Hi Folks,can anyone help me please?
I have had my MX5 for about 4 weeks now and have (or had) a permanent smile on my face ever since!
I was driving at about 40 mph and wanted to overtake a slower car in front. I put my foot down (in 4th gear) and I had some initial delay, which is unusual, as it has huge amounts of torque from 1500 up to 6500 rpm - I then discovered that I could not gain any more power - in fact, the car went smoother/faster by just touching the throttle. The other thing to note, was that the exhuast note had all but vanished and was replaced by a massive amount of induction roar!
Similar still happens now, but when cold started, the exhaust note is okay for a few seconds and the car runs smooth on tickover, but when any power is demanded, the exhaust note vanishes and the induction roar starts.
Really wierd! Anyone got any ideas please? Is it a flow-meter on the air-box or something more sinister?
BTW - it is fitted with some kind of induction kit (certainily non-standard air filter) and a (nice) noisy exhaust. 
5imon
Just tried something I spotted on the forum, removed the air intake on the front of the airbox and sprayed liberally with UB40 (as my wife calls it) and lots of smoke from the exhaust (that got the nieghbour thinking the house was on fire) but the flow of gases from the exhaust was not what I would expect. No throaty sound and huge induction roar from the inlet port.
The car ticks over okay, but I noticed that the car get hot very quickly - up to temp in 2 mins on tickover!
Still a very strange one me thinks!
Just taken it out for a run - to get more info and feel for this issue and it seems the more power you demand, the worse it gets. For example, I can travel happily (ish) at 25 mph in 4th gear, but drop a cog (and increase RPM) and you may as well put on the brakes.
So in summary, accelerate over 1500 rpm and it starts decelerating, change up (providing you are not on a hill) and it will drive.
When I returned to the driveway, the engine was rather hot, but the exhaust was almost glowing.
Fuel delivery / starvation???
I already have the service parts on order (should be here tomorrow) inc a new fuel filter.
Hi
Sounds like ignition timing, you haven’t disturbed the CAS have you.
Alan
Alan, this first happened while driving along minding my own business.
I have carried out no work on the car (up to now). It also came good after that (for a very short time) and then the issue returned again.
If it was timing it wouldn’t start easily either - but it starts just like it used to and ticks over ok.
Thanks anyway
Sounds like the cat has collapsed
This is normal.
By “almost glowing” do you mean “not actually glowing”? Because a normal exhaust manifold will get to over 300 degrees just from idling for a few minutes, and will get to 600 degrees just from being driven around a bit. The engine too will get rather hot.
That would explain it. The cat in a mk1 is a ceramic brick full of holes which can either erode and fill up with it’s own detritus, or crack (following an impact or thermal shock) and again become blocked up. You’ll be able to see this if you take it off and look down it, which will take half an hour (if you get lucky with undoing the nuts).
Captain Muppet,
Thanks for that - lucky with the nuts apart from one! That took the strength and determination of 10 mad badgers to undo.
Worth it in the end, my cat was a tale of 2 halves, but one half had detatched itself and turned 180 deg and was blocking the gas flow, therefore losing 90% of the flow!
Good advice!
Have sourced another from MX Heaven (1 hour from me) and will be fitting it later.
I am amazed that the ECU would allow the car to start and run with such a restriction in place. It was almost like someone had folded-over the tailpipe.
Just as a note to other owners - if you hear a rattle from the exhaust as I did, don’t mistake it for a loose heatshield (as I did)! That was obviously the ceramic rattling in the box, just waiting for an opportunity to block the flow with a 180 deg twist. Doh!
Yay! Internet advice turned out to be actually useful!
You can use a hammer and chisel to turn your knackered cat in to a de-cat pipe (I used an empty cat like this for a year or so) or sell it for a few quid as a decat pipe. Alternatively check with local scrap metal merchants - the presious metals in the cat brick can be worth a few quid and they don’t care if it doesn’t flow gas any more. I’ve known people to get £50 for an old cat, depending on current values of platinum…
Thanks to all - I would never have thought of looking at the cat!
The dog was not happy, I can tell you 
Anyway, went to Mx Heaven to pick up my replacement and it was a bit uncertain on the phone if he actually had one, but I guess the amount of after-market cats available it could have been any number of different types. He was also non-commital on the price - he kept telling me it “depends on the model”…
It turned out to be a 435mm long aftermarket cat, that been fitted some time ago (6 years +).
As I wanted the car back on the road quickly, I decided to drive to MX Heaven and explained to him that I would be there in an hour or so.
He had the cat off when I arrived, it was an original one that was fitted to a 1.8 roadster from '94 ish.
My destiny was in his hands, knowing how much these things can be (replacement for my audi a6 is £1098.00) - I waited for the price and he told me £50.00 !!!
I was then told that was what the scrap merchant would pay for it!
Hmm. I paid him and left. I was just happy with the thought that I would have the 5 back on the road later in the day.
Finally managed to drill out the studs that he had cut off to remove it from the car, put loads of exhaust assembly compound on the joints and tightened it all up.
Fired up the 5 and ran it up to normal temp and she is good again (apart from the blow in the cat beneath the heatshield
)
I then checked on t’interweb and found a direct replacement (exactly the same as I had previously) for £50 with a 2 year warranty!!!
That’ll learn me
Captain, this forum is great - I have learned more than I could have even if I had stripped the car down myself!
You won’t believe It, but I threw my old cat in his scrap skip!
Now who is a muppet?
Thanks for your help, very much appreciated.