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Old 03-02-2010, 12:22 AM   #1
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Default 160* Thermostat on '94

Installed 160* thermostat. After installing, I am idling at 2000 rpm even after the car warms up (5-10 minutes). I checked vacuum leaks, jumpered and checked idle screw, and rechecked that the 'stat was installed in the right direction. Is this simply because the Idle Speed Control Motor senses lower coolant temps due to the thermostat opening earlier? Car ran perfect prior to the change from 180* to 160* thermostat (180* is stock on the '94 correct?). Do I need to make any adjustments to accomidate the lower temp thermostat?
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Old 03-02-2010, 12:24 AM   #2
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I know this doesn't help, but it didn't effect mine.
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Old 03-02-2010, 12:25 AM   #3
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I had mixed answers in the past, what's the factory 180, 190, 195 ?
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Old 03-02-2010, 01:01 AM   #4
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service manual for the 01 says initial opening at 188-193.

why would you run a cooler thermostat? it wont change how hot the car runs...
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Old 03-02-2010, 12:09 PM   #5
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Quote:
Originally Posted by cymx5 View Post
Is this simply because the Idle Speed Control Motor senses lower coolant temps due to the thermostat opening earlier?
Is the engine in fact running cooler?

Actually, it's probably irrelevant. 2,000 RPM is much higher than the ISC system should ever reasonably target, even on a cold day on Neptune. And even if you are running at 160F (which seems unlikely), that's pretty much the end of the warmup cycle anyway.

I'd go ahead and re-attach the vacuum hose that you disconnected and forgot about during the procedure.

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Originally Posted by cymx5 View Post
I had mixed answers in the past, what's the factory 180, 190, 195 ?
According to the '92 FSM (the '94 should be more or less identical):



So, bearing in mind that the OEM thermostat actually has two valves, the smaller of the valves should begin to open between 182-188F, the larger valve should begin to open at between 188-193F, and both valves should be fully open by 212F.

For purposes of comparison, this is roughly equivalent to an aftermarket thermostat with a rating of 195F.
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Old 03-02-2010, 08:47 PM   #6
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Thanks Joe. I did a double check on the vacuum hoses visually, but I can go back and check my vacuum/boost gauge with the engine running (didn't do that the first time). My best bet will probably be to hit up the dealership on my way back to the garage this weekend to pick up a factory replacement....and be more weary of some of the advice I get.
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Old 03-02-2010, 08:49 PM   #7
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$20.95 after tax with a gasket. just picked one up today.
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Old 03-02-2010, 08:59 PM   #8
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$16.76 with 20% off club discount!
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Old 03-05-2010, 03:38 PM   #9
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Something still isn't right. Went back to factory thermostat. The idle remains at 2000 rpm constant with or without the diagnostic box jumpered TEN & GRD after the car is warmed up.

The work I performed prior to this issue was just the thermostat replacement and grounding the LC-1 to the engine ground next to the idle air control motor. I don't think the ground would cause this, but maybe a wire popped loose for the idle air motor while I was fixing the ground???
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Old 03-05-2010, 03:51 PM   #10
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wait did you think changing the thermostat would fix your vacuum leak?
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Old 03-05-2010, 09:51 PM   #11
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Joe, This would not be a proper thermostat discussion without a 'thermostat comparison.'




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Old 03-05-2010, 10:45 PM   #12
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Quote:
Originally Posted by Braineack View Post
wait did you think changing the thermostat would fix your vacuum leak?
I never had a vacuum leak. My boost gauge validates that. Car ran perfect before changing the thermostat. Throttle body sticking open maybe? I have no idea at this point. If the idle air control valve fails, does it remain open? Is 2000rpm realistic if it did fail?
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Old 03-06-2010, 09:22 AM   #13
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Quote:
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I never had a vacuum leak. My boost gauge validates that.
With a vacuum leak your motor will still pull the maximum vacuum possible for it's air supply and VE...thus the reason the revs climb with a leak. Your boost gauge could still show 25in hg vacuum, but that doesn't mean you don't have a BB-sized hole sucking air in somewhere. My gauge did wobble a bit more when I had a leak though...

