Sometimes hesitates and bucks on lpg

[pgtips]

Hi, I've got an intermittent fault that I'd like to understand and fix. This weekend I drove for miles on LPG no problems but on way home the car felt as though I was lifting my foot, then pushing down on accelerator, back off, down again etc so I switched to petrol and it was ok.

5 mins later switch back to LPG and it's ok for a bit then it happens again. Rinse and repeat. Petrol good, LPG 'bucks'.

Doesn't seem to start happening due to a reason like under load, heavy acceleration engine warmed, engine temps good, nothing different. It just does it.

Stopped for some food and hour later drove rest of journey home on LPG no problem.

My filters are all recently changed. It's a Subaru 2.0 n/a.

I'm going to plug in laptop later and see if any fault codes show but maybe it's something people know about.

Many thanks, Kevin

[pgtips]

I'm hoping a thread bump maybe someone else out there can help me with this, its really driving me a bit potty.

Seems mainly under load, accelerating uphill etc. On a level surface it will run for miles without problem. The car fully warmed up.

I have changed my filters, unsure what else to try

[Gilbertd]

Reducer coooling down? Lack of coolant flow could cause it.

[pgtips]

theres good flow, its inline and the LPG comes on at 25deg, the problem occurs when temps are in the upper 80s and I def don't have airlocks so I think I can rule that out. Also happens on the hottest of days when ambient temps are upper 20s.

The car fully warmed up.

[LPGC]

If it's a misfire would expect ignition system or injectors if nothing else has changed, but misfires are obvious and don't feel like moving the throttle.

Does the same happen when the engine is still warming up (soon after changing to LPG but engine not yet up to normal temperature)?

What LPG system parts are fitted? Some systems (including e.g. some Lovato) have default ECU settings which see the ECU add a percentage of petrol to the mix when gas injectors approach 100% duty cycle, lots of ECU's are capable of petrol addition but some (like Lovato) supposedly automatically calculate the percentage of petrol addition needed to make up the shortfall of fuelling that gas injectors cannot supply. The following can happen with a wide range of ECUs if the system and ECU are setup/calibrated in a certain way but is more likely with ECUs like Lovato that calculate the petrol addition themselves... If LPG calibration is wrong you'll likely get less torque/power on LPG than on petrol. With that in mind if the LPG system adds a percentage of petrol to the mix (reason for this starting to happen intermittently could be the ECU has started seeing lower gas pressure than it used to so has to compensate by pulsing gas injectors for longer, now long enough to be approaching 100% dc, now long enough to start adding petrol to the mix) the driver could feel a surge in power when petrol is being added to the mix. In this situation there'd probably be two problems, incorrect calibration (or at least gas pressure compensation) and reducer pressure that has gone down from what it was before the intermittent problem started.

What year Subaru is it? Stop & Start model or earlier? IIRC some of the stop/start models need a fuel return, if a fuel return (physical or emulated) is fitted and starts playing up causing the petrol system to read varying petrol pressure it will cause the petrol system to start compensating petrol injector pulse duration for the varying fuel pressure, and since the LPG ECU's primary input used to calculate gas injector pulse duration is petrol injector duration this would cause the LPG mixture to go rich/lean as the petrol ECU responds to varying petrol pressure.

[pgtips]

hi and thank you for your detailed reply.

could feel a surge in power when petrol is being added to the mix

its def not a surge, more like momentary starvation (lack of fuel) turn the LPG off so petrol kicks in, but of course the temp is warm enough so then it switches LPG back on and round it goes, starvation rinse and repeat etc

So I intervene and turn on petrol to stop this, run for couple miles, put LPG back on and all good.. until the next time.

The subaru engine is 1994, done about 97k now fitted into a VW camper (although its still a 1994 subaru . I have a stag 300-4 ecu.

Most of the time it all runs lovely, its just that odd moment when it lurches/bucks which is starting to grate me.

I have got a old laptop and I'm going to try to see what the reducer temp is when/if it happens, maybe first post was close but I am convinced it is hot enough. I was actually tending the other way thinking fuel vapourisation (had that before in a normal petrol classic car) so I have added some insulation to the LPG lines that feed to the reducer but I don't think it is that.
This LPG kit was on my original flat 4 VW engine which only had about 80hp, this engine has maybe 115hp. The thing is there is absolutely nothing showing on the AC software, nothing at all, as far as the software is concerned everything is perfect and spot on.. and most of the time it is.