Hi all, my Jaguar has failed its MoT on very high emissions readings. Its CO is around 5.4%, when it needs to be down to 0.3%. The Lambda reading was actually too low, surprisingly, and hydrocarbons were within allowed bounds.

So I need to lower CO, and raise Lambda a bit.

What would cause the CO to spike significantly without also raising hydrocarbons? I'm thinking O2 sensors, maybe the small holes and cracks in my exhaust manifold. Has anyone else had this issue, where emissions are massively out of balance?

Any suggestions about what might be broken, what I can test, and what to replace would be greatly appreciated.

 

Most likely the oxygen sensors

 

That was my first thought too. Can these be tested easily, or is it just a replacement needed?

 

They don't last forever, I replaced them on both of my Jaguars.

 

Thanks, I'll order a couple of those then.

Hopefully the difference is noticeable, I don't want to keep booking and then failing expensive emissions tests as a diagnostic tool!

 

On the O2 return wire to the ECU off the top of my head will read around 3.5 volts DC as the coolant is above 88 C ( closed loop )

The valve cover is not a good ground for the meter lead

See page 56 pin Papa Indy 105 -16 and 18 ( at the ECU ) as you can read it at the O2 sensor connectors , this will be the 4th different color wires Blue and other connectors Red

Notice the word swing

jagxj1996.pdf (jagrepair.com)



This will be different then the more common O2 sensors

The unburnt hydrocarbons being in range indicate fuel trim ( mixture ) is correctly regulated by the present O2 sensors , so a good sign


Grounds and connectors cleaned and fuse checked ?

Sensor connector positions swapped ( hydrocarbons value say that's OK ) ?

Unless the testing emissions tail pipe sniffer is a separate device from the car's ECU , the testing device may default to the more common type of sensor as it is looking at your car's ECU

The X300 O2 sensors are titania based which works differently than the more common

 

I'm confused now, the CO is way up which indicates a rich mixture and suggests O2 sensors are faulty, but the hydrocarbons being ok suggest that the O2 sensors are working fine?

O2 heater fuse is ok, grounds have been cleaned and checked. Surely if these were bad the hydrocarbons would be high also?

Plugs are new and the proper Champion ones, coils have been checked, cleaned, and had the springs stretched. Should be a nice efficient burn, hence low hydrocarbons.

 

Aside from the unburnt hydrocarbons the other after combustion gasses like nitrates and versions of CO2's will be converted by the Cats for your final exhaust result at the tail pipe which your O2 sensors will not see

The unburnt hydrocarbon just after combustion do play another role in the Cats feed stock so the into the engine fuel ratio is different than the ideal 14.7 to 1 mass ratio so the target the ECU regulates to is X

Lamdia 1 is 14.7 fuel ratio

So in a way if the O2 sensors are working correctly then it points to Cats effeceincy , but you don't have definitive proof of that yet

There is a graph that let's you picture it better

Exhaust manifold leaks ( cracked casting or loose downpipe donut gaskets , yep it has been seen ) , but your mixtue is not the usual rich as the other's case

Gotta runs some errands

 

I do have some small pinholes in the manifold and a couple of hairline cracks, and I have recently replaced my downpipe so it's possible the gasket there is leaking a bit.

 

The hairline cracks change as the manifolds heat up during different phases of engine load ( heat cycles )

On installation of the downpipe moving up to the manifold castings the 4 downward pointing studs can damage the donut gasket seat flare on the downpipe side

 

What would you recommend I start with then? O2 sensors seem the likely candidate to me, either they aren't working properly or they are getting too much fresh air from leaks. Failing cats would be annoying, and harder to prove so I'm more inclined to tackle the sensors first.

Option 1) Slather manifolds in exhaust putty and reseat downpipe to manifolds with new gaskets.
Option 2) Replace O2 sensors.
Option 3) Replace main cats.
Option 4) Further diagnosis.
Option 5) Replace cheaper, possible related things first: CTS, thermostat.

I just really want it to be a one problem problem. Replace one thing, and it's done. That way I don't have to keep going back for more tests in between replacing various things.

 

lambda too low (too much petrol injected) causing your CO to be too high. With a defective catalytic converter the lambda is (almost) good.

 

Well that’s good, another vote towards the O2 sensors then.

Here was the emissions test result for more info.

 

" I just really want it to be a one problem problem. Replace one thing, and it's done. That way I don't have to keep going back for more tests in between replacing various things. "


You're looking for the magic bullet without the detective work

The cheap and easy things to do is ensure the donut gasket nuts ( replace gaskets if old ) are tight and putty the cracks in the manifolds

You can read the 2 wire ECT sensor at 270 ohms or lower at operating temp to prove both the thermostat and ECT sensor is putting the engine regulation in closed loop ( the only time the O2 sensors are used )

Your unbunrt hydrocarbons value say it's OK

 

Coolant temp sensor has been tested for voltage and resistance, all within the normal operating range. Thermostat seems to open as the top radiator hose become hot to the touch and the expansion tank warms up.

What’s the best way to check the CTS at the ECU? Perhaps the signal isn’t reaching all the way.

 

The problem with a thermostat is being too open , In the old days they would fail safe in the full open position as you didn't need warm coolant to trigger O2 sensor engine regulation, today they can fail direction either way

The CTS wires have no splices or shared sensors to interfere with the 2 wires at the ECU

Your final result Hy - way milage value somewhat points to the sensors regulating correctly the engine along with CTS value getting to the ECU

 

Thanks Parker. So if it appears that the O2 sensors are communicating ok, fuel mileage is ok, and temperature appears normal, what else would be the most likely issue?

I'm still leaning towards O2 sensors, as these have been removed previously when I replaced the downpipe, and the wires were twisted upon reinstallation as I didn't know they were brittle. This is why it remains high on my list of suspects.

I should clarify about the thermostat. The temp gauge rises normally to the just left of centre position, the top hose is cool and then rapidly warms, and the expansion tank is similar. It seems that the thermostat is closed when cold then opens as it should.

Has anyone got a photo of the downstream O2 sensors when they are installed?
I'm not sure if mine are threaded in all the way as there are some threads showing on the sensor still, but they don't want to go any further in.

 

My plan now is to fit two used OEM O2 sensors from a reputable Jag specialist here in the UK as they're much cheaper than any new aftermarket options. I can then see if these make a difference. I'll take care with the wires this time. I think I'll also chuck on a new coolant temp sensor and a new thermostat as these are also cheap.

 

When replacing the thermostat , it does not take a large o - ring , even if it comes with one , cracks the housing as you clamp down

 

O2 sensor crush washers , 12 mm , can come from a cheap 12 mm spark plug , the X300 spark plug is 14 mm

Someone seemed to have had a difference when his crush washers were missing

Don't forget anti - seize on installation and to not get any on the sensor tip

17 mm hex on the O2 sensor

Be careful to not twist the O2 sensor pigtails as they are very brittle galvanized coated , mine had wires sticking out of the insulation when I got mine

in the worst case the threaded bong hole can be replaced

the instrument cluster coolant temp gauge is a " dummie gauge " that the senser is the single wire sender on the thermostat housing next to the 2 wire ECT sensor