I had this exact same problem for 2 years on my 97 4.0 Grand Cherokee, and
at last a few months ago I managed to get the problem solved after the LPG
system was not properly installed.

The problem is that the engine ECU is learning whenever you switch on to LPG
that the parameters such as ignition timing etc, are different to the
factory settings for petrol, so therefore the ECU adapts these settings to
those suited for gas (although it still thinks that it is running on
petrol). So this means that whenever you switch back on to petrol, that the
parameters are clearly not right for petrol, hence the misfiring and rough
running!

The solution to this problem is that the ECU needs to be reset from these
"learnt" settings back to those of the factory settings whenever switched
back on to petrol. The way to do this is to go to a reputable LPG installer
where he should be able to order and install a little electronic box (approx
£100) that fits inline to the ECU and resets it whenever the ignition is
switched off.

In other words the ECU must always be reset back to the factory (petrol)
settings each time the engine is turned off.

Good Luck

"Raymond Ramsey" wrote in message
...
My '95 4.0 Cherokee has an LPG conversion. There is a selector switch
giving
the following options 1-Petrol..2-LPG..3-Start using Petrol automatically
switch to LPG shortly after starting. I've been advised selecting 3 is the
best option.

Cold starts do not seem to be a problem. However starting when engine is
warm either switch position 1 or 3 is murder! The engine miss-fires and
runs
rough. Obviously a problem with the Petrol feed. Switching to 2 and all
seems fine (so far) as does switching to 3 after the engine has been
running
for a couple of minutes.

It seems obvious the problem is running using petrol. Have any of you any
ideas on where to start looking? I assume the electrics are fine as all
runs
well using LPG.

Any suggestions would be appreciated, along with where to look for the
parts
that might be faulty/blocked. I've only had the motor a week and a bit so
the geography under the bonnet is alien to me

I need help!

Cheers, Ray