flier129

iTrader: (7)

I've got a nasty 'knock' noise going on. It basically started after I was a quart to a quart and a half low on oil and then when the weather got warmer and my 30 degree tune didn't work out so well. Changed my oil and it got slightly better but is still there.

It doesn't seem like knock because it idles with no indication of the sound. But I could be completely wrong there. Its a weird clank/knock noise that starts at 2800ish rpm and continues at WOT. I can cruise at 3700 rpm and hear nothing, but if I give it more throttle it will make the sound. It won't make the sound if the engine is cold in the mornings, this part throws me off.

The sound doesn't seem to speed up with the engine speed. It does get a tad faster after 4500 rpm. Although I can't hear much because of my stupid loud exhaust. Hit 97.9dB at the last autox :-\. Hoping the turbo will muffle that once I get it on but i really want to sort out this noise before hand, of course.

I've got a 2000 head on my 97 block, running MSPNP self butt-tuned never running any leaner than 15.3 afr while cruising and sitting at 12.9-.13.2 afr at WOT. I run full synthetic and the block has around 80k and the head has about 38k mi now. Although my engine has seen hell and back with its different tunes from running the headswap with the stock ecu not knowing what to do and having MS base tune aka rich as hell

So I'm just looking for some extra opinions on what it could be. Help the lost nub out :-D

gospeed81

iTrader: (51)

Since you're still N/A I'd say it's probably not motor internals.

What else have you touched? > exhaust maybe?



It's really nerve racking as you play with tunes. I'm doing the same thing, and get different buzzes and vibrations, and oddly my HLAs are now audible where they weren't before.

I think we're just making different amounts of power at different places in the rev range, which shakes bolts/etc in new ways.

flier129

iTrader: (7)

well the noise is coming from the intake side of the engine, top of the block, bottom of the head. maybe valve lash and it out of line?

Sound is definately coming from the engine, if pull the TB cable itll make the sound around the said 2800 rpm.

On a good note, the engine still pulls nicely . I've got some other vibrations,clinks and shakes but thats just from old diff bushings. Going to replace those soon, my clutch definately brought out the flaws in my chassis, including my open diff lol. torsen goes on soon!

gospeed81

iTrader: (51)

In that case...the running it low on oil part may have something to do with it. Valvetrains generally don't like that. You and my little brother...he's a dipstick that can't find a dipstick, but sure as hell knows where the accelerator is.

Have you tried holding either a section of hose or a looong socket extension to different parts of the motor in that area and listening? Maybe the CAS get's noisy on their way out? I've got a vacuum leak in that area I can't find for the life of me.

Sorry I'm not more help, but I feel for ya.

flier129

iTrader: (7)

I havn't 'stethoscoped' anything yet. I've been gentle on my engine except for autox which its already seen two events with this noise.

I've just got to check all possible lose brackets,etc on my next day off.

Thx for the feed back though

dynokiller90

spun rod bearing. when the oil is cold it has a higher viscosity and masks the knock. that is until you put a load on it or the inertia reaches a point that induces knock again. ie 2800 rpm. I would assume that at cruize it still knocks slightly but is not audible due to road noise and then when you step on the throttle it knocks loud enough to be able to hear. I have also seen this happen where the knock completely disapears upon decel.

I say do yourself a favor and pull the oilpan off and check for slop. if you catch it soon enough you can get away with polishing the crank and going with a slighly oversized bearing. if you waste a bunch of time trying to track down phantom noises that you hope it is you could destroy a lot more than just the bearings. . . . obviously.

flier129

iTrader: (7)

Hrmm, makes sense. would it be easier to just get a new block instead of bandaiding my current motor?

this is frustrating, sounds starts a week after my turbo gets in haha.

dynokiller90

well don't just go throwing money at a new engine without diagnosing first. for the conditions you posted I feel that it is the most likely problem. seen it a thousand times.
so like I said you should pull the pan and check to see. best case you are out the cost of a gasket and some labor. worst case you've destroyed the crank.

best solution though is: slightly scarred crank, so you take the money you were going spend on a new engine and invest it in pistons, rods and machine work. if you take your crank, rods and pistons to a machine shop the first thing they are going to do is polish out the crank journals and mate some bearings to it any way. it's not a bandaid per say it's just a propper machinig job. if done within specs it will have no ill effects on anything.


