I've got a 1991 4.9L EFI

I've heard the rule of thumb is that on a cold start with the radiator cap off, the coolant should stay put and not bubble or spray. Is there any exception to this rule? When starting the engine cold with the cap off coolant is spraying violently from the radiator...

If this is a sure sign of a combustion leak into the cooling system, is there any clever way to test where the leak is occurring without taking the engine apart? I had the head rebuilt and machined flat and I installed a new fel-pro head gasket about 35,000 miles ago. This repair was made after small amounts of coolant were visibly leaking from the head/block joint. I obviously did not fix the problem, and I don't wanna tear into the engine yet without more information if it can be had first. I can't be certain the machinist who worked on my head did a perfect job, so I don't want to jump to the conclusion that the block is warped or cracked.

I'm losing a cup or more of coolant from the plastic-to-aluminum seams of my radiator every time I start the truck, presumably from the excessive pressure in the coolant system, and this has gotten worse and worse in the last month/5,000 miles. Would it be worth replacing the radiator for now, just so I can drive it until I have the time and money to take the motor apart again? I put a pressure gauge on the radiator yesterday and the highest reading was 25psi, and the lowest about 12psi once the engine warmed up and I turned the heat on. Will these pressures just ruin a new radiator quickly, or do you think it'll hold up for a while? I need to drive about another 1,500 miles for a big move, and then I can park the truck and wait until I have the time and $ to do the repairs (or engine replacement, *gulp*). I guess I'm wondering if the radiator has always been under these pressures and is just now failing from the strain, or if the alleged combustion leak is getting worse in a hurry and causing an acute failure of the radiator.

Coolant and oil are always clean, and the truck never overheats.

Thanks ya'll.

-Paul

 

25 PSI is awful high... cooling systems are usually around 10 PSI but they can go up or down a few.

I know what it's like to NEED your truck and not have the time (or money as it was with me) to fix it right... been there, done that, got the t-shirt. So if it doesn't overheat, and you HAVE to drive it, then I'd just keep an eye on the oil and coolant condition and run it. If the radiator blows, get a cheapo lifetime warranty one from AutoZone and keep exchanging it til you can fix that engine up right.

Or, something else I've done is just leave the cap loose and run it so it won't pressurize. When you put your cap on you feel 2 detents right? The first one is the cap seating past the detent that keeps it from falling off, the second one is what seals it down. Just put the cap on and turn it to the first detent, then it won't build pressure. Don't know if in your case you might lose too much coolant (and it WILL evaporate/leak so keep an eye on the level... check it every morning) but it's worth a try.

 

25psi is that absolute or gauge?
Sounds about right if you are talking absolute pressure but it should be 10-15psi gauge pressure.

Absolute pressure would be the 10-15psi cap plus atmospheric pressure for a total of 25-30psi absolute pressure.

If you are talking gauge why would you cap it to test pressure of a sealed cooling system? The cap should regulated it to 10 or 15psi gauge depending on the cap that is on it.

 

How dix you test the pressure in the radiator? The cap is the pressure regulator so if you removed it and installed a gauge in its place it will build pressure until something pops.

 

Dixie 460 - I bought a cheapo lifetime warranty radiator from AutoZone today before I even read your post! It seems like a good plan for the interim. It's going in tomorrow. Got a new cap too.

Bashby - I was thinking this myself. I used one of those hand pump tools/gauges that you can pressurize a system with to find leaks. I connected it to the neck of the radiator in place of the cap, did not pump it at all, and just ran the truck for a while with it on there, trying different things like playing with the heater and throttle. The readings I noted in my original post just came off of this. I had a suspicion that this was the wrong way to take a reading though, because yeah, the system's pressure regulator, (the cap), is now gone and in its place is a fixed and permanent seal (the pump/gauge). So should I just throw away this information from this gauge then? Useless?

The question remains why coolant sprays out of my cold radiator with the cap off upon start-up. Could air in the system from my badly leaking radiator cause this?

Thanks for your input ya'll.

-Paul

 

Sounds like you do not have thermostat in the system.
Yes that was useless information.

 

Are you saying that while you had the pressure tester you did not pump it up to say 16psi, then let it sit there a while to see if it held? or if it leaked down? If found it was leaking down didn't look around attempt to locate the source of the leak, where the pressure the coolant was going?

Your belt routed the correct way around at the water pump?

Replace the radiator cap?

Tank seals will leak if pressure is too high, not necessary anything wrong with your radiator. Even under normal cooling system internal pressures/conditions those o ring seals can weep a little bit of coolant during wide ambient temperature swings, yet never leak again after had done so or least not until next temperature swing (usually first start right after steep and rapid drop in temp).

 

Subford - there is a thermostat. I put it in there myself when I did the head gasket many moons ago. It's not backwards either, because I remember putting it in backwards the first time and then cursing myself for having to take it apart again and re-do!

danr1 - Brand new radiator cap put on today. Brand new radiator going in tomorrow. I'm loosing coolant big time from my current radiator, it is obvious and I can see it. It is legitimately flowing from where the plastic sides meet the aluminum middle section. And yes, the belt is routed correctly.

Leaking coolant isn't really my worry, as that's an easy fix. What I was afraid of at first is that I'm leaking coolant because of an over-pressurized coolant system, from a leaking head gasket, cracked head, cracked cylinder sleeve, et. al. Since I have to throw away this presumption at the moment because I did the test wrong with the wrong tool, I have to hope that all I need is a new radiator and cap and I'll be good to go. I need these things anyways, so in they go.

I guess a cylinder compression check and testing the coolant for hydrocarbons are in order to really test for head gasket troubles?

I'll let ya'll know what happens tomorrow when I know more.

-Paul

 

My '94 with the 4.9 inline six sprays antifreeze when it's running with the cap off, too.

Mine has a 3/8" hose from one of the heater core lines back to the radiator to help get air out of the system. That's where the spray water is coming from.

 

Hey, mine has one of those too Holmesuser01! Is this really it?

 

Well, my radiator is not leaking from the tanks, and it stays full, and has the 16 pound radiator cap.

Mine is working air out of the system now, after I replaced the thermostat the other day.

I wonder if your pressure tester gauge just went high because the water was heating up?

How is your oil? Milky looking, and do you smell antifreeze in the exhaust fumes?

I would think that the moment you start the engine the water pressure would rise fast due to the combustion pressures if you have a bad head gasket.

 

Well, I suppose I'll know better after a few days of driving around with a brand new radiator and radiator cap and I get all the air out of the system.

The pressure tester gauge rose pretty slowly. In hindsight it makes sense it got so high and rose and fell and rose and fell the way it did in response to throttle or turning the heat on full blast.

My oil is always spotless, and the coolant too, and I think there is no coolant smell from tailpipe, though I can't say with certainty I would know what burning coolant smells like.