78 f250 400 no mods

Well I was driving to work today and the heater hose split on me and drained the entire cooling system dry before I could pull over safely. I patched it up and filled it back up with water and tried to limp it back home about 4 miles. the hose split again and emptied out a second time. I managed to get it home but it was really running bad. I replaced the following when it finally cooled down. The bad hose, fuel filter, thermostat, and all spark plugs. I checked the cylinder pressures and they all still read the same as they did the last time I checked (116 to 118) But the truck will not run right, the vacuum shows about 7 inches at idle (normally it is about 18) and the truck will not idle or start well. It is not blowing white smoke and the oil looks good.

I am wondering if its possible with the amount of antifreeze flying around under the hood that some got sucked into the carb? Some other issue? head gasket, burnt valve, or ????

I cant see any obvious vacuum hose split but could a bad vacuum leak cause it to act like this?

 

This happened to my 66. It fixed itself after sitting.

 

You are very lucky that you did not ruin the engine by twice running it with "the entire cooling system dry". Such action can warp blocks & heads permanently. If your compression is normal, I doubt that a head gasket is blown, a valve burnt, or collapsed piston rings. Always let an overheated engine cool before replacing the coolant. If the vacuum went from 18 inches to 7, that is your most powerful clue to the problem. Apparently the overheating made a problem with the intake side of the engine. Look for leaks somewhere, as failed gaskets, cracked components, hoses, etc.
It would be really nice to have a vehicle as mountain dewd's that fixes itself.

 

When the motor is cold does it idle better than when it's warmed up?

It is totally possible to have a blown head gasket or cracked head that will give you normal compression readings. Cylinder pressures when running are way higher than the compression test readings.

Did your idle rpm change? Have you checked your timing? What about your idle mixture screws? Are you blowing coolant out the overflow now? I doubt these things changed but odd things happen when a motor over heats.

 

I was on a very busy very narrow 2 lane road and there was no safe place to pull over right away, I was kind of stuck driving it a little ways. I did let it cool down before doing my roadside repair which of course didn't hold and I was back on that @#^%$ road again.

I have not driven the truck since this happened, and I didn't want to break it more since something is clearly not right with it. It idles very slow and loopy now, I have not had time to check much else. Tonight I will go through the vacuum hoses and look for issues.

My dizzy is stuck so I cant change timing (it was fine prior to yesterday) Maybe I found a new way to unstuck a dizzy

 

Yeah, overheating can serve to loosen the stuck distributors in these engines. But more than likely it's still stuck.
Even if you're not planning to change the timing or the distributor anytime soon, I would still give it a shot of penetrating oil every time you open the hood. Took a LONG time before mine finally broke loose so I could play with timing. Didn't want to bust it, so I took the long easy road.
But I did still resort to a hammer occasionally!

I would certainly check ignition spark energy too, just in case the coil died from the heat. Not fully dead as it still sparks, but a weak spark will give you weak vacuum numbers.
If the vacuum was smooth and not pulsating, I agree with it not being a valve issue. But a weak valve spring "could" still have started with the overheating process and is now weaker than normal even though it's still enough to close the valve and give you a good compression reading. Just a thought.

But definitely give it the once-over tune up stuff. Check timing, check plugs, wires, cap and rotor, etc.
Maybe even the ignition module didn't like the excess heat? You never know. But that's why I like to look at an exposed plug end sparking to see just how much energy the ignition has. If it's a sharp, snappy blue/white spark, you should be good in that department at least.

Good luck.

Paul