It has now been sitting in the driveway idling for about half an hour. Gauge at 11:00, fan is coming on every so often. In other words, I can't get it to overheat under normal overheating conditions.
Did'nt they ever teach you not to second guess yourself in school?
The temperature sending unit is made up of two dis-similar metals that create their own voltage by way of, for lack of a better term controled corrosion. The heat applied increases the amount of voltage by the two metal speeding up the 'corrosion process. IOW Hook up the positive to the place on the sending unit and the negative on the ground(unless it is a two wire sending unit) it will probably peek out around 5 volts.
05 MX-5 Mazdaspeed #1024 Titanium Gray Mica
It has now been sitting in the driveway idling for about half an hour. Gauge at 11:00, fan is coming on every so often. In other words, I can't get it to overheat under normal overheating conditions.
From a similar experience with my BMW, I'd say the problem is in the gauge cluster. The solder joints on the pins that plugged into the back of the gauge would get corroded and crack, causing some funkiness in the contact at that point. The gauge would whip back and forth, then settle down... or go dead... whatever it felt like. I could sometimes whack the dash and get it straightened out... or not. I just learned to live with it after replacing the whole cluster twice (junkyard bits, of course).
Sounds like you don't have an overheating problem... and maybe the previous owner never had an overheating problem. I've heard some pretty goofy diagnoses from those that don't know what they're talking about...
'94 Black & Black & Tan
'99 head swap, JR header, TDR intake & header blanket, MegaSquirt, RB hollow bar, Tein Flex, 15x8 6ULs, HD M2 Sport, FM cat, Borla cat-back, black '95M interior, MOMO Zebrano, IL Motorsport console...
Dyno Days
8/16/08 (bone stock): 103.1 hp/99.0 lb-ft - Dynojet
8/23/08 (Borla cat-back): 108.2 hp/104.1 lb-ft - Dynojet
8/13/11 (more stuff...): 126 hp/116 lb-ft - Mustang dyno
Roger Moore: the Danny White of James Bonds
Wish I had an extra gauge to test that theory. It's unnerving to try to drive with the needle pegged past H.
'94 Black & Black & Tan
'99 head swap, JR header, TDR intake & header blanket, MegaSquirt, RB hollow bar, Tein Flex, 15x8 6ULs, HD M2 Sport, FM cat, Borla cat-back, black '95M interior, MOMO Zebrano, IL Motorsport console...
Dyno Days
8/16/08 (bone stock): 103.1 hp/99.0 lb-ft - Dynojet
8/23/08 (Borla cat-back): 108.2 hp/104.1 lb-ft - Dynojet
8/13/11 (more stuff...): 126 hp/116 lb-ft - Mustang dyno
Roger Moore: the Danny White of James Bonds
Okay time to approach this from another angle since voltmeter test was inconclusive. Would one of those infrared thermometers aimed at the engine give an accurate temperature of what the coolant temp is? Or maybe a meat thermometer sitting on top of the manifold while driving?
I guess you could train an IR thermometer on the upper hose or thermostat housing and get a reading. You might not get an accurate temp reading (but you could probably extrapolate one). At any rate, if you had a cooling problem, I think you should be able to see any wild fluctuations in temp. If you didn't see any huge changes, you could probably rule out a cooling problem...
At least it makes sense to me...
'94 Black & Black & Tan
'99 head swap, JR header, TDR intake & header blanket, MegaSquirt, RB hollow bar, Tein Flex, 15x8 6ULs, HD M2 Sport, FM cat, Borla cat-back, black '95M interior, MOMO Zebrano, IL Motorsport console...
Dyno Days
8/16/08 (bone stock): 103.1 hp/99.0 lb-ft - Dynojet
8/23/08 (Borla cat-back): 108.2 hp/104.1 lb-ft - Dynojet
8/13/11 (more stuff...): 126 hp/116 lb-ft - Mustang dyno
Roger Moore: the Danny White of James Bonds
Problem solved!? At least I'm about 90% sure. I had taken the instrument cluster out and probed voltmeter into the lead for the temp gauge, but didn't really get anywhere that way. So I went to the 99 cent store and bought a 99 cent meat thermometer. Then I wedged it in between the head and exhaust manifold. Drove the 2.5 miles to the nearest highway and checked temp at 175 degrees. Then I got on it up and down 121 several times, but the needle just stayed at 11:00! It just wouldn't budge, and I was doggin' it. Temp still at 175.
I think it was the gauge, or connections thereof, and it looks like taking things apart and putting it back fixed it. Trickyrix was probably right, most likely never had an overheating problem. Attempts by previous owners showed they had tried all the usual right things, all new hoses, the temp sender unit had been fiddled with, and there's a crack in the plastic across the instrument panel where I'm guessing somebody was trying to get to the gauge, or was banging on it till it cracked. I'm not sure but it looks to be a fairly new radiator as well. Kind of like a puzzle where everything falls into place.
That means you've got $800 worth of new parts and labor already done for ya!
Hope that was the only problem... if so, you're a lucky dog and got a hell of a deal! BTW, those instrument hoods are about $100-120 - I'd JB Weld the sucker and paint it!
*Of course, I had to embellish my role in this...
'94 Black & Black & Tan
'99 head swap, JR header, TDR intake & header blanket, MegaSquirt, RB hollow bar, Tein Flex, 15x8 6ULs, HD M2 Sport, FM cat, Borla cat-back, black '95M interior, MOMO Zebrano, IL Motorsport console...
Dyno Days
8/16/08 (bone stock): 103.1 hp/99.0 lb-ft - Dynojet
8/23/08 (Borla cat-back): 108.2 hp/104.1 lb-ft - Dynojet
8/13/11 (more stuff...): 126 hp/116 lb-ft - Mustang dyno
Roger Moore: the Danny White of James Bonds