Water temperature weirdess. I need your thoughts please!

[mitch_f1]

Okay, so I am having difficulties with my water temperature gauge (I think). I have a VDO mechanical gauge, which has it's bulb in the top radiator hose. Several weeks back on my way to a trackday on the M2 my car did not get past about 60*C (on the gauge) until after about 30 minutes or more of driving. Initially I thought it was because of the cold weather (not rugging up cold, but was jumper cold), and/or dead thermostat. I didn't have time to change it, but a week later doing the same trip I found that the temperature went to about 110*C while stuck in a 40kmh zone. I was in third gear, always am, and was not stuck behind anyone. The temp gauge on the dash did not move.

After these events I thought I would pull the thermostat and check its functioning. I put it in a saucepan and boiled the water. It started opening at low 80*C, and was fully open by about 84*C, which is well within specification I believe (correct me if I am wrong). So I put it back in and went for a drive. This drive happened to be a couple of trips to Nemo's house in the hills (30mins from my house). On the way back I went through a twisty road (by no means pushing the car hard, as I did not exceed the speed limit at all), with the last stretch being a long stint on Mona Vale Road (a long, straight road near home). When I parked the car in the driveway I noticed that the temperature gauge was reading beyond 120*C. Upon seeing this I was like FECK!!! I turned the ignition back on, but the fan didn't cut on, neither did the motor/radiator feel hotter than normal, and the dash temperature gauge was reading just cooler than middle as well. I watched the gauge and over the space of about 5 minutes it went from 120*C to 80*C to 100*C.

Today I took the gauge out and went back to Rare Spares at Prospect, where I purchased it. They had the smart idea of boiling it in a kettle, and it worked fine (read 100*C and did not go beyond), so it would appear that it is fine. This is really frustrating, because the main reason I got a mechanical gauge is because there is so little to go wrong on it!!

So I have some questions now:
-what could cause these unrealistic temperature readouts
-rare spares guy mentioned that it could be airlock in the pipes, but then the system hasn't been drained for a year now, and nothing has been changed until last week when I drained the top tank to remove the thermostat. Would an airlock create those readings?
-also, ever since I got the gauge it reads about 60*C when it first gives a reading (being in the top hose, no hot water would reach the bulb until the thermostat opens, which suggests that the thermostat is opening at 60*C, but I know that this is not the case). I would expect the mixing of the hot water from thermostat and cold water in top hose to create some difference in temperature, but 20*C, is that reasonable?

Thanks for any of your help

[NitroDann]

Heat soak?

Dann

[mitch_f1]

If that was the case it would read that temperature at the track, but this is the first time I have ever seen it happen. At the track the gauge will read 80*C while racing, back in the pits it will jump to about 90*C where the fan turns on, and it won't go any higher.

[Okibi]

I'm guessing the initial 60"c is because of the mix between hot coolant from the engine and cold coolant in the radiator hose, especially considering it normally hovers between 80" (thermostat open) and 90" (fans kicking in etc).

The higher temperature at 40 km/h could just be the balance between higher revs than parked at idle (when fans are doing all the work) and low speed where airflow through the front might not exceed the airflow of your 1 fan (if it was jumper weather no AC on I guess?).

You can wire both fans to come on and get the temperatures down faster, that could be one option.

Does your numberplate obscure the flow to the radiator at all? I remember one club member had overheating at the track until he moved the licence plate from blocking some flow to the radiator.

My NB thermostat opens at 180"F (82"C) and fans kick in at 210"F (99"C).

[mitch_f1]

That's what I thought, but then daily driving the temperature sits at around 60-70*C.

No AC on, because I have no AC. But I have dríven in plenty of 40kmh zones before and since with no overheating

I only have one fan now, and that has been plenty so far, even whilst at the track

Number plate is well out of the way.

Thanks

[NitroDann]

I have no fans and all is good. The car should never sit at 60-70 when cruising, your thermostat/guage is stuffed.

Dann

[mitch_f1]

ANd that is exactly what I thought, but both appear to be working fine when removed from the car. *confused*

[manga_blue]

I have a VDO mechanical gauge, which has it's bulb in the top radiator hose.

I can't see the point in measuring temp downstream of the thermostat. It's the temp in the head that you need to know.

[RX-747]

I have a VDO mechanical gauge, which has it's bulb in the top radiator hose.

I can't see the point in measuring temp downstream of the thermostat. It's the temp in the head that you need to know.

