote  Post ReplyReply Direct Link To This Post Posted: 13 Nov 10 at 17:26
Originally posted by Beaker Beaker wrote:

removing lambda means the ecu isn't looking to adjust anything based on lambda it just runs default map which is a bit rich anyway

Yep. I've got my head round that bit Smile

Quote I'd say that your gas is either rich or lean to quite an extent from optimum;

I reckon Steve @ gasure would disagree Wink Fettled by him last weekend and set up IIRC to around 1.1% for running on lambda - wouldn't switch over happily. Long story short: ended up disconnecting lambda and running around 1.6% on both - switches fine. Seems to be a bit richer than that now and I'm not sure why, but that's another story.

Quote as described when I come over to petrol from gas it's a bit down on power and won't accelerate if you try giving it some pedal but about 5-10 seconds later, it understands where it is at and then is completely driveable. If switching from gas to petrol or vice versa at idle it won't always change over without stalling,

Yep, found that too...

Quote but above 1500 rpm it will change over happily;

Well there's where your experience differs from mine, unfortunately Cry Mine will only switch (from lpg to petrol) if I hold the revs to c.3000rpm for a good few seconds at no load, and then it's touch and go whether it'll be happy...

Quote the lpg is a compromise setting , not too rich at idle but mustn't be too lean at high revs. Remember to disconnect lambda before restarting when setting idle mixture on gas, generally set 1 - 1.5% lambda off but check with Steve

Why disconnect the lambda? Surely it doesn't affect gas mixture anyway? Confused

Steve reckons 1.5-2% for both petrol and lpg running without the lambda. With the lambda IIRC the lpg was originally set to around 1.1% and the petrol was around 0.8% (no cat y'see).

Quote If lambda is connected it must rank very highly in the ecu's logic, especially at idle it's all about emmisions, MV can make 0.3% just like a modern engine, your 2.1% is acceptable for a DJ which is allowed 3%, generally the MOT doesn't know the difference between MV and DJ so you are tested with 3% limit - first time my garage tested mine they got 0.3% reading and told me the engine was running very lean until pointed out that it was lambda/cat engine; similarly if lambda is not connected then it will just run as a sort of DJ by map alone

Yep, got my head round that too Thumbs Up

Quote If the lambda is working well and the ecu is happy with what it's hearing lpg open loop works fine - remember I told you about the Thompson lambdas that even though the test meter shows it scaling correctly at idle as soon as you got properly hot on a run it would be as if it had failed lean. If you restarted immediately and tested the output it was fine, but the ECU didn't like it and would struggle to get the mixture and timing right - hence the subsequent use of the Bosch ones with no problems at all in last 5 years

And that's where I suspect I may have to go if I want to resolve this running with a lambda. Beru probe checks out fine and works fine on petrol when tested, but seems to be the cause of the switchover problem in a way that I can't identify. I may just have to stump up for a Bosch probe and see if that solves the issue...

Thanks for the detailed replies, Aidan.

Cheers!


Edited by SebRogers - 13 Nov 10 at 19:37
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ..lee.. Quote  Post ReplyReply Direct Link To This Post Posted: 13 Nov 10 at 18:54
so does the lambda influance ing timing then. i`ve read the pro training manuals " not nessessarily correctly Embarrassed " and there was no suggestion there that it does.
is the gas system injection pulse based
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SebRogers Quote  Post ReplyReply Direct Link To This Post Posted: 13 Nov 10 at 19:36
Originally posted by ..lee.. ..lee.. wrote:

so does the lambda influance ing timing then. i`ve read the pro training manuals " not nessessarily correctly Embarrassed " and there was no suggestion there that it does.
is the gas system injection pulse based

I'm on a steep learning curve here so I'm probably wrong, but... can't see how ignition timing can be varied by the ecu, cos surely it's a mechanical system? However it seems to me that in principle injection timing could be varied via feedback from the lambda. Or something Confused

