Clutch Fluid Burnt/black after 2,000 miles
 

I flushed/bleed the brakes and clutch system with some new bottle, cheap DOT3 fluid back in April or May. Since then the car has about 2,000 miles on it, and 3 nights at the drag strip, about 25 passes total. In 2,000 miles, the brake fluid is still as clear as the day I poured it in. The clutch fluid is dark and nasty looking. Clutch slave/master were replaced in 2013, and have maybe 6000 miles on them. The clutch system probably has water in it, as after making 6-7 passes at the drag strip, I start missing gears as the clutch isn't fully disengaging when I stab the pedal. This has happened before and new fluid always fixes it, but it's a short lived fix before it happens again.

Why would the clutch fluid have such a short life? I bought some DOT4 fluid today and I'm going to flush it and fill with that, but I was curious WHY this is happening and how I can fix it. I searched and didn't find anything, feel free to point me to the thread if this has already been covered/solved.

bad slave or bad master, or both. I'm going to guess slave... although last time this happened to me it was a bad master but I replaced both anyway.

It's rubber o-ring material turning the fluid black, the line is ran free and clear of heat so I doubt it's being burned.

I sucked my reservoir dry as I was bleeding my brakes a week or two ago, the refilled it with clear fluid. It's already going black again, and my master and slave are less than a year old, stainless clutch line.

The clutch goes through a lot larger range of motion, more often than the brakes and with less fluid capacity. I'm guessing my $14 master and slave are just a yearly maintenance item at this point.

Rubber O-ring turning it black seems very plausible. But why? Just a crappy O-ring? Or got too hot? I really want to find the cause of the problem and fix it. Bleeding the clutch system ever 3 months is not my idea of a solution.

I have my old slave and master, I was going to see about polishing the bores and potentially sourcing a better quality o-ring, seal or whatever somewhere other than the $14 atuozone kit. I just bought rebuilt units instead of the rebuild kits last time.

The clutch fluid can't get that hot, it'd be interesting to check it when the clutch goes soft.

I have new (not rebuilt) slave and master on my car. Mazda parts.

What would you check when it goes soft? I'm going to flush the fluid in a few minutes, and going to the track tomorrow. I can check whatever is needed before/during/after running it on the track to see if anything changes.

Just thought of this. So I have an ACT Extreme clutch, about the stiffest pressure plate you can buy for a miata. So I suppose my Slave is seeing a lot higher load than it would stock since pedal effort is much higher.

You might be on to something, perhaps the bore isn't smooth enough and the high load is smashing the rubber O-ring against the bore and rubbing it away. This is assuming the bore isn't that smooth, but I don't know how mazda finishes them.

Interesting though, maybe I'll pull it apart this weekend and see what it looks like. That could be the problem.

I'd just check the fluid temp in the reservoir, see if it is exceptionally hot or anything. It does get cycled a lot and compression will heat it, moreso with heavier parts, but I still can't see it getting soft because of the fluid boiling.

It moves a pretty decent volume on every stroke if you've ever seen a failed master bypassing straight to the resevour, so the temperature might stay kinda even throughout?

I'm interested to see what you find on the bores if you take them apart though, maybe someone needs to make stainless pistons for them or something, slightly larger diameter so the housings can be bored slightly.

I flushed it out. The old fluid literally looked like dirty black water. Thin and dark, with the consistency of water. Flushed the crap out of it, cleaned the reservoir, etc. It's got super clean DOT 4 in it now. We'll see how that holds up for now.

Could be the problem:facepalm:

I replaced the entire system at one time including the master, slave, and switched to a stainless steel line. The system was bled and new fluid provided by 949 was added. The fluid was black again within 3 weeks. I just run it like that because that is how it was before and I have no problems with it and it doesn't seem to have an effect on anything. I'll bleed and switch out the fluid on a regular interval unless I notice leaks or performance degradation.

Interesting. Looks like I'm not alone.

But in my case, it does affect the shifting. I can't get it to shift once the car gets really heatsoaked at the drag strip. New fluid is always the fix.

