Funny gearbox issue.

[Rasputin]

Hey guys, not sure if you can be of any help; but I have this odd gearbox issue.

When the gearbox and engine is cold, entering and exiting gears is very fluid and smooth.

As it starts to heat up, entering gears becomes much much more notchier and crunchier. There is no audible crunch, but if I were to explain it, I'd do so in saying it feels like there is a wall between neutral and the gear of choice. So when I put it in gear, there's free play (neutral) then this small wall, and then it pops into gear.

Occasionally when entering first from a stand stop, this "crunch" feeling occurs, and then the car lurches forward a millimetre or two.

When I mesh the gears by rev matching, it slides into gear without a problem.

Could it be that the clutch isn't disengaging enough? Even so, it doesn't explain why the gears are so fluid and smooth when cold.. Unless the heat is causing a pooof tenth of stretch in the clutch cable.


Dan.

[22Silver]

Problem goes away when revmatching maybe related to syncro?
^Highly uneducated guess with no reasoning research or warrant to claim.

[hks_kansei]

The fact that rev matching stops the issue would suggest worn synchros.
But the fact it only happens when warm is odd, a stuffed synchro would cause issues regardless of temperature.


First I'd be checking it's the right type and amount of gearbox oil in there.

If you have the ability, I'd also suggest changing it, if for no reason other than to check for particles in the oil. (there's always deposits on the drain plug, so don;t worry about them, just rub them between your fingers, they should be a really fine powder. If there's bigger chunks, then there's a potential worry, but again, not the end of the world)

[Blade_Hunter]

My guess is a) Synchros are stuffed and b) to explain the cold operation your gearbox oil is old and goopy, once it warms up it would become more viscous making the synchro issue more obvious.

This response is based on "I think", there is no real mechanical knowledge in this brain of mine

[TTT]

Occasionally when entering first from a stand stop, this "crunch" feeling occurs, and then the car lurches forward a millimetre or two.

Could it be that the clutch isn't disengaging enough? Even so, it doesn't explain why the gears are so fluid and smooth when cold.. Unless the heat is causing a pooof tenth of stretch in the clutch cable.

Definitely clutch related.
Get it adjusted to grab a bit higher.

Worst case it's been adjusted many times before and the clutch is on it's last legs.

[project.r.racing]

My guess is a) Synchros are stuffed and b) to explain the cold operation your gearbox oil is old and goopy, once it warms up it would become more viscous making the synchro issue more obvious.

This response is based on "I think", there is no real mechanical knowledge in this brain of mine

i'd be inclined to think the same.

[Pokmx5]

change your gearbox oils. When draining it check it to see if there is any shards in there while your at it. I assume the clutch is not slipping? But i would primarily drain the oil and top it up and see how you go..

[Rasputin]

Clutch was just changed out to an exedy hd button clutch. Was occurring before and after.

I fear it's the synchros too. I'll check logbooks on last gearbox oil replace.

I'm getting the short shifter kit installed soon by mania. Might book in for a routine gearbox check while it's there for shits and giggles.

Thanks for the feedback guys.

Dan.

[sailaholic]

No clutch cable, it's a hydraulic unit.


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[Rasputin]

No cable going from pedal to the master cylinder?

I'll let you guys know what happens after I get the clutch to disengage a bit mire, drain and replace oil and get the short shifter installed.

Many thanks!

Dan.

[Fra66L]

I'd also be tempted to flush the clutch fluid if it hasn't been done yet and check/clean the clutch slave cylinder in the process. I've observed differing cold/hot gearbox issues before on another (non-mx5) car and it turned out to be the clutch hydraulics.
Gearbox oil and clutch fluid easy and cheap(er) places to start.

[Rasputin]

I'd also be tempted to flush the clutch fluid if it hasn't been done yet and check/clean the clutch slave cylinder in the process. I've observed differing cold/hot gearbox issues before on another (non-mx5) car and it turned out to be the clutch hydraulics.
Gearbox oil and clutch fluid easy and cheap(er) places to start.

I can't believe I didn't think of this.

Your issue was probably caused by water/humidity being trapped inside the hydraulic system.

Brake fluid is extremely resistant to compression - while water is not.

Could definitely be losing some travel in the system if water is present in the line somewhere.


Thanks!!

Dan.

[Fra66L]

Your issue was probably caused by water/humidity being trapped inside the hydraulic system.

Actually, it was a heat related expansion/contraction issue that was exacerbated by old clutch fluid (that was most likely moisture contaminated).

Good luck and keep us posted.

[Mr Morlock]

If you have just changed the clutch and it was done by a professional then there should be a warranty. Its unlikely that a pro would not have set it up incorrectly and may well have changed the clutch fluid. I would get perhaps the clutch fitter to drive the car for a diagnosis. If the synchros are worn then it will need to be dríven sympathetically. Repairing the synchros is another story- a job I have never done.