Catalytic convertor or not

[MalcQV]

I had Mrs QV's MX5 MOT'd just over a week ago and it failed on two things, one was the brakes the other was emissions too high at idle. They can I understand rev the car a little which usually drops the emissions and allows it to pass. Not sure if this is strictly true but it was just as bad in this case. The tester said his equipment was not suggesting any Lambda sensor problems.

I had diagnostic done on the car and the IAT (Inlet Air Temp) sensor was giving a fault and specifying 39 degrees C; not sure how relevant that was, but the ECT (Engine Coolant Temp) was just plain faulty and no signal. I replaced both had diagnostics again - OK, took the car back after a good Italian tune and it passed (I fixed the brakes too).

The tester did note that at higher revs it was still reading slightly high. I have decided to run an injector cleaner through the car and have it re-checked. I am prepared to put a new CAT on, but if there is another problem it may only damage a new CAT.
There is a rattling sound at about 1500rpm under load which I have read could be a symptom of a damaged CAT, however it may just be a heat protection plate or something.

Any ideas?

[anonymous-user]

The coolant sensor fault would mean the car thought it was always cold, trying to warm up so would be putting more fuel in and running rich. Now that's fixed you're running right, hence the pass - the high readings at higher revs may well just be down to the fact the MX5 runs pretty rich - over 4k rpm the ECU doesn't adjust fuelling by checking the O2 (lambda) sensor and just dumps fuel in (this is why you get much worse economy sitting at 4.5k rpm on the motorway than at 4k rpm). The rattle could be a dying cat, but it could also just be the heat shield around it.

[franv8]

There's a lot of heat shielding that gets loose and rattles/cracks/fals off as the car get's older (assume you're talking Mk 1).

Interesting about 4k vs 4.5k - must remember that one given the price of fuel...

[MalcQV]

Sorry should of said it is a MkII 1.6 1998.

Thanks for the help so far.

[MX-5 Lazza]

At 1500rpm with a loaded engine you will probably hear a lot of gearbox noise - especially if you are trying to pull 4th at those revs. The synchro rings and selector forks tend to rattle together. This is normal with most cars but in MX-5s it can be quite audible as the drain-tubes transmit the noise into the cabin.