Hi everyone,
I`ve got a `97 4L petrol cherokee sport auto, converted to LPG in the UK some years ago. It has started to "kangaroo/surge" consistently on LPG but is ok when switched back to petrol. A mechanic changed the LPG converter & it ran fine for 5 weeks, then prob started again. A different garage changed air filter and box, replaced spark plugs and road tested with the LPG system connected to his laptop....he said the LPG side of things was fine so must be an ignition fault (could not get my head round this statement as the vehicle runs fine when switched to unleaded petrol). Anyway, he changed the distributor, rotar arm and plug leads and re-tested and hey ho, success! but now 2 days later and not more than 30 miles since i picked it up.....it`s doing it again!! I will be contacting the garage but thought i`d check with you guys first. I`ve had the the old beast for 6 years with no real issues and it`s still in great nick so I really want to resolve the problem. thanks for any suggestions.
Mick from the south of England.

 

Hi, welcome to the forum from fantic.
What exactly do you mean with kangaroo surge? Backfireing in the intake?
Is it a venturi system or a multipoint LPG injected?
What brand? Lovato, Landi, Brc, Stargas, Icom....

 

hi fantic, thanks for replying. it feels like momentary fuel starvation, which occurs every 2/3 seconds...imagine sitting in a chair and rocking backwards and forwards....that`s the feeling as you drive, if that makes sense! I knew i`d be asked some techy questions (i`m not techy at all!)....not sure how i tell if it`s "multipoint" LPG injected or what brand...gonna have to look at some paperwork i think i`ve got at home re. the LPG conversion. will get back to you. mick

 

Ok.
Does it backfire when it dies?
Does the reducer freeze?

You can easily say if it's a venturi or not.
If you have the aluminium mixer (a "donut" in the intake big rubber hose) with the single lpg hose attached to it.
The inj. has the little hoses (6) that go from the injection unit/s to the nozzles on the manifold.

In both cases you should have, (Euro Norm 67 r01) a cut out electro-valve on the copper line before the reducer (what you call converter).
This valve has a filter in the bottom of the brass housing that needs to be cleaned/changed.

Posting some pictures would help identifying your system and diagnose eventual mounting mistakes.

 

I know nothing about lp systems but
their must be a regulator in the system some were did it get replace during any of the serves

 

There is no pressure regulator on a LPG system. CNG systems have a pressure regulator.
LPG is a compressed gas that liquefies at room temperature once it is compressed over 4 bar.
(like in a lighter)
Until it is kept in a bottle together with an aeriform part, it stays at 4-10 bar (depending on the temperature), that is it's natural vapour pression.
The liquid gas is conveyed (at the same press. of the bottle) to the reducer via copper lines, and is transformed back into aeriform gas at atmospheric pressure.
Since the state transition is highly endothermic (it freezes everything), the reducer needs to be heated with the hot fluid of the cooling system.
The gas is then sucked in the engine via the intake. Flow is controlled by two screw valves, one on the reducer that regulates the main idle and one on the hose that regulates max flow.
Very simple, that's all.

 

cheers mike37. fantic, i`ve taken some pics (i`m a philostine with most things techy and that incs downloading them onto this site! so waiting for my mate to arrive to instruct me!....what a sad xxxxxx i am!....gotta get myself into this century). love yer bike. I`m slightly older than you and the speed thing has gone.....i`ve got a 1980 yam xs750se us custom (shaft driven)....really good nick and does me for fair weather outings.....wife not keen though....but a man`s gotta do etc!! ****....run out of typing space in yjis quick reply section! looks like the venturi sys you describe. will come back once pics are on. cheers.

 

Hi Fantic

Pics as requested

pic1





pic2




Pic 3



Pic 4




No it doesn't backfire when it dies
No the reducer is hot

From the pictures I assume it is Venturi

I think Pic3 shows the valve with the filter that needs cleaning, please confirm - if so do I just disconnect & clean?

