Excessive Coolant Pressure

[chappie]

1988 F350, 7.3l IDI, 4x4, crew cab, 5 speed manual.

History- I just bought and I am trying to diagnose an excessive coolant pressure problem. I don't know too much about truck's history other than about 2 months before I bought it, the owner had a trip where it started to overheat. He said it got to 3/4 on gauge and he pulled over and let it cool off and drove a little more until it got 3/4 on gauge repeating this multiple times until he got to his destination. He had it towed home and then replaced thermostat and radiator. He then had a problem with it having excessive pressure in the coolant system pouring coolant out of reservoir by the liters. I bought it thinking it would need a head and or head gasket replacement.

Current problem - I have tried to diagnose the excessive pressure problem and I am not sure if it is either heads or gaskets. There is no white smoke out of exhaust, the block test (exhaust test) was negative on multiple occasions, and the pressure is right after starting it and it will gush huge amounts out of radiator immediately with having pressure remain in system for a long time after shut down. The thermostat housing had a lot of rust on inner walls and check valve ball was all gummed up.

Fixes - I replaced all hoses, coolant reservoir, thermostat, cleaned out check valve ball and ran multiple block tests. If I run it while continually releasing pressure through the clean out valve on heater core hose, I can keep it from pushing coolant into reservoir. I am running half distilled water and half coolant.

Questions - could it still be a head and/or head gasket if block test was negative?
Does it sound more like a blockage coolant system? Where at? How to test/fix?
What can I do to further test head/head gasket leak?

Thank you.

 

[G. Mann]

What you describe is all classic symptoms of a blown head gasket.

With the radiator cap off, start the engine.. Do you get bubbles out the top of radiator.. surge of coolant blowing out?

If you do, Compression pressure is is leaking into the coolant passages because there is a leak path. Since you stated the PO had an overheat issue.. it's likely he blew a head gasket, was pumping combustion gasses into the coolant, and blowing out coolant. Combustion gasses insert far more pressure and temperature into the cooling system than it is designed to handle..

So... remove heads, Soon as you do, you'll see either a blown gasket.. or perhaps head crack.

 

[MTKirk]

Yep, blown head gasket at least. Coolant doesn't show up in exhaust because the leak is pretty much one way (combustion chamber to coolant port). If you pressure test the coolant system (engine off) a head gasket will be a pretty slow leak or even hold at low pressure. A cracked head (or cavitation) will be a more severe, steady leak that will usually leak all the way to zero. http://www.oilburners.net/forums/sh...d-gasket-Cavitation-Something-else&highlight=

 

[chappie]

Should I pull engine or replace gaskets on chassis? It is a non-turbo IDI. Also, if I pull engine, does ZF5 transmission need to be removed before engine or left on? Thank you.

 

[IDIoit]

I would pull the engine.
makes it a lot easier to work on it.
I would also re-seal everything.

I would also pull the transfer case then trans.
leaving a manual trans in the truck and trying to install the engine is a royal pita.

 

[smithman]

I would pull the engine.
makes it a lot easier to work on it.
I would also re-seal everything.

I would also pull the transfer case then trans.
leaving a manual trans in the truck and trying to install the engine is a royal pita.

Yes. Pull TC off trans, pull trans off. Even with an engine leveler it's a tight fit to get the engine out - would be near impossible I think with the trans hanging off the back. Pulling heads off the engine while in the truck is difficult but even harder to get them back onto the dowels. Consider having the heads rebuilt with a valve job while out at a competent local machine shop. If you don't want to rebuild the heads, check your exhaust valve guides for wear.