mx5cartalk.com

Hey legends,

Mikey here again - the muppet that always seems to cause himself problems Speaking of which I've got a serious one! One I've been dealing with for far too long and has me borderline about to throw in the towel which I'm sure no one here would love me for

I. CANNOT. Stop. This damn car. From overheating!!!

This is going to be big write up so I'll try to summarize as much as possible but please ANY help at all, ANY ideas at all I'll take. I simply cannot figure this one out; neither can 3 mechanics, neither can my mechanically inclined friends and family.


=== CAR HISTORY AND SYMPTOMS ===
Owned the car for about 2 years. Started overheating around February this year. It won't overheat if it's running from cold and not dríven. As soon as it's dríven at any speed for a minute or more and then comes into stop-start traffic or just sits at idle again, it WILL overheat. It will overheat faster if I'm moving through stop-start traffic. It will cool down if I'm cruising in 4th and 5th gears going 70km/h+ and it will also cool down if I'm going downhill doing 50km/h+ and sitting in 3rd or higher. I'll 95% of the time have the heater and fan on max, A/C off, and this will slow the overheating down but not stop it.

I've been driving it almost daily since then and I naturally try to minimize how often it gets hot but most days it will get to 12 or 1 o'clock or more. On bad days, sh*t traffic and the like, it'll get up to 2o'clock and right up to the H. Not good. I was concerned I've ruined my head gasket and softened my head but we've run a CO2 test, read on.

SO! What's been done since this started happening?

Attempt No. 1 : First thing I did was flush and change the coolant. No effect.
Attempt No. 2 : I then assumed it was the radiator since it was rather old and I went to town. This was March/April likely. It's now got an oversize Mishimoto aluminium radiator with the matching slim fan shroud. All wired correctly and working. I replaced the thermostat, flushed it again and put new coolant in it. I replaced all of the hoses except 2. First was the one of the heater hoses at the back of the engine - too hard. Second one was one going out the back of the throttle body and under the intake manifold - again too hard.
Attempt No. 3 : I noticed the car was leaking coolant very very minutely and so I tightened all the clamps and drove it for another month or so. Wasn't convinced that I'd done it 'properly' so I dropped the coolant, took the hoses off, sanded and smoothed the lips of the steel pipes to ensure a proper seal/seat and put it all back together, flushed it, burped it for an hour and still NOTHING. No more leaks though so that was grand. This was October.
Attempt No. 4 : This was completed last night. I met a mechanic just down the road from me 2 weeks ago and started chatting to him. Top bloke and genuinely cares about doing things once and doing them right. He offered me his shop, his hoist, help and supervision. He ran some diagnostics and some proper man hours simply trying to figure it out with me. He also had a auto-electrician/cooling expert with him on the day. Checked the head gasket doing a CO2 test. Came back all clear. He discovered that one heater hose was rock hard and hot, and the other was cool and not under pressure. Originally he said it was a blocked heater core. Moving on we took the cap off the radiator and left the car running. They noticed there was no flow in the coolant whilst idling but while revving there was. He also said there was 'tiny metal shavings' in the coolant. Something similar to seal-up. Diagnostic: screwed water pump. SO! Over the next three days with his help I changed; the water pump, timing belt, idler pully, tensioner pully. Dropped and changed the coolant. Checked the rocker cover for leaks, nothing. Put a new gasket in and resealed it. Checked all the hoses again. Basically gave the entire engine bay a checkover and clean in addition to a major service. Kicked it over and drove it to work this morning... AND I F**KING OVERHEATED AGAIN.

Someone please just put me out of my misery

Last few important things that I think are unrelated but worth mentioning. In the process of Attempt No.4 we've broken the thermostat temperature sensor. We tried gluing it with a 2-stage silica solution but it's definitely gone so I have to replace it. I know it's dead because as soon as you turn the car to ON but it's not running the fans will kick on. This never happened prior to Attempt No.4. And lastly, in relation to Attempt No.3; I was fiddling with my throttle body briefly while I did this in an attempt to fix my low idle (was sitting at 750-800 hot) I turned the flat bung using a flathead screw driver underneath the rubber plug on the the throttlebody. Since I've done this the car won't start warm unless I prime the fuel pump 3 times. I know this is probably a whole another drama but unless it's related to the overheating it's not my concern at the moment. The car starts with a little bit of care and it runs fine.

with the idle screw, what do you mean prime the fuel pump?
The fuel pump only start pumping on crank, and stays on once the engine starts.