Gas is open loop mixer - no injectors.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ..lee.. Quote  Post ReplyReply Direct Link To This Post Posted: 13 Nov 10 at 20:14

i`ve never opened up the dizzy on an mv so i cant be sure but i`d guess that there are no advance weights in there. the dizzy has a basic setting which all ecu adjustments are made from. so you set the dizzy to 5 degrees @2250rpm if irc. then the ecu will advance and possibly retard from this base setting depending on engine rpm which is sensed from the dizzy and engine load which is sensed from the air meter. possibly influanced by lambda but i`ve not read anything related to that only rpm and load. the map is altered by the temp 2 sensor untill its warmed up fully.

this info doesn`t really help you though. does the van drive fine on petrol. if so then i wouldn`t say there was anything wrong with the timing. saying this i` dont know much about gas systems so dont take too much notice of what i say in relation to the gas. have the plug gaps been re set for gas. has the emissions been checked i`d guess that the gas system is not right somehow, when you do the change over does the emission readings lean out. have you tried turning the controll valve on the gas in and out to see if it makes a differance. "obviously mark it first so you know where you started".
 
aiden mentioned a problem with some oxygen sensors being not so friendly with gas so id seriously look in to that as first hand experiance is difficult to beat.
keep us up to date seb as i`m quite interested in what you find.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ..lee.. Quote  Post ReplyReply Direct Link To This Post Posted: 13 Nov 10 at 20:24
re reading i see you have the issue going from gas back to petrol. is that right?
 
if so then i`d say aiden is probally spot on. i would want to scope the lambda sensor on petrol to confirm its working range and then scope it on gas and compare. i wouldn`t know what exactly to expect but would have thought the lambda sensor would still be switching even on gas. " maybe steve can confirm this " if it doesn`t and it just flat lines then when you change back to petrol the lambda sensor will be influencing the fuel system in a very bad way and will cause problems.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SebRogers Quote  Post ReplyReply Direct Link To This Post Posted: 13 Nov 10 at 21:01
Originally posted by ..lee.. ..lee.. wrote:

i`ve never opened up the dizzy on an mv so i cant be sure but i`d guess that there are no advance weights in there. the dizzy has a basic setting which all ecu adjustments are made from. so you set the dizzy to 5 degrees @2250rpm if irc. then the ecu will advance and possibly retard from this base setting depending on engine rpm which is sensed from the dizzy and engine load which is sensed from the air meter. possibly influanced by lambda but i`ve not read anything related to that only rpm and load. the map is altered by the temp 2 sensor untill its warmed up fully.

That all makes sense from where I'm sitting (at the back of the class LOL)

Quote this info doesn`t really help you though. does the van drive fine on petrol.

Yes. Though with the lambda hooked up and running c. 0.8% CO at idle (no cat remember) it's a tad lumpy at idle. Other than that, runs fine.

Quote if so then i wouldn`t say there was anything wrong with the timing.

That's what I'm thinking, though it'd be simple enough to check. I was more interested in whether the lambda influenced injection timing on the fly and if that would contribute to the engine being a bit unhappy switching from gas to petrol.

Quote saying this i` dont know much about gas systems so dont take too much notice of what i say in relation to the gas. have the plug gaps been re set for gas. has the emissions been checked

No, and yes. Plugs only very recently fitted, appear to run fine on gas (new leads recently too). Emissions are fine, though I'm going to have to revisit the idle CO on both gas and petrol once I've decided whether to go down the lambda route or not.

Quote i`d guess that the gas system is not right somehow, when you do the change over does the emission readings lean out. have you tried turning the controll valve on the gas in and out to see if it makes a differance. "obviously mark it first so you know where you started".

There is, as far as I can tell, absolutely nothing wrong with the gas system. This is all about me fiddling, and not being satisfied having to unplug a component that VW designed as an integral part of the engine management system. Tried tweaking the power valve on the gas - makes no difference to the switchover.
 
Quote aiden mentioned a problem with some oxygen sensors being not so friendly with gas so id seriously look in to that as first hand experiance is difficult to beat.

