Engine 'Popping' Sound

  1. My model of MX-5 is: 1.8 NA '93, with '97 engine
  2. I’m based near: Devon
  3. I’m looking for technical help or recommendations on: Possible causes for engine popping, and what else I can check.

Hi all. Since a post I made a while back documenting everything from bad compression test results to an engine swap and issues along the way, I hope it’s ok that I’ve made a new Topic fresh with the one issue I have remaining, which should be easier for any helpers to navigate as the other was rammed with info and over 50 posts.

I’m getting an engine popping sound from idol and up through the revs, with the popping getting faster with the revs. There is no major misfire, and car seems to pull well, but the engine will judder every few seconds at idle. Here is a link to the noise being amplified through the air intake - https://youtu.be/376r1z7AxT0

Suspecting possible vacuum leak, I put my original '93 inlet manifold on to the newly swapped '97 engine, where there is now no potential ‘EGR blanked’ issue. I’ve put my original injectors back in, with new seals all round.

I’ve put in new spark plugs and new HT leads. I’ve tested the coil pack for ohms between each lead port (getting good results), but that is the only test on coil pack I have carried out.

I havent yet put my original throttle body on the new engine, however everything else (bar the engine) is how it was before the swap, where no popping noise was evident. The o2 sensor cable was on the exhaust manifold causing the sheath, which is used to keep the four wires together, to melt, but the wire insulation itself looks fine. Unplugging the sensor for a minute changed nothing, but I’m not sure if that’s normal or not. Compression test results were all good, at 180 each, but could it still be a leaky valve which isn’t being picked up?

I’m not sure what else I can check, and I just wanna get back on the road! Any help is greatly appreciated, and again I hope it’s ok that I’ve created a new post, but will add this to the old if it’s a problem. Cheers.

1 Like

Never heard that noise before. Rather than a leaky valve could it be a sticking inlet valve allowing pressure pulses into the inlet manifold? Try fitting a vacuum gauge to any rubber hose coming from your inlet manifold and see if the vacuum fluctuates.

I have a similar, hiss/pop with my motorbike. It only occurs after initial start and on idle. Like you I checked for leaks on the exhaust and inlet, and double checked spark plugs and even replaced a coil but still does it. I too was thinking it might be a sticky valve, but as it doesn’t give me any problems driving I’m just leaving it! Other than taking it all apart not sure how you can determine if it is a sticky valve, especially as it is intermittent?

Or a weak/broken valve spring?
If so with the cam cover off it might be possible to push down the offender more easily than the others, just by finger tips.
Also it will sound different from the others with the mechanics screwdriver/stethoscope, if you can get it to bear on the head metal next to the offending valve.

Many years ago at Uni, we had one on a nearly new Austin Mini that refused to rev. With just two screws and a reusable thick cork rocker cover gasket we you could see the problems in seconds! Fortunately that car was a doddle to work on if you had supple fingers and wrists. With care, cunning, some improvised tools and four hands, we replaced the spring without taking the head off or losing any bits inside.

Be careful!

Thanks for the replies. I’ll order a cam cover gasket and have a peer in and see what I can see. Might try a set of NGK plugs and leads just to be sure the ones I bought weren’t duff. They were the Bosch Super 4 plugs, which I’ve heard a few people having issues with.

I just took it for a spin and it wasn’t smoking at all, seemed to pull really well again, with just the popping noise getting a little bit louder when the engine warmed up.

And then it’s just hoping it passes the MOT, and maybe driving it will help the issue as it has certainly been sat up for a while. I’ll keep you updated.

Hi again. Just learned that the older Mk1s have 8.8:1 compression ratio and the newer have 9:1 due to domed pistons… I could be entirely wrong here due to limited knowledge on the subject, but could the ECU be chucking in incorrect mixtures of fuel for the ‘new’ engine as car is '93 and engine '97? Not sure how much difference it could make though.

Bit of an info bank if you are:
1)Stuck with snow.
2)Tier 4 prisoner.
3) Bored.
4) Suffering from Prime & Netflix syndrome.
5) Run out of beer.
6) Waiting for beer delivery.
7) Working from home but you just cannot be motivated.


Cheers for that, makes for some interesting reading. Sadly doesnt give the 8.8:1 ratio I have read about due to domed pistons. But realistically, I’m not gonna take the engine apart any time soon so I’ll just keep driving it 'til I cant any longer…!

You don’t say whether you fitted a new cam belt on your replacement engine. If you have it would be worth checking the valve timing is spot on. If you haven’t replaced the cam belt and if you don’t know the engine history I would replace it and again make sure again that the valve timing is spot on.

I haven’t done yet, I’ve been working on it so much recently I was gonna save this til it gets a bit warmer. But yeh, that will be a job along with the water pump. Maybe it’ll be best to do it sooner rather than later…