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Discussion Starter #1
Greetings, ya'll.

I have a 2007 Mazda 3i that's giving me fits with the temp sensor and heater. I've owned the car since about 2012, and bought it from an individual dealer.

In May I had a massive coolant leak, lurked you fine people and was tipped to the water outlet. Sure enough, it was cracked and replacing it sorted out my leak.

In June I had to replace the coolant reservoir because it was cracked above the seam on the passenger side. Starting a few weeks ago, I've had my temp gauge spiking at odd intervals, my heater blows mostly cold, and my coolant reservoir levels will drop to low over a series of weeks. As soon as I top it off I'll get -some- heat at highway speed, but none when idling or coasting.

Two days ago I replaced the thermostat and housing. Since then my 15 minute commute with a top speed of 65 mph for 7 minutes has managed to peak the temperature gauge twice each way, if I start coasting when it gets high the gauge level will fall in under 30 seconds. The heater still works when accelerating or maintaining speed, dies off when I coast and is completely cold when at idle.

This morning when I got to work, after watching the gauge dance, the larger hose on the thermostat housing was stone cold. The larger hose on the water outlet (opposite end of the engine block) was hot to the touch. Coolant levels are perfect.

Help?
 

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Any sign of coolant on the oil dipstick, or oil in the coolant? Do you see any bubbles in the coolant tank? But regardless of those answers, I recommend a compression and leak down test, in order to either confirm or eliminate an internal engine issue.
 

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Greetings, ya'll.

I have a 2007 Mazda 3i that's giving me fits with the temp sensor and heater. I've owned the car since about 2012, and bought it from an individual dealer.

In May I had a massive coolant leak, lurked you fine people and was tipped to the water outlet. Sure enough, it was cracked and replacing it sorted out my leak.

In June I had to replace the coolant reservoir because it was cracked above the seam on the passenger side. Starting a few weeks ago, I've had my temp gauge spiking at odd intervals, my heater blows mostly cold, and my coolant reservoir levels will drop to low over a series of weeks. As soon as I top it off I'll get -some- heat at highway speed, but none when idling or coasting.

Two days ago I replaced the thermostat and housing. Since then my 15 minute commute with a top speed of 65 mph for 7 minutes has managed to peak the temperature gauge twice each way, if I start coasting when it gets high the gauge level will fall in under 30 seconds. The heater still works when accelerating or maintaining speed, dies off when I coast and is completely cold when at idle.

This morning when I got to work, after watching the gauge dance, the larger hose on the thermostat housing was stone cold. The larger hose on the water outlet (opposite end of the engine block) was hot to the touch. Coolant levels are perfect.

Help?
The question is - Is the thermostat/water pump working properly ?
 

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Discussion Starter #4
I have no evidence of oil/coolant mixing. Part of me wonders if I got a faulty thermostat or somehow I managed to get it sticky before installation.

I saw another thread suggest idling on an incline with the cap off the coolant to let potentially trapped air out, I’m trying that today.
 

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You'd think if it was a head gasket you'd see white steam out of the exhaust? I have a 3i as well (2004) and also just replaced my reservoir as it was cracked and venting steam (glad I happened to pop the hood one day after parking it) and performed a full flush (performed drain/fill/drive several times with distilled water to make sure old stuff diluted out). I didn't run into any issues with trapped air but the system is pressurized so it should vent the air out of the reservoir cap as you drive it? The fact your heater was intermittent tells me that your heater core wasn't getting proper circulation since that is what feeds the hot air; you may have a blockage (or air). Also, try an OBD II reading on the thermostat temperature if you think the gauge is bad. I drove with mine for a week to make sure the temperature leveled off.

Did you notice if your cooling fan ever came on?
 

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This week I was just having an overheating problem myself swapped out thermostat refilled coolant was still having the problem then my heat stopped working unless I was on the gas car would sit idel and stay at normal temp but when I was driving temp would still spike after thermostat instal, so I changed the coolant temp sensor still was giving me a problem i then had someone look at it for me and he told me only thing he found wrong with it was that I needed about a half gallon of coolant which I was almost positive it was full. So I am only mentioning my issues to you because it sounds like you have had you coolant definitely flushed and replaced are you sure you don't have an air bible and that your coolant is completely full if I was you I would unhook the coolant line to the throttle body as it is the highest point in the coolant system and then fill it up until coolant comes out that hose, and see if the problem still occurs because that's what seemed to do the trick with my system if that doesn't work a compression test if definitely needed.
 

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This week I was just having an overheating problem myself swapped out thermostat refilled coolant was still having the problem then my heat stopped working unless I was on the gas car would sit idel and stay at normal temp but when I was driving temp would still spike after thermostat instal, so I changed the coolant temp sensor still was giving me a problem i then had someone look at it for me and he told me only thing he found wrong with it was that I needed about a half gallon of coolant which I was almost positive it was full. So I am only mentioning my issues to you because it sounds like you have had you coolant definitely flushed and replaced are you sure you don't have an air bible and that your coolant is completely full if I was you I would unhook the coolant line to the throttle body as it is the highest point in the coolant system and then fill it up until coolant comes out that hose, and see if the problem still occurs because that's what seemed to do the trick with my system if that doesn't work a compression test if definitely needed.
+1. I'm betting the OP has an air pocket.
 
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