Top end knocking coming from N2 cylinder when revving. 2007 2.0 NC mx5 (UK)

Technical Area Engine, Transmission, Exhaust etc
1

Hi there. I just purchased a 2007 2.0 mx5 NC. A little story behind it.

It was for sale as spares or repairs. I went to see it. The car itself is solid. Sills original and rust / hole free and subframe is solid. Couple of grubby bits on the wings. Nothing bad. Alloys aren’t the best. It has been looked after. 5 years ago Owner said the car rattled and the mechanic said “scrap it or get another engine”. She didn’t know what to do so it was parked up in a car port untouched for 5 years. Me and my dad looked at it the next day, we got the engine running and yes… There was more than a rattle. It sounded like valve hitting a piston. But only above 2k rpm. Idle, it was silent. We negotiated and the owner said “if you don’t take it I’m getting it scrapped tomorrow”. I helped the owner fill out forms for scrapping etc. Basically they will get £100 to scrap the car. That’s including pick up charge. I offered £150 and that’s me… Now a proud owner of broken solid mx5. Best financial decision of my life… So far.

It’s last mot was in 2019 at 76k miles. It’s currently on 102k miles… Oil looks crap, spark plugs are… I’m surprised they can spark over such a big gap and the air filter… Is there… It was left with the hand brake on and we had to smash the pads off with a hammer.

Now here we are trying to “properly” diagnose the issue. Here are the steps we have taken.

1: compression test and top of piston visual inspection . Cylinders 1,3 and 4. 178psi was the lowest and 198psi was the highest.
Cylinder number 2 would spike to 140psi but quickly lose compression.
Using an endoscope. We looked down the plug hole. Piston 2 is solid. No cracks, nothing that tells us a valve has hit the piston. Everything looks clean. Happy times? So far…

2: we removed the rocker cover to check the valve lifters. On cylinder 2, the exhaust valve lifter closest to cylinder 1 was significantly softer than the rest of the lifters. This is with the engine turned over so the valve is closed. My dad knows his stuff about cars but, he has never worked on a car with hydraulic lifters so he/we don’t know if this is normal? The way I see it, if one is different to the rest of the 23 valves then something is wrong. Could this be the knocking/loud rattle issue? When you use a screwdriver and listen when the engine is running, you can clearly hear something up on the exhaust of cylinder 2.

3: On Tuesday we will be borrowing our family members valve checker. This tool apparently puts pressure into the engine and you should be able to hear if the valves are leaking. This is something to do before removing the head so we know what to expect.

We currently have the rocker cover off, airbox, fuel rail, injectors and throttle body off. Exhaust manifold is still on, we want to soak the bolts first a few times before attempting this. We are almost ready to take the head off after we get this tool. So now I just want to ask around to see if anyone can give any suggestions on what it could be, what we could check and maybe any advice on removing the head? My dad is confident. But he’s used to older cars. Pre 80s. Anything engine related on a “modern” car is usually “screw this I’m getting someone else to do it”

Listening to other people it’s all doom and gloom. “it’s dropped a valve” " piston skirt has broken" “the rings have broken and it’s piston slap”. For £150 it was worth a shot. If the body wasn’t clean on the underside I don’t think it was worth the risk… But here we go!

1000000205
10000002051920×1446 427 KB

2

1000000214
10000002141920×1446 432 KB

3

With regards point 2, the NC has solid cam followers, not hydraulic ones, so if one is “soft” then it probably points to a valve spring issue rather than a follower issue.

4

Thank you! We thought they were hydraulic. Would this be the cause of the rattle/knocking noise?

5

FYI…I would have DEFINITELY bought that car. It would be criminal to send it to the scrap yard.
Best of luck with it. :+1:

6

Just an update. Engine is dead. Cylinder 2 rod bearing is shot. When the piston is at the bottom, there is A LOT of play. At TDC, it felt okay. Piston 1 at TDC has play, piston 3 at bottom has play but not as much as 2. Piston 4 is solid. It has clearly been run for to long on old oil. Might have to scrap it. We don’t have the tools or facilities to do an engine change or rebuild.

7

Well that is a shame. In your position i would be dismayed but not disheartened. You have options depending on your budget/enthusiasm etc. My first engine rebuild was a V8 Rover, i knew nothing about engines at the time but as i got the car cheap i thought, sod it what’s the worst that can happen! I only had basic tools but i changed the big end and main shells, piston rings, etc. The shells were fine and didn’t really need doing but as it was in bits, might as well etc. This was after people telling me, engine dead, dropped valve, broken rings, much the same as you and none of it was true, it turned out to be a couple of hydraulic lifters. If your problem was big end bearings then it wouldn’t be silent at idle, there would be a definite rumble, and if there was a lot of play it would be clonking like mad. I don’t know about the MX5 but you can sometimes remove the sump with the engine in situ and then remove the number 2 big end cap and take a look. I don’t think it’s necessary. Number 2 cylinder compression spiking to 140 then dropping sounds like sticking valve to me. In your position i would buy one of those temporary garages from Machine Mart etc and whip the head off, i mean, the car was cheap so what’s the worst that can happen!
P.S. I also rebuilt an engine in my spare room once, this obviously depends on the tolerance of your better half. :wink:

1 Like
8

I’m 33 and still live with my parents. We have plenty of space but where the car is under a carport, it takes up a lot of space which makes it difficult for my mum to go out for a smoke without getting rained on. This is why we gave up with classic cars. My mum didn’t agree with it.

So in the last 10 minutes, a friend came over with his pressure tester (scrappy is on there way to pick the car up at 8pm, I told them to hold off while I did some tests) and said that all cylinders are fine. No sticky valve… Cylinder 2 however, if you turn it over and go back a bit on the pistons up cycle, you can nudge the piston down 2.4mm. Bore and top of the piston looks fine. My friend said drop the sump and replace the piston, rod and rod bearings.

Probably the only question I need answered.

Will it be worth the hassle lifting the engine to get the sump off, remove the head and replace either just the rod bearings or the rod bearing and piston?

It’s doable but a lot of people have access to lifts, pulleys etc. All we have is 30 year old metal car ramps and 6 different sizes of axle stands and various bits of wood.

9

Yeah i feel your pain. Having said that, i have a set of 40 year old ramps, some new axle stands and any number of bits of wood. I would be inside that engine just for the hell of it. You may be surprised at how easy a job like that can be, and conversely, how many new swear words you learn. :laughing: Also, that car is worth a hell of a lot more in spare parts.

1 Like
10

If you need inspiration, have a look at this topic…

1 Like
11

It needs a complete engine rebuild, more than likely a new crankshaft too.

12

Can we assume that you have the sump off at this stage. If that’s the case, have you looked at the big end shells.