MOT Failure

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MOT Failure

Postby elaverick » Tue 19.12.2017, 08:56

Hi all,
Looking for a bit of advice please. My SE failed the MOT with an emissions issue, it's blowing a CO reading of 5.3%. I've got it booked in with Williams Automobiles in Chipping Sodbury (seems to be the closest Lotus specialists), but they're not confident working on M100's as they don't get many through and apparently Lotus themselves aren't much help. They are however willing to have a look which is infinitely better than most of the garages I've spoken to so far.

As such I'm looking to provide them some suggestions to to try to help things along. My mechanical knowledge is pretty much 0, though owning a Lotus it seems to be increasing surprisingly rapidly... I don't necessarily have to do all of these, so I'd like to know how effective each is likely to be:
    I understand there's meant to be a CO pot somewhere that can be adjusted (anyone able to point me to where and how much adjustment it can take?)
    New air filters sound like a good idea, are these inside the trashcan? Also is the following a drop in part or is this part for an open filter conversion - here
    ECU change - I understand Lotus released a new ECU that lowered emissions, my concern is that Williams won't have the equipment to read which PROM id my current ECU has to know whether to do this. If I did go this route I was thinking of going for a Mountain chip (which I understand features the same reduction) as it's probably easier to get than the stock Lotus ECU
    Dump valve - I've seen some places suggesting that a dump valve improves emissions... is this true? Best guess as to why is simply that it vents those gases into the engine compartment rather than out of the exhaust, can anyone enlighten me on this point?
    Fitting an O2 sensor and going closed loop, seems drastic and there's a bit of debate as to whether this approach is worth it from what I've seen. What's involved in this aside from the O2 probe and an ECU that understands it?

Any and all suggestions welcome.

Many thanks

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Re: MOT Failure

Postby dapinky » Tue 19.12.2017, 10:43

Hi Edward,

When I went to Williams' I found them very helpful, but not at all familiar with the Elan - you won't be short of enthusiasm to get you going again, and they were happy to seek help - that was a fair few years ago (when they were down at St Phillips), but expect them to be the same now.... they should be able to read the Prom ID with their 'standard' equipment, but if not, it should be physically marked on the chip inside the ECU.

To try and address your questions:-

The CO pot is a small blue rectangular thing behind the glovebox/dashboard up by the ECU - it is about 1" x 1/2" x 1/2" wirh a shielded screw on one face (for the adjustment). Be aware that there is no end 'stop' on the thread, and so the screw will turn until you get bored and give up twiddling it - but it will have long given up changing anything as the adjustment bit is only a small range of screw movement. Basically, to adjust it, ideally you need to be plugged into a CO meter and do it in real time so you can observe the results.... turn it about 1/8 turn one way or the other and see what happens to the readings - if they change you know you are in the 'adjustment range' if not, then you have to go a lot further with the screw until it actually does something (count the turns so you can go back again!)... then it is a case of getting it in the optimum place.

There may not be enough adjustment to get you into range for the MOT, so you may need to go onto other things on your list.....

.... a change of chip would help, but if you are going to do that, I would suggest that you went for an Everest, or even a 'standard' spec Federal chip (1499 prom) and fit an O2 sensor - that will bring the CO down to a level where you wil never have to worry about it again - mine reads low enough to pass a test for a Cat-fitted car let alone a 1991 SE - and it will pay for itself with better fuel economy (I worked out that it payed for itself in just over a year - ie, about 6000 miles).

If the air filter is dirty it may help to clean it up (if a washable one), or replace it (if a standard paper one) - you should be able to get one locally - it is a BL part for a Maestro/Metro/SD1 etc etc and can be got for about £7 - the K&N version you link to at SJ's is worth having if you want 'the best' and intend to keep the car and/or do higher mileages, as it is a one-time cost.... the K&N part number is E-2360 and they can be got from lots of places, but SJ's are one of the cheapest - especially if you order some other bits as their p&p tends to whack the price up for single items.... they are a simple drop-in item.

One other thing to think about - is there a vacuum leak somewhere (a split pipe or pipe come off the plennum chamber) - or more specifically, did there used to be a leak, but the pipe has been replaced?? The reason being, that if a pipe comes off (usually the one which goes on the top rear centre of the plenum and works the heater valve - but it could be any!) it will cause the engine to run lean.... if this has gone un-noticed, a previous owner/worker may have adjusted to CO pot to get it into 'range', and then when the leak is fixed, it will be far too rich. This is unlikely, but possible :? {if you induce such a leak whilst connected to the CO reader, you will see how low the reading will go.... it won't drive perfectly like that, but it will run - I've even known (or heard stories) of people whose pipes have fallen off on the way to an MOT test, and discovered by the owner on the way home}.

Just the one now, but this one's mine! - and it will be finished eventually.....

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Re: MOT Failure

Postby Fredjohn » Tue 19.12.2017, 12:33


Just to add to Dave's comments, I believe the engine needs to be in diagnostic mode before you adjust the CO. See the Engine Management Manual available to download from this link:


See instructions in Section EMK 1 F and then EMK 3 E. Pay particular attention to the warming up of the engine before taking any readings.
Even if your knowledge doesn't reach the manual's level, the garage should get it.

