Manic/Depressive Miata running overly warm

I'm still in the throes of trying to get the Manic/Depressive Miata through inspection. It's currently in the Depressive "stock" condition which basically means stock computer, stock sensors, and stock fuel injectors, although admittedly the car is pretty rigged in this state. However, this issue started before the switchover from Manic phase.

The car has an aftermarket aluminum radiator and a coolant reroute kit on it. When I first bought it 3 or 4 years ago, the temp sensor was in the reroute fitting at the back of the head. However, as part of the switchover, I replaced the temp sensor and relocated it to the front blockoff plate.

Because of the reroute kit, the temperature would initially spike, and then drop to 185-190 as soon as the thermostat opened. After that it would hold temperature pretty well. However, after a year or two, it started showing spikes while just running normally, and the 'stat was taking longer to open initially. So I assumed the stat was bad, and so I replaced it last weekend while I was relocating the temp sensor. Although the old stat didn't have it done, I drilled out the flapper hole (about 5/32") to allow a little coolant flow from startup. I made sure this was oriented at the top of the housing when I reinstalled it in line. Refilled with about 1.5 gallons of 50/50 and ran it for 20 minutes with the cap loose, the heat on, and the front end of the car picked up about 8". Closed the cap and topped off the reservoir to the "hot" level.

Now, as I hoped, the temperature rises smoothly rather than having the initial spike, and stabilizes for a few minutes at 180-185 (185dF thermostat). However, as you drive the car, the temperature edges up to 200 or so, and stays around there. It may drift up to 203-204 while sitting at a stoplight, or down to 197 when you've been moving at an easy cruise, but it pretty much hovers around 200. It's fairly independent of what the car is doing, actually.

This is taken from reading the aftermarket gauge / sensor that's been installed, as well as using my scan tool to read real time data from the OEM sensor (right next to the aftermarket one in the blockoff plate). They agree within a couple degrees of each other.

I did the diagnostic jumper trick, and both fans come on when the accelerator is pressed. With the car running, the main fan comes on when the temp is in this 195-205 range, and it doesn't get hot enough to force the AC condensor fan to come on. I'm thinking of jumpering the feed to the AC fan so they both come on, but I'm also thinking I shouldn't have to; it should stay closer to the thermostat setting.

The car idles at about 8d ignition advance, and moves up to 27-28d during cruise. Is that too far advanced? I know my old '67 Poncho used to run warm until I realized the timing was too far ahead.

Sorry about the length. Is this normal? I was really hoping that the car would hold 185 and drift up to 195 occasionally. The ambient has only been in the 65-70dF range. I'm worried about an autocross in August when it's pushing 100 out on the grid. Any thoughts are appreciated.

Wait, are you saying that the timing is set to 8d advance at idle, with the diagnostic jumper set? I set mine between 12 (factory) and 14 (where I made the best power on the ol' arse-dyno).

No, this is without the diagnostic jumper set. That's just reading off my scan tool. But yes, it is advanced about 8d at rest and idles just over 1000 rpm. TPS is showing 10.5% open at idle. Under sustained 30-50% cruise throttle, the advance goes up to the 26-28d range.

Understand, the car is a very odd duck. It is supercharged and normally runs a standalone ECU, so the timing advance and idle as it sits now are actually set up for when the car is running with the full monty, wideband O2, supercharger, big injectors, etc. Right now all that stuff is off the car (except the blower, but the blower bypass is wired open).

Ah, gotcha, that throws all that right out the window :) Trying to pass an inspection, I take it? :)

It really sounds to me like the water pump is going, though.. Any idea how old it is?

Just for dumb curiosity, have you tried running with no thermostat?

WonkoTheSane wrote: Ah, gotcha, that throws all that right out the window :) Trying to pass an inspection, I take it? :) It really sounds to me like the water pump is going, though.. Any idea how old it is? Just for dumb curiosity, have you tried running with no thermostat?

1.) You bet. If only the PO had started with a '95, I'd only have to worry about coaxing it through a sniffer test. As a '96, it needs to pass the plug-in.

2.) That was my next thought. Well, after the inspection, it needs to go back under the wrench and be put back into the Manic phase. Guess I'll try a new water pump. It could probably use a new set of accessory belts anyway.

3.) I have not. Because of the blower location and the reroute kit, the thermostat housing is buried under a crap ton of parts to be removed. It's the better part of an hour's work to get down to it. So I just replaced it and put it back in, assuming that would fix the problem. And it did fix the spiking problem, but not the steady-state problem.

Thanks for the input!

Ah, cursed sniffer... Is it possible to just slap a big cat on it next time and glide through?

Remember that the water pump is behind the timing belt, so might as well do that, too, if you don't know how long it's been..

You're welcome!

Well, it might pass the sniffer, even in manic mode. At least if I drove carefully there and back, I could probably set the Link to a fuel table that would pass.

But the Link ECU isn't OBD-II compliant so it definitely will not pass the plug-in test, which is required for '96-up cars, without any age limit. And the stock ECU just rolls its eyes all funny and goes bluh-bluh-bluh-bluh-bluh-bluh if you plug it into the manic configuration. You can see the CEL from orbit.

All the OBD-II discussion is now tabled, because I no longer have to pass the plug-in test due to special dispensation. YAY!

However, since we have Keith's attention at the moment, I'd still like input on the original question. I'm about to put the car back in Manic mode and I need to know if I should replace the water pump. Thanks!

Which temp sensor is in the blockoff plate? ECU or gauge?

Maybe I'm mistaken, but around 200F on a Miata with a 185F thermostat doesn't sound too off to me unless you're running a massive capacity radiator?

Keith Tanner wrote: Which temp sensor is in the blockoff plate? ECU or gauge?

Both. The OE temp sensor was giving me intermittent P0115 errors. With all the plumbing it was a PITA to get to. So I bought a brand new coolant temp sensor over the Mazda parts counter, bunged it into the blockoff plate at the front of the head, and extended the connection. The aftermarket temp sensor is bunged into the same plate, about 1/2" away.

BoxheadTim wrote: Maybe I'm mistaken, but around 200F on a Miata with a 185F thermostat doesn't sound too off to me unless you're running a massive capacity radiator?

It's got an unknown-brand aftermarket aluminum radiator, but it's not massive. Pretty much a same-size replacement.

Sounds as if you're just reading stable temps at a different location than at the thermostat.

So stable temps at +20dF over the thermostat (admittedly at the hot end of the head) are not unexpected? I would have figured that under easy highway cruise, there would be enough circulation that the temperature would settle right about the thermostat set point.

Is a consistent and stable 200-205dF a comfortable coolant temperature for the Manic Miata? I don't want to drive it in anger if that's too hot.

Thanks for the input.

Easy enough to tell, move one of those sensors back to the thermostat. I never measure the temp at a stagnant point like that.

OK, thanks!

Are you using the M Tuned bypass kit? If so, make sure the thermostat has pilot hole(s) drilled into it for flow. If you have no pilot flow it will behave just as you described. They shipped kits with undrilled thermos a few years back.

The OEM T-stat had pilot flow in the original configuration/location. Now there's less DP with the turbo in line (low flow = laggy data to the ecu). You should consider moving your temp sensors back to the correct location with that bypass kit in place.

Yes, it's an M-tuned kit. When I replaced the stat, I drilled a 5/32"-3/16" pilot hole in it, and made sure it was in the 12:00 position when I installed the housing.

DP = ???

Differential pressure. The restriction of the turbo orifice lowers flow through the old t-stat bowl.