Miata A/C and Cooling issues with questions

[balactm]

Ya'll...I'm confused.

Yesterday I recharged the a/c in my car (feels great!). Then I proceeded to have some overheating issues ALL day. At speed, at idle, everywhere. on a clock-type guage, normally the needle is just to the left of 12, it went to about 12:30/1 position at most. I had to go over 2 mountains on my way home (rte 211). I overheated on first one, pulled off, and I had no coolant leaking anywhere. I did notice quite a bit of clear liquid coming from around the heater core area, and then noticed that the a/c lines were condensing like crazy. Which is weird, because the a/c switch hadn't been on in over an hour. I had noticed some water dripping the other day, but didn't take much note of it, but now it got me thinking...Is my a/c on despite my switch being off? So I unplugged the a/c comp from the harness, started the car, and then plugged it in with the switch off again, and boom, a/c comp kicks in. Also, both fans are running....So, a couple questions:

Would a constantly running (not cycling, just ON) a/c comp cause the car to overheat? I know these have relatively weak cooling systems, but I feel like the cooling system should be able to keep up with the a/c. but what about if the a/c is constantly running?

Why is the a/c comp on, when the switch is off? Relay? interior switch? A/c clutch or comp?

Moreover, now the car feels completely different. Like it has a few extra hp (makes sense, since the a/c is DEFINITELY off), idles quieter, runs smoother, doesn't stall sometimes coming to a stop.

I also noted this morning that the heater seems to work differently. Up until this morning, if I had the heat on anything other than full heat, the upper vents would blow colder while the feet vents were blowing hot. Now, there is much more variation in the heat level on the upper vents.

I believe the a/c has been stuck on since I bought it, but it hasn't been a problem since it's been cooler. Suprisingly, there is very little on a/c's being stuck ON in miatas, usually they are stuck off apparently. Has anyone had experience with this type of issue?

[Scott]

Out of curiosity, what Miata's have relatively weak cooling system's?   I've owned at least one of every year, 90-04's, daily driven them, raced them for 15 years and never had an issue unless something was wrong in the cooling system.  If the needle ever moves from where it normally lives, you have a problem.  


With that in mind, going over a mountain with the AC on will not cause it to over heat, if anything having the second fan on should help.  The first issue you need to figure out is what is wrong with your cooling system, then diagnose the AC staying on.  

What is your coolant level with the car is cool?


Also sounds like you have a clogged heater core.

[Senor_Taylor]

I agree with Scott. Unless there is some weird edge case that changes my understanding of the AC system, I think these issues are unrelated.

Perhaps running the heat was bringing enough air through the heater core (combined with that air being cooled from the AC) that it kept it from overheating until you switched it away?

I'd start simple with new coolant and confirm there aren't leaks/the water pump is spinning appropriately. Perhaps your belt isn't tight enough or you goobered something up when you were fiddling with the AC?

[balactm]

Scott, I don't have any first hand knowledge of the weak cooling system.Just something I've read/heard.

Upon further diagnosis, I discovered a tiny pin hole on the bottom side of one of the heater hoses; replaced both, cooling system is now operating normally. I had done the water pump, timing belt, cam seals, thermostat, and all radiator hoses a couple months ago; it was leaking around the timing cover, and the water pump pulley was slinging coolant all over the engine bay.

The a/c system is still acting weird. Definitely stuck in the on position regardless of what the a/c button is doing. I've got the compressor unplugged (from the single conductor plug near the driver side headlight) to keep it from constantly running. Even with it unplugged there though, it's still go the secondary fan on, leading me to believe that the a/c switch itself is malfunctioning. Maybe stuck closed, instead of stuck open?

[balactm]

Some More interesting data:

I've been working in charlottesville recently and am making the dreaded drive over afton mountain during rush hour.

On Monday of last week, I reconnected my ac system at the single conductor plug closest to the compressor. It blew so cold and so nice. On my way home, I left it plugged in, and sure enough about 1/2 up afton, the temp guage crept up over the 12 oclock position. I quickly changed from cold to heat, full blast on the defrost, and windows down to vent the extra heat. Never did it again the whole ride home, despite turning temp selector back to cold.

On Tuesday, very similar conditions. ~70-80F with HIGH humidity, this time with the compressor unplugged. I downshifted, I accelerated from 45-80 (typical afton bs), COULD NOT GET THE CAR TO OVERHEAT.

I've now gone to the effort of removing the "ad fan" fuse from the fuse panel in the engine bay to prevent the ac fan from being on. I'm waiting for a couple hot days to see if only having the cooling fan (instead of both the ac and cooling fan - still stuck on this problem) willl cause the car to overheat. I'm also seeing the cooling fan on even with the engine cold. Only while the engine is running though. With the key in the "on" position, no fans on. I've done the procedure to test the fan by grounding the tfa and gnd terminals in the diagnostics box, it works. The other test, where the engine is running, and remove the plug, and ground to chassis won't work, because my fan is always on, even unplugged. Faulty switch or ground fault in wiring somewhere upstream?

I'm beginning to think that maybe the previous owners weren't as honest as I originally thought. Have I mentioned that I found out the engine has been replaced?? Wonder wtf else has been meddled with...

[Senor_Taylor]

Everyone "clean" Miata ends up being a shit show for me. Hidden rust, weird botch jobs to cobble things together.

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[Evan]

Have you checked the timing? Are you getting any pinging?

Are you sure your timing belt isnt off a tooth? It would probably run like crap if it was, but its worth checking.

