MOT Fail… engine emissions

[Craig]

Hi, I’ve just failed an MOT due to exhaust emissions, printout above. It was very high on carbon monoxide and slightly high on hydrocarbons. The tester seemed to think replacing the engine coolant temperature sensor and cat might be the place to start- at least when he wasn’t dramatically telling me the engine might be knackered or welding not good enough….

I’ve installed new sparks, HT leads, distributor cap and rotor and second hand exhaust manifolds (inc lambda) and a cat. I knew the risks of a second hand cat… didn’t pay off. It has 64k miles, so probably lower than healthy. It’s a 97 Mpi, so don’t think my cheap fault sensor will work. The head gasket has been done before, and there’s no obvious signs of smoke. The scale of failure makes me think it can’t be the cat alone.

Does anyone have any advice on what the likely cause is before I potentially waste my money? Are there any key symptoms to look for/tests to try? Thanks for any advice given.

[Reckless Rat]

Not my field of expertise but I also have a 97 Mpi, which has over 120,000 miles on the clock. A few MOTs ago it struggled to get through the emissions, and a new cat was fitted. Last MOT it returned CO figures of 0% on both idle and fast idle test and the Lambda (which is original) showed 1.004.

I reckon a replacement cat would not go amiss.

Use stainless steel bolts/nuts when you fit the new cat.

[Craig]

Thanks. I’d definitely decided on a new cat too. Bought this one second hand off eBay by someone who claimed they bought it new, used it for a bit, then decatted their F for racing, and it was practically unused. Was obviously a bit of a porkie or it’s been damaged at some point. I went cheap and knew the risks.

Do you have any record of your CO figures before the new cat? Mine are over double the limits.

I’m not sure if that alone will do it though, I think it might sort the hydrocarbons as they’re only a bit high. Don’t want to buy waste 40 quid on an MOT and find it is something else causing the CO.

[Reckless Rat]

In France it's every two years, so here they are:
2010 - idle 0.11% fast idle 0.07%
2012 - idle 0.13% fast idle 0.15%
2016 (after new cat fitted) idle 0% fast idle 0.12%
2018 (after second new cat fitted as the first one fell apart) idle 0% fast idle 0%
2020 idle 0% fast idle 0%

It is worth while taking the car for a run prior to the MOT and get it properly up to temperature (coolant AND oil) and also give it an Italian tune-up (ie give it a good spanking) to clear everything out just before you present it.

[Reckless Rat]

If in doubt, take the car to someone who has a PSCAN to see if there's anything else. Normal fault code readers won't work on your car because it's not OBDII compliant.

[Craig]

2 year intervals seem much more reasonable!

Ah ok, I think my CO is much higher than yours was before the new cat, so I probably do have a duff sensor somewhere.

I’m going to do exactly what you’ve suggested and take it to get diagnosed at a garage near me which specialises in MG Rovers. Won’t be any doubt that way. He’s even said he’ll give it a good run before to clear the rubbish out as I obviously can’t really be doing that without an MOT.

Really appreciate your help.

[Rob Bell]

I am way too late to this thread, but it is clear that your engine was running too rich.

The common explanations for this are as Craig says, a sensor error (usually a failing water temperature sensor on earlier cars) or a Lambda sensor on the way out. The other explanation to exclude is a vacuum leak - so check all the vacuum lines, as this can be a very easy and quick fix

[jonaf]

Clogged air filter? Catalyst core loose?