Miata 1.8 getting light “death rattle”. Help me sleep tonight.

So I’ve been noticing (mostly hot days) the Miata will get what sounds like pinging, either at low rpm with a lot of throttle, or, sometimes, near redline. Everything works great, only cell code is p0420 (I don’t have a secondary o2, just straight pipe). It has Honda CBR COP conversion. Other than that it’s stock, no forced induction. 

***IT IS NOT A HEAT SHIELD*** definitely, 100%, from the engine. I know what knock sounds like  

It does have 198,000mi, so I’m sure the little engine has had it, if it’s the original. My thoughts were 1) throw premium in the tank. If it goes away, it’s knock. 2) if knocking, chuck in some sea foam and run it hard. 

The car is new to me, so I still need to check engine timing. 

Thoughts?

Check the timing :)

Start running it on 93 octane and give it an Italian tune up.

NermalSnert said:

Check the timing :)

What he said. Didn't you just have alarming handling issues which were addressed by getting an alignment? :)

It's sounding a bit like this particular car could use a once-over to get everything baselined.

In reply to Ransom :

Yep. I’m starting to wonder if this was a former autocross car. Lots of little things done to it that are...mysterious. For instance, it came with the lightest OEM Miata nb wheels. The body has some damage that looks suspiciously like cone marks. The COP conversion. The water wetter only in the radiator. Hmm..

I gotta Check timing, thanks for confirming folks. 

A couple of friends with turbo cars had trouble with COP conversions breaking up at high rpm.

Don't suppose you have an AFR gauge?

I do not. 

This forum is so awesome sometimes. I’ll run seafoam first and see if that helps. This 1.8 has good power between 3-5k rpms. Makes me wonder if it is the ignition timing. I’ll check tomorrow. 

What year is the car? If it's an NB, there's no timing adjustment, but the sensor can be a bit too far away from the crank pulley and you can get bad readings at high rpm.  Here's the adjustment procedure if you need it. 

If it's an NA, don't forget to jumper the diagnostic Port before adjusting the CAS.  That always gets me :)

In reply to WonkoTheSane :

Thanks. It’s a ‘97 na. Good to know about the diagnostic port. 

Is a '97 still in the lifter tap age range?

MrJoshua said:

Is a '97 still in the lifter tap age range?

Yes, yes it is. When I heard Miata with "light" and "rattle", rattly HLAs were my first thought.

Check crank nose.

i lost a motor to it and two friends have.  (all 1.8's)  bad prior service will kill it.

mine could have been described as you are

I second what Apexcarver said. I had a similar experience in my 96 Miata. I thought the engine was done for and went as far as listing the car for sale with a bad motor. Then I started taking parts off and quickly realized that 3 of the 4 bolts that hold the pulley off of the crank were sheared off. A quick and easy fix and it was as good as new. Here's a video of how mine sounded: https://youtu.be/YDPT7fFwpZg

Worth taking a look.