I have driven it this morning and guess what - it's running fine again but no doubt it will start surging in due course!

It feels like something is clogging the fuel which unblocks every now and then and allows correct flow, but then goes back to blocking supply - does this sound like something that cleaning the filter might cure.

Although I'm not tekky I'm more than capable of giving the filter a clean!!

Thanks for all your help so far

 

AAHHHHH... Romano! The factory is just a couple of miles from my house in Italy. I know the owner very well. They build old stylish stuff but very reliable.
Unfortunately Romano doesn't do the injected system for the cherokee and you have a the very dated lambda control system.
There is a stepper motor on the main LPG hose (pic. 2, near the valve cover) that is controlled by the O2 sensor. If the O2 sensor is not perfect, it will malfunction (the stepper motor will cicle opening and closing the LPG flow, causing the surge you have).
The jeep will work well on petrol though, even with the bad O2 sensor.

You can easily check if it's this by putting the traditional manual "screw" instead. (picture).
It is a very cheap part and eliminates all that junk of the lambda system, basically useless (the purpose of this system is to close the LPG when you step off the pedal and slow down, in order to save a "little" more fuel).

If you don't want to spend any money at all, you can just take it off and put a piece of copper tube (3/4) and restrict it with some pliers.

Yes, the filter i say is where you have shown.

The system is well mounted overall, except for the position of the reducer that should be mounted as low as possible, or it will be the first to run out of water, and the copper line from the filter valve to the reducer should have a spiral with two or three loops (euro norm) to absorb vibrational stress.

 

Fantic. well done my man! I decided to use your 3/4 copper pipe method first and it has resolved the problem. I know you said the part was inexpensive but I figured rather than frig about finding the part, I would use the no cost version (had some pipe in the shed) first and if it works properly over the next couple of month`s I will get the proper part. I really appreciate your time helping me....what a great idea this forum is! I definately intend to visit it regularly, although, I am not a petrol head, so my contribution to problem solving will be as good as useless! The wife and I have a 22ft motorhome and we intend an Italy trip next year.... your neck of the woods looks good to me so we may look you up on our way down for a thank you beer(s). Will try and chuck some mug shot pics + cherokee onto the site. Thanks once again mate. Mick

 

Happy to hear it works, mate.
Whenever you decide to come to Italy i will be happy to accept that beer/s.

The only issue you will have now is a higher fuel consumption. The purpose of the screw is to restrict flow at max rpms. With the copper pipe you can't register it very well.
I suggest you a 80% of restriction and a max flow test.
The test can be done by using two or three pipes with different restrictions.
Start with a pipe with 90% of restriction and go test drive it.
If the engine dies around 4000/ 4500 rpm it is too restricted and you need to open it a little bit. (that will give you a very good mileage though)
If it doesn't die all the way up to 5000/ 5500 rpm, restrict it more.
I strongly recommend you to purchase one of these screws, every LPG installer should have one lying around in the shop, it will allow you a much more fine tuning of the flow.
If you don't find any you can buy a common 3/4 water valve at any hardware shop.
When you will have the correct restriction you might have to register your main idle with the black screw on the reducer. But only if the idle is low. Never turn that screw more than 1/4 of turn per time, and it takes a while to affect idle speed after registration. It is not immediate like with petrol.
Same story of the pipe, close (screw) a couple of 1/4 and wait. If the idle is too low open (unscrew) a 1/4 and so on. When you see that unscrewing it doesn't affect idle speed anymore, close it back and you're done. Don't unscrew it too much or it will pop out and you will lose the little spring behind it.
Remember the original position of the screw in case you mess up with the idle.

..and to honor the good ideas i will add this:
"If I have a dollar and you have one dollar and exchange them in the end we'll always have a dollar each.
If you have an idea and I have an idea and we exchange, in the end we will have two ideas each ...."

 

Cheers fantic, all understood. a very profound but true quote to finish....you are obviously a deep thinker. will try and touch base with you next year in your beautiful country. all the best, Mick.