Has anyone experienced zero coolant pressure while driving?

What would cause a zero, or for that matter a negative pressure reading while driving?

This is under any driving conditions too. City or HWY. It doesn't make a difference?

Puzzled........

Eddy

 

A loose degas cap, a hidden leak, thermostat issue.

 

My guess would be a pinhole in the vapor space of your degas bottle.

or a bad gauge.

 

My first guess would be a bad coolant cap, followed by bad gauge.

 

Pressurize it and find the leak.

 

You can borrow the kit from o'reilly, just make sure you get both pieces.

 

That's a great idea, also Autozone, they carry OEM tools, much better than buying crap from Harbor Freight.

 

Howdy,

A little more info might help. What type of gage are you using, does it ever indicate above zero?

By "negative" pressure, do you mean less than zero on the gage or does the gage actually have negative numbers on it (or does it have a vacuum scale? 0-30 inches of mercury etc?)

What is the cooling system temp? (Scanguage II fWT ...or ECT)

Most 50/50 coolant operating at around 200 degrees F would produce a pressure of around 10 PSI

Since there's hotter spots in all engines engines, a radiator cap will always be rated higher for a 190F system (16psi for our 6.0 engines)...... and because there's hotter areas (than indicated) in any engine, the actual pressure will likely be somewhat higher between than the approx 10psi vapor pressure for a 50/50 ethylene glycol water mix

If you are driving around and your (verified) pressure gage is actually indicating zero (or less than zero for a poorly calibrated gage) you probably have a fairly significant leak in the system.

If not, you probably have a defective gage.

You can usually check by grabbing the upper radiator hose and squeezing it at normal operating temp (verified by a real temp gage....NOT the "idot" gage on the instrument panel)

If you feel pressure on the hose, the system is pressurized. If the hose collapses with little or no effort, you should be looking for a leak.

If the temp is making it to normal temp (190F or so) and you DO NOT have a puddle under the truck, you might be dangerously LOW on coolant.

In any case, there should also be a strong coolant smell when you raise the hood (or you're downwind of the truck) .....

Regards,


Rick

 

Sounds like I might have a significant leak. Got home from work the other night and squeezed the upper rad hose. Didn't have any pressure at all. If it's an internal leak. I'm not sure how I would find it. The truck doesn't leak any coolant. I have a way to pressurize the system.

This is the gauge I'm using.

 

Looks like we posted at the same time!

 

Repost with picture

 

Going to be installing a new degas bottle. Maybe this one is cracked. Bought back in March. Warranty on it is still good. So is the T-Stat. Got both today.

Got home today. Raised the hood and smelled coolants. Degas was covered. But, coolant level was just below the minimum mark. I was able to easily squeeze the rad hose. No pressure at all.

Temps varied between 193-202. EOT stayed between 197-199.

Going to run another pressure test tomorrow after installing the degas and cap.

 

I see this often in the winter. I pour the ice cold -5 degree coolant from the back of my truck into a generator with 80 degree coolant in the block and maybe 40 degree coolant in the radiator. Put on the cap and the upper hose collapses.

Now try that with a hot 190 degree engine with low coolant level. Apparently some radiator caps pop off at 14#, but must require 10# of vacuum before pulling from the overflow bottle. Or maybe they never pull from that bottle. Whatever it is, they certainly allow a vacuum to build.

It's no different than grabbing a gas jug you put in your trucks tool box in the summer, and now it's all collapsed because it's winter.

Generally my systems are designed to be filled when cool. Then run the gen and let it bleed out whatever excess coolant it has. Upper hose should be firm until it cools. You can feel 10# of pressure.

 

Looking for clarification. Vapor pressure is usually given in psia. I had a buddy at work run a bubble point curve for a 50/50 mix and it shows a vapor pressure of 10 psia around 200 F. Unless I'm interpreting this incorrectly, 10 psia vapor pressure won't be a significant component of cooling system pressure generation. I think our pressures are a thermal expansion effect. This would also explain why little pressure generation is seen after venting the cap at pressure.

 

But they're still due to vapor pressure of the 50/50 eg and the partial pressure of the volume of ordinary air present in the system. (which I so conveniently left out in my previous diatribe above!!!)

The temp of the solution at the temp sender (near the T-stat) is very likely close to 190F or so..... The temp at the hotter places in the head and block will be MUCH higher and will produce over all much higher vapor pressures.

SO for areas that may get up to 250F or more, the local vapor pressures (Steam pockets) could be as high as 23.3 psia (or more)

The actual pressure on the system in a case like that might be 23.3 -14.7 (standard day) = around 8-9 psig PLUS the partial pressure of the air trapped in the system (due to the Ideal Gas Law ) (the expansion you mentioned!)

I took the above numbers from the Dowtherm 4000 Engr and Operating Guide

And you're implications are right! simply placing a 50/50 mix in a pressure cooker and bringing it up to 190F wouldn't produce anything more than about 10 psia (which is less than ambient......as long as you had NO air or other gasses in the system)

You would have to know what volume of air is present and add the partial pressure of the final temp of that air to the partial pressure of the 50:50 mix of water/EG (vapor) THAT gives you the pressure of the entire system.

My wife won't ask me what time it is anymore........she's tired of me telling her how to build a clock....

And you (better) explained as "Expansion Effect"

Ok I'll stop!

You need to find the leak. You might be able to pressurize the system using compressed air. With the engine off, you should be able to hear air escaping someplace... If it produces NO pressure at all, it's not going to be a small leak

Regards,

Rick