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[BigM]

Here's a challenge for the online pool of knowledge 

I have a Jeep Grand Cherokee 4.7 1999 with a prins multipoint LPG system and I'm ready to admit defeat at trying on my own to cure this! Don't think it's anything to do with the LPG tho.

My car is slightly misfiring when at motorway speed (70-80) and only when under load.... the misfire gets worse when going up hill or switching the air con on or having the car full of people.
I have changed the spark plugs twice (now have a cooler set in) and also changed the coils (changed 4 and no difference, so changed the other 4 and still no difference).
I have had it plugged into a Jeep diagnostic unit and no faults were shown.
If I have the car under a very small load the misfire seems to be about once every second, but if I then put my foot down it misfires loads.
Not completely sure, but seems to be better when the car is first started.

As I said earlier it seems to happen on either gas or petrol so I'm pressuming it won't be anything to do with the fuel system, but did wonder if any part of my fitted LPG system could cause this?

Many thanks in advance to anybody that can help me.

Mark

[Bubba]

does it do it on petrolium distilate or just fart vapour

[BigM]

Both 

[Bubba]

i would think o sensor problem but i stand to be corected by those who know much more about later models

[Mike Pavelin]

If it's a regular misfire on one cylinder I would suspect the ignition coil.

[Lornaben]

I'm with the irrefutable Mike Pavelin.

An Engine misfiring under load is always a spark problem and your problem points to a coil failing under duress 

[donk498]

Not familiar with this engine, but had a mates v8 rangie missfire when driving and it turned out the alternator was weak. Would keep the battery
topped up enough to start it, then after a while would start to miss. Was worse with the lights on for obvious reasons. Might be worth checking and eliminating it.

Dave 

[JamesH]

I would go through all of the coil packs, leads and plugs and make sure all is spot-on. My 4.0 on LPG has to have a new distributor cap and rotor arm about once a year and plugs and leads every 2 years for perfect running. The other thing to try is the LPG diagnostic software - this can give you the option of advancing or retarding the injector timing and normally shows the output of the O2 sensor(s) it connects to.

[PtP]

I had a misfire problem with my 4.0 TJ and it turned out to be the plug leads, but I don’t know if you automatically change them when you fitted new coils on your engine.

Have a look at these threads:

http://www.birtydastards.com/frm/index.php?topic=7902.0

http://www.birtydastards.com/frm/index.php?topic=9777.0

[Mike Pavelin]

You won't find plug leads on a 4.7. it has an individual coil on top of each plug.

[Bubba]

i think he has changed the coil packs and plugs gas conversions have to stimulate the 02 sensor readings and i am still thiking therein lies a story

[PtP]

You won't find plug leads on a 4.7. it has an individual coil on top of each plug.

I wasn't sure so it rules out my theory of plug leads then!