As Dave concurs, changing the air filter will help: the K&N one is re-usable provided you wash and thoroughly clean it: they sell a "cleaning kit" to do this.

Also read the WikiLEC page on CO Pot.....see WikiLEC tab at the top if the page.

If the car is not used a lot, give it a good run and blast away all the crap that may have accumulated.

I agree putting in a closed loop Mountain chip with an O2 sensor would also provide a permanenet long term solution: it's on my to-do list, but only as a last measure if above procedures don't work.

Good luck
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Re: MOT Failure

Postby theelanman » Tue 19.12.2017, 13:34

as stated....CO pot adjust....
car in diagnostic mode......long thin screwdriver......
put yourself upside down in the passenger seat and get a emissions tester......and someone to watch it......coz the CO pot screw is a long one......
IIRC its about 40 turns from all in to all out.......
you will only need 1/4 of a turn to find the 'sweet' spot.....its just a case of finding that 1/4 turn lol...... gets there in the end........
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Re: MOT Failure

Postby elaverick » Tue 19.12.2017, 14:43

Thanks for all the advice, always pleased to see how helpful this forum is.

I think I'll go the route of the CO pot adjustment along with a new air filter. With the closed loop as my plan B. Aside from attaching the O2 sensor and fitting the appropriate ECU is there anything else required for this? Mine is a Nov 1991 SE, and I understand some of the earlier Elans need extra wiring (I'm hoping mine doesn't fall into that category).
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Re: MOT Failure

Postby lotusflasherman » Tue 19.12.2017, 14:52

Just to add to the comprehensive info from Dave and others..

Early CO pot' s could run out of range so pot with a revised range was introduced at the start of '91 if I remember correctly. There was a Technical Bulletin about that time and many pages of the Electrical Service Manual were reissued dated 1991/01 on the footer. One of my early SE's needed a new pot in the mid '90's and I see SJS still have them (A100E6151F ) - but at around £110 ... ouch! That may be cheaper and quicker fix than changing chip and fitting O2 sensor.

CO Pot adjustment procedure is on pages 6 & 7 of section EMK3 in the Service Notes - might help to supply a copy to your garage if they are not familiar with M100's.

Before I changed my CAS and reset base timing I was borderline on emissions at the MOT and MOT guy's screen was flashing between green & red. I suggested I could adjust the CO pot but MOT guy suggested I remove the air filter - and it dropped by just the required amount and stayed green - and mine is a washable foam filter and was clean as when it was new! Might be an idea to get garage to check base timing and remove air filter and see what you've got..

Dump Valves - I've got a recirculating dump valve (RDV) but it won't be doing anything to emissions, particularly for the MOT. The 'gas' it vents is just air so it's the ECU that gets the fuel/air ratio for emissions, nowt to do with DV/RDV. Lot of bullshit written on the web...

Purpose of DV/RDV is to avoid compressor stall or surge when the throttle is closed against a turbo providing boost, such as when changing up a gear or for a corner. When the throttle is closed against high boost there will be a pressure wave going back down the induction pipe, through the intercooler and increasing the load on the induction side of the turbine to slow it down unless there is somewhere to vent the compressed air. The M100 doesn't run a lot of boost in standard form and the volume of the induction tract - pipes to intercooler, intercooler and more pipes to throttle plenum is quite high so pressure change is not as extreme as some. The exhaust gas slows after the throttle is closed so there is a reduction in input to the exhaust side of the turbine anyway but having a DV/RDV provides the venting and makes life easier for the turbine and it's bearings. There is a small bore vacuum hose from the inlet manifold to the DV/RDV so when throttle is closed and a vacuum in the inlet manifold is created it sucks the valve open to vent compressed air. DV vents to atmosphere creating a hiss that I find annoying and intrusive but an RDV silently vents into the air filter canister, on the filtered air side. So when the throttle is closed, the RDV opens and compressed air is fed straight back to the inlet side of the turbo allowing it to keep spinning and circulating air round the loop, depending on what's happening to exhaust side input. Without an RDV, when the throttle is opened quickly again the turbo starts to spin up and shift the compressed air through the intercooler and pipe to the plenum and boost comes back as it builds; with an RDV that air is already circulating so full boost is back much quicker ... :bananasex:

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Re: MOT Failure

Postby alan e » Tue 19.12.2017, 20:16

elaverick wrote:Thanks for all the advice, always pleased to see how helpful this forum is.

I think I'll go the route of the CO pot adjustment along with a new air filter. With the closed loop as my plan B. Aside from attaching the O2 sensor and fitting the appropriate ECU is there anything else required for this? Mine is a Nov 1991 SE, and I understand some of the earlier Elans need extra wiring (I'm hoping mine doesn't fall into that category).

If you go closed loop you don't change the ECU just the chip and fit a O2 senor.
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