excessive coolant usage-- '98, 3.8.

I have done a search for this, but I'm not coming up with much. Our '98 Windstar, 3.8, has recently started going through about a gallon of coolant every 4 weeks or so. What are all the probable causes of this? I'm assuming a leak would be one obvious problem that would cause this, but I'm wondering if it could also be a blown head gasket...? We have detected no coolant leakage in the driveway -- don't know if that's helpful or not. Also, I don't know if this is related or not in any way, but there is a very significant knocking/banging/rattle kind of noise when the van rocks from side to side as when backing out of the driveway and dropping over the elevated bottom of the driveway or maneuvering the van with the steering wheel side to side. Probably not related, but it's so significant, that I thought I'd throw it out on the table anyway, just in case. It almost sounds like a baseball rolling around in a tin can, banging on the sides, when the van is being turned or maneuvered from side to side. I have no recollection though of having dropped anything in previous repairs that wasn't retrieved so this is a mystery noise -- related to the coolant issue or not?

Oh, one more thing: No lights are on on the dashboard. These are just things we've noticed on our own.

The 2 most likely causes of coolant leakage on your '98 3.8L that are not out of the engine are the front cover gasket, aka timing cover gasket and the lower intake manifold gaskets.
The head gasket is not a high failure item on your year.

Both of these leaks will cause serious damage if not corrected.....in the form of destroying the bearings inside the engine.
If you have one of these 2 leaks, most likely oil is getting into the oil.....which will destroy bearings long before you can detect it by any of the "old school" eyeball methods.
I have used Blackstone Labs for testing my oil...... www.blackstone-labs.com
Their website will provide all the information you will need to take a sample and mail it in.

Lower intake manifold gaskets are easy to misdiagnose as a head gasket failure as they leak coolant into the intake.....which is drawn into the cylinder(s).
This is a common failure.....on a number of vehicles...
I have pictures posted of what is involved in the lower intake manifold gasket job.

The timing cover gasket is a much more involved job than the lower intake manifold gasket repair.

Wiswind, thank you SO MUCH for your continued excellent advice. You are helping so many people via this site and we all really appreciate it. We are awaiting the testing kit from blackstone labs to send out oil in for analysis. In the meantime, codes P1537 and P1538 (IMRC banks 1 and 2 stuck open) popped on for a while today. Interestingly enough, my husband filled the coolant back up again today and then the lights went back off. We've had these codes come off and on in the past, and my husband has been under the hood for long periods of time trying to hunt down the potential vacuum leak, but he just can't find anything. He's tried all the tricks mentioned on this site for finding the leak. I read a post in another thread by busboy and he had said that he has used a "hand vacuum pump" to help troubleshoot where a vacuum leak is. Can anyone give me a brief rundown on how to use one of these things to help track down the source of my codes? Also, what do these codes tell me in regard to the coolant issue? Are the codes and the excessive coolant issue related? We are a family of six and we are paranoid that we are about to ruin the engine while we wait on the results from the lab to get in!! Please HELP!!!

The hand pump is very helpful for finding vacuum leaks ... you merely hook the pump up to a suspected leaker and pump up a vacuum (shown on the included gauge) ... if the vacuum "holds" with no further pumping: no leak has been found.

The (two) vacuum IMRC actuators on the front of the engine are prone to be vacuum leakers. I would check them early on. Ford discontinued the vacuum actuators in '99.

I suspect the vacuum leak and the high coolant usage are not related ... but never say "never"! Is it possible that someone hooked up a small vacuum line to the coolant system in error?

The 2 IMRC actuators are fed off of the same vaccum line, so 1 leaking actuator can rob both of the needed vaccum......
If you need to replace one or both, I would get Motorcraft ones, as several people have reported issues with the aftermarket brand ones.

The only connection between the IMRC issue and coolant leakage would be a buildup of junk in the lower intake manifold....that could cause the runners (butterflies) to hang up.
You can see this in my pictures......the IMR butterflies next to the cylinders that the lower intake manifold gasket leakage was near had much more of a buildup on them.