Mazda3 Forums banner
1 - 11 of 11 Posts

·
Registered
Joined
·
30 Posts
Discussion Starter · #1 ·
Now that the weather is warming up I am having some puzzling issues with my '08 Mazda 3 2.0L.

It started as a suspected AC issue. After 10 - 15 minutes of city driving and at idle my AC is short cycling to the point of blowing warm air. However, on open interstate about about 50 mph the AC works fine. I suspected low refrigerant but a set of AC manifold gauges showed normal pressures. That suggests fan, but the fan is operating while the AC is on. I also wondered if a pressure switch went bad.

However, I then noticed that the very low sensitivity coolant heat gauge on the cluster sometimes ticks up to maybe the 2/3 position from it's normal 1/2 mark. That's never happened before to my memory. I threw on an OBD2 scanner and drove around in slow traffic on a very hot day and saw temps as high as 238 within 5 - 10 minutes (and the AC started to short cycle). It wasn't enough to throw a CEL but--that isn't a normal temp for this car so far as I can tell. Testing on the interstate got it down to 220 and the AC came back on. The coolant hose to the radiator was extremely hot after the test drive but the return hose via the thermostat was only ambient temp.

It seems like the thermostat is broken in the closed position, but I'm not certain of that because I thought this car's thermostat normally fails in the open position (the thermostat is OEM and the car has about 140K miles) and the AC cutout is strange. Does anyone have experience with a similar issue? Does it sound plausible that the ECU keeps cutting power to the AC compressor because of the heat or that excessive engine bay heat means the condenser isn't working right? The compressor isn't seized up, the belt turns freely, and at idle with the AC on (before it starts short cycling) idle is a normal 750 rpms. All thoughts welcome!
 

·
Registered
Joined
·
30 Posts
Discussion Starter · #3 ·
I don't have an IR gun but I have felt the hoses and there is a massive temp difference between them when warmed up. There is also no flow activity visible in the expansion reservoir (although the level did rise). A test yesterday got temps to 240 in 10 minutes of moderate street driving (with no AC use), which I believe is just under the level of throwing a CEL.
 

·
Registered
Joined
·
387 Posts
The 3 likely suspects that come to mind are T-stat stuck closed, broken water pump, and radiator/hose blockage. If the T-stat was in a friendly location, I'd just swap it out because it's such an easy and very inexpensive thing to do. However, if the location of the T-stat on your 2008 is like my daughter's 2011, then it's probably not a fast and easy bit of work. I haven't needed to mess around with it yet, but it MIGHT be possible to remove and reinstall the T-stat housing without first doing all of the stuff the FSM says to do, however I'm not sure at all about that.

So if the T-stat removal doesn't appear to be an easy thing to try on your vehicle, then you might want to first check for air flow through the radiator, after draining the coolant. Attach compressed air hose or maybe even a shop vac hose to one of the (detached) big rad hoses, and see if there's decent air flow at the other detached hose. This test should take very little time to produce a result, one way or the other.

If the air flow through the rad is ok, then next up would probably be removing the T-stat. However, definitely do a hot water test first, instead of just replacing it and reassembling, because you don't want to have to drain it again if the problem isn't the T-stat. And if the T-stat does surprisingly test ok, then that leaves the water pump as the final suspect, and IMO you might as well just replace it while the coolant is out.
 

·
Registered
Joined
·
30 Posts
Discussion Starter · #5 ·
Thanks for the radiator flow test idea--I would hate to go through the trouble of pulling and replacing the thermostat just to find it's something else. A you suggest, the thermostat is in a tough location--most people end up removing the belt tensioner for better access.
 

·
Registered
Joined
·
420 Posts
Also you might want to stop driving it until this is fixed, otherwise your next post might be titled "Does anybody near me want to help me swap out my overheated engine?"
 

·
Registered
Joined
·
387 Posts
... A you suggest, the thermostat is in a tough location--most people end up removing the belt tensioner for better access.
Yes, that's listed as a part of the procedure for my daughter's 2011 3i as well. While I was in the process of doing another job on that vehicle a few years ago, I spent a few minutes poking around at the T-stat housing, trying to evaluate the possibility of replacing the T-stat by going directly at it without removing anything else.

I remember being able to able get a socket attached to each housing bolt using the right combination of extensions, which allowed good access to remove all of them. And it also appeared to me that there would be just enough swing room and clearance to be able to get the T-stat and gasket out and back in as well. And if the T-stat, gasket, and housing are able to be set back in correctly, then with a helper holding the housing in place by pushing on the rad hose that attached to it, there should be no problem reinstalling the housing bolts, including torquing them with a torque wrench.

Now I'm in no way saying that I'm sure what I wrote above can be successfully done, only that I think it might have a reasonable chance to work. And one good aspect about this idea is that I don't think there's a risk of anything bad happening, except for losing a bit of time if it doesn't work. Not saying that you should try it - just something for you to consider.
 

·
Registered
Joined
·
420 Posts
When working on cars, it is almost always easier and faster to just go ahead and remove objects that are obstructing access to the part needing replacement, than it is to try to wrestle around awkwardly trying to reach fasteners and jimmy parts through very restricted spaces.
 

·
Registered
Joined
·
30 Posts
Discussion Starter · #9 ·
I'm going to tackle the thermostat replacement today. I'll let you guys know if that solves my overheating and AC woes. I have seen only a couple cases in these forums of the Mazda 3 thermostat getting stuck closed, so it'll be another data point for others if that turns out to be the case.
 

·
Registered
Joined
·
30 Posts
Discussion Starter · #10 ·
Well, the replacement thermostat seems to have fixed both the overheating and the associated AC problem. I'll push it harder tomorrow in the heat, but meanwhile the boiling test of the old thermostat and the newly correct engine temps seem conclusive that the old part was stuck closed.
 
1 - 11 of 11 Posts
Top