As most of you know, my GEMS P38 runs on LPG. Unlike the later multipoint systems it has a single point so LPG enters just before the throttle body and the output from a lambda sensor adjusts a stepper motor valve in the LPG feed to keep the mixture correct. Because it doesn't slave off the petrol system, it can run on LPG from stone cold. It is set to change over at 1,100rpm on deceleration which means it starts on petrol, the revs rise initially and as they fall, it changes over and runs on LPG from that moment onwards. I keep some petrol in the tank for starting and in case I run out of LPG. Which I did a couple of days ago. My local Flogas charges 78p per litre and the two other filling stations nearby are £1.05 and £1.09 so I try to avoid them. I ran out of LPG a couple of days ago, so ran on petrol to Flogas but was too late, they had just closed. As I needed to use the car that evening, I bunged £20 of petrol in only to find it runs like a dog!

The idle on LPG is perfectly smooth but on petrol it is lumpy, accelerating up a slip road it could only just hold the same speed rather than accelerating as it would normally so seems down on power, although once I had got it up to cruising speed it felt normal. As I stopped at a roundabout, it stumbled and died but restarted without problem. The lumpiness clears once the revs are up to 1,000 rpm or so, so it is primarily an idle problem. As idle is controlled by the idle air valve, I gave that a clean (but thinking about it, that is still used when on LPG where there isn't a problem) and the petrol filter was replaced about 10k miles ago (I bought it when I first got the car 13 years and almost 300k miles ago and found it in the garage recently so decided to change it as I could). There is a separate 0-1V lambda sensor in the right bank exhaust downpipe solely used to drive the LPG system so the only part of the petrol injection system used by the LPG install is the throttle position sensor and that is only used to shut off the LPG on the overrun.

When running on LPG the petrol injectors are switched off but the fuel pump stays on and just circulates the fuel back to the tank. On petrol the RH bank lambda sensor switches as it should but the LH bank one stays pinned at 5V suggesting a lean mixture, yet the exhaust smells rich. Which means the only thing left would appear to be the injectors. My theory is that one of the LH bank injectors is partially clogged so the exhaust on that bank is showing as lean, the ECU is richening the mixture (Nanocom shows short term fuel trim on that bank pinned at +38.5%) but not all are getting their full quota of fuel so some cylinders are running lean while others are running rich. Hence me thinking the injectors could do with a clean. So the question is, has anyone used an injector cleaner, which one and did it make a noticeable difference? Or do I need to strip the injectors off and clean them for the rare occasion I actually run on petrol?

It could be a petrol injector problem or could be the failed lambda sensor / learned fuel trims / less likely learned position for the IAV that's the problem?

Or even something (Pitagora?) that's not connecting all the petrol injectors when it's running on petrol?

I've forgotten what brand(s) of petrol injector cleaning fluids I've used in the past, Wynn's rings a bell. Maybe ask Dai Brace (ClassicSwede), I had a chat with him last night (first chat in maybe a year), I know he highly rates certain brands of injector cleaning fluids but I forget which. I've just sent him a txt so he might see this thread.

Recently I've been working for another firm a few days a week, they have a very well equipped workshop including petrol injector testing / ultrasonic cleaning equipment (which I've used while I've been there). If you'd asked me last week or the week before you could have maybe sent me your injectors and I'd have put them on the machine for you but I got a message from the firm's boss this week to say they don't need me this week... and I doubt they'll be needing me again soon.

Edit - Dai txted me back and sent me this link https://www.forteuk.co.uk/product/petrol-specialist-injector-cleaner/

Lpgc wrote:

It could be a petrol injector problem or could be the failed lambda sensor / learned fuel trims / less likely learned position for the IAV that's the problem?

First thing I did was reset the adaptive values in case that was the problem and it made no difference.

Or even something (Pitagora?) that's not connecting all the petrol injectors when it's running on petrol?

If that was the case it would be running on 7 when on petrol but it isn't. All cylinders are firing but the idle is lumpy rather than a consistent misfire.

I might give the stuff Dai recommends unless any has any better ideas.

if you want to do it in car you can get injector cleaners in a bottle from the auto stores, the biggest decision to make is how much you want to spend on them as some are very cheap and some aren't but you get what you pay for , gas has wax in it so one that dissolves wax would be hi on my list . even a liter or two of diesel can help as it's the base ingredient in the cleaners is diesel (defiantly smells like diesel IMO) they say add to full tank of fuel , i put it in as least amount as it's more concentrated and works faster. other way is to remove the injectors and use a 9volt battery and some carbi cleaner to flush them out little bit more work but better result . use a puss button as the coils in the injectors only need pulsing , dont open for long periods as it will burn them out, piece of tube and a syringe full of cleaner , you know where this is going

Did that about 210k miles ago when the engine was rebuilt so it might need doing again but something I can chuck in the tank would be a lot easier. I think the most it has ever had in the tank since I've owned it is just under half a tank and that was only because I was setting off to drive across Europe and wasn't sure if I would be able to get LPG. As it turned out it is easier to find in Europe than here so I think that half a tank lasted the best part of a year!

