Dieselish sounding 1.8 Miata motor

Chad Culbertson · **Posted:** Mon Apr 25, 2016 10:35 am

So I used a 2001 bottom end to replace the broken non-VVT original bottom end in my car.

It runs great and drove it to Danville yesterday and did all my runs and drove back with no issues. The noise goes away over idle.

It sounds pretty much exactly like the guy in this video:

https://m.youtube.com/watch?v=5OPVC9Eo0DI

Doesn't seem to be hurting anything but it's kind of a strange sound. The head is a non-VVT 2000 head so there was a compression bump and supposedly premium fuel is required. I had a little under half a tank of 87 in the car when it got park for the work and once it was about to a 1/4 tank I filled up with 93, so guessing the octane should be about 91 at least now.

Any thoughts? It doesn't seem to be detrimental so i'm not too concerned the motor is going to blow up or anything but it does seem unusual to me.

Scott Johnson · **Posted:** Mon Apr 25, 2016 3:58 pm

Lifters?

Chad Culbertson · **Posted:** Mon Apr 25, 2016 5:40 pm

Scott Johnson wrote:

Lifters?

Solid lifters in this motor. The head came off the original motor anyway and I don't recall this sound.

Richard Casto · **Posted:** Mon Apr 25, 2016 6:17 pm

My 2015 Fit has gas direct injection and it can be very diesel like and loud at low RPM due to noise from both the high pressure fuel pump and injectors. So yours would not be GDI, but maybe noisy injectors is part of it? Noisy rockers? Need valve adjustment? Does it change when it warms up?

Richard

Chad Culbertson · **Posted:** Mon Apr 25, 2016 8:39 pm

Richard Casto wrote:

My 2015 Fit has gas direct injection and it can be very diesel like and loud at low RPM due to noise from both the high pressure fuel pump and injectors. So yours would not be GDI, but maybe noisy injectors is part of it? Noisy rockers? Need valve adjustment? Does it change when it warms up?

Richard

It doesn't really change once it's warmed up.

My focus and the Fiesta are both direct injected and the Focus is certainly noisy when cold.

Usually lifters without hydraulic lifters usually sound a bit like a sewing machine in my experience and also a bit faster paced as well I would think. I guess I could pop the valve cover off though to listen while it's running. I need to replace the plug seal at the back of the cover anyway.

Chad Culbertson · **Posted:** Mon Apr 25, 2016 8:42 pm

One thing I didn't mention is that this car has a california ECU but no longer has a california exhaust manifold. I wonder if it was tuned to run a little extra lean at idle maybe? I've considered trying to find a 49 state ECU to throw in.

Aaron Buckley · **Posted:** Mon Apr 25, 2016 9:38 pm

Chad Culbertson wrote:

One thing I didn't mention is that this car has a california ECU but no longer has a california exhaust manifold. I wonder if it was tuned to run a little extra lean at idle maybe? I've considered trying to find a 49 state ECU to throw in.

Are you sure it's not the engine protesting the finger prints on the valve cover? :sing:

I would agree that the California ECU is a bad thing for a car with all of the California emissions stuff removed. - AB

Roger McDaniels · **Posted:** Tue Apr 26, 2016 10:18 am

Has the head been shaved or the block decked? Has the block been bored? With the newer bottom end, you're getting a .5 compression bump, and if someone's taken a few thousandths off of the head and/or block and overbored it, then you could be approaching 11:1, which would thump at idle. It's probably too late now, but what was the piston-deck height? I think that stock on those is something huge, like .5mm. Someone could have easily decked more than half of that, bumping you up another .25 or more, depending on the bore size.

Have you verified the cam positions? Getting one of them off can reduce valve overlap and raise static compression, causing it to sound like a diesel at idle.

Chad Culbertson · **Posted:** Tue Apr 26, 2016 1:59 pm

Roger McDaniels wrote:

Has the head been shaved or the block decked? Has the block been bored? With the newer bottom end, you're getting a .5 compression bump, and if someone's taken a few thousandths off of the head and/or block and overbored it, then you could be approaching 11:1, which would thump at idle. It's probably too late now, but what was the piston-deck height? I think that stock on those is something huge, like .5mm. Someone could have easily decked more than half of that, bumping you up another .25 or more, depending on the bore size.

Have you verified the cam positions? Getting one of them off can reduce valve overlap and raise static compression, causing it to sound like a diesel at idle.

Cam position was pretty much spot on when I put it together but it's not a bad idea to double check it again after it's run. That is a good point that could be raising static compression, perhaps I should pickup a compression tester and just see what it reads.

This was Dustin's old car and I know he never did engine work and that's the head i'm using. The block as far as I know came from a car Alec and Andrew Jonell bought and to my knowledge had never been apart. Although, it did kind of look like the oil pan may have been off based on the excess RTV, it was sealed really well, so possibly still factory.

The block is the unknown here, it came out of a 2001 with 110k miles, so I would surely think it was all stock, but I guess anything is possible. I didn't bother checking anything, the head hadn't been decked at all. I seriously doubt the block was, but anything is possible.

Richard Casto · **Posted:** Tue Apr 26, 2016 2:14 pm

Chad Culbertson wrote:

...perhaps I should pickup a compression tester and just see what it reads.

Someone correct me if I am wrong, but it doesn't work that way. To get the compression "ratio", you need a number of specific measurement that are determined prior to assembly. Stuff like deck height, volume of combustion chamber, bore, stroke, etc. I guess that hypothetically you could arrive at some type of compression ratio given some of those number above, the cranking compression pressure (via compression gauge), gas temp in cylinder (PV=nRT), etc. But I can't imagine it would be very accurate.

Richard

Roger McDaniels · **Posted:** Tue Apr 26, 2016 2:33 pm

You can make some assumptions based on a compression reading based on other similar motors. If I see 280 PSI out of a Civic motor, then I know that something ain't stock.

Chad Culbertson · **Posted:** Tue Apr 26, 2016 2:39 pm

Roger McDaniels wrote:

You can make some assumptions based on a compression reading based on other similar motors. If I see 280 PSI out of a Civic motor, then I know that something ain't stock.

Right, I can't determine the compression ratio, but I can see if the static compression in PSI appears to be some abnormally high number from what a normal stock 1.8L runs. Or if it's outside of what the FSM tells me should be normal.

Roger McDaniels · **Posted:** Tue Apr 26, 2016 2:56 pm

Looking it up, it should be around 200 for an NB.

Richard Casto · **Posted:** Tue Apr 26, 2016 3:42 pm

Roger McDaniels wrote:

You can make some assumptions based on a compression reading based on other similar motors. If I see 280 PSI out of a Civic motor, then I know that something ain't stock.

Wouldn't this be also making very broad assumptions about ring and valve sealing ability? Meaning a tight motor with low compression ratio might show higher compression "pressure" than a loose motor with an actually higher compression ratio? And can you actually really see minor compression differences? Then you have the volume of the tester hose, etc. It seems like a lot of variables to be worrying about.

Richard

PS: I really don't know. Just thinking out loud on this.

Roger McDaniels · **Posted:** Tue Apr 26, 2016 4:00 pm

The rings and valves would have to be pretty bad to cause significant losses on a compression test. I'd do a leakdown test for that. Mostly I'd be looking for compression numbers somewhere in the normal range for that motor and without too much variance. I'd think that if it is sounding like a diesel on account of compression, then the numbers should be much higher than normal.

THSCC Forums

Dieselish sounding 1.8 Miata motor

Who is online