I'm definitely seriously considering tracking down a Bosch probe to try, even though the fitted Beru probe appears to be switching (tested on a pukka lambda test setup) properly.

Quote keep us up to date seb as i`m quite interested in what you find.

Will do Smile Although it's a bit frustrating, I'm also finding the whole process pretty interesting and I'm learning a lot about how my engine works Thumbs Up

Gary, who owns the local garage here (top bloke btw), has taken an interest and has put a good couple of hours of his own time (with me peering over his shoulder) into this so far. Definitely a work in progress. 

The bus runs great on gas and fine on petrol (always been a tiny bit lumpy... haven't got to the bottom of that yet but having the injectors properly cleaned is next on the list). My goal is simple - to set it up so it switches cleanly between the two, in either direction. I can already do that with the lambda unplugged, but it'd be great if I could get it working with the lambda operational, as VW intended...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SebRogers Quote  Post ReplyReply Direct Link To This Post Posted: 13 Nov 10 at 21:11
Originally posted by ..lee.. ..lee.. wrote:

re reading i see you have the issue going from gas back to petrol. is that right?

Yep.
 
Quote if so then i`d say aiden is probally spot on. i would want to scope the lambda sensor on petrol to confirm its working range and then scope it on gas and compare. i wouldn`t know what exactly to expect but would have thought the lambda sensor would still be switching even on gas. " maybe steve can confirm this " if it doesn`t and it just flat lines then when you change back to petrol the lambda sensor will be influencing the fuel system in a very bad way and will cause problems.

The lambda can only switch if the mixture is changing between rich and lean. Since the ecu controls the mixture on petrol and uses the lambda to switch from just rich to just lean, the lambda's output on petrol shows the variation you'd expect (we've tested it). But on gas the ecu only thinks it's controlling the mixture, and this is where it gets interesting. The gas mixture isn't ecu controlled so the lambda output is constant when the engine's running on gas. So yes, it's flatlining like you suggest (again, tested and verified) and the ecu only gets the opportunity to actually respond when you switch to petrol. No wonder it's confused Wink There's no way the lambda signal on gas can vary like it does on petrol because the mixture isn't varied.

Aidan's already said his MV takes a while to settle down on switchover. The difference is that my SS barely seems able to switch at all with the lambda plugged in. Getting to the bottom of why that is - and finding out if there's anything I can do about it - is what this is all about Smile


Edited by SebRogers - 13 Nov 10 at 21:26
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ..lee.. Quote  Post ReplyReply Direct Link To This Post Posted: 13 Nov 10 at 21:32
just a quick thought.
does the gas system switch off the fuel pump. could be low fuel pressure temporarily as it switches back on and the fuel pressure catches up ????????
 
i would have expected to see lambda switch with gas when the engine was revved. air fuel ratio is bound to change slightly regardless of fuel type????
 
is the lambda flatlining at 0.1v or 0.9v. and if so how long does it take after switch over to start working properly
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Baxter Quote  Post ReplyReply Direct Link To This Post Posted: 13 Nov 10 at 21:55
Having a quick read (I'm crap at reading lots of text, so i just skim through and may have missed something but...)
what Aidan says makes the most sense to me.
 
Engine is fine on Gas.
Engine is fine on petrol.
Switching gas to petrol, is problematic.
 
Please remember CO is nothing, it's just one of a few gasses that matter, setting CO is a load of bollocks, setting the mixture however take into account ALL the constituents of the exhaust gas, so the CO readings are a little missleading in my opinion, However....
 
As Aidan says, the engine can be working fine on gas but the Lambda sensor is seeing a uber lean mixture, so as far as the ECU is concerned it is richening up the petrol mixture to compensate, it's only doing what it's told (the injectors are however switched off by the gas system, so not actually injecting any fuel)
You flip it to petrol and the injectors are switched back on they are stupidly rich because the lambda sensor has been telling the ECU to run rich, result is a engine that runs like a bag of shit.
Turning the engine off, then back on again will reset the ECU, by which time the exhaust gas has settled down and everything is behaving.
 
