LPG ECU piggy-backing causing main ECU failure? V6

[Ron Miel]

Ron, sorry if I have missed something as I just speed read most of the posts.
Going back to first principals, why are they certain of the cylinder that is the problem? As replacing the injector didn't cure the problem could the connector be causing the misfire or could it be another cylinder with a dicky injector?
Sometimes you get blinkered and look to deep for problems.

Yep, you've missed something. It's all set out above

In a nutshell, after inspecting all cylinders, the technician advised that number 4 is not receiving fuel from its injector, hence the obvious idling misfire and driving power reduction - at which stage though, due to flywheel momentum, the V6 is again smooth and the misfire as such is no longer obvious except under attempted hard acceleration.

A simple injection pulse detector shows that the injector in question is being ECU-pulsed (but it does not measure the pulse duration), hence the injector swap, assuming either a failed injector solenoid or a completely stubborn output blockage. Until I read up some more last night, it was assumed that what they had detected was the pulsed ground switching output directly generated by the main engine ECU.

So when the injector swap failed to cure the problem, it finally seemed to the garage in question (and to me) that the the main ECU's injector driver (see http://www.autodiagnosticsandpublishing ... esting.htm) might be the source of the trouble, perhaps pulsing that particular petrol injector for too short a duration.

However, in my eureka moment last night, I realised that the LPG ECU intercepts and then regenerates twin injector pulses during LPG operation, sending a full operating pulse to the LPG injectors and a separate diminished (duration?) pulse to the petrol injectors to prevent them operating but somehow fooling the main ECU into not actuating an engine failure fail warning light. Therefore, as the LPG injector pulsing is clearly fine, with no LPG-running misfire or power loss, the problem now seems more likely to lie in either the LPG ECU itself or in its output wiring to the petrol injector - probably giving too high a switched ground resistance for the petrol injector's solenoid to operate properly. This seems consistent with the garage's observation that they have sometimes been able to cure petrol misfire problems in LPG converted engines, by "fiddling" with the LPG system wiring loom - so, fingers crossed, I'm going to try to test the cylinder 4 circuit, this morning

[Rhinoman]

(The only bit I perhaps question is this: "A greatly reduced signal is sent to the petrol injector which is not strong enough to open the injector, but is enough for the petrol ECU to not see a fault." Doesn't he really mean not long enough, in duration, to open the petrol injector, rather than not strong enough? After all, the main ECU just provides a negative to an already live solenoid, so unless resistance is added, the applied voltage isn't altered - or is that what it does do?)

Injector fault diagnosis is often done by measuring the voltage on the low side switch. When the switch (transistor) is off the voltage should be battery, if it isn't then there is a short circuit to ground or an open circuit. I've worked on a lot of Denso ECUs but not one from a Bongo.

[Ron Miel]

.........in my eureka moment last night, I realised that the LPG ECU intercepts and then regenerates twin injector pulses during LPG operation, sending a full operating pulse to the LPG injectors and a separate diminished (duration?) pulse to the petrol injectors to prevent them operating but somehow fooling the main ECU into not actuating an engine failure fail warning light. Therefore, as the LPG injector pulsing is clearly fine, with no LPG-running misfire or power loss, the problem now seems more likely to lie in either the LPG ECU itself or in its output wiring to the petrol injector - probably giving too high a switched ground resistance for the petrol injector's solenoid to operate properly. This seems consistent with the garage's observation that they have sometimes been able to cure petrol misfire problems in LPG converted engines, by "fiddling" with the LPG system wiring loom - so, fingers crossed, I'm going to try to test the cylinder 4 circuit, this morning

Too much engine bay and under dash dismantling needed to get in and directly identify/trouble-shoot the number 4 injector's signal line. However, it's now sorted anyway, at least for the time being. There's a dodgy connection (presumably high resistance, in view of all the foregoing) in the main multi-way connector at the very hard to access LPG ECU. Moved it about a bit, and bingo, no more misfiring on petrol. Who would have thought it?

Needs pulling out, disconnecting, and cleaning properly to try to ensure a permanent fix - but that can wait until the weather improves. Meanwhile, after months of misfiring, she's running sweet again on either fuel; smoother and quieter on LPG but that was always the case. Even at tickover, you can hear that individual petrol detonations are deeper and louder than those on LPG but heh! Six bangs in every engine cycle again

Owzat!

Now agree with Geoff/g8dhe, BTW, that my "Bongo-friendly" should have gone the extra mile and worked that out for me, especially as they had obviously had similar cases before, and could have sorted this while they had the engine bay top right off and could have easily withdrawn the loom and the connector, for a permanent clean up.

Interesting how even the best "Bongo-friendlies" usually don't provide a full service. I had previously discovered that one of the best-regarded on diesel Bongos wouldn't tackle a V6, and now here's one who knows the V6 well but won't touch even the LPG/petrol interface in LPG-converted ones - as most petrol Bongos will surely soon be.

Anyway, hope the info in this thread might now help other LPG Bongo peeps as well as me. interposing the LPG ECU and two additional in-line signal connections per petrol injector, with hindsight, greatly increases the potential for connection-related petrol running issues, albeit easily tweaked, if you know about it.)

[g8dhe]

Where about is the extra ECU fitted ? I had sort of assumed that it might be inside the Cab piggy backed on the original ECU, but from the sound of it its in the engine compartment ? Connectors in that environment need to be very rugged!

[Ron Miel]

Where about is the extra ECU fitted ? I had sort of assumed that it might be inside the Cab piggy backed on the original ECU, but from the sound of it its in the engine compartment ? Connectors in that environment need to be very rugged!

