Hoping for a little help in figuring something out. My daughter's '91 Miata (original 1.6 with 108k mi) started making a horrible racket. It's quite violent, and I'm trying to figure out what it is. She says it started on one drive, but I haven't been driving it, and she plays her music loud enough to hear it over the car, so I don't know if it started all at once or developed over time. I have a short video with the noise, but I don't know where to upload it so I can link it here. It's similar but louder than some videos I've seen claiming to be lifter noise. The mechanic's stethoscope also made it sound like it's on the top, but I've never had great success with that tool. So, I pulled the cams and cleaned out all the lifters, but the noise didn't change. Also on the lifter front, there's a lot of talk about oil, and the oil was changed 1k mi ago with the same exact oil that was used for the previous 5k. We did have an occasional light lifter tick on startup, but that went away over time as the car started to be driven regularly.
So the other source that the internet seems to point to is rod knock. I've never tried to diagnose that. One video showed dropping the oil pan to get at the rods and mains, and I can't think of another way to figure it out. Is that my next step? Anything else I should look at or do first?
One other data point is that we replaced the clutch/flywheel/pressure plate/TO bearing/pilot bearing just a couple hundred miles ago. I can't see how that could matter, but
If anyone can point me to where to upload the video, I'd be happy to add it. Or I could email it to someone (17Mb) if anyone wanted to volunteer to do it for me. The noise is very easy to hear on the video.
I uploaded the video to Vimeo. Hopefully this link works.
https://vimeo.com/562271122
In reply to pkingham (Forum Supporter) :
I've not personally experienced rod knock on a Miata, but that doesn't sound like lifters to me. Unless you possibly have more than one lifter that's totally collapsed.
In reply to Pete Gossett (Forum Supporter) :
Yeah, the lifter angle was mostly wishful thinking on my part, unless I'm missing something. More than one collapsed lifter would produce a higher frequency, I think. I've never experienced rod knock on anything so I'm in uncharted territory if that's the issue.
In reply to pkingham (Forum Supporter) :
Did you put your ear under the car while it was running. If it's lifters it should definitely be louder up top.
I haven't actually been under the car while running. I can try that. I did use a simple stethoscope and poked around, and it seemed like it was louder on top. Unfortunately I think this fits in the category of not knowing enough about how to interpret the data. The oil pan should be a good speaker for a bottom end noise, though, so I'll put it on the lift and try listening under it.
I think it's lifters from the sound. Pull the valve cover and look for one that's not quite making contact with the cam.
Pulling the pan is not easy, the subframe has to come out and it's not simple to seal with the engine in the car.
In reply to Keith Tanner :
I would love for it to be lifters, but I've pulled the cams and cleaned all the lifters, and I have the same problem. Is there something else to look at that affects the lifters?
Just to be complete, cleaning the lifters consisted of disassembly (all except the check valve), cleaning everything with brake cleaner and confirming free movement of the check valve, and then reassembly oiling liberally but still producing springy movement of everything. I also pumped them up submersed in oil until no bubbles came out.
A problem with oil feed to a lifter can cause problems, and I've also had healthy ones rattle for a while after a cam seal change. I'd pop that cover off and check for gaps - in large part because it's much, much easier than going into the bottom end.
In reply to Keith Tanner :
Ok, let me describe what I think you're saying.
1. Run the engine for a minute to be sure whatever is going to pump up is pumped up
2. Pull the cover and check the gap between the cam and lifter for every valve with the cam at the base circle. Anything anomalous indicates a potentially bad lifter or oil supply.
Is that correct? Presumably a properly working lifter would have zero gap, right?
That sounds like lifters to me.
If it's a rod knock, it will usually get progressively louder with RPM. Oil pressure is usually very low, especially at idle.
In reply to Carl Heideman :
Hmm, interesting point about oil pressure. Cold start oil pressure is just under 60 psi per the gauge on the dash at ~1400 rpm. I'm afraid to run it long enough to get it warm and at a normal hot idle, though.
pkingham (Forum Supporter) said:
In reply to Keith Tanner :
Ok, let me describe what I think you're saying.