here's also something you can try. pull all of the spark plugs out and rotate the engine. stop when you have a pair of pistons @ bdc. don't rotate backwards, this is very important. the idea here is to pull the rod and piston down, but to not let the crank move anywhere past bdc. now take a wooden dowel and push down on the top of the pistons. check for any movement. remember even the slightest movement means bad things so don't try to fool yourself. repeat for the other pair of pistons. repeat a couple of times just to rule out user error. this way you're only out a few bucks at the hardware store for a dowel and half an hours time.

flier129

iTrader: (7)

Thx for the feedback dynokiller, I'll be checking this tuesday when I'm off work. In the meantime my car is guna be parked haha. You said you've seen this lots so I take it you've had some mechanic job time or just been a monkey-wrench your whole life? This problem definately makes me think about investing into some good rods,pistons, good machine work, and maybe studs. Could run on my hp goal very reliably and I think that has been discussed lots on this forum haha.

dynokiller90

yeah, although I'm no longer a professional, I still turn wrenches on the reg. Had to give up the profession due to a pretty serious injury. started working on cars at the ripe old age of 13, and spent the next 15yrs as a professional mechanic / fabricator at a speed and custom shop.

check it out and post back. good luck

gospeed81

iTrader: (51)

ouch, bad news flier. Hope it's not the case, but like you, I trust the voice of experience. Good excuse for a build though.

I worry about this happening soon after I start boosting. I'm already lining up a back-up motor.

flier129

iTrader: (7)

Well had a local mechanic/friend take a look at it, he has lots of experience with miata's and is a master technician for landrover/jaguar dealership. Never the less, i trust his judgement.

Said it was detonation ping, maybe from a plug running too hot or too cold.Found out my engine ground nut where the oil dip-stick is also connected to came off, I need a new CAS seal...... badly. Talked about the headswap, had no idea head studs are a one time use because they'll stretch. That could lead to an coolant/oil leak or blown oil out and then when my wideband is reading rich, its not actually rich but is just reading the oil. So its all kinds of fucked up atm.

Probly going to buy this block off a local guy for cheap, rings,seals,new studs. If i had the money on hand i'd just get some forged rods and pistons. Cant wait for cash since its my DD. But there is this built 1.9 gtx motor on m.net classifieds haha.

I told him what you said dynokiller, he said it didn't sound internal from outside or inside the car. I'm sure if you heard it IRL you'd have a different diagnosis too, considering my explanations are pretty shitty. Thx for all the feedback tho .

dynokiller90

welp, hope it all works out for ya. some times that's the problem with the internet. actualy being able to put your hands on it is allways better whether it be a good or shitty explanation. it was a great explanation for a rod bearing though. haha

DXO

iTrader: (2)

My 2000 has this same problem, I had recently changed the oil then it sat for 2-3 weeks. One morning I cranked it up and the same thing you describe starts. I shut the engine off right away and check the oil and see none on the dip stick. I fill it with oil and still does it. I used a stethoscope and it seems to be in the valve train. More specifically rear cylinder intake side. I have not driven the car and it has been sitting. ATM I don't have the money to get head work done. So, I have not taken the valve cover off to check things out.

flier129

iTrader: (7)

Well I suck at basic maintenance and im dripping a **** ton of oil out my CAS seal. Going to replace that ASAP lol. It never really hit the ground but I put 4 quarts in, 600-700 miles later, lost too quarts, put my finger on the back of the head under the CAS and tada, wet oil.

Still shooting for a rebuild though, found a local block for $200 so we'll see how that goes.

flier129

iTrader: (7)

After further thinking my friend says rod, little end or big end. So back to what dynokiller was saying haha.

flier129

iTrader: (7)

Now leaning toward the bushing that goes around the wristpin and that making it rattle. Getting the block from a local here soon, tearing it down, getting new bearings,seals,rings,new head studs, and gaskets. Yanking my motor tearing it down, seeing whats wrong with it and if its what we think I'll be able to salvage it and save money and buy forged internals . Thats if i dont come across a built engine for a good price

AlexandertheOk

iTrader: (1)

If im not mistaken that bushing is a press fit, do they fail often?

flier129

iTrader: (7)

Yeah i was hoping someone else could confirm that lol.

dynokiller90

you know I can't say I've heard of many failing in miatas. it's not an uncommon thing in general. it is more often the big end that fails though.