I hope that makes sense!
:)
 
I would try making the LPG system richer, similar to that of petrol and see if the transition between the 2 is better.
 
I think this is one of the limitations of the cheap end of the LPG systems, and the fact that the VW injection systems of the era wasn't the best.
Unplugging the Lambda sensor would work and give better transitions as the ECU is not seeing a over rich or over lean mixture as the lambda sensor is taken out of the equasion meaning that when you flip the switch the default map is already running and it's not being corrected in leiu of exhaust gas composition.
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Aidan Quote  Post ReplyReply Direct Link To This Post Posted: 13 Nov 10 at 21:58
lpg system just cuts out the injector loom so that the injectors aren't being flashed to earth (ie opened) by the ecu, but the ecu still thinks it's controlling the fuelling, but the fuel system is pressurised and fuel is flowing through the regulator and back to the tank as normal


I've never checked to see what my lambda does when it's on lpg

The digifant is adaptive though so it gets used to how things are running and adjusts the map accordingly, hence it can cope better with small intake air leaks much better than the DJ can, it just gets used to the extra air and adjusts the map accordingly, because it can vary fuel and timing to get the best output. But not good with exhaust leaks as the weak mix registered by the lambda increases fuelling to over rich without it knowing it. I regularly have to go over my very old rusty exhaust with the paste to block off the small cracks and rusted out flange joints that let in air upstream of the lambda ; even if I can't particularly hear the leak on the exhaust a drop off in fuel economy is noticeable and signals need to check. Best time to check is on start up stone cold when you can see the moisture in the system being pushed out of any weak spots and you see drops of water, once the exhaust is hot you won't see that as it all evaporates - on a real cold clear morning you may also see the exhaust gas/steam coming out of the cracks

When it's so compromised it's not happy you have to disconnect the battery and when you reconnect the power the ECU will default back to base factory settings and start learning again

Maybe flat line lambda on gas is confusing the ECU so it stops listening (ie goes all DJ), but that doesn't explain why when it's suddenly given petrol (ie control of fuelling) it is able to go "oh, ah it's varied, er what do I need to do to get myself running sweet" and gets on with it (or not)

Running out of gas usually on a run, at speed under load, when I switch over the revs don't drop, usually they pick up because it was already starting to drop off as the lpg ran out and goes a bit lean, returning to petrol gives it enough fuel to pick up but throttle response is dulled for a few seconds till it catches up with itself and then it's fine

I'd agree the lambda must vary a bit on lpg as you vary the throttle, there's noticeably a slower response on gas compared to petrol and no full throttle enrichment either so pedal to the metal doesn't have the same effect
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Baxter Quote  Post ReplyReply Direct Link To This Post Posted: 13 Nov 10 at 22:19
There Seb, it's fixed.
 
1. - Richen the gas.
2. - Live with it.
3. - Buy a more expensive gas system.
 
LOL
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SebRogers Quote  Post ReplyReply Direct Link To This Post Posted: 13 Nov 10 at 22:27
Originally posted by ..lee.. ..lee.. wrote:

just a quick thought.
does the gas system switch off the fuel pump. could be low fuel pressure temporarily as it switches back on and the fuel pressure catches up ????????

Fuel pump is running continuously and as Aidan says the injectors are all primed and ready to go...
 
Quote i  would have expected to see lambda switch with gas when the engine was revved. air fuel ratio is bound to change slightly regardless of fuel type????

Yes, true. Lambda responds to engine revs on gas as you'd expect, but flatines because the mixture isn't being cycled.
 
Quote is the lambda flatlining at 0.1v or 0.9v. and if so how long does it take after switch over to start working properly

Didn't take a note of the readings. Higher at idle and drops when the engine's being revved. Hard to say how long it takes to switch over because it's tough to get the engine to switch from gas to petrol with the lambda plugged in! LOL
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