Ok so for the longest time (about 5 years) i have had an oil leaking onto spark plug problem. No biggie just every 6 months change plugs and truck runs nice. Recently took truck on 500 mile trip no problem . but then my heater control valve went out throwing antifreeze everywhere and the truck over heated. after i fixed that the truck will randomly over heat. had a leaky radiator. fixed that. had a bad thermostat. fixed that. truck still over heats!! And it runs like it is misfiring pretty bad. Also i smell burnt fireworks smell. And white smoke out tail pipe is BAD on cold start. Could this be a blown head gasket? oil is not milky. Could it be the water pump? Might I try running engine with no thermostat? any ideas greatly appreciated! truck runs the WORST at idle. give it some gas and it responds nicely and things sorta smooth out. O yeah i also noticed i have bad exhaust leak at the muffler. Like the pipe is trying to fall off. Can a clogged muffler or cat cause over heating? Thanks for all your help!

 

Welcome to the forums.
The white smoke at start up is coolant in the combustion chamber.
I've been here now for a little while and I've discovered that theses trucks can't tolerate any over heating.
You're in the 4 litre V6 forum, so I'm going to assume that is what your engine is.
What year is your truck ?

Pull the plugs, one will probably be very clean from water getting into that cylinder.
At any rate head gaskets just don't fail on there own, they fail because the head is warped and or the head is cracked allowing water into the combustion chamber.
Time for some new heads.
It's still over heating because you're loosing coolant through the combustion chamber.
The oil leaking onto the plug tells you that your valve guides/seals are worn, probably time for some new heads anyway.
Also, when adding coolant, remove the rad cap and make sure that it's full up to the neck, because you have a leak, the system won't suck fluid from the over-flow container.
At this point, most likely not the water pump, if it was failing, you would see water coming out of the "weep" hole.
You can try running it with out the thermostat, but in the end that's not going to stop your coolant loss problem _ keep on eye on the level by removing the rad cap when it's cold.
On a cold engine it should be right full up to the neck.

A leak in the muffler will not cause over heating, but that should be dealt with depending upon where the leak is and its relation to the O2 censors.

 

What year and engine, OHV or SOHC?

Do The Glove Test, its free and easy

Cold engine
Remove rad cap and overflow hose from rad
Plug overflow hose connector, vacuum cap, putty, gum, ect.......

Put Latex glove over rad cap opening and seal it with rubber band, or use a balloon or even a condom will work.

Disconnect Coil Pack, unplug the 4 wire connector so there is NO spark and NO start

Crank engine(try to start)
Watch the glove
If it bounces up and down you have a cracked head or blown head gasket

If it just lays there you don't

If it does bounce then remove 1 spark plug at a time and crank engine again
When it stops bouncing the last spark plug removed was from the leaking cylinder
Put it back in to confirm


No, clogged exhaust can't cause over heating, it will limit RPM, and you will lose power as back pressure builds up.

The 4.0l OHV engine(1990-2000) was known for heads cracking if overheated just 1 time, there was a weak spot in the casting between valve seats

 

thanks guys! Yes it's a 99 4.0. Sux to have to do heads but i will try that glove test first! I really appreciate you guys. It's been kicking my *** trying to figure out what to do. But if the heads need replacing does the whole engine need replacing? i'm at 240,000 miles. Is it running rough from coolant leaking into cylinder? Thanks again!

 

If Ron's glove test fails, then you'll be taking the heads off.
Once there off you can have a look at the bores and see how much wear there is and how much of a ridge is at the top of the bore.

300,000 miles is around what one would expect out of these engines, so you're getting close to that mark, but that's a general statement.

If those were long highway miles, then conceivably you could another 200,000 out of it.
You will know more if the heads have to come off.

There was travelling salesman who had a big 70's Chrysler New-Yorker with around a million miles on the bottom end.
It was driven all over the continent _ long highway miles, and all he did was change his fluids and filters.

If the truck isn't rusted and is in good shape, then spending money on a complete overhaul is money well spent.