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[chrisgixer]

Why is it, when setting up lpg and fine tuning, do fuel trims show opposite plus and minus on each bank? Drive by wire 3.2 engine.

I would expect both banks to show + or - if the set up was out due to vap pressure , for example +15 and +14 if it's adding injector duration. As I understand it.

But what appears to happen is one bank shows eg +15 while the other shows -14 say. Then as vap pressure is adjusted they might both get further apart, say +20 and -21. So adjust the vap the other way and the numbers start to get nearer to zero. Say +7 and -6 ....And so on until something like +1 and -1 is reached.

It will then vary a bit on checking over the next couple of days. You might see -3 and plus +5 one week, then back the other way +2 and -3 the next.

My point is, why are they never both showing + .... Or - as the case may be?

Does it really matter? It's just a symbol. Or is it the software I'm using? Is it the lpg Ecu software ? Does tech 2 show the same. And if so does it show the same behaviour on petrol? 

I've mentioned this before so I thought I'd ask on a specific thread.



Surely it's not possible for each bank to actually be rich and lean with the same intake pressure?

[chrisgixer]

I would add its not just my car. Iirc it's the same on all the stag set ups I've seen.

[Marks DTM Calib]

Sounds like an oddity of the LPG system/software.

Not something you see on the engine/tech 2. 

[chrisgixer]

This is engine ecu via the software that will not be named  fuel trims on live data.

[Lazydocker]

Sounds like an oddity of the LPG system/software.

Not something you see on the engine/tech 2. 

We've set up with a genuine Tech 2 plugged in too (a certain TBE to name 1) and it shows one +ve and one -ve. 

[Marks DTM Calib]

So you are referring to the long term trims in the Motronic?

I have for some time considered the LPG ECU's to be missing a trick or have a some what basic error somewhere as they don't support O2 feedback and always do what you describe for some unknown reason.

Yet on Petrol, the trims remain solid.

Is it because there is a slightly longer run to the 2-4-6 bank of injectors in some installs (although I know some are equal with a Y connector) so the pressure maybe slightly lower? Is it down to where the Vac tap is taken from (e.g. it seems to often be the 1-3-5 half of the plenum or a T from the brake servo).

[Kevin Wood]

I did try to chase this on mine once. I think I came to the conclusion that the plumbing layout made no difference and that the trim shifted with the injectors, suggesting that their flow rates are not very well matched.

I keep meaning to knock up some crude means of calibrating their flow rates.

The whole system is pretty crude, to be honest, so we shouldn't be surprised that the trims in the petrol ECU have to work a bit harder to keep it under control.

[chrisgixer]

I'm fairly certain, although it's a long time since I had a petrol only omega, that tech2 shows both trims as either plus or minus if the trims are out.

If so, why does lpg reverse one bank to the opposite symbol? This implies that one bank is rich and the other lean by a similar amount, but I can't see how this is possible. Unless something is programmed or wired arse backwards somewhere.


Mark, over the years I'm certain that we have eliminated things like feed lengths and vac references to make little odds, provided the pressure reaching each bank of injectors is balanced, and provided the feed from injectors to manifold is similar length and not trapped of kinked, then we won't have any issues as far as achieving a usable set up is concerned.
 The car runs fine. No lag. No errors. Curve matches petrol perfectly. Although we know it's prety basic and struggles to cope with wide variances in ambient temp.

The question is purely one of the symbols + - being correct on the petrol ecu. I suspect it's purely a symbol, and doesn't actually matter which way the scale goes provided it's within limits. ....

Thought occurs to try it on petrol, and see what the trims do, then on gas and see what occurs then. Although I'm sure we tried that once upon a time.

As LD says, we've often noted the trims seems to go in opposite directions.

I'd ask master to play but I know he hates working on gas cars.

[chrisgixer]

Maybe the title is misleading.

To clarify, the trims are currently within 1 or 2 of zero on both banks. So there is no actual imbalance as such. Purely the symbols suggest as much. Eg...

Where you might see petrol show;
Bank 1 -2
Bank 2 -3

Lpg would show;
Bank 1 -2
Bank 2 +3

Same code reader. Same petrol ecu. Just differant fuel.


Hopefully that explains it better...? And I've explained it correctly as others remember it too?

[Rods2]

Does the software using a floating zero, so the difference is a zero centred range between the two banks?

[chrisgixer]

Does the software using a floating zero, so the difference is a zero centred range between the two banks?

I see what you mean. But the thing is each bank could be anywhere in the scale, say, if an O2 sensor was Wonkey, or an air leak on one bank.

So each bank has a range. I dont know how wide, but I do know the light comes on at plus or minus 25 eather side of zero for each bank. Differant codes for each bank as well.

[TheBoy]

It happens on TBE (Motronic 3.1.1, KME Gold vap), but not on the Silver Bullet (Motronic 2.8.3, Christ knows what vap, and the car isn't here to check, but its single outlet).

I have seen this same issue on chrisgixer's black one and lazydocker's estate, both of which use the twin outlet KME Gold.

My theory has always been around the vapouriser...  ...but as other than trim imbalance that's mostly controllable the vapouriser works well, is it the lesser of evils?


Yes MDTM, its the Motronic 3.1.1's LTFTs that drift off... ...in opposite directions.

[chrisgixer]

So wile all tyre example have kme gold, they also have Motronic.

[TheBoy]

So wile all tyre example have kme gold, they also have Motronic.

If you run on petrol, you will see that the Motronic (of all versions) will kept the LTFT at 0% (or idle and partial load FT on Motronic 2.x.x at 128).

I did toy with trying to swap the vap to injector block pipes over to see if fault moved with vap output...  ...but mine mostly behaves. Plus I never get to drive it anyway

[chrisgixer]

Myself Kevin and LD have all played with differant configurations re lpg feed.

Swapping injector blocks. Swapping fuel feed to injector block. Swapping pipes on the vap so the outlets feed the other bank. Etc.

Made no odds.

I think it's an enterpretation the data that's displayed arse backwards. (Although that does seem implausible )

Certainly its not a fueling issue directly.