( I know its not AC related, but even if the AC was stuck on, the car shouldnt overheat)

[Senor_Taylor]

Have you checked the timing? Are you getting any pinging?

Are you sure your timing belt isnt off a tooth? It would probably run like crap if it was, but its worth checking.

( I know its not AC related, but even if the AC was stuck on, the car shouldnt overheat)

It's very much possible it's off timing and still feels alright. I had my cars all turned around and it felt better at the top end than stock. That'd be a good thing to check and would offer a chance to get his hands on all of the plugs and accessories and make sure they're plugged correctly.

If the engine has been replaced, maybe check to see if all of the cooling lines are ran in the correct orientation?

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[balactm]

Have you checked the timing?   Are you getting any pinging?

Are you sure your timing belt isnt off a tooth?   It would probably run like crap if it was,  but its worth checking.

( I know its not AC related,  but even if the AC was stuck on,  the car shouldnt overheat)

It's very much possible it's off timing and still feels alright. I had my cars all turned around and it felt better at the top end than stock. That'd be a good thing to check and would offer a chance to get his hands on all of the plugs and accessories and make sure they're plugged correctly.

If the engine has been replaced, maybe check to see if all of the cooling lines are ran in the correct orientation?

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I followed the service manual getting the engine back in time, spun the engine a couple times to confirm everything lined up again. First time on a miata, but definitely not my first rodeo with a timing belt. There are no unplugged plugs. Car feels the same before and after timing belt. And Taylor, I was anticipating needing to change fluids, fiilters, hoses, etc; the car was only driven 12k miles in the last 12 years. But this seems to be more of a wiring gremlin type thing. God I hope they didn't feed this thing after midnight...

[balactm]

Well another update:

The car started overheating again on the same stretch of 64 yesterday on my way home, no a/c on. It happened when I got stuck behind a truck, and as soon as the truck moved over, the temp dropped immediately.

Clogged Radiator fins? Time for a new one? that's the only part of the cooling system I haven't replaced.

[Senor_Taylor]

It's a possibility somebody at some point ran stop leak through the cooling system or used non distilled water and it's clogged. If you're on the original radiator, it's time to replace it anyway.

You don't have anything in the mouth of the grill right? Some people have little mesh screens there or a license plate and it blocks air flow.

It could also be a sign that your cooking system is just barely getting the job done, so I'm not 100% sold on it just being the radiator.

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[Evan]

Do the fans work? If you're behind a truck and the temps climb the fan should turn on.

Failed thermostat in half open state restricting flow?

I hate to throw parts at problems, but if the radiator is old then put a new one in, along with a new thermostat.

Make sure to burp/bleed the coolant really well too. Miatas are more stubborn with that than some cars in my experience.

[ScottyB]

it may not be much help, but doing coolant flushes with this Lisle coolant funnel tool were extremely successful for me and allows you to get a very solid "burp" out of the system. $20 on amazon and i'll never be without one. might be worth doing it this way if you plan to yank the thermostat and have to refill the system anyway.

[Senor_Taylor]

I forget, where do you live?

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[balactm]

Do the fans work?   If you're behind a truck and the temps climb the fan should turn on.

Failed thermostat in half open state restricting flow?  

I hate to throw parts at problems,  but if the radiator is old then put a new one in, along with a new thermostat.

Make sure to burp/bleed the coolant really well too.  Miatas are more stubborn with that than some cars in my experience.

The fan is a weird story. It's always running, even when the car is dead cold from sitting overnight. But it does run. I've done several of the "thermoswitch tests" that I've read about on some of the miata forums, and based off the results the thermoswitch is good.

I had to replace the thermostat shortly after buying the car, and when I did the water pump, timing belt, etc, I also replaced the thermostat again. I have no idea how old the radiator is. With the low mileage on the car, just about everything looks "new-ish." 

Taylor, I live in Harrisonburg. I have my license plate mounted in the stock location, the stock air dam under the car is still in place to direct air through the radiator, and there's nothing physically blocking the radiator. At this point, the radiator and thermoswitch are the only 2 components of the cooling system outside of the engine that I haven't replaced. Should I be using a less than 50/50 coolant to water ratio? Maybe the coolant is holding too much heat? I've read about people using a 60/40, or even 70/30 split depending on where they live. I figured with how mild virginia weather was that 50/50 would be fine...

It seems to me that if the problem is at speed, then it would be indicative of the radiator not being able to pull enough heat away from the coolant while its flowing through the system. And under load (going up afton mtn) adds just enough extra heat that the radiator can't keep up.

I'm going to do the radiator this weekend since I'm also simultaneously trying to figure out why my crank is so wobbly (thanks Scott for the replacement pulley set!). To car's future is at stake pending the results of this weekend's work. If the crank is messed up (instead of just tired drive pulley), it means a rebuild on my daily driver, which is....less than ideal.

[Senor_Taylor]

Wobbly crank? Is this a short nose car? Oh no.

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[JPolen01]

Wobbly crank? Is this a short nose car? Oh no.

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That was mostly 1990 cars but I think some of the 91s did have the issue as well.

[Senor_Taylor]

Wobbly crank? Is this a short nose car? Oh no.

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That was mostly 1990 cars but I think some of the 91s did have the issue as well.

I'm aware. I don't know what year his car is.

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[JPolen01]

His is a 91. I could have made that more clear. I had to find his project thread to figure it out.

[Senor_Taylor]

Other Taylor, you're aware of the keyway issues, right?

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