My 1990 3.0L has a little over 10k+ miles on a full engine rebuild. It runs great with no apparent problems. However, I'm having an issue with coolant disappearing. It hasn't done this since the rebuild, but over the last six months or so it's become an issue.

What I've observed is that I top off the overflow reservoir and then drive it for a week or so (probably ~120 miles/week); then when I check it again, without fail the overflow reservoir will be empty. But when I open the radiator, it is never below full, even after running the car for several weeks or multiple hundreds of miles. For example, I took it to Big Bend and back a couple of weekends ago; 500 miles one way with plenty of 80+ mph driving en-route. I only topped the overflow off once while I was in the park.

I've never seen the radiator anything short of full; in fact frequently when I go to open the cap it "pops" off as if it's still under pressure. And sometimes it's (over) full: i.e. all the way to the brim. The cap is brand new (AutoZone) and I even replaced it with another new one to make sure it wasn't the cause of the problem

I've put a pressure tester on the system and let it sit for several hours. It loses pressure at the rate of a couple of lbs/hour. But I'm not seeing it drip anywhere (skid plates were off). I know all of the places where it's likely to drip (heater hoses, TB hoses) and even getting my hand around there I don't feel it getting wet anywhere. It did have some leaks around some of the hose clamps, but I've closed all of those that I could find.

The only ideas I can come up with are that it's leaking in a place where I can't see or feel. I guess that would mean internal (e.g. head gasket) or in the valley under the intake and between the heads?

I also think that it's weird that it always seems to slurp every bit of coolant out of the overflow but then never seems to go down in the radiator. Does that make sense in anyone's experience? Any other suggestions?

thanks,
steve

My first trick would be to retorque the head bolts. Soon.

I had a similar issue with coolant going missing and was able to have it pressure tested. They injected some dye into the system and looked for it with a black light. They were are to trace it to a tiny pin hole in the one of the heater core hoses.

I recently went through something more extreme. As it turned out my Intake Manifold was not torqued on to specification. I fixed it easily once I figured that out. I'm definitely not saying that is what you are experiencing, but it may be worth a look.

Do you get puff of white smoke from your exhaust pipe? If so, that is an internal leak that is getting into the combustion chamber and ultimately makes its way out the exhaust. You may be able to smell it too.

Oh CRAP!!!! 1989 3.0L Overhaul Fail

as fasteddy said,,,retorq heads.
Did you use silicone at the ends od you manifold gaskets. I've had slow leaks there , just enough so you smell it but don't see it cause silicone sealant wasn't used.

I have had the same thing on mine.... for the last 10K I have re tq the heads no change and its never low.. I wonder if I have a leak in my hose that goes to the reservoir...

Maybe its a freeze plug on the back of the block. This would be extremely hard to detect. And you did have a rebuild. Maybe a bad seal on the freeze plug. If your loosing coolant from your head gaskets and retorquing them fix's the problem, i would highly consider redoing the head gaskets. If it is leaking coolant from there then the damage is already done. You wont get a very good amount of mileage out of that.

Thanks, Eddy. When you say to retorque, you mean to pop the valve covers and put a torque wrench on the bolts and then tighten them to spec (80 lbs, as I remember)? No backing out involved, I assume?

Sound like another fun Saturday in store. I'm getting pretty good at tearing the top off of my engine.

thanks,
steve

I would do it thus. Start at the center of the head and follow the bolt tightening sequence, and back off each bolt, then retorque to 80. Do not fully loosen, just take about a quarter turn and then retorque tight.

That's a good tip fasteddy. Hey stevec, if you are going to take the valve covers off, you will essentailly be at the same point as my picture from earlier. You may want to see if you have coolant in the intake as you see in my photos.

According to your post, it seems like it is a really slow leak, so you may or may not find anything. It wouldn't hurt to look though.