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Aug 29, 2012 13:55:30 GMT
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Really looking for some vague pointers here, as I have absolutely no experience of LPG systems.
One of my friends is about to replace his Lexus LS400, and I've offered to help him with that - both selling the LS (it'll be on here) and inspecting/looking for new cars for him. The Lexus is a 1993 one with stratospheric mileage - LS400 with V8 and a sequential LPG system.
Recently it's been misfiring on petrol and very poor on LPG, from his description. He's had garages look at it, but that ends up costing more than the car's worth; it's had the obvious new leads and so forth.
I drove it myself and it feels very much like a crank sensor conking out, except without the inevitable total failure to start when properly hot. On petrol it feels very much like missing sparks, whereas on LPG it feels more like a lack of fuel - and apparently after the air filter was found to be totally clogged, the LPG was 'remapped' - is it normal for the LPG ECUs to learn new maps somehow?
As both sides play up, I was really wondering how the engine management info is shared between two ECUs - are the sensors tapped, or do the ECUs talk, or is the LPG system using a different setup?
Appreciate it's vague, but I like a challenge. The car bogged down at idle, went okay when booted, spluttered at cruising speeds. Haven't had a chance to check for fault codes.
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Aug 29, 2012 14:12:00 GMT
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If it runs poorly on idle and cruise (ie, closed loop) but runs fine when you give it throttle (ie, open loop) then perhaps it's the lambda/oxygen sensor? Really it could be anything, based on that description and not having seen the car. I would probably suspect a sensor of some kind though.
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Aug 29, 2012 14:16:48 GMT
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^ Sounds possible - likely even.
I think most LPG systems 'piggyback' on the petrol ECU - they allow the petrol system to do all the hard work, and just convert the petrol injector signals to what the gas system needs.
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Aug 29, 2012 19:04:46 GMT
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That makes sense, I wasn't thinking hard about that as the previous stuff had been around timing and ignition and the petrol mode FELT very like ignition issues.
The car's been run with a clogged air filter - it passed the MOT in March, but with the miles it has done... If it's run overly rich for a period, that presumably would mess things up a fair bit regardless. I'd initially suspected throttle position sensor style weirdness but out of gear, it revved smoothly up the range quite happily - once in gear, it would be fine when booted, but missed, spluttered when driven normally.
No fault lights were on apart from a low coolant warning crying wolf.
It'll probably be here tomorrow, at which point the question of what becomes of the Lexus would appear to be entirely up to me. So many crazy ideas, none of which will happen, but I might have a go at fixing it and drive it for a little while.
I read somewhere that if there's a problem with a car, even minor, LPG will make it far more obvious.
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gruss
Part of things
Posts: 242
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Aug 29, 2012 19:24:18 GMT
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Is there any way of checking for codes, either with a suitable scanner or bridging the pins to count the eml flashes? If the air filter has been blocked for some time then it may have poisoned the lambda sensor as mentioned. It could be fixed rich.... Where abouts are you? A quick look with a scope will allow you to check things.
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Daihatsu Mira TR-XX
Suzuki Alto Works
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Aug 29, 2012 20:35:14 GMT
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I've got a scope, a code reader, but the car won't be back up here till tomorrow - reading codes is bridging pins apparently, so yes, I will do that as a matter of priority!
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Aug 29, 2012 22:28:01 GMT
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try unplugging the injector emulators and see if it runs ok if they do unplug with what you have. Also a mate of mine had one of these but petrol only which missfired in exactly this way and the fault was a leaking rocker gasket to a couple of the plugs. Does it run ok from cold for the first minute?
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10mpg
Club Retro Rides Member
Posts: 4,253
Club RR Member Number: 204
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If you find otu what kind of lpg system it is (make and model) should be written on ecu, i will be able to tell you how it supposed to work, some emulate from the injectors some older ones have a standalone map, some take a crank trigger pulse others take igntion/coil source many will do both and emulate ontop of these base parameters depending on how the ecu's been setup...
Sounds more like a fault with the car than the lpg system, cm and crank sensors need checking, resistance values are a good start, given its been run with a blocked air filter i'd suspect the AFM if it has one as these an get cooked buy a completely clogged afm also check the higher vacuum hasn't collapsed a vacuum pipe or knackered a pcv/one way valve....
