Thought is was rod knock, now i'm not so sure. - Miata Content

Jumping to the point, I have 99 Miata, with a AVO turbo kit, I just got that I've started to tear into for what sounded like a rod knock.

I started disassembling the car and found a large crack on top of the exhaust manifold. It was cast and I thought that might be the culprit. For an experiment I grabbed the TIG welder and sealed the manifold as beast as I could, but it kept poping back open because I could not do any preheating on the car. The result was a still cracked manifold, but a much smaller crack then what was present before.

I reassembled the car and the sound was significantly different, but still present. Another note is the sound did not get louder when I removed the oil fill cap. I don't know how to describe the sound other than a very hollow noise that changed with RPM. It never really got louder, just different.

Without a definer root cause I dug back into the car. I've removed the radiator, A/C compressor, P/s Pump, charge piping, and started removing the turbo. Once I got the turbo off I noticed the manifold had cracked not only on the top, but had an equally large crack on the bottom.

The engine only has 50K miles on it and I'd prefer not to pull it if I can help it. I'm looking for a productive direction from here... I have already called FM and am on the wait list for one of their turbo manifolds. I will replace the manifold and fabricate a new down-pipe for the turbo (FM manifold is a slightly different placement).

In the mean time is there any way to figure out if the manifold is the only problem?

Obligatory picture.

P.S. - yes, I know the turbo needs to be rebuilt.

Miatas are often accused of having a rod knock when they have sticky lifters.

SVreX wrote: Miatas are often accused of having a rod knock when they have sticky lifters.

I've heard that. I don't think that is the problem here, but you never know. Is there a common fix?

In reply to NordicSaab:

Yeah- clean the lifters. Plenty of YouTube how-tos.

It's not difficult.

With their aluminum heads I've seen a problem on Miata that I'd never seen on another car.....loose spark plugs! I thought it had the worst lifter tap of all time only to discover the spark plug in #3 was two full turns loose.

In your case I'd say it's the manifold.

Can you coat the cracks with some jb weld/epoxy or something that will last just long enough to run it for a short time with no leaks and see if the noise is gone?

GAA! I read your post thinking the crack must have be pretty small. Sheez, that's ready to break in two.

Send off an oil sample to blackstone. Can give you a pretty definitive answer if you have problems on your wear surfaces. Its worth the price in saved work otherwise.

Check the crank nose. I had one letting go that sounded like a nasty knock. if that thing is loose, be prepared for either loctite fixing, crank replacement, or a new motor. (rest assured, it happens to 1.8's too, just not as frequently. My 1.8 did it, as well as a friends)

Stuck lifters are pretty obvious, perhaps listen around with a mechanics stethoscope (big screwdriver held to part and your head) to see if you can center on where the noise is coming from.

It might just be exhaust manifold, so dont fret too much yet.

NordicSaab wrote:

SVreX wrote: Miatas are often accused of having a rod knock when they have sticky lifters.

I've heard that. I don't think that is the problem here, but you never know. Is there a common fix?

I had a potential buyer say that my Challenge Miata had rod knock. I fixed the rod knock due to sticky lifters with Rislone this morning.

This only applies to 90-97 Miatas with hydraulic lifters. 99+ Miatas used solid lifters with shims to adjust the lash.

SVreX wrote: Miatas are often accused of having a rod knock when they have sticky lifters.

SVreX wrote: In reply to NordicSaab: Yeah- clean the lifters. Plenty of YouTube how-tos. It's not difficult.

There's one really easy way to preheat the manifold on the car... HOT LAPS!

RossD wrote: There's one really easy way to preheat the manifold on the car... HOT LAPS!

LOL.

Thanks for the feedback all. I have ordered a kit from Blackstone.

In the mean time I think I am in a holding pattern until FM has turbo manifolds back in stock. I think the plan is the fix the turbo system and put the car back together enough to start it and go from there.

This might sound strange, but check your alternator bolt. On these cars, they can strip and back out super easy. However, there is a bracket behind that prevents it from backing out completely, so it just rattles like a mother lover and sometimes sounds like rod knock.

I agree with Desmond here. I had a similar sounding issue with a fiat 124 once. Engine was making a terrible knocking sound.. until I noticed that the alternator was rattling all over the place because the bushings had gone

NordicSaab wrote:

RossD wrote: There's one really easy way to preheat the manifold on the car... HOT LAPS!

LOL. Thanks for the feedback all. I have ordered a kit from Blackstone. In the mean time I think I am in a holding pattern until FM has turbo manifolds back in stock. I think the plan is the fix the turbo system and put the car back together enough to start it and go from there.

I would still fire a sample off to Blackstone

http://www.blackstone-labs.com/standard-analysis.php

its only $28 and a lot of piece of mind.

If you have the parts, you could put the car back together with the stock exhaust manifold/downpipe (i.e., non-turbo) while waiting for the FM bits to show up and see if the noise has gone away.

codrus wrote: If you have the parts, you could put the car back together with the stock exhaust manifold/downpipe (i.e., non-turbo) while waiting for the FM bits to show up and see if the noise has gone away.

Unfortunately, I don't have the stock parts