OK I posted a while back about this and though I had it fixed. Well I thought wrong

88 F-150

Ok the engine is still missing throughout range but most noticably in lower rpm or under light to steady throttle. If you put and load on the engine it seems to clear up. Say if I change from 3rd t 4th gear it will stumble/miss several times while accelearating. then it will also do it whn I am holding a constant speed.
Also, the engine idles absolutely horrible when it is cold. you cant hardly even move the truck. It idles ok when warm though. I have checked the coolant temp sensor and air charge temp sensor (along with about everything else) and they are well within limits.

I have changed the spark plaugs,dist.cap, rotor, and fuel pump. I am going to get new spark plug wires here in a few.

also sprayed some cleaner around looking for a vacuume leak, No suck luck.

So what might I be missing here?

Thanks!!

 

Dihydrogen monoxide!! Get 2 bottles of heet and 2 bottles of iso heet and throw it in
the tank 3/4, 1/2 whatever run it until you get to 1/8 tank then refill with fresh fuel
and a bottle of stp fuel treatment or that lucas fuel treatment. Add a bottle of heet
every oil change to keep the water out of the fuel tank. Don't replace the wires yet
test them they should be 5000 ohms per foot if they are close to that they are fine
unless they have cracks or abrasions etc. Another good test it to spray the wires
from dist to plugs with a mist of water from a spray bottle and turn off all lights and
start the truck, If you see any sparks jumping between wires, jumping to metal or
to low voltage wires you need to reroute the wires add seperators or cover the wires
with split loom!! Of course be careful around a running motor especially at night since
you can't see the fan or belts you might not think about them either!! I would also
check the timing and make sure it is in spec. I would check the wiring going to the
coil and check the resistance on the primary and secondary windings. A performance
coil may be a good direction to go to since it can build up the energy quicker and more
to boot which will help more completely burn the fuel, You could open the gap a bit
too!!

 

Check the timing.

 

Could also be a sticking EGR Valve.

 

I think a stuck EGR valve or vacuum leak is most likely. Ignition misfires are typically OK at low load and get worse as load increases, not the other way around. I have heard terrible tales of water in the gas, but I have never ever seen it happen on an automobile unless someone put it there. Almosty all modern fuel has alcohol in it when it comes out of the pump, so there really isn't any need to add more unless you can confirm that there's water in your gas.

 

I strongly second the vaccuum leak, my 300 is such a pain when I think about the vaccuum lines...(mine was doing the same thing also, intil I ripped off the whole system and made it a three line setup >.<)

 

Thanks for the suggestion guys!!

I think that I have gotten it fixed. I put new plug wires on and it cleared right up. It runs like a champ now.
Funny thing is, I checked all of the wires and they all had really low resistance on them. (<4,000) and book says between 3,000 and 5,000 is acceptable.

After I replaced them and it ran good I decided to check them again and this time I could not get a reading on the main wire from the coil to dist. cap!! I must have had it in the right position when I checked it the first time.

I am in the process of replacing a few vacuum lines just for safe keeping.

And I forgot to mention that the EGR valve went out several years ago and I just fixed it by putting a restrict or plate between it. (Thanks to the suggestions from this board) But, you would be correct to check this, it produced almost exacrly the same symptoms then.

Thanks again