Unexplainable Coolant Loss 1991 22RE 4x4 Pickup
 

Hello All,
My truck is about to hit 100k miles on the odometer and I have owned since new. I usually do all routine maintenance myself and began to notice coolant loss at a rate of 1 cup every 200 miles. This has been going on for two years and I cannot find any external leaks or significant residue. There is never a drop of coolant on my garage floor. I had a block test performed last year with negative results. I finally took it to my trusted dealership and had them perform a pressure test on the coolant system which also passed. Dealership found minor indications of past leakage around water pump weep hole but nothing to indicate the amount of loss I am seeing. Also, I had the timing chain and guides inspected with borescope and those were in like new condition. I have examined the spark plugs and all look the same with the expected color.
The oil is like dark honey and non milky, but I do have minor mayonnaise like film on inside of oil cap and there is also a wisp of steam when I first remove oil cap when engine is at temperature. No exhaust steam after warmup. I am hoping someone here can tell me something else I should investigate. Thanks in advance.
Keith

Add a fluorescent dye to the cooling system then it should be more obvious where it's going.

Test, and replace if necessary the radiator cap

Have you run a hydrocarbon test? If not I would see if you can rent/borrow a tester from your local parts store. I found one on Amazon that seemed cheap enough.

Unexplainable Coolant Loss 1991 22RE 4x4 Pickup
 

Thanks for the replies, yes I have replaced the radiator cap and I have also performed the block(hydrocarbon) test.

I have not added the dye yet but will put that next on the list.

Slow coolant loss can be hard to find. I would start with replacing all hoses, the big 3 radiator and "U pipe" to water pump. (The one above the alternator has leaked on my truck, taking out the alts bearing.)
Then there are 3 coolant hoses under the intake plenum. They love to fail with the years.
Then there is a oring gasket from a pipe that goes around the back of the engine and connects to the intake plenum (underneath)
Also you have that radiator cap to overflow tank stuff, make sure they are ok (in the tank as well. Check the length of the one in the tank)

After that, probably the sign of a starting head gasket. Which is a fun weekend project, so don't worry.

Leaking intake gasket is possible.

If all the antifreeze is going out the exhaust, the O2 sensor is going to fail sooner or later.

https://www.yotatech.com/forums/f116...pickup-307177/

SomdayJ had a similar issue. Lots of pics in that thread.

#1 DON'T just start replacing stuff. you are more likely to create another small leak in doing so.
you need to find and fix the leak you have before replacing hoses that may be just fine, still at 100K miles you should be planning to replace them.
#2 1 cup every 200 miles is going to be difficult to find with either a pressure test (because most pressure testers slowly bleed off even on a good system) or a block test (needs to be exhaust fumes in the coolant/radiator to change the color of the test fluid and there won't be much of that).
What I would recommend is to put a pressure tester on the radiator with the engine not running, pump it up to 15 psi and leave it to sit for a while, come back every 1/2 hour or so and pump it back up to 15 psi.
after a couple of hours I would remove the test pump and see if you lost any level in the radiator.
Then look for puddles under the truck, signs of liquid running down the bottom of the water pump or around the intake.
Check your freeze plugs for corrosion or signs of leakage.
If you suspect a leak into the intake you could try opening the throttle body and sticking a piece of paper towel in one of those flexable grabber tools and run it down into the intake runners to do a "swab test".
then check the floorboard under the dash for wet carpet and signs of fluid dripping from the bottom of the heater box (leaking heater core).
Then pull all the spark plugs and see if any coolant leaked into the cylinders by hand cranking the engine and watching for fluid to come out of the spark plug holes.
if you still haven't found a leak you could drain the oil and see if you get any green or red coolant if that's what you are using when you first pull the plug.
Also keep in mind that it may leak worse or only leak when the engine is up to operating temperature, so some of the tests should be repeated with a hot motor.
good luck!
If you still haven't found a leak I would recommend that you continue monitoring it until the fluid loss gets bad enough to positively find the leak.

This?:
https://www.yotatech.com/forums/f116...82/index2.html