Hey fellas, looking for some serious help here on a 97 Ford F150 4x4 4.6. While at idle the thermostat will read normal (blows cold air out the vents), start driving it the temp gage will get on HOT and then it will start blowing hot air out of vents for about 20 seconds then thermostat will come back down to normal and will start blowing cold air again. When the temp rises again it doesn't go as hot then comes back down and about the third or forth time it does that it stays at normal and blows hot air like normal. Replaced thermostat three times (ford motorcraft) and it continues, no water pump leak. Took it to the dealer and they said number 5 piston is losing 30 percent compression and exhaust is leaking into the water and causing a "burp". Hard to believe with no knock, no oil pressure loss or smoke (runs good) -- They said I need a new motor. I ran the RPMs up about 2500 in the driveway and watched and noticed the coolant in the overflow coming out of the cap when it got hot then stopped once the temp came down. Please help -- thanks john 660-563-1801

 

If your down 30% compression in one cylinder then your kinda screwed.......... Hopefully just a head gasket?

 

Had the same exact problem. Mechanic said i had a blown head gasket.They wanted $1200.00 to replace gasket. Had motor rebuilt for $1789.00. I picked up the motor today. I have 97 F150 4.6 it had 191000 miles on motor.

 

big fred -- thanks for the reply -- I have the same miles. Was yours doing exactly like mine? Mine is not smoking and still holding great oil pressure, was it #5 for you as well? It just seems to me its like the water pump or the radiator (hoping). So, when the rebuilt your motor did they say it was the head or the gasket? What bore did you get, 30 over and did they mic the crank? Are you putting it in or was 1700 for pulling it, rebuild and dropping it in?

thanks
missouriguy1010

 

I pulled the motor myself. I replaced the water pump first did not fix problem.Then took radiator and had it cleaned and new tanks. Still had problem. That is when I took it to a shop.They checked it out and said I had a blown head gasket. I had C M Engines rebuild it. They said blown head gasket and 1 bad rod. Do not know what they bored it. The shops around here all wanted $900 to $1200 to pull motor and reinstall.

 

Big Fred, they told me that exhaust is getting into the coolant through a cracked head or head gasket and causing the system to "burp". After the temp gets high a few times it stops and blows hot air and ever over heats again, but once you stop and cool the truck down it does it all over again. I just find it hard to believe when I still maintain great oil pressure and the truck runs strong, no smoke. I have babied that truck and it makes me so sick -- but its a keeper and a new motor isn't looking that bad anymore. Not sure about having it rebuilt or just buying a new motor -- thoughts?

thanks Fred

 

face it you have a blown head/head gasket.

you have "Good" oil pressure becuase the oil pressure guage is a fake and rarely does a blown head gasket affect oil pressure.

you have no smoke for two reasons: you have 4 cats burning off anything that comes their way and the exhaust pressure is keeping the coolant out of the oil. (for now)

all you have to do (Which I'm surprised the mechanic didn't do this) is put an exhaust gas analyzer probe in the coolant overflow tank while the engine is running if it sniffs anything well there ya go!

 

thanks for the replies

Should I just replace the head gasket and take a chance that it's not a cracked head -- or is it coming from the exhaust valve through the head into the water (just not sure where the water ports are on the head). I have like 200k on the truck and I think the lower end will go if I rebuild the heads -- its a beautiful truck and I'm going to keep it so maybe a new motor would be good. Should I go brand new from ford or rebuild?

 

My 98 ford 150 was doing just what you descibed , Heres my theory If you indead have a blown headgasket the water flow, plus combustion gas is to much for thermostat to open .I removed the thermostat broke off the trickler valve . approx 1/16 hole This allows any Blurping to simply pass thru back to radiator so you don't create an air packet around water pump. I know this isnt fixing the root cause but I have added another 500 miles to mine and it has'nt had any problem . Im gona run it until it blows completely then r/r motor.I hope this helps

SD

 

I would check the bone yards to see if I could get a lower mileage engine from a wreck - just a thought. Sometimes you can get lucky that way.

 

start this test with a cold engine and a full cooling system

Pinch off the hose that leads from the radiator to the coolant recovery bottle.
disconnect the hose at the coolant recovery bottle
stick the end of the hose in a 2 liter clear palstic bottle filled with water and un pinch the hose
start engine and observe the bottle for gas /air bubbles. any air in the systen should be purged quickly if the system is completely full
if the system has air in it it should get purged once the thermostat if fully open
if there are still bubbles coming out the hose 5-10 minutes after the thermostat is open you are leaking combustion gases into the cooling system- crack in a head or blown head gasket