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Sooo... it's a third year in a row failing mot due to high emissions. This time it's running quite rich, lambda shows 0.6. Co% and HC about 10 tenfold past the limits. And can't seem to find the source. Here's my investigation so far:


- no vacuum leaks

- compression test ok

- no visible leaks in exhaust system

- plugs in need of change but nothing critical about their condition

- plug wires ok

- timing ok

- ecu reseted

- worn cap + rotor, a little bit of play in dizzy axle + dizzy a bit noisy = gonna replace distributor. But not in such a bad condition that would explain the richness.

- coolant reservoir was empty, only a bit of dark brown muck on the bottom. At first thought it was oil, but when i rinsed it turned out was (atleast mostly) dirt etc. Discovered a small crack on the reservoir lid, might be a cause. Besides the reservoir no other clues pointing to a possibly leaking head gasket.


Any ideas what to look for? One suggestion was that a faulty heat sensor etc. could be feeding the ecu wrong info and messing up the mix? Valve stem leak also one possible cause. In addition to the dizzy i'm planning on replacing the ecu with a p28 with Hondata to get better data about what's happening. And ofcourse once it's fixed, then it's tuning time 8-).

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On 6/25/2018 at 10:35 PM, dmrichards said:

it could be the coolant temp sensor, might be worth putting a fresh one in. Can't you slide the mot guy a few quid for a pass? 


Luckily they're pretty cheap, i'll give it a shot. And i'd be ready to slip quite a lot more than a few quid to get a pass, this is getting a bit boring year after year! But on the other hand, otherwise the deck is working spot on i'd like to keep it that way. Been allready 9 years under my ownership.

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Mine failed emissions last year, but it was tested before it was serviced. 

I ran some injector cleaner through it, then had the service Inc plug change and went through okay. 

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  • 1 year later...

My problem got sorted by replacing the o2 sensor. It's become a yearly thing now, this fall i swapped it before mot and passed without any notifications. The old sensor was totally black.

But in your case it has to be something different. Have you checked compression and for vacuum leaks? Dizzy bearing?

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  • 4 weeks later...
On 12/9/2019 at 11:05 PM, dr_broon said:

That's interesting.. I wonder why they're failing so often. 


I'm suspecting an oil leak somewhere inside the engine which causes the car to burn even more oil than a vtec honda normally would. It feels like i'm topping up more often than before. I've also had oil frequently inside one of the spark plug wells even after changing the gaskets, for a few years now.

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1 minute ago, KiNK43 said:

Oil in the spark plug hole can only come from one place. Clean the gasket up and use silicone.



You mean silicone the gasket to make a more secure fit? What kind of silicone? I've used hondabond when changing rocker cover gaskets.

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