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[BigCat]

My 3.2 V6 has an LPG conversion which I had put in about 3.5 years or 45K miles ago. For sometime now it's been hard to start after running on LPG.

If I start it within a few minutes of stopping it starts ok, otherwise it requires at least 20 seconds of cranking, even after a few days of having last used it.

If I remember to turn off the LPG say 10 or 20 seconds before stopping the engine (and then turn it on again) it starts straight away.

If I run it on petrol only, the problem doesn't occur.

I had a misfiring issue which has now been solved (oil in the plug wells), but the hard starting issue is still with me.

Does anyone have any suggestions?

Dominic

[albitz]

Coil packs not as healthy as they once were ? :-/
Showing any codes ?

[BigCat]

Coil packs were new just before the conversion.

I've only just got the car back for the oil-in-the-plug-well thing, so haven't had a chance to check codes yet.

What I don't understand is why running it on petrol for a few seconds (10 -20) before shutting it off makes a difference. Either way it starts on petrol.

I have this wild idea that the system leaks LPG into the plenum after shut down which then "floods" the engine at startup. The logic being that by changing back to petrol before shutting it off, I'm purging the leak out.

Does that sound plausible?

[BigCat]

BTW: I'm not getting e-mail notifications for my posts.

[albitz]

Coil packs have been soaked in oil, which does them no good at all. LPG is much more sensitive to HT weaknesses than petrol. :-/

[feeutfo]

Any codes? Pedal trick? Lights on dash? Fuel trims ok? Did it get progressively get worse over time....?

[BigCat]

LPG is much more sensitive to HT weaknesses than petrol.

I accept that, but whether I do my trick or not, the system starts on petrol.

Surely that rules out the HT system?

[BigCat]

Any codes? Pedal trick? Lights on dash? Fuel trims ok? Did it get progressively get worse over time....?

I've just picked it up from my mechanic to correct a misfire which was down to oil in the plug wells, so haven't had a chance to check for codes.

It does suffer from the V6 pre-cat problem so the emission light is normally on (reset at the moment).

What are fuel trims? is that the offset in the LPG ECU?

It did progressively get worse, although it now seem to have stabilised i.e. at first, it didn't happen every single time , but now it does - unless I do my trick.

[Lazydocker]

LPG is much more sensitive to HT weaknesses than petrol.

I accept that, but whether I do my trick or not, the system starts on petrol.

Surely that rules out the HT system?

Petrol doesn't need as strong a start to ignite so it doesn't rule it out... However, it should start on petrol before switching to LPG anyway so I don't think it's a HT issue with regards the starting.

Your theory of an injector flooding the cylinder is possible but I'd start off looking for a petrol leak. Although that would probably occur regardless of fuel system when switched off :-/

What LPG system is it?

[feeutfo]

Fuel trims are the amount of positive or negative duration applied to the fuel injectors on each firing cycle. The engine ecu adjusts the duration according to the mixture that the o2 sensors are seeing in the exhaust. If it's rich it trims back the injector duration giveing less squirt per firing cycle giving negative fuel trim. Lean it applies positive fuel trim to give more fuel. Fuel trims.

With LPG, if the set up has an issue, causing the excessive fuel trim on gas only, then when it switches to petrol it immediately has to go back to the correct fuel trim which takes time as the trim level is worked out as an average over a period. This is long term fuel trim. Which, if it has say +25 % of lt fuel trim on gas will bring the engine light on, and will run very grumpy on first stater up on petrol first thing.

Switching the gas off 5 mins from home will bring the fuel trims in line enough to make petrol running better. Although it takes longer for the trims to be correct due to it's average learning.

It's possible the trims are far enough out to cause a starting issue IMO. It needs a look at live data and codes on petrol, then see how the gas affects it and adjust accordingly.

Work to do. Sortable though.

[tunnie]

my thought is maybe an injector too, sticking open when shut off? dropping fuel into the pot making it harder to turn over?  :-/

Or is it trying to start on LPG, not petrol?  :-/

[feeutfo]

Might be worth getting your mech to get a code reader on it. See what fuel trims and codes are doing first. Clear codes and see how it goes, assuming the faults are fixed.

There are other threads from the op discussing other issues iirc...?

[Kevin Wood]

Switching the gas off 5 mins from home will bring the fuel trims in line enough to make petrol running better. Although it takes longer for the trims to be correct due to it's average learning.

It's possible the trims are far enough out to cause a starting issue IMO. It needs a look at live data and codes on petrol, then see how the gas affects it and adjust accordingly.

Yep, I'd go along with this. It means the fuelling on LPG is wrong, either due to a fault in the LPG system or because the system just needs mapping again. The problem is, the software, lead, dongle, etc. needed to talk to the LPG ECU will be required to get much further although you could confirm this by checking the fuel trim values in the petrol ECU using an OBDII cable. Do you know what LPG system is fitted?

[BigCat]

Yes, I can see what you're saying and it sounds quite reasonable. When I had the LPG installed, they did ask me whether I wanted it set up for power or economy; I chose economy reasoning if I wanted more power I could simply switch to petrol.

So, just freethinking here: I guess my LPG ECU is setup lean, the Engine ECU compensates by richening the mixture; therefore when it stops on LPG and then starts on petrol, it's running rich...

...except, despite everything I said earlier, I have just realised it doesn't start as though it "has the choke out", rather the opposite. Perhaps it's all the inverse of what I said, which still points to fuel trims.

I forgot to mention one other thing, As it starts, warms up and changes to LPG, it progressively runs rough and nearly stalls (the engine light flashes). If I immediately change back to Petrol then back to LPG, it runs smooth. If I don't interfere it eventually sorts itself out.

Definitely sound like fuel trims.

I'll do a pedal check on my way to work tonight.

Next step: Are there other readers that can read fuel trims live or just a Tech II?

[feeutfo]

I think your LPG installer is either being lazy, or know full well the kit has an issue they can't be arsed to sort out. If driving reservedly  a slightly lean mixture will bo covered no problem.

Something is amiss. Certainly. It needs checking and mapping correctly so the lambdas never go lean, but so it's not rich either. LPG injector nozzles may very well need re sizing.

There are cheap code readers capable if code reading and live data. Most basic function of which is lambda readings/fuel trims.

But once again...WHAT KIT IS IT?