As has been suggested a decent diagnostic is a good start, may Toyotas of this era had a facility to short out two pins in the diagnostic to get the engine warning light to flask out the fault code..
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The Internet, like all tools, if used improperly, can make a complete bo**cks of even the simplest jobs...
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ChasR
RR Helper
motivation
Posts: 10,194
Club RR Member Number: 170
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^ Sounds possible - likely even. I think most LPG systems 'piggyback' on the petrol ECU - they allow the petrol system to do all the hard work, and just convert the petrol injector signals to what the gas system needs. Most work off by piggybacking indeed, at least the AEB based system do. When he says it has been remapped, the chances are that the LPG ECU was recalibrated. IMO there is not much point doing that if the car is working poorly on petrol, and they do not need recalibrating as much as people say (something else generally causes the issues). When I had a similar issue with my sister's 2003 Mondeo (it would misfire at the bottom end slightly on petrol, but worse on LPG to the point where it would start to backfire if you really demanded acceleration at the low end (e.g junctions)) it was down to an iffy ignition system. The pattern HT leads came off for some Magnecor replacements (Ford wanted £100 for 2.0 Duratec leads!) in addition to getting a genuine coilpack fitted (and taking off the Intermotor item). That along with a new set of plugs had the car running like it had not been doing so for quite some time (it even felt peppier on LPG but I'm sure that was more down to the fact that the car worked!). I would also check things like as 10mpg said on the basis that it has been running with a clogged air filter. |
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Last Edit: Aug 30, 2012 19:34:49 GMT by ChasR
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I know it's had plugs and leads, but caps weren't mentioned - lots to be looking at, thanks guys. It'll be here later today and I'll start with checking for codes - it had been run with a couple of plugs not working due to damaged leads, and a clogged air filter.
I don't THINK it has MAF, but MAP - I'll google, but I base that on it looking like it has the flappy metering thing on the way to the airbox with a quick glance under the bonnet on a wet dark evening. So I'm not claiming to know what it has - and I will check before delving properly.
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Aug 30, 2012 10:19:54 GMT
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LPG is very sensitive to poor ignition - but you'd tend to find it more of an issue under high engine load (when the cylinder pressures are higher) rather than at light load.
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No codes... would an LPG ECU suppress codes? It's so packed in the engine bay (and I'm very wary of disturbing pipes, it does have a faint whiff of gas about it) that getting to anything is a bit of a nuisance.
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Aug 31, 2012 18:38:17 GMT
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LPG is very sensitive to poor ignition - but you'd tend to find it more of an issue under high engine load (when the cylinder pressures are higher) rather than at light load. ^what James said. Check what the new leads and plugs are - if they're cheap curse word, replace them with std parts. Also check the lpg filters as they catch a lot of crud if the cars been driven low on gas. Ive known bad gas too. Nothing I could do would make it run, til I drained the tank and re-filled with fresh gas. |
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Koos
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ChasR
RR Helper
motivation
Posts: 10,194
Club RR Member Number: 170
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Aug 31, 2012 19:00:15 GMT
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LPG is very sensitive to poor ignition - but you'd tend to find it more of an issue under high engine load (when the cylinder pressures are higher) rather than at light load. ^what James said. Check what the new leads and plugs are - if they're cheap curse word, replace them with std parts. Also check the lpg filters as they catch a lot of crud if the cars been driven low on gas. Ive known bad gas too. Nothing I could do would make it run, til I drained the tank and re-filled with fresh gas. You got away lightly  . On my previous V6 Mondeo (which came converted to me) I would have been better off ripping the entire lot out and starting from scratch. The weekends I wasted on that POS. To be fair it is still running... |
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Aug 31, 2012 21:52:22 GMT
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Oh I know that feeling!
I bought an expensive kit from a reputable supplier and found it was utter curse word!
I went through replacing it bit by bit with good stuff til the only original part left was the injector emulators! :-(
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Koos
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. On my previous V6 Mondeo (which came converted to me) I would have been better off ripping the entire lot out and starting from scratch. The weekends I wasted on that POS. To be fair it is still running...