Outer Limits 4x4 Board

Got an F 150 5.8EFI,It will run fine then will go to shit and start back firing and nearly stalling then will come good again,it does it on petrol and gas
Ive change leads and plugs,still does it,Ive changed dizzy cap and rotor and still does
Took it to a machenic he said leads again,done that and still does it
what else can i look at?

Could be a cracked pipe between the airbox and inlet manifold. Doesnt need muck just a hairline split.

If it's not spark then maybe fuel? Could be a fuel pump problem or just sucking up crap occasionally in the tank. Does it have bad fuel? Intake air leaks usually are a constant problem rather than intermitant. If the pump is easy enough to get to and cheap enough try replacing it to see if that fixes the problem, otherwise you will be stuck doing some diagnosing to find the problem. The other, more painful option, is sensor failure. And for that you will need good electrical knowledge and scan tools.

guzzla wrote:Could be a cracked pipe between the airbox and inlet manifold. Doesnt need muck just a hairline split.

I`ll have a look

ScrawnC wrote:If it's not spark then maybe fuel? Could be a fuel pump problem or just sucking up crap occasionally in the tank. Does it have bad fuel? Intake air leaks usually are a constant problem rather than intermitant. If the pump is easy enough to get to and cheap enough try replacing it to see if that fixes the problem, otherwise you will be stuck doing some diagnosing to find the problem. The other, more painful option, is sensor failure. And for that you will need good electrical knowledge and scan tools.

Its not fuel related cause it does it on LPG to

my chev used to missfire at idle once warmed up. Turned out to be a blown intake manifold that leaked on the inside of the valley, so it was almost imposible to find.

Just put it down to luck and a bit of cluey guesswork to find it

4sum4 wrote:

ScrawnC wrote:If it's not spark then maybe fuel? Could be a fuel pump problem or just sucking up crap occasionally in the tank. Does it have bad fuel? Intake air leaks usually are a constant problem rather than intermitant. If the pump is easy enough to get to and cheap enough try replacing it to see if that fixes the problem, otherwise you will be stuck doing some diagnosing to find the problem. The other, more painful option, is sensor failure. And for that you will need good electrical knowledge and scan tools.

Its not fuel related cause it does it on LPG to

Just reread the original post, ignore my first thought/rambling.
It back fires because it is running lean. Does it run an air flow meter or MAP sensor? Could be an air leak if it runs an air flow meter and/or sensor problems.
I'm interested in the outcome of this, keep us posted.

I think maybe distributor.
If its points, check the lobes and gap.
If electronic, check the inductor thingo isn't loose and has correct gap.
Pull it out and check the roll pin in the drive gear.
Check the vac advance mechanism moves freely when you suck on the hose.
May not be dizzy, but if you find and fix anything here it WILL run better.
Good luck.

was Burnt wiring to the dizzy

Did you find the cause of the burnt wires or just repair them?