having an annoying backfire problem with my ute....

 

if i hold a fairly open throttle (75%+) for awhile (30 seconds plus) then try to increase the throttle even more it will occacionally pop in rapid succession through the intake with a major drop in power and a rather rough ride...it bucks like a mule....continueing to hold the throttle open results in a loud hissing sound and a total loss of power....and the paint pen writing on the intake being burnt leads me to think that i have been having a fire in the manifold...closeing the throttle to about halfway then reopening it clears the problem...until i hold the throttle fairly wide open for awhile again.

once it has done it once and been allowed to continue until it starts hissing it will do it ever time i try open the throttle past about 75%...until i slow down for about a minute to half throttle or so

 

it doesn't start to do it every time either....but seems to be more common when the air is cold... below 2-3 deg C

 

the A/F ratio is showing rich when it does it and doesn't change with the backfire/manifold fire

 

any ideas?

I had a problem like this and it turned out to be that my cat was clogged. Check and make sure you have a good flow through your exhaust system.