Pressure in cooling system

[datlo]

I started out with a leak in the radiator of my 91 4.0 ranger. Replaced it only to have that one blow before I realized the the system was building up to much pressure, could hear a faint percolating sound that you could also feel when holding the upper rad. hose. I was told that a crack head was most likely the cause.
I have another motor out of a 94 explorer and my question would be, can I just put the heads off of it onto the 91 motor. or would I need to strip the 94 down to a long block and swap the whole thing. I don't have the harness or computer for the 94 so I would need to eliminate a few sensors that the 91 doesn't have like the cam sensor.
Any help would be great.

Thanks.

 

[iammattchew]

Hmmm... I would be interested to find out what caused it. My AC line was close to 100%, but I'm trying to figure out how it could be higher than atmosphere without a leak...

 

[BrooklynBay]

Welcome to this forum! How many pounds does it say on your radiator cap?

 

[datlo]

I bought a 7# cap for it. didn't want to go through another radiator.

 

[glfredrick]

If your system is showing all those signs of pressure, you might want to check your head gaskets! Leaking head gaskets or a cracked head can pressurize the cooling system and start blowing/overheating stuff.

Quick test is to run the engine with the radiator cap off. If it is bubbling and foaming like crazy, I'd suspect head problems. You should see some flow, but it should not be blowing out of the fill hole.

Lowering the PSI of the system will cause overheating. The pressure allows the anti-freeze/water solution (you are using a solution, right? Pure water corrodes the system, and overheats, too much anti-freeze also overheats -- 50/50% mix is best unless you live in a REAL cold area) to operate at a higher temprature than boiling without boiling. The pressure increases the boiling point.

 

[datlo]

No overheating problems so far. I was aware that the boiling point would be lower and plan to put the original cap back on once my ploblem is solved. With the cap off there isn't any foaming inside, pressure build-up is more of a gradual thing. Just trying to find out if the heads I have are interchangeable before I tear anything apart.

 

[Maniak]

. Just trying to find out if the heads I have are interchangeable before I tear anything apart.

The 91 4.0 heads from a ranger have a 90TM casting heads.. the 94 explorer has 93TM casting heads..

YES, they are interchangable.. the 93TM's are "supposed" to be less prone to cracking and are more desirable when your buying "rebuilt" heads. You could even have one 90TM head and one 93TM head on the same motor with no ill affects.

~Mark


EDIT: Good reading on what went into what years 4.0's.. Dead Link Removed

 

[datlo]

Thanks for the help. Lots of good info on the link.

 

[Maniak]

Just some extra info..

We were hearing a percolating noise on our X for a while too (for a few minutes after turning the engine off)..

In our case we were loosing pressure. I did a pressure test and found that the heater bypass valve was leaking. Without the pressure the boiling point drops and the hot coolant actually boils as it looses pressure. We found the leak by bypassing hoses and found once we blocked off the heater hoses the pressure drop went away.

Another thing.. You should not be able to build up pressure if your radiator cap is working properly.. If the pressure goes up the cap should release some. We run a 16 psi cap on our '92 and have done so for many years. The extra 3psi (most people run a 13psi cap) keeps the coolant from boiling for another 9 degrees. Each 1 psi increase in pressure will increase boiling point 3 degrees. That is something we need for living/wheeling in southern Arizona.

Just some more info before you tear apart the motor.. maybe it will help...

~Mark

 

[Cobraguy]

Take your car to a shop with an exhaust analyzer and have them probe your radiator. If it's a head gasket, it will have CO, etc in the cooling system that the analyzer can pick up.

You can also have them do a leak down test and check. However, that doesn't always work on cracked heads as the engine may need to be hot and operating for the crack to show and leak.