High emissions on MOT test

[Lowtechguy]

Hello again,

Backround
I had fitted a faulty throttle body by mistake (Idle control valve was broken) and as such have coked the engine.

I took the car for a mot and it failed on the emissions only just (should be 0.3 on the fast idle and was as high as 0.90 but as low as 0.45).

My question is what can I do to remove the deposits on the exhaust valves without dismantling the engine?

There are many products that claim the world such as power boost, redex and so on. I have been recommended Forte gas treatment from a local tuner but I wondered if you had any products you had good experiences with or perhaps another technique such as filling the spark plug holes with something and then turning the engine by hand or something?

Many thanks,

Cristian

[sumplug]

My old favorite is Redex. Pour down the plug holes, and turn engine over. Ive found it works a treat. Another unconventual way, is to put a pint or two of diesel in the tank. this will depense the muck quickly. there is also by Wynns, upper valve lubrication. This has a cleaning effect. I would try the Redex first as its my trusty friend at cleaning.

Andy.

[Guy Croft]

Cristian, hi

I'm not sure you're on the right lines with that.

Cylinder deposits will trap hydrocarbons but will definitely not cause the car to fail MOT on carbon monoxide emissions. Hydrocarbons are excessive out when the fuel is not burning. Like if a plug is way too cold, not firing or ignition timing is miles out.

CO goes up when it's burning too rich. But just because it's rich (high CO) doesn't always mean the HC will be high, there is quite a wide envelope for for high CO but with still relatively low HC. If you run so rich that the HC doesn't pass MOT (they are pretty generous limits I might add) the engine would not tend to run at all. EG: Lucas fuel injected engines like the TR6 can idle with over 7% CO and still be way under legal HC limit. Likewise any SU carb equipped Jaguar.

I'm reading some data here on a MOT on a Vx unit, quite highly tuned, 3.2% CO (much higher than yours) and only 156ppm HC. Yes, these are not 3-way catalytic converter equipped but take off your cat and the effect would be the same. A defective cat or Lambda could send the HC out of control limits, sure.

Best have a rethink on this, there could be another cause there. Give me all the MOT test data please.

GC

[Lowtechguy]

Hi Guy,

I didnt get a print out as I knew the emissions were an issue so the gentleman let me test only that first before the rest of the mot were to commence (to give me more time to do things should it fail).

Idle emissions are fine, just the fast idle (happest at 3,000rpm (0.45) otherwise it will rise).

The engine would have coked alot as it was running 7mpg with the dodgy icv until the throttlebody was fixed (about 60miles with 7mpg) and now it is running well over 30mpg with a heavy foot. (this also relates to my fuel trim problems, which after the throttlebody had been repaired is now acting normal (short term +-15% long term set at -79%)

I have heard a couple of bangs while driving as well which led me to that conclusion of the exhaust valves.

I am re-booking later this week so fingers crossed redex or the like will help, however if it doesnt I will scan the mot sheet for your examination.

In the short term however is there anything you think might help de-coke? I have yet to replace the plugs but this will be before the next mot, I wanted to give it a run before hand incase I foul them...

(ignition timing will be out as the camshaft is now 290degree instead of 244 when the car was last mapped which coninsidently was with too cold a plug - long story..) - My aim is to drive legally to Birmingham from London to get remapped at the end of the month.

[Guy Croft]

Couple of bangs? You better do a compression test, leaking ex valves can cause high HC too.

GC

[sumplug]

If the fueling was that rich, then i suspect you have damaged the cat, so your emissions will never be below the max. You could also have a sticking exhaust valve, which will cause the odd bang, so a good upper lubricant is recommended like Redex. I also before an MOT, stick 2 bottles of injector cleaner in the tank, and give the car an "Italian Tuneup" down the motorway for several miles. Its also advisable to get the cat red hot just before the test. its amazing how many cats fail, because the cat is not hot enough!! I would also, clean all breathers out, as this can make a marked difference on emissions too. Are you running an EGR valve? If so, i bet this is choked up with crud, and a good clean will really help the emissions. If your running a MAF sensor, take this off and give it a clean as well including pipes.

Andy.

[Lowtechguy]

Hello,

Just to give feedback.

It seems the MOT tester had the wrong defaults on the machine, my car should be 0.5 fast idle and 0.3 idle and it passed at 0.44 and 0.26.

The emissions were higher last time so a plug change and redex with 99octane I think helped.

Car now has MOT.

Many thanks for the responces received.

Cristian.