Just put the screw back where you started it. (also, 750-800 hot is not far off normal, OEM is 850+-50 from memory, and the factory tach is not accurate enough to check that little of a change)




Anyway, as far as the overheating, since you've replaced all that stuff already, have you confirmed that the temp sensor for the ecu/gauge cluster is actually reading correctly?

So the car's got an immobilizer in it. I deactivate the immobilizer and when I turn the car from ACC to ON you hear the fuel pump (I'm assuming it is) prime for 1-1.5 seconds or so and then shut off. If I try to start it from cold just straight up, it'll just keep cranking but won't start. If I turn it from ACC to ON 2-4 times and then crank it, it'll kick over.

I wish I could simply put it back where it started... problem is I played with it so much that I have no idea where it originally was. I put it all the way down to stop and all the way up to loose. Dumb, I know.

I haven't confirmed that the temp sensor for the ECU/Gauge is reading correctly. That's a brilliantly simple idea, perhaps should start there?

Also, it's worth noting that I've been doing a lot of reading and people always say put your A/C on to activate the second fan and get it cooler. This actually makes the car overheat worse. The car won't overheat when cruising on the highway like I said, no dramas whatsoever. If I turn the A/C on cold though, it will creep up to 12:30 or just past vertical.... interesting hey?

If you are in Ripley then you are not far from me. Happy to help problem solve, no guarantees!

1. Does it have the undertray installed?
2. If you turn the heater on (and the fan to full) whilst driving does it reduce the temperature?
3. Does the heater actually work?
4. Are the fans turning in the right direction, that is pulling air through the radiator. I know this sounds silly, but worth confirming. Another way is to use a differential pressure gauge and measure the pressure differences inside the engine bay with a reference at the front of the radiator at different speeds.
5. Confirm your fan(s) are working when driving, maybe a 'loose' wire is causing an intermittent fault.
6. As for hks_kansei have you confirmed the temp sensor for the ecu/gauge cluster is actually reading correctly? This can be done with a non contact temp gun pointed at a few places.
7. Have you changed the timing?

If the car cools when on the highway then it is more than likely an airflow issue at low speeds.

Does the car have the under tray installed?

Other things to check - that the thermostat opens fully in a pot on the stove, & that it's not installed backwards (if possible).

After 9 months I would be taking the car to someone who does this for a living.

I agree, give it to a cooling specialist.
I'm thinking something drastic, blocked coolant passages in motor?

bruce wrote:I agree, give it to a cooling specialist.
I'm thinking something drastic, blocked coolant passages in motor?

Same.

If a previous owner has used chemiweld or some similar "leak sealant in a can" stuff it's pretty likely.

It's bloody awful stuff that tends to just coat everything in a thin layer of sh*t, while the bulk of it just collects in a corner/narrow spot and blocks it.


and on top of that, it usually doesnt stop the leak anyway.

Mr Morlock wrote:After 9 months I would be taking the car to someone who does this for a living.

bruce wrote:I agree, give it to a cooling specialist. I'm thinking something drastic, blocked coolant passages in motor?

If after 9 months it has not blown up then I would say it is not something drastic

Sounds air cooled atm.
How do you know its overheating ? Is the temp gauge factory or aftermarket ?
Does the coolant system actually pressurize when hot ? Do you have the correct thermostat and radiator cap ?
Bypass the heater. That will determine if the heater core is blocked.
Replace those two old hoses.
Good luck.

