Failed MOT on emissions

The high lambda will be an exhaust leak after the catalyst.
If it was a leak before the catalyst the CO would be down as the extra oxygen would convert it to CO2 inside the catalyst.

The high idle is a concern, could be an air leak at the manifold- is the pipe from manifold to vacuum tank intact, and no leaks at the vacuum solenoids, you can try just capping the fitting at the manifold, obviously the vacuum hubs wont work with the pipe disconnected.

The High CO is overfuelling, very likely the manifold sensor.



You need to sort the post-cat sensor too, as an obviously defective sensor is an MOT fail (major), whether or not the emissions are correct.

Leak at the manifold is a strong possibility. I had an engine put in after the last one blew the head gasket. Since then it has been running very high revs at idle, over 1000rpm, goes to 2,000rpm at start up but drops to just above 1,000 after about 15 seconds. This points to leak at manifold yes?

Not necessarily.

Normally you only deal with one problem at a time.

I thought the problem was the emissions had drifted high and nothing had been touched, which is likely the manifold sensor. High lambda and an obviously broken/missing post cat sensor just needs leaks fixing and a new sensor, so a pair of sensors and an exhaust repair would sort it.

Now you are saying it is a replacement engine that has never been right since fitting......


High idle is probably extra air getting into the engine that the idle speed system can't reduce.
It could be an air leak at the inlet manifold flange, a leak in the vacuum hub system or a pipe broken/leaking/fitted incorrectly. It could be the charcoal filter purge valve leaking or a pipe off the purge system. It could be the pipe to the valve I forget the name of that goes into the tappet cover. It could be a leak at the brake servo, or in the pipe from manifold to servo.

If someone else fitted the engine you need to check every pipe for

a) being there
b) being in good condition/not leaking
c) connecting to the right place at both ends.

It could also be the throttle cable is too tight, someone has messed with the adjustment of the throttle butterfly, the pedal sensor is out of adjustment and it thinks the pedal is pressed rather than released so it doesn't go into idle control etc.


Has it ever MOT'd with this engine and the high idle?

If it has, sort the sensors & exhaust and get the MOT.

Then fix the idle problem.

I checked all the hoses and everything seems fine, no leaks. But I did find a leak in the fuel injection hose. I've ordered a replacement. Would this be a possible cause of high idling and emissions?

Not unless the leak is bad enough to significantly alter the fuel pressure, a weep won't matter.


High idle speed is caused by extra air getting in, by accident or design. This is because there is a leak for air to bypass the throttle butterfly and idle control valve, or the ecu is setting the idle speed too high, because it thinks the engine is cold (faulty temperature sensor??) or it thinks the air-con is on, or there is a large electrical load (I think there is some sort of load sensing to bump up the idle speed, I'm not 100% though)

High CO is too much fuel. It could be the faulty manifold oxygen sensor, which is the most common reason on a jimny, or it could be excessive fuel pressure, wrong injectors (from a bigger engine), or again the ecu thinking the engine is cold and basically running on choke. There is also another option- leaking exhaust manifold.

High HC is unburnt fuel, due to a missfire- could be caused by over lean or over rich, or ignition fault. (you haven't got this)

High lambda is too much oxygen in the exhaust , 99% it is due to an exhaust leak, I said post-cat because you said the oxygen sensor was broken and there was corrosion.

However, it could be a leak in the exhaust manifold before the first oxygen sensor.

This makes the sensor read lean, so the ecu increases the fuel, increasing the CO, but the lambda will still be too high as well!



What this boils down to is that there is a lot that can go wrong, and blindly chucking money at it changing stuff can be a waste of money.

However, the reason I said change the manifold oxygen sensor is because

1) They are cheap (generic ones that work anyway )- cheaper than paying for a code read, nevermind a session at a garage
2) Dead easy to change
3) Commonly fail.


If it isn't the manifold oxygen sensor, and you have eliminated all possible inlet and exhaust leaks, then you have to start reading and interpreting live data, to find out what is going on.

Lambda sensor is ordered and on the way. In the meantime I checked for leaks and removed the throttle body to clean the IAC. Everything looked clean and could not find leaks. Is the noise in the attached video normal?
youtube.com/shorts/WWSVr05AKZY?feature=share