Sudden and complete loss of power in SP

[MX5-SP]

The CAS failure on an NB8B that I witnessed was only happening on slowish uphill crawls (i.e. Springbrook and Mt Tamborine) in summer conditions. The engine would die then fire up again after a 10 minute rest with the bonnet open.

Sounds very similar to what happened to me, John, except it wasn't that hot. However, I was "stuck" behind a couple of very slow-pokes for quite a while due to the double white lines, but got a rush of blood when I could see a safe overtaking area. You won't believe the amount of flack I got from the BOV when I applied some right foot. I think I'll slip her some valium into her skinny-chai-latte the next time I take her with me.

Benny, I did take a look under the bonnet for any obvious signs of something coming adrift, but could find anything when tracing pipes, leads, wires to their respective targets. That's what made it so intriguing. I honestly thought I'd snapped the timing belt - BTW is there an easy way to check that? If it is the CAS (...and that's the current thinking) what's to stop it happening with a replacement unit? I was in a very awkward place and couldn't move in either direction, so I'd hate to think of it dying so quickly on something like Cunningham's Gap, GOR, or the Amalfi Coast.

Okibi, several chaps (...and also AJ) have suggested the coil packs, so I'll keep them on my list of possibles. I also agree with your comment about the usefulness of this forum.

[JBT]

You won't believe the amount of flack I got from the BOV when I applied some right foot.

Oh yes I would - get it all the time from mine

I honestly thought I'd snapped the timing belt - BTW is there an easy way to check that?

Well that hasn't happened to you. However, the easy way to check that is to look in the cam area by removing the 710 filler cap while turning the engine over on the starter. If the engine is cranking and the cam isn't turning the belt has broken.

If it is the CAS (...and that's the current thinking) what's to stop it happening with a replacement unit? I was in a very awkward place and couldn't move in either direction, so I'd hate to think of it dying so quickly on something like Cunningham's Gap, GOR, or the Amalfi Coast.

The NB8B CAS seems to give up at about 60,000 km. Maybe they can only put up with so much vibration and heat - a bit like knock sensors. Your concerns are fair enough - when it quits it is total power loss/engine failure.

[NMX516]

Andrew I have a spare set of coils/leads floating around here somewhere if you want to borrow for a trial...

I thought I had a spare CAS too, but I can't find it so might have tossed it out.

[Caffeine]

My cam angle sensor slowly started to die after 230,000km so there's definitely life in them. Excess heat like that produced by an SP may hasten failure.

Mine didn't die outright, but the car struggled with power. My coilpacks were also dodgy and once I replaced both the cam angle sensor and the coilpacks the car is back to normal.

Note that it may take 20km or so driving on the new parts before you see a benefit (especially with the coilpacks) as the ECU has to learn that it can advance the timing again.

[MX5-SP]

UPDATE 09-OCT-2010:

I sourced a replacement part from MX5-Plus which managed to get it to me in a couple of days (...I do have a concern about the way packaging used, but I'll raise that with the chaps out there). Whilst it's easy to replace, I almost started dismantling the wiring before I realised there was a harness clip-lock (...so maybe a trap for newbies?). So now just waiting for the rain to stop before going for a fang up Springbrook to see if I can replicate the same conditions as when the breakdown occurred.

I DID notice though, that the cap I used to block-off the dooflicky pipe when I did the T-Piece mod, was missing. So I'm not sure if that might have been the culprit, or maybe a contributing factor. Anyway I've replaced the cap and used a cable-tie to hold it there. I'll keep checking that and if necessary, I'll do something more permanent.

[Steampunk]

I DID notice though, that the cap I used to block-off the dooflicky pipe when I did the T-Piece mod, was missing. So I'm not sure if that might have been the culprit, or maybe a contributing factor.

In light of this, my new guess is that the EMS might have gone into "limp home" mode, hence you were able to drive home after giving the car a respite.

That pipe is post turbo and MAF, and pre MAP sensor (does the SP have one?) and Lambda sensor.
The MAF is telling the EMS that the car is sucking in "X" amount of air, but the MAP and Lambda sensor is not concurring and seeing "Y" amount of air/pressure, so it chucked a spazz (technical term).
In order to try to replicate it properly, drive it as you are going to do, and then do the same drive sans plug. I realise you would be hesitant to do the latter, but it will confirm if the plug was/wasn't the guilty party.

PS: bring a spare plug

[NMX516]

I DID notice though, that the cap I used to block-off the dooflicky pipe when I did the T-Piece mod, was missing.

That would have sent your boost levels through the roof, and also created an air leak, both of which should have caused the ECU to pull the plug on festivities. I say should have, because not 100% sure how the ECU is programmed to react to such conditions. I would be very surprised if there was no emergency over-boost cut facility. No MAP sensor on the SP, but the extra air flow needed for higher boost levels could well have sent the ECU into an emergency response which shut things down.

[MX5-SP]

Almost 12 months since I replaced the CAS and I haven't had any issues. I still get a wee bit nervous if I have to follow any cars slowly when going up hills, but I haven't had a repeat performance of the breakdown, so I'm going to go along with it being the CAS.

Thanks to all who responded. It really goes to show the value of these forums.



Andrew