Engine overheating in traffic after coolant flush and change 1992 NA 1.6

My coolant was previously brown and a bit rusty as well as containing silicates so I drained the old coolant and did a coolant flush with Holts Speedflush and deionised water. I then drained all that and filled the engine with around 50/50 of Comma Super Coldmaster and deionised water.

I have tried burping the engine three times now. Twice I had the front up on jack stands, let it run and the thermostat open and squeezed the pipes, and the other time I did just now on my driveway which is much steeper, just to truly let the radiator cap be the highest point. This is also with a funnel with coolant in to let air get replaced by coolant.

Now, when I idle my engine it seems fine, ran it for about 24 minutes yesterday and the temperate gauge stayed right before 12 o clock where it should be. I can also drive my car and rag it all I want, the temperature will be fine. The only time I get an issue is when I am in stop start traffic. The temperature gauge starts slowly overheating then before you know it you’re approaching H but not quite there. If I were to then exit traffic and drive normally, allowing the radiator to get air through it, the temperature will go down though not necessarily to the right spot before 12 o clock. Even when I just went to a carpark and left the engine running, but then exited out of the car part I found it started overheating again, and that’s with the clutch only being used a few times.

I thought that perhaps the coolant fan wasn’t working, but I ran the engine and found that it would periodically start and stop as it should when operating temperatures are reached. This was both last night just running it for 20 minutes and when trying to burp the coolant this morning. Yet, when my engine is overheating and I take it home and leave it, the fan is not turning on once when you would expect it to be going full blast. Even though I was stationary, the temperature gauge would still fluctuate as well, and did get quite hot again until I turned it off. I also don’t see why a coolant change would effect the fan, as I had absolutely no overheating issues before even if I did a bit of drifting, so whether the fan was working before or not I shouldn’t be getting these overheating issues right?

So this leads me to believe that it’s due to the coolant change, maybe an airlock, but as mentioned I have tried to really burp the engine. Squeezing the s*** out of the three big hoses, leaving the engine running for at least 20 minutes all at operating temperature so the thermostat should be open. Worth mentioning, the thermostat and waterpump were changed last April, though an aftermarket thermostat from mx5parts was used. Any help would be greatly appreciated as I currently can’t rely on my car to take me to uni as running into traffic could cause major overheating problems.

Which model?

Aside from not knowing the above have you got full heat into the cabin, could be an airlock/blockage there. Can only think maybe a faulty stat also, who knows?

Sorry, completely forgot other MX5 models existed. It’s an NA 1.6 1992. My heating works great,

I would change to a Mazda genuine thermostat.

I’m going to take the thermostat out and see if it is working but I may replace it with a genuine one regardless. Seems to be a few types though like 82 deg C or 88 deg C and having a bleeder valve vs not

On the 1.6, there is a thermostat bypass line, and that can block; its possible some crud was dislodged when you changed the coolant.

From Miata.net:

"I’d seen it mentioned in messages on Miata. net that there’s sometimes gunk in the thermostat housing that can cause weird cooling problems should it block the bypass port there. There was no blockage at that point in my car. However, based on what I found in troubleshooting my Miata, one should also check to be sure that there is good flow through the bypass line from the thermostat housing to the water pump inlet.

First gen. Miatas (1.6L engine) have a 90-degree metal barb for connection of the rubber bypass line at the water pump inlet. It’s a true 90-degree bend, rather than a radiused one as it probably should be (due to space constraints, I’m sure).

To check, remove the rubber bypass hose from the the thermostat housing to the water pump inlet. It runs almost vertically from the housing to the inlet. Coolant should flow freely from the barb on the water pump inlet once the hose is removed.

Mine proved clogged with bits of rubber–I suspect from a previous hose or water pump replacement or just accumulated over time. I cleared it using a straightened piece of coat-hanger as a “broach.” The symptoms have not recurred after this barb was cleared.

I expect the problem occurs as follows:

*>From a cold engine, without sufficient flow through the bypass line, the thermostat doesn’t heat up properly–in tandem with the coolant. The coolant, however, heats up normally. When the engine finally reaches a near overheat or overheat condition there’s enough pressure against the still closed thermostat to cause the brief leakage at the housing gasket. Then, the thermostat finally pops open from being heated by conduction through the housing itself, and the sudden resumption of proper flow rapidly cools the engine again."

I had a 1.6 that had overheating problems, but modifying the temperature gauge (search for Miata linearized temp gauge), I could better understand what was going on. The blockage (in my case, bits of metal), caused the engine to run hot at cruising speed, so the fans were cutting in too much. So eventually the fan didn’t turn so well, as the bushes were worn.

To test the fans (so you can properly observe their action, with ignition on, unplug connector on top of thermostat housing. Using a suitable conductor (piece of wire) short (touch together) connector to earth (housing casting). Fan should turn on. Jump GND, TFA, check for volts at the fan connector. You should see 12V. If this checks out, and the fan doesn’t spin, its the fan, or, probably more likely now, a bad connector.