stutters and back fires when throttle floored ...wrangler 4.0 mpi

Backfire usually means spark issues to me....

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

Mick Miller wrote:

wish it was.......it works fine on lpg so the spark side and timing is fine........feels fuel starvation

Do you live anywhere where it is cold enough to cause condensation to freeze up?

I note the use of the word petrol, but just around freezing temps and non or low use of the gas with less than a full tank in it can cause a lot of condensation in the gas tank. This can plug up the pickup sock/filter in the tank with ice beads.

Maybe try some gas line antifreeze?

Mike

Mick Miller wrote:

na.....no freezing

but lpg works fine ........no backfire

Wouldn't lpg & gasoline require different conditions to run well? My bet is that whatever setup you have (plugs, coil, etc.) would be optimized for one or the other, leaving the 2nd running a bit worse. Just my $.02.

I wrestled with a similar problem after installing a supercharger on my 4.0. Turns out the fuel pump was only puttting out about 30psi instead of the 45-50 it should. You can easily test the fuel line pressure at the shrader valve fitting on the fuel rail.

thats what i think it is.................but not checked the pressure yet

what i am getting it when floored.........revs pick up then stutter!!.....and back fire

gradual throttle much less of a problem

did.....reset the ecu .....by disconecting the battery .......and that sort of cleared it but its still there

Mick Miller did pass the time by typing:

What that tells me is you have a problem with WOT mode (Wide Open Throttle) That puts some sensors back in the loop, chief among them is the MAP sensor and MAT sensor.

will give you an idea. Throw an ohmmeter across the MAT sensor, it should give a value corresponding to the chart. or take. That table isn't always accurate for all years)(as long as it's not zero or infinite your fine) Then check the MAP sensor. Center pin should be 4-5 volts with ignition on, engine not running.

Next, your TPS (throttle position sensor). Ignition on, engine not running. Backprobe (don't disconnect, use a pin) the TPS center wire and measure voltage to ground. With the throttle plate at rest you should see >200mv. Rotate the plate to full open and you should see

I still think condensation could be an issue.

Maybe try some gas line antifreeze anyway, can't hurt. The antifreeze mixes with the water allowing it to burn through.

Water getting into injectors can cause strange things to happen.

Mike

Mick Miller wrote: