Clutch line heat soak

[NDNDNDND]

I have a Mk 1 MX-5. As the clutch wasn't feeling its best, the master cylinder was the original and the slave leaking, I replaced both the master and slave. While I was there, I removed the original, long hard-line and replaced it with a 'straight shot' style braided hose directly from master to slave.

The clutch feels great, when the car is cold. As things warm up, I seem to be getting heat-soak issues. Gears become progressively harder to engage, and last night, while manoeuvring into the garage, the clutch failed to disengage, I couldn't pull it out of gear and had to kill the engine before it stalled itself.

This morning, and driving to work, it's been fine, albeit a little grabby when parking up.

I've ordered some high-spec DOT4 fluid (Gulf RF1000), but I'm also considering using some heat-shielding tubing I have.

My question is: where is the majority of heat soak likely to be coming from? Will it be radiant heat from the engine, or could it be through heat soak from where the clutch slave is attached to the bell-housing? With the former, I'm thinking the heat tubing would help keep the line cool, but with the heatsoak from the bellhousing, I'm thinking the extra insulation might actually prevent the line from cooling down.

Any thoughts?

(Yes, I could re-install the hardline as it clearly had some cooling benefits, however the 'straight shot' line seems to have considerably sharpened the clutch, improved the gearshift, and made bleeding easier... so I'll see if I can make it work first!)

[stevieturbo]

The chances of heat being the problem are almost zero.

Ensure it is bled properly, ensure the parts you changed were of good quality and installed correctly.

[TooMany2cvs]

NDNDNDND said:

(Yes, I could re-install the hardline as it clearly had some cooling benefits, however the 'straight shot' line seems to have considerably sharpened the clutch, improved the gearshift, and made bleeding easier... so I'll see if I can make it work first!)

I find that hard to believe... Metal pipe won't bulge or anything like that. Hydraulics simply pass fluid from A to B, to let the cylinders do their work. Flexi certainly won't last as long as some new cupronickel, and won't be as easy to route.

stevieturbo said:

The chances of heat being the problem are almost zero.

Unless it's routed too close to the exhaust...

[PositronicRay]

If you're unsure put the original pipe back on, just to confirm or rule out.

[GreenV8S]

It may just be a bleeding issue, but I guess you have bled it a few times by now.

From the symptoms I wonder whether the hydraulics don't have much spare travel and the hose is simply expanding under pressure when it is softened by heat. If that was me I'd check whether the hose is moving (deflecting) when pressure is applied to the clutch and also measure the OD accurately to see whether it is expanding under pressure when hot. Even a slight expansion could add up to a significant volume over the length of the hose.

Is the hose designed for use in hydraulic systems?

[NDNDNDND]

The clutch line I'm using is this:

https://www.mx5parts.co.uk/clutch-line-master-slav...

A stainless steel, braided hose by HEL.

I agree with you Stevie about the heat being generated not seeming to be enough - when the clutch actuation disappeared before, I jumped out and felt the clutch hose, and while it was hot, it certainly wasn't too hot to touch. That said, I could only get to the part nearest the master cylinder, so it may have been considerably hotter deeper in the engine bay, nearer the bellhousing.

What's making me think it's heat are that the clutch seems to be at it's worst after coming to a halt after a run - when I meet traffic after a short dual-carriageway spat, or when idling the car as I open the garage doors. Plus the complicated design of the original hardline - it runs back and forth across the bulkhead, then through a short length of hose to another hardline with a double-loop in it before the slave - makes me wonder if it was laid out like that to provide fluid volume and surface area to help dissipate heat. My 'straight shot' line has considerably reduced the volume of fluid and, being a rubber-sheathed hose, doesn't provide the opportunity to radiate heat like a copper/nickel line would.

I tested my theory a little by sitting with my foot on the clutch for 30secs or so when I got home, thereby cutting off the reservoir and limiting further the volume of fluid to dissipate heat. Sure enough, the clutch became grabby and the biting point dropped to barely a centimetre off the floor.

[stevieturbo]

Sounds more like there is either air in it, or the master/slave combo were of poor quality and allowing fluid to leak past.

it will be nothing to do with heat in the fluid.

You've changed major parts of the system, unless you changed them with genuine OEM parts...they would be my first port of call. In fact...if there was nothing wrong with the old I'd be refitting them !

[TooMany2cvs]

NDNDNDND said:

What's making me think it's heat are that the clutch seems to be at it's worst after coming to a halt after a run - when I meet traffic after a short dual-carriageway spat, or when idling the car as I open the garage doors. Plus the complicated design of the original hardline - it runs back and forth across the bulkhead, then through a short length of hose to another hardline with a double-loop in it before the slave - makes me wonder if it was laid out like that to provide fluid volume and surface area to help dissipate heat. My 'straight shot' line has considerably reduced the volume of fluid and, being a rubber-sheathed hose, doesn't provide the opportunity to radiate heat like a copper/nickel line would.

The back-and-forth sounds odd, but things like double loops and short flexis are there to allow movement.

I know the MX5 is a Japanese design, so nominally "intended for" RHD, but I wonder what proportion of the design process had LHD firmly in mind. Perhaps the odd routing is a legacy of some strange compromises for that?

[NDNDNDND]

stevieturbo said:

Sounds more like there is either air in it, or the master/slave combo were of poor quality and allowing fluid to leak past.

it will be nothing to do with heat in the fluid.

You've changed major parts of the system, unless you changed them with genuine OEM parts...they would be my first port of call. In fact...if there was nothing wrong with the old I'd be refitting them !

The master cylinder is a Blue Print one, and seemed to be reasonable quality. I saw no evidence of leaking as I bench-bled it, anyway. The old one didn't show much evidence of failure and has been stored 'just in case'.

The slave is an IL Motorsports one, as supplied by MX5parts. The old one had blown its seals.

I'll start with a good bleed with good-quality fluid, and then start swapping components - I could try a bench-bleed with the old master and see if it shows any signs of seal failture.

[GreenV8S]

If you've got long runs back and forth and loops and so on, bleeding might be quite tricky.

[NDNDNDND]

TooMany2cvs said:

The back-and-forth sounds odd, but things like double loops and short flexis are there to allow movement.

I know the MX5 is a Japanese design, so nominally "intended for" RHD, but I wonder what proportion of the design process had LHD firmly in mind. Perhaps the odd routing is a legacy of some strange compromises for that?

Well, that was my first thought too - that the extra looping is somehow allowing for the LHD market. The only possibly explanation I could think off was as cooling - it reminds me of the looping PAS pipes on my Chevy, running from the pump across the radiator and then back again to the steering box.

The loop looks like it would be to allow for movement, but there's already a rubber hose to take up differential movement between the body and engine... so the loop must only be there for the sake of making the pipe longer (oh, and to provide a nice air-lock to make it trickier to bleed).

[NDNDNDND]

GreenV8S said:

If you've got long runs back and forth and loops and so on, bleeding might be quite tricky.

The 'straight shot' hose eliminates all the complexity of the OEM pipework and runs directly from the Master cylinder straight down to the Slave - it's one of the big selling points over the stock setup!

[stevieturbo]

TooMany2cvs said:

The back-and-forth sounds odd, but things like double loops and short flexis are there to allow movement.

I know the MX5 is a Japanese design, so nominally "intended for" RHD, but I wonder what proportion of the design process had LHD firmly in mind. Perhaps the odd routing is a legacy of some strange compromises for that?

I've seen teh long runs of pipe on a few cars....and see no reason for it to be a RHD or LHD thing, but it is clearly deliberate. Haven't a clue why they would do it though unless a long thin line s to try and soften/dampen pedal movements.
Much in the same way some Euro cars would have a restriction built into the line at some point

[GreenV8S]

That sounds plausible - a long line could have a significant drag given the length of travel involved and that might avoid people dropping the clutch too fast. A few feet of pipe would probably work out cheaper than adding a separate restricter to the circuit.

[DrDeAtH]

I reckon a braided teflon line would be better than a braided rubber one...

[NDNDNDND]

stevieturbo said:

I've seen teh long runs of pipe on a few cars....and see no reason for it to be a RHD or LHD thing, but it is clearly deliberate. Haven't a clue why they would do it though unless a long thin line s to try and soften/dampen pedal movements.
Much in the same way some Euro cars would have a restriction built into the line at some point

GreenV8S said:

That sounds plausible - a long line could have a significant drag given the length of travel involved and that might avoid people dropping the clutch too fast. A few feet of pipe would probably work out cheaper than adding a separate restricter to the circuit.

Hmm... I think my heatsoak theory is more likely. I'd be interested to be proved wrong, but I'd be surprised if the length of the pipe would have any dampening effect at all. As far as I know hydraulics (when properly bled!) don't work like that.

As for cheapness, I'll bet a restrictor grommet is substantially cheaper than an extra metre of metal tubing, plus half a dozen mountings and fixings and labour to install it all...

[NDNDNDND]

Thinking about this coldly, if someone presented me with this evidence, I'd probably tell them it was either an hydraulic fault, or the clutch hadn't been bled properly.

If I'm honest, this is probably where I am too! It would be reasonable to think that either my new clutch master or slave are faulty. The slave will be a quick visual check, but I've realised I can check the master by clamping the slave closed and then checking for resistance at the pedal - if the pedal still descends it's likely fluid is getting past the seal and returning to the reservoir and my new master cylinder is faulty.

In the meantime, I just read about an old boys' trick of placing a brick on the clutch pedal and leaving it depressed overnight to squeeze any air out of the pipe. Sounds a long shot, but no harm in trying. I shall listen carefully for a gurgle from the MC when I take the brick off the pedal tomorrow!

[stevieturbo]

There is nothing whatsoever plausible about heat soak.....largely because there is nowhere for the heat to come from that's hot enough to affect it !

And Green already explained how a long thin length of hose could affect clutch operation.

[Mr2Mike]

NDNDNDND said:

In the meantime, I just read about an old boys' trick of placing a brick on the clutch pedal and leaving it depressed overnight to squeeze any air out of the pipe.

You can't squeeze air out of the pipe, if it's not leaking then it's a sealed system so how can air get out? This method allows the air to dissolve into the fluid, like CO2 in a fizzy drink, and just like a fizzy drink it can escape the fluid amd expand again.