Lumpy 97 grand cherokee motor runs fine on LPG but on petrol its a bit lumpy when but into drive, while on tickover its ok

Hi dave hill, not really sure on your problem but I have a VN commodore that I purchased that had the fuel pump crook and did the same thing of not starting on petrol. Owner said to hold in lpg button when starting . I got the fuel pump replaced. Mechanic said it was caused by not running petrol through enough and running always on LPG.

not sure here well to be totally honest i have no idea other than a thought and thats maybe it has an air leak on the inlet manifold so its a bit weak ,but i have similar problems with these cars here in spain on the few petrol ones i have seen as most are diesel but i have one customer with a petrol/LPG gas one and nothing but problems like this but on LPG its perfect and i do not know the answer but it seems these early models do suffer from this

try air mass meter

Check out the fan, maybe is not working, about 1 year ago same happened to me, changed many parts but keep getting hot and finally found that the fan had a short circuit and sometimes didnt work.

There is a connector between the fan motor and the fuse box eliminate the connector instead directly join wire to wire

If you mean the monster 4 litre petrol engine then yes because i have changed one as they are all the same block but you will ahve to sort the later to earlier ignition system but not a problem but use the parts off the old one,fitted a diesel into one as well for a car dealer as the petrol ones here are worthless pieces of junk,but a diesel with a manual box ?? around 5000 euros but a petrol with no LPG gas conversion even in good condition you might be lucky to get 500 euros ,scrap man will give you 50/100 euros .Highest value vehicles here when old are merc diesels or a peugeot /citreon/renault diesel cars ,petrol you cannot give them away least off all a 4litre gas guzzler with a auto box

You need to take it to an authorized dealer for OBD-reading and see what's wrong.

Look for a vacuum leak on the motor intake system, such as perished hose, loose inlet manifold or air intake fitting.
Check operation of the PCV valve and associated hoses.

Whats the difference between petrol and gas? Dual fuel ? lpg

1. My suspicion is your electronic ignition
2. If your car is a bit old, there might be a leak through your valves
3. Distributor timing gear is damaged (replace)

You will have to recheck that all the wires were reinstaled.The noise is the fuel pump.The lpg system has its own fuses so you will have to check them also.The only thing you touched was the battery so the problem is there somewhere..Earth wire is also important just double check all the things you undid.Let me know how you go