High emissions with intake manifold leak?

Technical Area Engine, Transmission, Exhaust etc
1
  1. My model of MX-5 is: 1999 NB 1.8
  2. I’m based near: Cheltenham
  3. I’m looking for technical help or recommendations on: A potential leak from the intake manifold causing slight vibration on idle and poor emissions on CO.

For 18 months or so I’ve been struggling with poor emissions on MOT tests, which the car has struggled to pass on. Idle has been a bit lumpy and is noticeable worse when electrical load is on (IE headlights and heaters on).

Just about scraped through my MOT last month, but not without a lot of work to bring the CO down below 0.3.

Anyway after seeking some advice, someone mentioned to me that they had a similar issue when they had a leaking intake manifold.

Then I remembered that shortly before my MOT woes, I removed the intake manifold, both upper and lower, cleaned and unblocked them, even vapour blasting the top part, and put it all back together.

However my fatal error was to reused the metal gasket on both the main manifold to the engine and the EGR Valve.

How likely is it that a small leak on this would be the cause of poor emissions and slightly lumpy idle?

I’ve ordered new gaskets already, as well as new spark plugs and I’m going to remove it and fiy the new gaskets anyway just for peace of mind.

My lazy ■■■■ 18 months ago might have come back to bite me now, all for £15 worth of gasket.

2

High CO is usually an indicator of incorrect Air/Fuel ration hence your potential proble with leaking gaskets allowing excessive air into the system, which the car attempts to correct by putting more (too much?) fuel into the system. What was the lambda reading as if this is well below 1 then it could confirm the diagnosis.

3

lambda was about 0.99, which was a Pass.

I’m hoping that it is the cause as I was going to change the catalytic converter, but this is much cheaper to fix if it is the cause.

4

Out of interest is it still considered a viable leak test to spray brake cleaner around the intake pipework while car is ticking over to see the revs change?

5

I’d do that yes.