Talk Morgan Forums Morgan Sports Cars Maintenance & Servicing GDI MOT Update

I went to the dealer today. My new Catalyst did not produce an easy pass. CO was the problem. The idle was OK, limit is 0.300, score was 0.285. But the fast idle, limit is 0.200, and the score was 0.516, so way over. The lambda was good.

The issue was that increasing the idle speed, was richening the mixture, off the scale. This was not due to the catalyst but could have been a throttle body issue, an engine temp sensor issue, or the ECU mapping. The diagnostic showed that the engine temperature sensor indicated 103c, so that appeared to be OK.

So I had a dilemma. The dealer doesn't have access to the Aero Racing ECU upgrade, without sending the box through the post to Morgan, and it was suggested a while ago, that this may not be possible soon, or even 'now'. So I could either return home with a second fail, or have the standard ECU map re-applied, but lose the AR mapping.

Way back in 2018, I first picked up my AR Clubsport manifold and exhaust system from the factory, and fitted it at home. There was a distinctive improvement just with that change. So I then returning to the factory to have the ECU flash, and immediately found stronger revving at the top end. With this experience I decided to go back (risky) to the standard re-map today, and that gave an immediate MOT pass. The idle limit is 0.300, score was 0.264. The fast idle, limit is 0.200, and the score was 0.019, so well under.

As standard, 2016-2018, the throttle was hair trigger, and the tailpipe was black soot. The AR flash softened the throttle and cleaned the tailpipe, with hardly any deposits, but the colour turned black after my bad running experience in October 2024. The new catalyst cleaned the tailpipe again with hardly any deposits.

After todays changes, the throttle is not hair trigger, but maybe slightly harsher. The tailpipe is now black and sooty again.

This has been my experience today, and if I need to keep the standard ECU set-up to get an MOT pass, so be it. On the drive home, 55 miles, quite honestly, there was no discernable difference. The clean revving was still apparent.

Just a wild thought Dave. With your preferred ECU installed and previously left off battery for a while to ensure adaptive mapping is lost could you just pop straight over, after suitable warm up, to have your emissions tested again - a different test station is another thought. Other GDI owners don't seem to suffer as much as yourself at MOT time even those with full factory Clubsport upgrade

This may or may not be applicable, but I was over with Matt at Life's today and I was chatting about whether he had experienced any difficulties getting a club sport cars thru the MOT. He felt the exhaust was too big, so his MOT station put a cone that narrows on the end of the exhaust when testing the emissions.

I think that running an inlet valve cleaner damaged the cat. I think that the bad running last October was a partial internal collapse of the cat. It's certainly curious that now I can't really feel any difference but the tailpipe is black, as it always was in the first two years. Really this is typical of Morgan where two identical cars behave differently. Just look at the rev hang that some Roadsters get, but not all of them!

Where did you manage to get a new cat from. and what spec is it ?

I'm currently in negotiations to purchase a club sport spec Morgan Plus 4, time will tell if I get it, hence my discussions with Matt whether he had experienced any MOT problems with those spec models.

One of the considerations for this particular car is the car had a quite light clutch by comparison to my Plus 4 and others manufactured around the same time, that I've tried recently. Why the clutch should vary so much is interesting to say the least. I was down at Melvyn Rutters yesterday and he mentioned a heavy clutch being one of the main reasons (besides mobility) for people moving their Morgan on.

Out of interest I asked whether using an inlet cleaner could damage the cat and the AI answer was

Yes, an intake cleaner can damage a catalytic converter if unburnt cleaner enters it, causing it to overheat, or if it's used incorrectly or excessively, leading to potential internal cracking or substrate failure. Overuse can also cause heavy carbon deposits to break off and block the converter's honeycomb structure, though smaller particles often burn off during normal driving.

https://www.stainless-exhaust.com/

Ask for Steve. I took the cat section. They jig it and cut the old cat out. Replacing with a 300cell.

Two other specialists were not interested.

My previous car had sports cats that always struggled at mot time.

I stumbled on these things..



To my utter amazement they actually worked. The ‘logic’ being that sports cats flow too much gas and the downstream picks this up and throws and eml. The extender moves the sensor out of the flow and makes it less ‘sensitive’. Cheap as chips from amazon

https://amzn.eu/d/iiF5QzS

Grumpy you are spot on there, Originally made by the specialist stainless exhaust bods for systems that when fitted obstruct the fitment of the lambda in the car. You can also get right angled versions. I feel that the cats might well be adequate to do the job but when fitting a larger bore exhaust system it allows the gasses to flow through faster due to decreased back pressure and the lambda sensor picks up quantity of Co2 and therefor sends fuelling messages to the ECU. This adjusts everything re. the fueling side of things and ups the Co2 through flow even more. However if it is still within the ECU parameters (which are functional parameters) it won't flag an engine warning light but will show high on testing.

I had an error code on my Plus8....downstream O2 sensor heater fault. Basically saying the sensor was not being preheated properly. The heater is there to get the sensor up to temperature quickly to get proper readings ( similar to a choke when cold starting a car). This sensor is there to check the cat is working properly. The upstream sensor is the really important one as it manages the engine. Mine were ok and the engine ran well. Using gomog I found the sensor ID from the Gems manual in there. Ordered two sensors from Rimmer Bros. After struggling to remove the faulty one I replaced it. Problem solved. The pre-heater fail fault sorted. No other faults so I assume the cat was ok. I still have one spare tucked away somewhere. These extensions look like a cheeky cheat that appears to work. I wonder if an eagle-eyed MOT tech would pick it up?
It prevents the ecu finding a fault so no MIL or code and a failed MOT. What about the " up the tailpipe" sensor during the test? Surely that would pick up a failed cat? The extenders cheat the ecu by reducing gas flow past the on-board sensor but the gases would still exit pass the test sensor?

Apologies if I've misunderstood your post sewin, but isn't the post CAT lambda (oxygen) sensor present in a purely diagnostic role despite also reporting to the ECU and raising an EML if out of spec. Its the pre CAT sensor(s) that control fuelling via the ECU surely.