While I understand your reasoning behind the thermostat being the only variable you changed, it is a variable which has minimal to no effect on your idle speed. As pointed out earlier, even 160* F is outside of warm-up range where the ECU plays with idle and enrichments, and even at that your 160* F thermostat isn't keeping the motor at that temp, but simply staying open at any temp above that.

Since this is the only manner in which your single supposed variable could possibly effect idle, and it's now out of the way, it's time to reassess the situation. Also, the fact that you reversed the modification, without change in idle speed, further supports the likelihood that there is a seperate, unrelated problem causing your high idle.

That problem is most likely a hole, somewhere in your intake tract at or after the throttle body, which your ECU is not in control of.
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Old 03-06-2010, 01:24 PM   #14
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Thanks for the help. I had a doubt at Braineack's response considering the car has been in the garage and not driven for a week prior and after changing the thermostat (hence my confusion over the issue). I'll get some Snoop and go to town on the vacuum hoses and see what I find.
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Old 03-06-2010, 03:49 PM   #15
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think about what happens when you crank the throttle, you're opening a valve allow the engnie to pull more air and thus allow it to increase RPMs. It cannot increase RPMs without increasing the airflow it ingests.

so you have a vacuum leak or you unplugged the idle valve when you fiddled around.
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Old 03-06-2010, 04:10 PM   #16
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reposting the **** out of this, holy lol
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Old 03-07-2010, 03:10 PM   #17
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Ok it is time to tap the brains of the gurus. I got out to the car today in the garage. Vacuum proved leakless, but I have a bigger problem I imagine:

I check the timing and the tick marks were no where in sight. Adjusting the CAS, the idle came back down around 1000rpm and I was able to see that the tick marks were 150-180* out of phase. Is this the sign of a slipped toof on the timing belt?
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Old 03-07-2010, 03:44 PM   #18
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Quote:
Originally Posted by 94mx5red View Post
Joe, This would not be a proper thermostat discussion without a 'thermostat comparison.'
Ha. I think that may be the most fun tutorial video I've ever made.


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Originally Posted by cymx5 View Post
I check the timing and the tick marks were no where in sight. Adjusting the CAS, the idle came back down around 1000rpm and I was able to see that the tick marks were 150-180* out of phase. Is this the sign of a slipped toof on the timing belt?
Something seems very wrong with the test procedure here. If the timing was in fact 150-180 out, then engine simply would not be running. Perhaps you've clipped the timing gun onto the #2 or #3 plug wires?

It's also possible that the crank pulley itself has failed. The pulley is made in two halves separated by a rubber coupling, and I have heard (though never witnessed firsthand) that it's possible for the outer section, which is where the timing marks are, to rotate with respect to the inner section.

Remove the #1 spark plug, and also the oil dipstick. Insert the dipstick into the plug hole, and rotate the crankshaft clockwise by hand while observing the relative positions of the dipstick and the crank marks. The dipstick should reach the apex of its movement just as the timing mark comes to align with the 0 reference. If it does not, the crank pulley is bad. If it does, you're measuring something wrong.

It's entirely possible that the timing belt may have jumped a tooth or two, however that's not what is causing the timing marks on the pulley to appear 150-180 out under inspection with the timing light.


I'm also not sure that any of this is relevant to your idle speed problem. You're starting to change too many variables all at once here.
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Old 03-07-2010, 07:21 PM   #19
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The way my tests comes out on my posts is a lot less clear than the way I'm actually going about it. You're right about the timing. I had the wrong wire accidentally. Got the right wire, and timing is fine. I need to have a cup of coffee and stop thinking for a while. My brain hurts right now.

Going to borrow a multimeter later this week so that I can test the idle air motor.
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Old 03-13-2010, 06:13 PM   #20
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I figured it out. The car's got a hotside MP62, so the throttle body is mounted relatively close to the brake lines against the firewall. Apparently something shifted and caused a very subtle interference between one of the lines and the throttle body. It was not shutting completely.

During the week, I removed the throttle body, cleaned it, and reset the TPS. I also checked the resistance on the idle air control motor; it was good. While reinstalling, I found the interference with the brake line. Temporarily, I have the brake line tweaked out of the way. It will be replaced by a brake proportioning valve, so the temporary fix will do.

Restarted the car and got the idle back to normal.
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