Compression/ head gasket check

  1. My model of MX-5 is: 93 eunos
  2. I’m based near: Stratford upon Avon
  3. I’m looking for technical help or recommendations on: checking compression/ head gasket

So a couple of weeks ago I was left with no choice than to drive my Eunos to work for 2 days as my other car was playing up. With about 120 miles each day.

On the last day I got home only to find steam coming from under the bonnet with the temp gauge still in the middle. Stopped the car and diagnosed that the heater matrix hose had given up. I was worried that I had ruined the engine, but when fixing the hose I drained about 2.5 ltrs of coolant out, so I was optimistic I hadn’t ran it dry.

Replaced the hose, flushed and replaced coolant. I’ve just checked it over about a month after and there’s some mayo on the coolant cap. No smoke on start up, and no drop in coolant or oil level, no milk on dip stick. Car doesn’t over heat at all either.

Would a compression test be beneficial in checking the head gasket? And if so what is the correct procedure as I’ve seen people disconnect different to things to do the test.

To be honest, from your symptoms I don’t think that your head gasket has failed. A small amount on mayo in the oil filler cap is not really a cause for concern if everything else is OK.

If you want to carry out a compression test, then the workshop manual procedure is to disconnect the ignition coil connector. I think that there is one connector just to the driver’s side of the ignition coils that disconnects both. Remove the spark plugs, fully depress the throttle and crank the engine.

Does disconnecting the coil plug also cut the fuel? Or do you complete a compression test with fuel being added?

I was also wondering if I’ve got a slight coolant mix causing the mayo. As I never properly cleared the system. Only ran water through twice. Would a mix of coolant colours cause the mayo?

Holding the throttle fully open cuts fuel injecton.

Did a quick compression test, I’ll probably do it again at some point in more detail. But this is what I got this evening.

Cyl 1. 175
Cyl 2. 175
Cyl 3. 155
Cyl 4. 170

This was cold, I was gonna do a wet test on number 3 but then it started raining.

Should I be worried about cylinder 3?

So I decided to do another coolant flush and refill, just to make sure I hadn’t accidentally mixed coolants and cause he mayo on the filler cap.

Ran that through and all toped back up. However I only could fill up about 4-4.5 ltrs in the rad + the expansion.

Taken the car for about an hour drive and temp is all stable and fluid levels stable, however I’m not getting a lot of heat out of the heater inside. It’s warm ish but not as warm as I remember.

All the hoses seem hot so fluid must be getting round, is it possible the matrix is gunked up, got a block, or something like dodgy thermostat even though it’s less than 6 months old.

Normally MX-5 heaters are pretty hot, sounds like you may still have air in the system somewhere.

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What’s the best way of getting any out, I was hoping that a decent drive would clear it but doesn’t seem to be the case.

Some suggestions here:

'Spill Free NO Spill Cooling System Filling Funnel KIT ’

and this vid. from about 17:50 onwards

I had the same issue after changing the thermostat on my mk1 overheating blowing warm air. Purchased an other thermostat cheked it in hot water working as it should removed the one in the car and checked that, not opening swapped for one that worked all good now. Bleeding the system raise the car up at the front as said leave the radiator cap off run the car you should here a burp dont let the engine get to hot or the water will blow out top of radiator. Worked for me.

Ill give that a go, guessing to do the thermostat you need to drain the coolant out, which will be my third lot in like 6 weeks.

hopefully can get it to burp any air out.

So tried burping it. No luck. There blower still comes out cold and the exit pipe for the matrix is cold when compared to the inlet.

I didn’t here the thermostat open or the fan come on at all, is my thermostat dead and not letting the system run?

Also how is the coolant system set up, is the heater matrix apart of the loop, or like a separate system that only pulls water through when the heater is on?


The cooling and heating system is all as one works together. Did you turn heating on in car and run the car also sounds to me the thermostat is not opening if one side of the radiator is hot and the other side cold new thermostat and Gasket check the new and old thermostat in hot water to see if they are opening and closing correctly.

Also you could try connecting up hose pipe to inlet of heater to see if water is exiting the outlet that will tell you if the matrix is not blocked.

I drained it all again last night, checked the thermostat and it didn’t open in boiling water. That definitely can’t be helping. Going to back flush the matrix just to be sure.

Definitely new thermostat required. When car is running squeeze the hoses by hand to try snd release air lock. Let me know if your matrix is clear.