Engine Advice

This is gonna be a pretty long post so, if anyone has personal experience or advice that is willing to read all this I would appreciate it a lot.

2000 nb miata 1.8 4 cylinder engine. Roughly 143k miles. It’s a rust bucket with no new parts maintenance wise. Besides some small but consistent leaks and a slightly frayed belt nothing looks bad. Only codes are for evap things. No o2 sensor. No egr, so intake manifold does have an open hole, as well as the exhaust side at the rear of the engine next to cylinder 4. Exhaust is only headers and half of the mid pipe so it is very loud (I don’t like it as a daily and it doesn’t sound the best but I’ve used to it so I just haven’t put it back on fully). Ik, it’s a piece of shit but it’s my piece of shit.

So yesterday afternoon I was driving to my friend’s house to help him with his truck. As I was on the highway I just barely noticed something that felt off. You could maybe describe it as a misfire but it was so slight and I didn’t hear it, I just felt it ever so slightly and I wasn’t completely sure if it was a problem but that’s when my suspicion started. I drove for about 5 more minutes fine then I got to an incline and I was looking at my rpms(my speedo doesn’t work because the sensor isn’t in) and I could tell that I was losing power slowly. Rpms were slowly dropping. Once I got over the hill, I was going to put it into neutral to listen better then downshift to 4th. Once I put it into neutral I didn’t hear anything but then I hit the gas to rev match for the shift and I also didn’t hear anything. I looked at my rpms and I realized that the engine shut off on me.

I pulled over and called my one friend, called another incase I needed a tow. I try to start it while on the side of the highway and it doesn’t crank, plus dash lights are very dim. My one friend comes with jumper cables and we charge it enough for it to crank but it still doesn’t start. My initial thought was that my alternator and/or my battery were shot, me thinking that since I couldn’t even crank it without assistance. So we just end up towing it to his house.

There I use his battery charger that using an outlet and the battery reads dead. Once it charged we go for start and just crank no start. I pull the spark plugs and verified they all have spark and there wasn’t anything unusual about them expect the insulator of the electrode in the middle had like a V groove(I can send pictures later to show and the previous owner supposedly changed them and that was about 10k miles ago). So then I change ideas and check fuel. I could hear the fuel pump prime when I turn the key(I also have like 3/4 of a tank). We try using starting fluid and nothing. While I was checking spark a cylinder or two did fire.

Here comes deeper diag. My fuses aren’t blown. My main relay should be good because it can still spark. Thoughts are crankshaft position sensor, camshaft position sensor, timing, and compression. I didn’t check the sensors yet.

Compression test showed, cylinder 1- 125psi, cylinder 2- 120psi, cylinder 3- 60psi, cylinder 4- 35psi. I know that the factory cooling route isn’t good for the back of the engine. I have a coolant leak that’s not too bad but it’s also not always topped off. But I’ve also never seen my coolant temp gauge go more than 5/8ths. There’s no signs of coolant in the oil. We pull the valve cover off and only the timing on the exhaust camshaft gear is off by one or two teeth. The oil is fairly clean but to my surprise there’s a fair bit of metal specs in there. Nothing crazy big and the cams aren’t chewed out and they don’t have side play. I will be looking closer at them tomorrow because at the time I was unmotivated and depressed as it was just getting worse and worse.

So that’s where I’m at right now. If you’ve read this far thank you at least for reading. I plan on pulling the head and checking the gasket and for any divots on the surface and warpage. It just confuses me how the engine has been running pretty good until now then just dies mid drive. And how only cylinder 3 and 4 have low compression, I’m thinking maybe a break in the gasket between them that hasn’t reached an oil or coolant passage. Or maybe the heat from the open egr on the exhaust.

I’ll update soon. I usually don’t get a lot of views on reddit so if you can share, especially to someone that’s smarter than me it’d be nice. (this was longer then i thought it was gonna be)