+1, no point measuring the temp after the thermostat by the time you know the motor is too hot, its too late... I know you said there was no air pockets in your system but if the thermostat and gauge work, thats what i would be trying next even if it might not be it...

[RX-747]

I couldn't edit my previous post so ill just put what i was going to put in my last post here...

Also check to see if your cooling system has corroded, it happened to one of my previous cars and it would get really hot when not moving/moving slow unless i had the aircon on but when driving would read below the thermostat opening temp... just squeeze your radiator hoses and if they feel crusty/crunchy then they are most likely corroded, also smell your coolant and if it smells funny its most likely old and burnt like my car that had the corroded system smelt like yoghurt instead of coolants usual sweet smell.. If its corroded just get it pressure cleaned at somewhere like ultra tune or somewhere else that has one... I dont know if any of this will help but its worth a try...

[mitch_f1]

You make a good point about it being in the top radiator hose. Is there any way to mount it in the head? Like can I remove the sensor for the dash gauge (does it go through the ECU, or to any switches), and replace it with the VDO sender?

As for corroded system, I know it is some kind of chronically rusted. Check my thread, but this is just a sample:


I have replaced every single hose and flushed and reverse flushed it. Could the internals being corroded affect this?

[Okibi]

I'm guessing thermostat is starting to open at 80, mixing with cold coolant and creating the 60ish " around your in hose temp sensor.

Cooler coolant starts to mix into the system and the thermostat starts to close, coolant heats up inside engine to 80"c and the thermostat starts to open again.

It could be at regular cruising speeds that you're on a fine balance were the thermostat isn't either fully open or closed and the "mix" of hot engine coolant and colder radiator temps sits about 60" in your upper hose.

Or your sensor / gauge is out of whack but the kettle test should have shown this.

[RX-747]

I don’t know if replacing the sensor will cause problems with the ECU and I also don’t know how accurate mx5 gauges are but if its anything like my old car half on the cars temp gauge was like 100 degrees on my mechanical temp gauge..

I highly doubt mx5 gauges are out by that much so I wouldn’t think that the 100-120 degree reading is correct and the only realistic thing that I can think is causing that would be an air pocket... When the gauge reads these unrealistic readouts does it quickly spike to that position then fall back to a realistic temp? Or does it gradually climb to that temp? If it spikes really quick then goes away quick 90% likely its an air pocket caught somewhere in the system.. You can usually get rid of air pockets by putting a bottle with the end cut off in the top of the radiator then fill it with coolant, run the motor until the thermostat opens and pump the top hose to make sure all bubbles come out..

Also looking at the picture you posted it seems that your system is corroded but it doesn’t seem as though there would be anything restricting flow judging from that picture.. If it was me with this problem the first thing I would do is try and get rid of any air bubbles even if you think there aren’t any, it doesn’t cost anything and it’s a starting point.. If that didn’t fix it then I would take it to a place that has a machine to pressure flush it/ clean it (you would be surprised how much corrosion a proper flushing machine will get out) then top it up with fresh coolant and see if the problem goes away...

The only thing that has me thinking part of it is corrosion is because my old car did the same thing, when you were on the open road the temp would read below thermostat opening temp and then in traffic it would run really hot.. When I squeezed my radiator hoses they were really hard and crusty inside which made me think how much that would restrict flow..

That’s just my 2 cents and I hope one of them works and that the problem isn’t too serious.. P.S. did you re bleed the cooling system when you installed the temp gauge and when you took out and replaced the thermostat? This would have possibly created an air pocket...

[mitch_f1]

I didn't notice if it rose to 120*C slow or fast, but it was fluctuating quite fast after I came to a stop (went from 120*C to 80*C to 100*C in about 5mins), so I think that you may be right about the air pocket. I did a lazy bleed of the coolant after re-installing, but probably should have done it properly.

A friend pointed me in the direction of this link (http://www.miata.net/garage/GaugeInstall/), and so I think that that is what I am going to have to do now; find such a t-junction and do that.

[bigdog]

From past experience with a temp sender that was mounted in a hose in my old Alfa, you will get exactly that sort of issue when the fluid levels drop and air enters the system. Most sensors are meant to be 'earthed' by contact with the head/block - when suspended in rubber the only way for it to complete it's 'circuit' is via the coolant - when it looses contact with the coolant the 'circuit' is broken and wierd readings will ensue. In my case the temp readings dropped as the coolant leaked out, and the first sign that that things were wrong was a drop in oil pressure (thank god for an accurate gauge!). When I pulled over to investigate the temp was through the roof, but thankfully no damage was done. I moved the sensor after that and resolved the problem.