Cherokee is grumbling.

[Fire99]

Hi folks,

Well as much as I love my Cherokee, it's having a few wobbles.
It runs a single point, closed-loop LPG system that is triggered from the O2 sensor.
It once had an issue where it was losing power on both petrol and LPG and in the end it was diagnosed as bad leads (the new replacement ones had only lasted 1k miles).

Anyway, a similar problem has occurred. Starts fine, runs ok on LPG but after a good run it loses power, to the point that you have to kick down a gear to get up a modest gradient.

I've replaced the replacement leads with 10mm Silicon Top Spec ones and the plugs, dizzy cap, rotor arm are all relatively new.
I got the Coil tested and apparently all appeared normal but the problem isn't all the time.

Anyway, it can't be the MAF as LPG doesn't use it. The LPG system has a new filter and that seems to operate ok.

Anyway, any thoughts. Intermittent Coil fault perhaps? Clogged Cat? What do folk think?

[300bhp/ton]

Does performance improve after it rests/cools down?

If so to me it would suggest a heat issue somewhere.

Is it just feeling less on power or is it missing? Does it freely rev ok?

[Fire99]

300bhp/ton said:

Does performance improve after it rests/cools down?

If so to me it would suggest a heat issue somewhere.

Is it just feeling less on power or is it missing? Does it freely rev ok?

Thanks for replying,

It's not a perfect science but it seems better for the first few miles (before the car gets hot) and was at its worst after a 2 hour motorway drive when it started to really struggle, so i'm with you on the heat issue.

It feels just a loss of power. The best comparison would be to feeling like driving with the handbrake slightly on or pulling a caravan. It will still rev cleanly, just much more of a struggle to get there. (if that makes sense)

The symptoms are not far off when I had bad HT leads before. (Fortunately i've now discounted that issue)

[300bhp/ton]

Fire99 said:

300bhp/ton said:

Does performance improve after it rests/cools down?

If so to me it would suggest a heat issue somewhere.

Is it just feeling less on power or is it missing? Does it freely rev ok?

Thanks for replying,

It's not a perfect science but it seems better for the first few miles (before the car gets hot) and was at its worst after a 2 hour motorway drive when it started to really struggle, so i'm with you on the heat issue.

It feels just a loss of power. The best comparison would be to feeling like driving with the handbrake slightly on or pulling a caravan. It will still rev cleanly, just much more of a struggle to get there. (if that makes sense)

The symptoms are not far off when I had bad HT leads before. (Fortunately i've now discounted that issue)

Does it roll freely? I suppose a sticky brake could cause this or some other drivetrain issue if it produces enough resistance.

And you say it's the same on both LPG and petrol? Certainly seems a bit odd. Ignition related does sound plausible. Not sure, do these have knock sensors at all?

[Fire99]

Yeah i've checked brakes etc and there's no rolling resistance (well no more than normal )

Petrol isn't great either with it being a bit stuttery but I've not used that as gospel since, with it being run on LPG for a long time before my ownership and no doubt regular low fuel levels, that could be a misnomer.
I don't think that year and engine config have knock sensors.

The LPG system uses less ignition components than on petrol (not using the MAF) so I thought it could either be the coil failing or the TPS which the LPG does appear to take a signal from.

I'm trying not to go down the 'enthusiast' route and randomly replace parts.

[pcn1]

Just a wild guess, but is it an exhaust problem do you think ? Im thinking a bad o2 (lambda?) sensor ??

[Fire99]

pcn1 said:

Just a wild guess, but is it an exhaust problem do you think ? Im thinking a bad o2 (lambda?) sensor ??

I've fitted a new O2 sensor unfortunately, so no go with that one.

[munroman]

Blocked/collapsed cat?

Clogged exhaust?

Symptoms would be hot running and quiet exhaust, as well as the power loss.

Edited by munroman on Friday 10th December 20:58

[Ray Luxury-Yacht]

I think I might know the answer to this. Are you running NGK plugs or something else?

As a long time LPG user, the only plugs which work well with gas are NGK. All the others fail miserably.

[Fire99]

munroman said:

Blocked/collapsed cat?

Clogged exhaust?

Symptoms would be hot running and quiet exhaust, as well as the power loss.

Edited by munroman on Friday 10th December 20:58

Thanks. It's possible. I have it as a possiblity but the exhaust seems pretty free flowing and revs cleanly on startup etc. Unless the CAT gets worse once hot.

[Fire99]

Ray Luxury-Yacht said:

I think I might know the answer to this. Are you running NGK plugs or something else?

As a long time LPG user, the only plugs which work well with gas are NGK. All the others fail miserably.

Good point. The funny thing is the JeepForum doesn't seem to rate the NGK's for normal petrol running but LPG seem to like em.

To be honest, i'm not running them and may be worth trying a set. It's relatively inexpensive and you can never have too many sparkplugs.


I'll have a go and report back.

On a side note, today i've started working through all the greasing points with the grease gun. Including the prop shafts and UJ's, there's quite a few points. Local spares place was impressed. It's interesting how many folk don't bother to do the job.