However, one thought has just occurred to me. If I put something in the tank to clean the injectors, I'm going to have to run on petrol to let it do its stuff, so maybe taking them out will be cheaper and more effective......

it could also be a bit of water in the tank or just old fuel if you haven't used petrol for a while, a cup of metho will remove the water and 20 liters of petrol is all i would mix with the cleaner fluid. might pay to run on petrol once in a while just to keep the system clean , the old rule of, if you don't use it you loose it
PS i stopped using standard fuel 91 octain here in Aust because it is made up with all sorts of rubbish where the premium 95 octain is more stable and causes a lot less issues eg blocked injectors from particulates in the fuels, our diesel is notorious for it ,i recommend fitting better after market filter system or it's a $5000 injector replacement bill if you like to buy supermarket diesel. todays fuels are rubbish , no other way of explaining it .

If you usually run on LPG, maybe the £20 of petrol has lifted something from the tank bottom ?

Does Nanocom say which cylinder is misfiring ? If so, I would take that one out & do a bench clean with the 9V battery & carb cleaner.

i imagine a lot of these cleaners are somewhat snakeoily...

On mine, i found the injectors from a Rover K series were a direct fit and had similar flow characteristics, while being a much more modern design with better atomisation etc, so i just swapped them out. I can dig out the part number if you like? Was cheaper than paying to get the old ones cleaned.

Pete12345 wrote:

If you usually run on LPG, maybe the £20 of petrol has lifted something from the tank bottom ?

Does Nanocom say which cylinder is misfiring ? If so, I would take that one out & do a bench clean with the 9V battery & carb cleaner.

Maybe, but it was running the same before I put the extra in so unlikely. Thinking about it, it ran much the same last year when I tried it on E85 (which Nigelbb has been running his Thor on) and I decided it didn't like it. But it may be that it wasn't the fuel but the fuel system. Nanocom doesn't give any P030x codes on a GEMS, so no idea which one, or ones, isn't running right.

I've still got the 3 Ohm wirewound resistor I used to limit the current when I did them last time, so may well do that.

Aragorn wrote:

i imagine a lot of these cleaners are somewhat snakeoily...

On mine, i found the injectors from a Rover K series were a direct fit and had similar flow characteristics, while being a much more modern design with better atomisation etc, so i just swapped them out. I can dig out the part number if you like? Was cheaper than paying to get the old ones cleaned.

That's why I asked in the first place, to try to avoid spending money on snake oil. I must admit I am a little sceptical anyway as when I did them with brake cleaner and a battery last time I found back flushing them got more dirt out and nothing you put in the tank is going to do that.

Interesting that K series injectors are a direct fit. Most K series engines used the earlier MEMS so that would suggest the electrical characteristics would be similar (if not the identical) and there isn't much difference in capacity per cylinder either.

Maybe consider just buying a set of second hand injectors? I know cleaning products can work to fix lesser injector issues such as duff spray pattern but if they're bad enough to cause a misfire I wouldn't fancy the chances of a cleaning product fixing them.

I recently bought a set of 6 second hand injectors (for a V6 vehicle I was tasked with fixing for the firm I worked for) for around £60 including shipping.

The firm runs mostly large vans and flatbacks that were originally diesel powered but they remove the diesel engines, fit LS3 petrol engines and convert them to LPG. But the V6 vehicle I mentioned above was the company owner's brother's own car, it had been parked in the works yard for 18months and had a constant misfire on at least 2 cylinders. Before I worked there they'd dropped the engine and fitted a new valve, the ECU had been left unplugged and had got full of water so they sent that off to have it fixed. Still the misfires remained. They blamed the ECU and had another ECU cloned to match the one they thought hadn't been fixed properly. Then it refused to start at all. I diagnosed and fixed it... The reason it wouldn't start at all was because the immobiliser had kicked in after the ECU didn't recognise the key after 5 attempts at starting, so I coded the keys to the ECU. Then it started but only ran on 4. Then it wouldn't crank... Both due to 2 cylinders being hydraulic locked with petrol due to stuck open injectors (no doubt due to dirt ingress because the guy who removed and refitted the engine didn't flush the fuel line or injector rails before reconnecting). Then it still only ran on 4, due to 2 failed coil packs. I was thrown a curveball on that because one of the mechanics there said he'd already ruled coil packs out by swapping the positions around... I expect either a coincidence or the coil packs failed due to something related with the fuel locks but the mechanic reckoned 2 failed coil packs had caused ECU failures, I doubt there was a problem with the first or second ECU after they came back from the ECU firm first time but they mistook stuck open injectors, failed coil packs and need to recode keys for an ECU problem.

I don't think any are actually clogged, but if the spray pattern is screwed or flow is restricted, then that could cause the lumpy idle. It doesn't sound to have a misfire as such, it just isn't smooth. I think taking them out and backflushing them with brake cleaner is my next step. I could just ignore it and do as I have since I've owned it and only run on petrol if I really have to. I've always said the petrol in the tank is regarded the same as the spare wheel, for use in emergencies only.

Gilbertd wrote:

I've always said the petrol in the tank is regarded the same as the spare wheel, for use in emergencies only.

I was just thinking the other day that in over fifty years of motoring I think that I have run out of fuel more times than I have needed to put on the spare wheel because of a puncture. This probably means that it would make more sense for me to carry a jerry can of petrol rather than a spare wheel.

It could be the injectors are sticking, usually a direct 12v feed frees them off, 2 wires and a battery make sure they are all clicking/working.

Had the same issue with an old Jeep ZJ that ran LPG its whole life, the injectors were intermittent sometimes they would fire, other times not.

exercising them with a direct 12v feed freed them off..

worth a try.