1. Run the engine for a minute to be sure whatever is going to pump up is pumped up
2. Pull the cover and check the gap between the cam and lifter for every valve with the cam at the base circle. Anything anomalous indicates a potentially bad lifter or oil supply.
Is that correct? Presumably a properly working lifter would have zero gap, right?
Zero gap on a hydraulic lifter, yes. If you'd like, I can check the manuals to see if Mazda has a procedure.
Probably no need to run it very long, it shouldn't make that noise for more that a second or two at start so cold pressure should be enough.
Note that a series of short start/stop cycles will make the car more prone to this noise - like backing in and out of the garage a few times.
Ok, I'll take a look at that, hopefully tomorrow. Thanks for responding.
Pull the plug wire off each plug one at a time while it's idling. If it's a rod it'll get noticeably quieter when you pull the plug on the cylinder with the bad bearing.
Just checked for cam to lifter gap, and none found. Pulling the plug wire on #4 seems to change the noise which is consistent with the noise sounding like it's more towards the back of the engine. Also the last time I started it, the starter struggled to crank it over. The battery is a little low at 12.4v, but I'm wondering if a bearing problem is adding drag. I didn't notice anything while turning it over by hand to check for lash.
I guess I'm convincing myself it's a bearing problem. Anything else I can look for or any other way to definitively prove/disprove rod bearing?
if not, given Keith's comments about getting to the bottom of the engine, maybe I should pull the engine. I'd do a leakdown first to decide how far to tear it apart once it's out.
I'd love to be wrong but I'm pretty sure that's a rod bearing. If that's the case you should really tear the whole thing down. Both the crank and the connecting rod will be damaged. I wouldn't bother trying to pull the pan in the car. I don't think it's possible and the come out in under an hour.
If you stick a dowel or long screwdriver down the plug hole with the piston roughly half way down the bore and rock the crank back and forth there may be enough play to be noticeable. Compare it with another cylinder to get a feel for what it's supposed to be like. Ideally you'd use a dial indicator for that test but a noisy as that is I don't think you need that precise a tool.
A note on oil pressure. I'm pretty sure the factory gauge on those is just a yes/no indicator and only reads the same at 10PSI or anything above whatever the cutoff is as it does at 60PSI. I've also seen 1.8L Miata engines carry decent oil pressure on a mechanical gauge with a spun rod bearing.
In reply to APEowner :
It's an early 1.6 NA so it has a real oil pressure gauge. The later NAs and all NBs have the dummy gauge.
DeadSkunk (Warren) said:
In reply to APEowner :
It's an early 1.6 NA so it has a real oil pressure gauge. The later NAs and all NBs have the dummy gauge.
You can quickly tell the difference by looking for units. If it's just L-H, it's a dummy. The change was 1996, I think.
It does sound like the next step here is an engine pull.
Btw, if the thrust bearing is damaged enough it will start to add drag when cranking with the pedal depressed.
APEowner said:
I'd love to be wrong but I'm pretty sure that's a rod bearing. If that's the case you should really tear the whole thing down. Both the crank and the connecting rod will be damaged. I wouldn't bother trying to pull the pan in the car. I don't think it's possible and the come out in under an hour.
It is possible (I've done it in my garage), but I agree that the right answer is to pull the engine.
I've done it too why which is why I don't recommend it :) It's no time-saver.
Cut open the oil filter before pulling the motor.
If there is no metal in the filter, it's not likely a bottom end problem.
I had one that carbon knocked....
Ok, engine is coming out. Thanks for all the feedback. I'm thinking engine and trans together unless someone has a reason I should just pull the engine by itself. I didn't enjoy the bell housing bolts when doing the clutch job, so it seems easier to pull both as a unit.
I've never cut open an oil filter, so I'll try that, too. I would think carbon buildup related knock wouldn't show up at idle, though, so I'm thinking that seems unlikely to be my problem.
It'll be 2-3 weeks before I can get back to this car. I'll update this thread when I get things apart.