What fluid did you use? Also, what brand parts?

Try a high temp fluid instead of the cheapo stuff. Bottle of motul rbf600 is cheap and would tell you if it was getting too hot.

Where is that fluid bought cheap? Amazon shows 18.65 for 0.5 L bottle.

But you're right, if this DOT4 I used isn't the fix, I'm putting in unicorn tear brake fluid to see if that fixes it.

Hmmm thought it was cheaper.

Try ATE Type 200. Similar unicorn tear content but you get 1L for $20 $17 (keeps dropping) on amazon.

I am using whichever fluid is provided by 949 kits. I believe it was a motul fluid but I couldn't tell you what type anymore.

Emilio provides Motul RBF600 with the overhaul kit.

all miata fluid does this on the clutch. Doesnt matter, new or old parts, cheap fluid, super blue, rbf600. Has no effect on anything besides looks. Now your problem, guessing the springs popped out of the center of the clutch or the friction material blew apart or just a random unfortunate failure of your new clutch parts.

Miata clutch masters do that. No big deal. Change the fluid if it makes you feel better, but it will continue to work fine if you don't. I change mine once a year between racing seasons. I've seen some Miatas where the clutch fluid was completely black. It still worked.

Made 14 passes, fluid is as black now as it was yesterday before I flushed it.

Also shifting issue when hot is still present.

Adjust the clutch?

I don't know how aggressive you've got yours, but I've redid mine a little looser to alleviate occasional hard shifts. Feels like the clutch hasn't let go entirely as you shove the shifter out into neutral, makes a nice loud bang. Most times I'd even hit the next gear still, like I was so close to the disengagement point that the little tiny bit of torque I had going on was enough to keep things in contact.

I'm not stupid and I know I've got my foot hard on the floor, but it would still only happen occasionally and not since I've made it a bit looser. Maybe you're out there heating up and growing the disc material or something, more room might help?

Isnt there an issue that people often run into with FM clutches about having it adjusted wrong that causes no disengage when hot because of a bleed port or something?

I think it could be the clutch adjustment. I can't even remember where it grabs, going to check that out today and probably adjust it.


Also you mention growing the disk. I'm launching the thing at 5,100 in boost, and slipping the clutch for about 10' out of the hole (so about 1/4 second) but I know that's throwing a ton of heat into it. Was cutting 1.76 60' times doing that. Someone at the track mentioned it getting to hot and the disk growing, so yeah maybe that's it.

But if it is, how do I fix that? Adjust the clutch so it disengages so much that it still disengages when super hot?

Leafy, I've never heard of that, but if you got any more info I'm all ears. I have an ACT Extreme clutch, but maybe I'm having a similar problem.

Ok so I just went and checked the clutch pedal out. As it was, it starts to grab a tiny bit about 1/2" off the floor. That is, I can feel the syncro's having to do work to get it to snap into gear after the pedal was 1/2" to 1" off the floor. But it didn't really grab until 2-3" off the floor.

I adjusted it about as much as possible. My adjuster rod is barely threaded all the way into the nut. Now it's about 1/2 way threaded into the nut, so can't really adjust it out much any more. I also adjusted the upper clutch switch out a touch so the pedal will come up a bit more so I'm not keeping pressure on the throw out bearing when the pedal is out.

This changed the pedal engagement for the better. Will have to run it at the drag strip to see if it's better or fixed. But I'm not sure if this is normal, seems odd. I hate to think my 8yr-old-bought-used ACT Xtreme could be compromised???

From looking at what it's doing, it seems a larger bore master cylinder for the clutch system would help, since that would move the slave cylinder more for the given clutch pedal movement. Or maybe a different clutch line to replace the rubber one, perhaps it's flexing and soaking up some of the travel by stretching the hose.

You still have that nasty rubber clutch and pigtail line?

Bees knees is the 1 piece stainless line, or the short stainless stub line like I've got.

I bet what little extra throw you added in the pushrod will keep the clutch further enough out when hot now, though. I went the opposite direction with the clutch switch, but I'm pretty sure it's not engaging the master at all, I can feel a bit of slack between the pushrod and piston at least.

