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PostPosted: Sat Jun 16, 2018 8:01 am 
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I may have an issue with my stag ECU unit I fitted it about 3 years ago in my wife's Isuzu trooper since then it's been fine done 20000 plus miles recently I had a miss on idle on both gas and petrol and some starting issues traced it to no 6 cylinder which was putting petrol in whilst on gas and petrol more than the timed pulse the gas ECU thinks it's fine timing on both fuels sonic thought a leaky petrol injector changed it and still the same when I disconnected the petrol fuel pump it runs normally on gas so.im thinking the gas ECU is holding the petrol injector open a bit all the time so it's flooding the cylinder with fuel question is are the newer stag ECUs compatible with the older wiring I'm not that keen to pull it all out and start again also has any one had one fail in so short time

Thanks Tom


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PostPosted: Sun Jun 17, 2018 10:04 am 
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I've seen a lot of Stag ECU's fail in a similar way to that you described but not exactly the same way (more on this below). I've known new ones have such fault or develop such fault after a few hours and I've known such faults take years to develop.

The first thing to mention is that your Stag ECU isn't capable of adding a petrol addition. To someone into electronics this can give some insight into the range of likely faults - If an ECU uses a simple relay to disconnect petrol injectors from the petrol ECU (and turn on emulation when the relay is switched) it's less likely that the ECU is the cause of a short to earth (short to earth would keep a petrol injector open constantly) than if the ECU had petrol addition ability (solid state electronics that are designed to be able to pull a petrol injector to earth for the petrol addition). With a relay setup we'd expect either the relay to work as it should or to fail to switch and thereby not turn off petrol injection pulses but also not turn on emulation... So if a relay fails we might get both gas and petrol pulses at the same time on the effected cylinder but wouldn't usually get the petrol injector open constantly.

The usual fault is that the engine will run OK on petrol but has a misfire on gas, this happens when an internal ECU fault means the ECU doesn't disconnect a petrol injector when running on gas... so the effected cylinder gets both petrol and gas injector pulses and a mixture that is double rich.

If you're getting the petrol injector open constantly regardless of whether the system is switched to petrol or gas (and since you've already tried a new petrol injector) it could point to the negative pulsing side of petrol injector wiring for the effected cylinder being shorted to earth. Such short could be either side of the Stag ECU petrol injector wiring (between Stag ECU and petrol ECU or between Stag ECU and petrol injector). Petrol injector pulse duration readings in Stag software might reveal which side of the Stag ECU such short is on - If the Stag ECU reads normal petrol injector duration for all cylinders when running on gas but unusual petrol injector duration on one cylinder when running on petrol it would point to a short on petrol injector break wiring between the Stag ECU and the petrol injector / If the Stag ECU reads abnormal petrol injector pulse duration on one cylinder regardless of whether it is running on petrol or gas it would point to a short between the Stag ECU and the petrol ECU / If you don't get any unusual petrol injector pulse duration readings such short could be between the Stag ECU and the petrol injector.

Before buying another ECU it would be a good idea to confirm that the fault is the ECU, not a wiring problem. You could confirm this by routing petrol injector wiring for the effected cylinder as it would be routed on the standard none LPG converted car - cut the effected cylinder's petrol injector pulsing wire at both sides of the Stag ECU and rejoin them, bypassing the Stag ECU altogether for the effected cylinder.

Another Stag ECU labelled Stag 300 6 Premium should be exactly the same as yours including plug pinouts, easy enough just to compare pics of one you might buy to the one you've already got paying attention to size/shape/material(plastic/aluminium) of the body of the ECU and it's loom connector.

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PostPosted: Sun Jun 17, 2018 1:50 pm 
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Thanks Simon just seems odd that on gas it's fine I currently have a relay that shuts off the fuel pump whilst on gas and it runs fine also it now seems to be dropping one of the other cylinders took me a while to work out that it was rich on the no 6 cylinder and it's only when there is petrol.pressure first I thought it was a coil pack but swapping them round did not move the fault to another cylinder I was thinking maybe of upgrading to a more modern stag ECU although my friend has a Volvo that is an not falier with a Emer system fitted thinking of robbing that perhaps


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PostPosted: Sun Jun 17, 2018 2:19 pm 
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Just to clarify then... Is it only fine when running on LPG when you've disconnected the fuel petrol pump? If so then my advice above is still correct and it's likely that there's short to earth for the petrol injector between the petrol injector and LPG ECU.

I'm not much of a fan of any Stag ECU but would rather have your Stag 300 than the more recent Stag Qbox ECU's and for your 6 cylinder application would rather fit the Stag ECU than Emer. Wouldn't advise either of those ECU's but out of the two for your model vehicle would opt for the Stag rather than the Emer. You're looking at the simplest / cheapest way to sort it and on balance if it did come to replacing the ECU (after confirming the problem is the ECU) I'd replace the Stag ECU like for like. Bearing in mind that if there's a short on the petrol injector wiring (where I've suggested there might be) swapping the LPG system / ECU won't change anything unless you accidentally find /fix a short while wiring in the different loom.

First step should be to confirm it runs OK on petrol with the LPG ECU out of the equation.

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PostPosted: Sun Jun 17, 2018 3:26 pm 
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Yes I will do some tests on it I will take out the gas ECU and try joining the injectors back to the original wiring loom IE not using the gas wiring to see if the fault replicates


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PostPosted: Sun Jun 17, 2018 3:43 pm 
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I am assuming that the ECU uses transistors to switch rather than relays maybe I will open up the ECU to have a look the ECU is mounted in the engine bay maybe the heat has got to it


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PostPosted: Sun Jun 17, 2018 4:35 pm 
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Earlier Stag ECUs used relays, dunno if yours is solid state or relays but can say it doesn't feature petrol addition... so high speed solid state switches rather than relays wouldn't be a must.

A simple test could be to check resistance between ground and petrol injector pulse wire, it shouldn't be shorted.
If it is shorted and unplugging the ECU removes the short try checking resistance on the petrol ECU side of the LPG ECU, still shouldn't be shorted.
If you find a short doing the above without the LPG ECU plugged in then it doesn't point to a dodgy LPG ECU, points to a short on the wire.
If no short except when you plug the LPG ECU into it's socket it would point to the LPG ECU.
Could also check resistance between petrol injector pulse wire (injector side of LPG ECU) and petrol injector positive, should be somewhere around 13 to 16 ohms.

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PostPosted: Sun Jun 17, 2018 8:06 pm 
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Just disconnected the gas ECU for now unsoldered the wires and re joined to the original loom still runs badly missed on 2 cylinders now but here's the interesting thing when it was running on gas the petrol injector pulses on idle were normal IE around 2ms and gas about 3ms on all cylinders so that seems normal maybe the fuel pressure regulator is gone and the fuel pressure rail is too high thus leaking petrol to the manifold whilst on gas does that seem plausible


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PostPosted: Tue Jun 19, 2018 6:48 pm 
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Making progress - If you've rejoined all wiring properly (so individual petrol injectors are getting pulses at the right time) you've ruled out your LPG ECU as the cause of the problem. If there's no vacuum leaks etc the only other way the LPG install could still be affecting things is if LPG calibration was previously very incorrect and fuel trims were affected and are still affected, very unlikely.

Have you done the electrical tests I suggested such as checked resistance between petrol injectors pulsing wires and earth yet?

Simon

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