I have a 1997 f350 7.5l (w/ 109k original miles) W/ a coolant burning issue only on cold starts and a never ending P0174 & P0171 CEL. I’ve done 6 leak down tests that have shown that none of the intake or head gaskets are leaking and I don’t see any signs of a cracked head/block. I have noticed a weeping freeze plug on the passenger side of the block, but doesn’t drip unless running. Additionally, I have heard a hissing sound that lasts 5-10 seconds after the engine has been shut off - but have no vacuum leaks. The coolant burning only happens when the engine is dead cold and goes away once the engine warms up (5-6 min after a cold start). Because of this it seems to me that the metal is expanding once the engine has been running for a bit sealing the leak - hence why my leak down and coolant pressure tests have come back with flying colors (I also don’t see any evidence of coolant in the crank or oil fill cap). Also with the CEL, I have taken a smoke machine to the whole vacuum system and haven't found any leaks, along with replacing the MAF, TPS, and fuel pressure regulator (all standard ignition brand parts) with no avail to the issue. I have seen that my ltft on both banks run about 25% and my stft on both banks run about 7-9%. Readings from my MAF read 7.5g/s at idle and double when rpm’s double so it is reading correctly. My O2 sensors also stay between the .1v-.9v. With the MAF connected, my engine will misfire when revved in park, but when the MAF is disconnected, the mis goes away. And my fuel pressure at idle is at 32 psi on both tanks. And finally, the icing on the cake, I am now dealing with a slight overheating issue when the truck is slowing down from highway speeds. While driving at highway speeds, the temp holds steady, and before highway speeds, it holds steady. Only when I come off the highway does the truck start to overheat. I took the truck to another shop to have the cooling system looked at - it got a new water pump, clutch fan, thermostat and rad cap (brands of parts unknown). I am at the point of pulling my hair out trying to figure this out… any help would be greatly appreciated

 

Are you having to add coolant?

It needs a cooling system pressure test covering overnight at the least. The pressure tester should be air operated to maintain a constant pressure on the system. If you come back and it has lost correct pressure, and there are no visible signs of coolant loss, you would use a boroscope to check for coolant in all the cylinders.

You pressure checked the radiator cap?

 

I had to burp the system when I got it back but I have not needed to add any coolant since. What would I need to do that test overnight?

 

And the rad cap pressure tested fine at I think 12 psi - that may be the wrong number but I do remember that it held at the proper pressure

 

What does your fuel pressure do when youre accelerating up an on-ramp at full power? Does it hold 40psi?
Testing your fuel pressure in your driveway only tells 1/2 the story.

 

If you’re not losing coolant and have no visible leaks, why do you suspect a coolant burning issue at cold start? Excess condensation from the exhaust pipe?

Does the hissing sound come from the area of the leaking freeze plug?

 

i suspect the hissing sound is from the airconditioner system. My truck does that too.

 

@prototypemech

I’ll have to run that test after I get a longer line for my mech fuel pressure gauge



@90project5.0

Only only a dead cold start, I get white smoke that’s sweet billowing out for 5-6 min, and I have drips coming out the exhaust that smell like coolant when I dip my fingers in it

Also no the hissing isn’t coming from the freeze plug, it’s near the ac dryer - I have heard that ac systems can hiss but I wasn’t sure if it was a bad intake gasket leaking, but hearing that prototype’s ac does the same makes sense



Also update, I finally got a code reader w/ live data and am seeing my upstream o2 sensors fluctuating from .1v-.9v like they’re supposed to, but I noticed that my downstream o2 sensor fluctuating sort like a upstream every 7-10 sec. It mostly reads at .055v (which should be very lean) then will abruptly shoot to .7v-.8v for a half a sec then slowly come back down to .055v - I believe this means a bad cat but in my mind doesn’t make sense as to why I’m getting a lean code since by my understanding if the downstream o2 is reading lean, the ecu should make the truck run rich. Also another note I’ve noticed (for the overheating issue) is on the live data, my ECT reads 185f-190f but my laser thermometer is reading the upper rad hose, block, and the physical temp senders at 195f-210f. Again my logic to make sense of this is the ecu thinks the engine is cold (@185f) so it leans itself to warm it but is tripped by the downstream o2 reading very lean (.055v). I checked the the stored codes again and only have P0171 and P0174

 

If you’re getting the smoke at start up, it sounds like a head gasket to me. Other option would be intake gasket leaking between coolant passage and runner. Sounds like it’s just starting which is good!

My AC can do the hissing as well, especially on a hot day.

Its possible that the steam from the burning coolant could be throwing off the cat or O2 sensor.

 

****, yea I never thought of that since the coolant burn is only on cold start… I’m going to do a cat delete anyway and get a dummy o2 sensor since when I bought the truck, half of the egr crap was already deleted

 

Just get a good aftermarket universal cat like a magnaflow. Being that you have a system running pre and post cat O2s, it would be more beneficial to have it operating correctly! Delete the EGR crap, but you’ll be happier with the O2s working. Even if it’s for the fuel economy. That’s my two cents, I don’t push global warming either..

 

No.1 - Do Not Worry About O2 DTC Until Cooling System Is Corrected

No.2 - Replace Leaking Core Plug - Check Others - You Have A Corrosion Problem

When first started and the exhaust clouds while standing @ the outlet pipe, do you smell coolant?


Vacuum Tester and Refilling Kit - MityVac 4533

 

Sorry, I got banned somehow - no reason implied so here’s a new account…
yes, when I stand in front of the exhaust on start up, it smells sweet and the drips coming out smell like coolant - it does this for about 5-6 min then it goes away

 

My opinion, it’s either head cracked, head gasket or intake gasket (least likely, but possible).
Welcome back haha

 

Do you think that this issue is causing a lean code on both banks? That doesn’t seem to make sense - I would think that it should only be lean on one bank for the leaking head/intake gasket