So, I've been following the Hustler's thread closely, but unfortunately, either none of the suggestions worked or they weren't applicable. I've been searching like hell on Google, nothin', and since the search here is ------- retarded, well, time to post.

I define overheating as the coolant needle going between the 12 and the 1, or hitting the 1. Normal operation has always been 11 o'clock on the coolant gauge through two engines, 3 reroutes, and the standard coolant route.

What I've done:

New (Well, junkyard) engine, new hoses, new everything except rad, rad cap, overflow hose, overflow tank, and water pump.
I just finished Coolant reroute #2, and am basically copying Brainy at this point with a few minor changes. I replaced all hoses, a new thermostat (Some Valucraft large single-hole @180F instead of the double-hole I had before), and put all new gaskets/etc. on.
Coolant Reroute #1 also suffered from the headaches with the overflow tank however.

What it's doing:
The coolant in the overflow tank disappears after I run it once for any prolonged period of time, leaving only maybe an inch or two of coolant in the bottom. However, it *never completely empties* as would be expected, there's always at least a little bit of coolant in the tank.
Car is overheating at highway speeds if I don't turn on my A/C fan, *but nowhere else*. It runs perfectly fine untill I go cruising at highway speeds for 15+ minutes, and if I turn on my A/C fan, it doesn't overheat anymore.

What I've tried:
Everything in the Hustler's thread and most stuff in prominent overheating threads in this forum.
Replacing everything in the coolant reroute.

My theories:
Waterpump (But no weeping that I've found....)
Headgasket
Radiator Cap
Crack in the overflow container
Radiator

Any suggestions? I don't want to start replacing everything on my list at random, especially not the headgasket. Debating just dumping a ---- ton of dye in it, running it, and taking a UV light to the sucker.

 

Check the actual temperature.
Do you need to keep filling the overflow tank?

 

If I discount my paranoia? No, there's no need to fill the overflow tank that I've noticed, it's just ridiculously low after operation.

 

Does you has a "magic funnel" to dump all air? Do you haave a belly pan? Have you blown out the trash in the condensor with high-pressure water? Have you touched a male? How hot is it and what is your anti-freeze/water ratio? Are you naked?

Step one is to get a coolant pressure tester and see if there is a leak or pressure drop.
Step two is a leak-down test
Step three is to deploy magical funnel of paramount success
Step four is to check fans
Step five is to put temp-strips on stuff to see if it's overheating

 

I use a different technique than that - it's something you have to learn as a rotary owner, as the high point of the system *is not the radiator* - but it works equally as well.

No. Belly pans don't affect rotary operation temperatures, I had assumed they operated similarly with the Miata since the radiator design and shrouding is damn near identical to the RX7 - although I have mangled the stock shrouding by cutting holes in it for intercooler piping.

I don't have a condensor. I've removed the A/C and wired it so that if I push the A/C button, it turns on the A/C fan.

Not lately, but I'm certain we can fix that!

100F-ish, and only 50/50 right now. I had water wetter and diluting the antifreeze down to 70/30 on my list, but I was uncertain if I was simply trying to put lipstick on a pig and hide a much worse underlying condition in the car.

Steps 3-5 are basically done (5 will be done ASAP [Tonight, if I can swing it even], that's an amazing idea there Hustler). Coolant Pressure Tester, I suppose I can hack one up, although I don't think I *can* get a leak-down test done here, I'd have to order in a kit for it.

 

First of all, the belly pan is required. Period.
Reduce the anti-freeze percentage, add some dye to help track the leak.

 

Interesting. I think I may take this opportunity to data log temperatures back-to-back on this, since I have to drive to get to where I have the belly pan stored anyways. I don't doubt you when you say that mind you, but I wonder what the difference is in a Miata? If it's 10F or 20F, I'll ---- my pants in shock.

Will do.

 

I do not have a belly pan (yet), and I have never overheated. It hit mid 90s around here last week, and it never crept up much higher than normal.

 

"Cleaning" the air under the car will always reduce pressure behind the bumper and heat exchangers, and therefor increase flow through the heat excahngers.

If you haven't ducted the front of the radiator to the mouth, now is the time to order some plastic and do it. I'll save you some effort; if I had to do this all over again I'd build a frame around the radiator, frame around the mouth of the bumper, then use a combination of plastic and rubber, edged in aluminum 90* strap and flat aluminum, to seal everything. My turbo car has never been over 200*f in it's life, even when "racing" a Panoz in 103* heat at HHR. You also probably want to make a new bally pan that is mushroom shaped and attaches from the bumper "chin" to the steering-rack area.

 

What radiator are you running on your turbo car?

I'm not ducting a ------- 20+ year old stock rad. I'll be picking up a new rad if I'm going to duct.

 

When you say belly pan, you are talking about the plastic tray under the engine compartment, yes? Also, got any pics of this duct you're trying to explain?

 

Mind...blown...

Tonight, I checked the coolant overflow after taking it for a spirited drive to get it Nice and Hot. Remember, I have taken it for several drives since I even touched the coolant last.

The coolant overflow was filled a little bit above the Full mark, which should be standard operating procedure of a mazda of that year for being full of hot coolant.

If what I think is going to happen happens...I'm going to have almost no coolant tomorrow again. If I refill it at all, it's just going to burp excess coolant, than leave me with the same almost-empty coolant overflow when cold. If it heats up, I'll be at almost full again.

What. The. ----. I'm pretty sure our coolant overflows aren't supposed to be *almost empty* when the car is cold, but what I'm seeing on the coolant overflow right now indicates I was overfilling the overflow. I don't know anymore. It makes no sense to leave a car with almost no reserve capacity however! I mean, the car shouldn't go from an almost empty overflow to a full overflow from just heating up the car to operating capacity, right? If it is supposed to do that, I've never seen it on *any* car I've owned or worked on!

 

What are your fans like, for ***** and giggles?

Are they wired together or just stock, which should be the main fan at 180*ish and the AC fan whenever the AC is on.

 

Stock setup, one fan is controlled, one is on whenever I push in the A/C button.

 

Checked the coolant overflow right before I went to bed.

Expected behavior happened - barely any coolant is left in the overflow. So it seems that the coolant level is somewhat static, it's just that it...goes to ridiculously low and high levels within the overflow tank I guess?

 

Are these symptoms of a bad radiator cap?

 

I believe you have a slow HG leak.

Do you have an unusually high capacity radiator?

If you have an unusually high water capacity, then simple heat expansion might fill the overflow tank, but I have extreme doubts about that possibility. Air expands at a significantly higher rate than water.

Got a go-pro? Take the hood off and mount it in the engine bay looking at the coolant tank. Take the car to full boost and see if you get air bubbles in the overflow tank.

A slow HG Leak might push out a pretty good amount of water over time while not being nearly as obvious as a slightly warped head. When the car cools, the air that is left in the system contracts enough to pull all of that water back into the system. The next time you start the car, that air, which now resides mostly in the radiator, is expelled first as the car heats up, which is why "air in the system" is normally self-curing, but if you have new air being introduced into the system, you'll continue to experience the problem.

 

Check the rotation of your main radiator fan. Another member had a similar problem years ago and found that when his main fan started at highway speed it was stopping the flow of air through the radiator.

 

Both should go on when you push the A/C button.

sorry, I am late to this thread.

 

I have a vintage Radiator Barn double-pass.

ducting is not radiator specific. It doesn't attach to the radiator, it attaches to the car. Go ahead and build the ducting now, the stock radiator is fine for any street car, even the turbo ones. I only replaced mine because it was plastic and I didn't want to burst the radiator with all my Dallas to Houston driving.