I’ve read on here for quite a while, but rarely has a problem that stumped me for too long… that ended recently with a cooling problem that my truck is having.

I’ve got a 2000 F-250 Super Duty with the 5.4l. The truck is losing coolant, but only when driven… the system is pushing coolant out the reservoir cap and the hose that attaches to the bottom of the reservoir. The problem cannot (so far) be replicated in the driveway even after a short drive where it did force some coolant out (revving at 4k rpm’s for five to six minutes immediately afterwards had no effect).

The engine is not overheating, the gauge doesn’t move from normal even when driving.

I’ve tested for combustion gases from the radiator and the tests come back negative, and I really don’t want to do the heads if that isn’t the problem.

I live at about 5,000 feet and I have a new thermostat (195F OEM temperature), it appears that the system does not hold the 16 psi pressure that it’s supposed to (although it appears to be a small leak) is this enough to cause a significant loss of coolant while driving?

I think my next step is to replace the coolant reservoir as I believe I’m losing pressure from the threads under the cap.

Has anyone else had an experience like this? If so, do you know what the culprit ended up being?

My truck has a bit more than 263k on the clock, but has been pretty well maintained and hasn’t been worked too hard.

Thanks in advance.

 

How are you checking for combustion gases in the radiator?

With something like this:
https://shop.snapon.com/product/Leak-Testers/Combustion-Leak-Tester/GDCT16

Have you pressured up the radiator with a test kit like this:
https://www.autozone.com/test-scan-a...ter/449952_0_0

 

Yes, I’ve tested with similar kits… from O’Reily’s….

For the pressure the following (with the adapter set: https://www.oreillyauto.com/detail/c...kit/ren1/67085

For the combustion gases: https://www.oreillyauto.com/detail/c...gas+test&pos=0

I seem to lose pressure slowly from the system right now. I think part of this might be the coolant reservoir and will be replacing that in the next day or so as the test adapter has to be cranked down pretty hard (past how hard you can tighten the cap due to its having the click lock feature, I’m pretty sure) to stop leaking pretty badly.

I think if there is a head gasket issue, I should be getting at least a little gas into the system even at idle as long as the engine is hot, is that right? The pressure within the cylinders shouldn’t change depending on revs or power produced, right?

I see no evidence of coolant into the cylinders (white smoke) or the oil… at least so far.

Mt most recent test drives were of short duration… a quick drive up to about 45 mph on almost flat road for around 5 miles round trip lead to a little leakage (very little movement of level in the reservoir)… another trip (to the grocery store) of maybe 6 miles but one that climbed maybe a couple of hundred feet (at most) led to much more water being forced out of the system and a noticeable drop in coolant level within the reservoir. The return trip down hill was, again, very little water lost.

 

When you remove the oil filler cap, is there a white foam, milky inside of the cap ?

 

You can’t go by the coolant temp gauge. Once it starts moving it’s typically way too hot and often past the point of no return. The normal ‘gauge sitting’ point has a wide temperature range. Check for bubbles in the degas bottle while a friend throttles the engine.

— Dave

 

Yep, I was about to say, hopefully the original poster used another means of testing coolant temperature. Either with a scan tool or IR gun at the top rad hose.

 

This is replies to a few posts…

First, I have not seen any milky buildup under the oil filler cap, but I haven’t checked since early on in my work of trying to figure out what this problem is, so I’ll check again tonight.

Second, I didn’t know that the temperature gauge was that inaccurate, so I’ll see if my brother has a thermometer that we can use our pick one up.

As for the bubbles, I’m assuming you mean with the coolant reservoir cap off? If you meant with it closed, I have done so and didn’t see anything. This is also a little tricky because my truck has the downflow radiator (tanks are on the top and the bottom instead of the sides)… I think that might keep me from seeing any bubbles as they would escape from the radiator through the little hose that is above the water line of the coolant reservoir, wouldn’t they?

Thanks.

 

Sounds like the Ram V10 I had. The heads are notorious for cracking into an exhaust port and the coolant disappears without a trace basically. Look extremely closely for coolant residue in places like hose connections,loose clamps, that cap gasket. You might pull the plugs and look for one with a odd color and signs of coolant contamination. How's the heater core, have you pulled up the mats or carpet and checked for wet floors?

 

I checked again when I got home tonight and the oil filler cap doesn’t have any white foam… just oil.

That is one of the reasons that I’m hesitant to do the heads… besides the coolant being forced out of the reservoir (and, the best I can tell, all of it is coming from the reservoir… no signs of water anywhere else despite repeated checking), there is no evidence of a problem with the truck.

I’ve got an IR thermometer coming so I can check the temperature of the top hose… and a camera I can hook up under the hood to actually see the loss in action… it’s like Schroedingers leak… I know it exists and have seen the water in the engine compartment, but haven’t been able to actually see it happen.

I wish I had a dyno that I could run it on for a few minutes.

Thanks.