Magpie wrote:

1. Does it have the undertray installed?
2. If you turn the heater on (and the fan to full) whilst driving does it reduce the temperature?
3. Does the heater actually work?
4. Are the fans turning in the right direction, that is pulling air through the radiator. I know this sounds silly....
5. Confirm your fan(s) are working when driving, maybe a 'loose' wire is causing an intermittent fault.
6. As for hks_kansei have you confirmed the temp sensor for the ecu/gauge cluster is actually reading correctly? This can be done with a non contact temp gun pointed at a few places.
7. Have you changed the timing?
   
   If the car cools when on the highway then it is more than likely an airflow issue at low speeds.

1. Yep
2. I would say yes but it doesn't reduce it enough to stop it completely, moreso it slows the overheating.
3. It blows hot as the pits of hell so I would say yes haha!
4. Doesn't sound silly at all but yes they're installed and turning the right direction. The Mishimoto radiator/fan setup can only be installed one orientation and direction so 100% this is correct
5. Definitely working, you can hear them going off their clappers
6. No idea how to do this but would be happy to do it
7. Nope, all is as it was. I've just replaced the timing belt with the water pump

This is something has been suggested to me but I've never known or heard of Mx5's simply overheating due to lack of airflow, is this a thing?

RS2000 wrote:Other things to check - that the thermostat opens fully in a pot on the stove, & that it's not installed backwards (if possible).

Thermostat has been semi-gutted and drilled out by the mechanic and it's not installed backwards. Haven't tested it in hot water but I'm confident it works.

Code4 wrote:Sounds air cooled atm.
How do you know its overheating ? Is the temp gauge factory or aftermarket ?
Does the coolant system actually pressurize when hot ? Do you have the correct thermostat and radiator cap ?
Bypass the heater. That will determine if the heater core is blocked.
Replace those two old hoses.
Good luck.

What do you mean by 'sounds air cooled atm'?
I'm confident it's overheating because the coolant overflow reservoir (when it's full which it currently isn't) will boil during the overheating. How can I test that it's pressurizing myself? I believe the mechanic checked this and said it was 100%
Correct thermostat and radiator cap confirmed.

How do I bypass the heater? I imagine there's a little guide somewhere?

You need to understand that the replacement radiator may be faulty and must be checked. Driving a car and overheating is a big no no.

What do you mean by 'sounds air cooled atm'?
It sounds like the cooling system is not working anywhere near how it should be. Its only cooling with air flowing past the block at speed and the water/coolant is not doing anything.

I'm confident it's overheating because the coolant overflow reservoir (when it's full which it currently isn't) will boil during the overheating.
How can I test that it's pressurizing myself? I believe the mechanic checked this and said it was 100%
He's probably wrong. A good mechanic or coolant specialist can check it. Check this out. It is one method. https://www.youtube.com/watch?v=2C4R96icPpI

Correct thermostat and radiator cap confirmed.
Cooling occurs not just with water in the system. The boiling temp is increased if the coolant is pressurised. This is the job of the radiator cap and if it is the wrong one, boiling water will bypass into the coolant reservoir.

How do I bypass the heater? I imagine there's a little guide somewhere? Take a length of heater hose. Attach it to the heater outlet at the back of the engine and loop it around and attach the other end to the heater inlet on the rear of the engine. The heater is no longer attached and is bypassed. If the heater is blocked then coolant wont circulate and just boil. If it doesn't overheat with the bypass hose in place then you definitely have a blocked heater core. Its one way to eliminate a possibility.[/quote]

Hope this helps.

Air has two givens.
1. Air flows from high to low pressure
2. Air will find the path of least resistance

Hence why to make sure all the air is going through the radiator, not around/under or over AND the engine bay is a low pressure zone to draw the air through the radiator.

At low speeds the fans do the job of pulling air through the radiator and out of the engine bay. This is why you want as much air as possible being pulled through the radiator.

As mentioned if it is fine at high speeds but bad at low then it is speed related.
Water pump issue at low speed, for example worn impeller. However at highway speeds the impeller has some efficiency
Heater core however you say it is working so the issue is not there. The heater is always on and it is only when you open the flaps is the heat let out. It can be bypassed however there are some that argue that you need to restrict the flow out to simulate the heater core.

Nothing short of pulling the motor down and inspecting the water passages could rule out a blockage or a piece of crud floating around causing blockages.