What clutch line do you have? I'm going to order one, it can only help.

I have this, it uses the firewall hardline still but ditches the pigtail and rubber.

Miata MX5 clutch line

There's a longer one piece deal that replaces the hardline, straight shot from master to slave.

MiataRoadster "Straight Shot" clutch lines MAZDA - MiataRoadster - High-performance customer service...and parts for Roadsters

There may be other places to grab both these.

Let me know what you find out...

I replaced the entire clutch hydraulic system two months ago, brand new master cylinder, slave, ss line all from FM, amsoil brake fluid, flushed it many times...

2500 miles later the fluid is black again. And the clutch pedal moans sometimes when its pushed in/coming out...

I ordered a stainless line to replace the stock rubber one, and a bronze shifter bushing and something else. I doubt that fixes anything but we'll see. I'm actually beginning to wonder if maybe the clutch is worn enough that the angle of the lever that actuates the PP to disengage is affecting how much "throw" it moves, causing it to not fully disengage. No idea, will have to climb under car to look, but that's a possible explanation I came up with.

If the clutch line doesn't fix it, I'm going to look into modding the pedal to gain a bit more travel.

Hm, that could be an explination for your pedal feel issues, but not the fluid darkening?

Yeah I can't explain the fluid getting dark so fast. Only guess is something is getting too hot causing it. I was launching at 5,100 that night so wasn't taking it easy by any means. Cut a 1.76 60'. Still one day it went from new out of the bottle to black.

Shifting issues, replace the master and slave.

I'm going to try the new line first. Both master and slave are new mazda parts installed in 2013. But I'll replace them again if nothing else works.

Curious, what failure inside a master or slave cylinder would cause this problem? Leaking o-ring? Something else?

pretty good 60' time. what tires?

I was reading that the slave cylinder is more prone to darkening the fluid due to it's proximity to the ground and dirt/dust, and its large movement. I haven't taken the slave apart other than taking the plunger out and packing the boot with high viscosity amsoil grease. I'm assuming there is a thick o-ring / gasket...

oh, and what were your 1/4 mile results? I'd guess in the 11s at 110? but I have no idea. I'm a subaru drag guy.

I adjusted the clutch pedal so that it disengages wayyyy before my foot hits the floor. Same result as before, if not worse. I'm starting to think it's not the clutch at all, maybe it's the transmission? And it affects all the gears, I missed 2nd, 3rd, and 4th last night once it got 8-10 passes in a row worth of heat into it.

I think I'm going to swap the Ford fluid for some Amsoil and see if that fixes it or at least has any affect.

This helped me.

1 quart of MTG, 1 of syncromesh. It didn't fix it completely but it was better than the ferd stuff.

I just picked up some mopar stuff that made my gf's jeep transmission go from shifting like a screwdriver in a box of rocks, to a screwdriver in something smooth.

Its like $22 a quart though. Might be worth a shot.

Also emailed ACT, told them I bought this clutch used 8 years ago, and asked if it's still under warranty. :)

Interesting. Did you try straight MTG before doing the mix? Curious why you went with the mix. My syncro's seem to quit working when it gets hot.

I did not try straight MTG. The mix was recommended by a few people in my thread.

Alright thanks. I think I'm gonna try the Amsoil, and then if that sucks a mix since I'd just have to drain 1 qt out and add a qt to do the mix. Thanks for the info though, I remember someone here running straight syncromesh now that you mention it.

The cap on the clutch reservoir is vented with a direct port open to atmosphere. This lets the clutch fluid absorb moisture much quicker than the brake fluid does. I replaced reservoir caps and masters/slaves several times trying to keep the clutch fluid as clean as my brake fluid. But could never keep the fluid from turning black in a month or two. The best option is to run fluid with the highest wet boiling point on the street and just live with it and then do a fresh flush before a track day weekend. I have been doing that for years now and have not had clutch issues on track.

Also make sure the clutch pedal is adjusted per FM instructions, that removes any contributions due to incorrect clutch pedal.