Unexplained Temp rise at 4k RPM

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karl.kittler@gmail.com - 10 Feb 2005 21:08 GMT

Hi, it's been long time since I've been in here.

I have a 90 miata, with a 92 engine (crankshaft failure) that I just
replaced the alternator and belt back in December. The temperature
would go up when ever I drove above 4000 rpm for more than a few
seconds. I assumed that the belt was slipping, so I tightened it around
a week ago. This was after checking the tension about a 1000 miles
after original installation and tightening it then. Later that day I'm
driving along and the belt snapped. I assumed that it was a defective
belt, becuase it seemed to stretch way too much. I limped my baby to
Sears (1 mi away?) where I bought a belt and installed it. (Sears
wanted way too much to install a belt, and I'm pretty well broke so I
couldn't afford to pay them if I thought it was reasonable right now
anyway.)
When I got home, i checked the tension, cause I did it in their parking
lot as fast as I could. Everything seemed fine.
So today, I'm driving to work (34 mi one way) and I have the same
problem happen again. Any thoughts?
This isn't the first time that I've installed a belt, but it is the
first time I've had a problem like this.
The odd thing is that this only seems to happen at highway speeds, it
does not happen if I drive 40 mph in second gear.

Thanks,
Karl & Maverik,
90 Red.

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Larry Gadbois - 10 Feb 2005 21:55 GMT

It is unusual for a MX-5 to overheat unless it is a very hot day, and you
are running the air conditioning.

Temperature increases at highway speeds can be caused by many different
things. The easiest is to change the belt and the thermostat. If a new
thermostat doesn't fix it you are looking at more money. You could have bad
plugs, wires, or coil pack. Pull the plugs and inspect them to see if any
are black or fouled. Maybe your fan is not working properly. If the water
pump has been changed you may have one with a plastic impeller. They get
damaged and need to be replaced. Are you running a high percentage of
antifreeze? If so, your mix should be about 50% water. Your radiator may be
plugged and need replacing. Was the head pulled when the replacement motor
was installed? The head gasket could be partially blocking the water
passages.

Good luck!

> Hi, it's been long time since I've been in here.
>
[quoted text clipped - 22 lines]
> Karl & Maverik,
> 90 Red.

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Lanny Chambers - 10 Feb 2005 23:12 GMT

> I have a 90 miata, with a 92 engine (crankshaft failure) that I just
> replaced the alternator and belt back in December. The temperature
> would go up when ever I drove above 4000 rpm for more than a few
> seconds. I assumed that the belt was slipping

I can't see where the alternator belt has anything to do with
overheating. It broke because you overtightened it.

Assuming you've already ruled out coolant loss, and your car is stock,
here are the likely causes of your overheating:

clogged radiator
stuck thermostat
bad thermosensor
bad fan motor

Signature

Lanny Chambers, St. Louis, USA
'94C
the alignment page:
http://www.hummingbirds.net/alignment.html

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karl.kittler@gmail.com - 21 Feb 2005 16:53 GMT

My guess was the temp rise was due to the alternator/water pump belt
slipping, that's why I tightened it.
Coolant is 50/50. The water pump was replaced when the engine was put
in, that was in 2002.
Clogged radiator/stuck thermostat should give me more trouble, not just
when I'm driving at highway speeds right?
My fan runs normal as far as I can tell.
Thermo sender should be an all the time thing, also right?

How should I go about testing these?

Lanny, glad to see you're still here.
Thanks.

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karl.kittler@gmail.com - 21 Feb 2005 18:33 GMT

As far as mods go:
95 open rear end (blew the old one in Apr. 2003, that was my best
option at the time) and 95 mazda bigger brake upgrade
Got rid of the stock air box, replaced it with a K&N filter for an RX-7
(planned to switch out the MAS but haven't yet.)
Added ammeter this last weekend, after the second alternator
inexplicably died, I wanted a little more electrical system scrutiny
than the idot light that didn't turn on until I was sitting on a DE
highway in the dark. First experience with dead alternator here:
http://www.miata.net/garage/alternator.html
So nothing that would encourage this problem.

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