Overheating issues on 1.6 NB

Technical Area Engine, Transmission, Exhaust etc
1

Hi, I’ve got a 2001 1.6 NB MX-5 and I’m having some overheating issues. I bought the car to learn to do some more complex mechanics as I enjoy working on cars. I’m regretting it already!

I’ve got an overheating issue which I’m struggling with, it comes in two parts. The car has an aftermarket digital temp gauge and japspeed radiator.

Part 1 - Initially the car would sit and idle fine with the temperature at around 80, but after 10-20 minutes driving it it would shoot up to 100+, so I haven’t driven it far. It would overheat every time I tried driving, the temperature would be fine for a while but then start rising quickly despite not working the engine hard. It also intermittently lost coolant. There was no milky residue in the oil or white smoke from the exhaust.

I suspected a coolant leak, so used UV tracer dye but there’s been no sign of leaks and it didn’t lose coolant. I thought maybe the leak only happened at high pressure when the engine got properly hot, so I let it get up over 90 degrees but still no sign of any leaks and it overheated again. I know the coolant is flowing as the top hose gets hot.

I then thought maybe there could be some blockages so I’ve been flushing the coolant using STP coolant flush. In the process of flushing the coolant I took it on a gentle test drive to let it get hot, but the temperature still rose up to 100 at which point I cut the drive short, got home and found the header tank nearly empty again but no signs of a leak, no white smoke, no milky oil residue.

Part 2 - On repeating the flush for a second time, there is now lots of white smoke from the exhaust which doesn’t clear up, and it overheated again. I suspect a head gasket but there’s a chance letting it overheat has damaged the head or block. Either way I think it’s head-off time, but this may not be the real cause of the overheating.

So to my questions:

What are the chances that a head gasket was the problem all along despite no white smoke initially?

If the gasket is a new symptom of the underlying problem, does anyone have any ideas what that problem could be?

What should the next steps be in finding the cause of the overheating?

Thanks in advance

2

Possibly the water pump bearing beginning to seize. There is a small hole on the underside of the pump housing behind the pulley wheel. If it is weeping, then the shaft seal is allowing coolant past it causing a slow drip usually visible on the front edge of the sump. I had a water pump do this and kept overheating until I replaced it.

3

Thanks for the tip. Since I’ll almost certainly be changing the head gasket now, may as well do the timing belt and water pump at the same time. It’ll be a new experience if nothing else!

4

Is the header tank intact, and the cap seating correctly?

5

sounds like head gasket to me. change all the water related items, thermostat, hoses etc at the same time as doing he water pump.

also are your fans working?

6

Yes it seems to be holding pressure, there were no signs of the UV dye around the header or anywhere else

7

Thanks. I’ve got a thermostat ready to fit so I’ll do that. All the hoses are silicone that were uprated before I got the car, but I’ll check them since I’ll be taking them off anyway.

Fan isn’t working, it works when hotwired from the battery though. I tried looking around the switch on the thermostat housing, but it’s a complete mess of bodged wiring under the bonnet so think I’ll be putting it on a separate switch in the cabin, that’s another job I need to do!

Thing is, it overheats when doing 60 on a clear road 15 minutes into driving with outside temps around 10 degrees so I doubt the fan would make a difference in that scenario since it’s getting good airflow.

8

got to be thermostat or water pump in that case bud. fans obviously isn’t ideal

9

Thanks. I’ve got a head gasket, water pump, timing belt & pulleys kit on order. Never taken the head off a working engine before, only one I was scrapping with a snapped timing belt just out of curiosity, so wish me luck! Any good idiot-proof guides to follow?

10

this guy.

11

Thanks I’ll give it a watch

12

Enjoy the adventure!

13

Have you checked radiator ? Mine was partially blocked. Easy to check, make sure it’s evenly hot all over once water flowing hot by feeling with hand. Obviously turn it off before putting hand near pulleys or fan.

14

I haven’t checked that, no. All I checked on the radiator was that the top hose got hot, the cap held pressure, and that it didn’t leak. I’ve run two lots of radiator flush through the system and it’s an upgraded japspeed radiator so I just assumed it would be fine!

15

Mine had good flow but was large section that wasn’t heating up so obviously had some blocked channels. Tried various methods to flush out but wouldn’t clear. Bought a new rad in the end . Wasn’t too expensive, about £100 I think and hasn’t overheated since.

16

Update:

I’ve replaced the head gasket and water pump. The head gasket looked fine but some of the coolant ports were clogged up.

I’ve replaced the thermostat housing as that was almost entirely blocked and contained no thermostat.

Put it all back together, flushed it 3 times and it held temp while ticking over. Filled coolant with radiator cap off and let any air bleed out.

Took it on a drive and it overheated massively. Coolant filled the expansion tank to the brim and the bottom of the radiator is cold. So I’m guessing something has blocked it up again. The radiator top hose is red hot so I think coolant was circulating until it got blocked and then overfilled the radiator into the expansion tank. So by my logic either the radiator is blocked or somewhere inside the engine. Does that sound logical?

17

Did you have the cylinder head flat tested and crack tested while it was off?

18

No, I put a straightedge on it and it seemed ok put it hasn’t been professionally checked. The old gasket looked fine, there was no damage to the old water pump, and there’s no mixing of oil and water.

19

Did you check thermostat is opening ? If not that then most likely blocked radiator. Mine was, tried flushing with various things but although had flow through rad a lot of the chanels wouldn’t clear so wasn’t getting an even flow leaving a large section of rad not getting hot. Bought new rad and 20k miles including a 15 hour jam in France with engine running (was the Alps in march so too cold without) and no sign of overheating.

20

I’ve done a bit more digging today.

I took the radiator bottom hose off from the engine side and ran water through the radiator, it had good flow.

I took the top hose off from the radiator side and ran the engine with the radiator full, barely a trickle came out.

I took the thermostat housing off and did the same, water poured out.

Also ran water through the heater and that’s running fine.

So somewhere between the thermostat housing and the top hose is blocked, which I think points to the cylinder head?

There’s also mayo on the oil cap now. So I think what’s happened is that there’s a blockage in the head and by overheating it again I’ve either blown the gasket or the head is warped.

Does that sound like a reasonable diagnosis?