No, it's not in the engine bay, although 90% of the loom is of course but that's also impossible to get at without a fair amount of dismantling, and there are some soldered connections somewhere along it that I now also want to check over. The ECU end of the LPG loom comes through the big grommet, top of the engine's forward bulkhead, together with pre-existing Mazda loom(s), up under the auto transmission shift linkage, and into the multi-way all along the side of the LPG ECU, which itself is crammed in immediately in front of the linkage.

There's a whole mass of strapped together cabling tightly jammed alongside the ECU multi-way, most of it running past it and forward, either up into the dash and the steering column, or through to the front compartment to the relays, fuses, etc. It's going to need a fair amount of dismantling, then careful unpicking, to get the multi-way connector out.

Glad you asked the question, as it had just occurred to me to post another message, advising anybody getting an LPG conversion done to take a very active interest in just where the LPG ECU, in particular, gets fitted. I would have preferred mine to have been made much more accessible by bolting it down, through the floorpan, in the carpeted area below the front heater "director control box" - where, in fact, I later fitted the controller for my cruise control system.

[Ron Miel]

Where about is the extra ECU fitted ? ................

No, it's not in the engine bay,............ The ECU end of the LPG loom comes through the big grommet, top of the engine's forward bulkhead, together with pre-existing Mazda loom(s), up under the auto transmission shift linkage, and into the multi-way all along the side of the LPG ECU, which itself is crammed in immediately in front of the linkage.

There's a whole mass of strapped together cabling tightly jammed alongside the ECU multi-way......... It's going to need a fair amount of dismantling, then careful unpicking, to get the multi-way connector out.

.........it had just occurred to me to post another message, advising anybody getting an LPG conversion done to take a very active interest in just where the LPG ECU, in particular, gets fitted. I would have preferred mine to have been made much more accessible by bolting it down, through the floorpan, in the carpeted area below the front heater "director control box" - where, in fact, I later fitted the controller for my cruise control system.

Here:





The cruise control box is only lightweight, so it's Velcro secured (100% box surface area, stuck-on male-female strips - carpet and box surfaces), but the LPG ECU is a bigger weightier item which would need at least strong self-tappers through the floorpan, then undersealed of course - but it's the best place I can see outside the engine bay and within the loom's length. The cruise control cables were subsequently Rescue Tape bound, BTW.

As it is, my LPG ECU is, virtually inaccessibly buried in the under-dash area above there.

[westonwarrior]

amazing thread and so pleased you found the problem was in the end a simple one.
Could help some one with the same problems

[Ron Miel]

amazing thread and so pleased you found the problem was in the end a simple one.
Could help some one with the same problems

Thanks, I was more than a bit relieved! I also hope it might help somebody else, although I can't pretend to have been purely altruistic in my postings - i.e., no sainthood yet for me

Simple soul that I am, I only find out how well I really understand something (or don't) when I try to explain it to other people, so it really helped by sort of thinking on my feet in public as the plot thickened

[mikeWalsall]

I have 'religiously' read all your posts and depth level of dedication as been amazing ...

To me, as an LPG user, it would be a shame if all your work (and heart ache?) gets lost in the general forum posts ..

It's one of those scenarios ... which I hope never happens to me ... but if it did ... at least I now have knowledge of a 'maybe it's the ' fixit that I would not have (initially) thought about ..!!

[The Great Pretender]

Glad you got it sorted.

[Ron Miel]

Thanks, both.

Cheers, David

[Ron Miel]

...........I was just considering taking the plunge with the Bongo when I read this. I will certainly hold off untill your problem is resolved.......

.........fear not, and get the Bongo converted as well asap

It's only 67.7p/litre locally today BTW, and, according to Go-Autogas, it's as low as 59.7p at Flogas, St Austell - best in the UK right now.......

http://www.guardian.co.uk/money/2011/se ... en-cheaper

[mikeWalsall]

Damn ... they could have converted mine as a freebie .. and I could have given the system long term feed back ..!!

[lpgimports]

Haven't been on Bongofury in a week so missed out on this one, have just read it with interest.

I had it down as a bad connection where the negative pulse wire was soldered back into the main loom after the LPG ECU, and to test would have recommended that the loom be opened up and signal and return from the LPG ECU be cut out and wired direct again and see if it ran spot on again on petrol, which is would have, then cut and remade connection and return from the LPG ECU and the problem would have returned, so problem would have been in loom to LPG ECU or connection loom to ECU or ECU. Breaking the ECU to loom connection, and cleaning with switch cleaner may have resolved that.

For understanding the negative pulse feed to each petrol injector is intercepted and send to the LPG ECU and then returned to the petrol Injector, when on gas the pulse is multiplied by whatever setting is in the software for RPM load etc and send to the LPG injector, nothing is returned to the petrol injector, on petrol the signal should return just as sent to the LPG ECU directly back to the petrol injector.

LPG ECU's are designed to be mounted in an engine bay but should be mounted wiring loom down or away from direction of water ingress i.e backwards if vertical is not possible, they should be mounted so they cannot come loose of be interfered with by feet and such, I have seen some in bongos mounted in the passenger foot well!! looms should be loose enough to allow unplugging.

Personally I fit them under the passenger seat vertically mounted against the door ideal spot, means only power and control cable need to come through to cabin, I use an existing mazda hole for this, I have seen others where you could fit your fingers into the huge hole they drilled in the bulkhead to get cables through and did nothing to seal up.

Most V6 systems are actually a 8 cylinder system and so there are 2 redundant injector drivers and wiring and plugs some installers cut all this off but I leave it present and heat shrink them inside looms and tape up external plugs so if you have a fault on a v6 and only one injector driver or wiring goes down it is possible to go around this problem.

Glad you have solved the problem.

Paul

[lpgimports]

If anyone ever wants me paul at lpgimports.co.uk or a PM which triggers a email anyhow. Sometimes am really busy and when I get home computering is last thing to do, other times I am online till early hours.

Paul