MOT pass after AFM adjustment - advice please

[wrightp]

Hi there

I have owned my Chimaera 500 for over 2 years now and enjoyed it greatly without issue and with a smile. In that time I have upgraded the brakes and the suspension - many thanks to Joo at TrackCar Solutions for the set up!

My question follows a recent MOT visit.

Last Friday I took the car for MOT at a friendly garage where everything passed as normal but the emissions were out of spec and caused a fail. It was advised that the Lambda sensors may be the issue (Both the garage and a TVR mechanic in Yorkshire advised the same).
The CO was at 5.2+ and the Lambda was at 0.870 ish

I had the Lambdas changed (Thanks for the quick job at Gordon Walker's garage near Huddersfield) and returned to the MOT garage on Saturday. I had given the car a longer higher revs run before the retest. The emissions were checked again - but the results were almost identical to the previous day Bugger!

The friendly garage chap started to test the plenum for leaks and looked around and under the engine. He then started to adjust the Air Flow Meter mixture screw (If that is what it is), and the readings started to change. He was testing at about 2500 revs and he continued to adjust and retest the readings.

To cut a longer story short he adjusted to a point were the readings on the computer were in spec at fast idle and normal, and we got a test - Relief!!

What I would like to understand is : If he has adjusted the mixture to prevent it running so rich - how can I tell if the car is now too lean (I have tried to listen for pinking but cannot). The car seems to go just as well as before and will rev cleanly to 6000+ The power seems to be good and not noticably different than before when it was deemed to be running rich.

What caused the issue, and is adjusting the AFM something to be concerned about?

I hope there may be some people who can advise.
Cheers
Phil.

[scotty_d]

Are you talking about the adjustment on the plenum? the hex screw?

[wrightp]

Hi

No he was adjusting a screw on the AFM itself - as far as I could see.

Thanks

[supadik]

If he was adjusting the screw on the airflow meter itself then he has been fiddling with the idle CO setting! The ecu should be making those adjustments automatically provided it's running the catalyst map. It's possible that your tune resistor is broken or disconnected causing the ecu to be running the wrong map. Slide your ecu out and check that the tune resistor (white, on the loom near the ecu) is still fitted. To check the tune resistor disconnect power from the battery, disconnect the multiplug from the ecu and check resistance between pins 5 and 27 of the multiplug, it should be giving a reading of around 3900 ohms on a multimeter.

If you find the tune resistor is faulty and you replace it you'll need to re-adjust the AFM CO base setting. Start the engine and let it warm up completely (take it for a 10 min run). Adjust the hex screw on the AFM until you get a reading of 1.7 volts between the red/black (ground) wire and the blue/red (signal) wires on the AFM.

I'd then take it back to your "friendly" mot tester and ask him to stick the probe back up the cars bum and check the co readings again for you.

It may not be the tune resistor at all but if the ecu is running the correct map (catalyst) it should be keeping the mixture correct automatically and would have (I think) ignored the adjustments made by your mot tester.

[blitzracing]

What Superdik says is correct 90% correct, but as far I as found with all my testing the AFM setting is totally ignored on the catalyst fuel map, so setting the DC voltages should have no effect and its only there for adjustments on cars without lambda probes. Get a test meter on the connector to the lambda probes (difficult I know). Measure across the black and white wire with the engine warm and ticking over. It should constantly cycle at about 1/2 second intervals between about .2 and .8 volts (these are approximate- its the cycling thats more important). This will show the lambda feedback is correct and working, and the emmissions should be OK. If the voltages are stuck at .8 - 1.2 volts there is a problem and its running rich, and it will fail the MOT. The ECU will generate an error code in this case to point at the failing item, so you need to borrow a fault code reader. Each bank is controlled separately, so one side might work, and the other not. Like wise a reading of less than .2 volts indicates a highly lean mixture. The cycling of the voltage from the probes is critical to get the catalysts to work, you cant simply get the mixture near right and hope the catayst will do the rest. You can simply remove the tune resistor and it will also default to the catalys map. If someone has been messing around with the non cat map it has a 400 ohm resistor, fitted with green plastic over the resistor wires. The catalyst resistor has white sleave wires.

Mark

Mark

[That Daddy]

supadik said:

If he was adjusting the screw on the airflow meter itself then he has been fiddling with the idle CO setting! The ecu should be making those adjustments automatically provided it's running the catalyst map. It's possible that your tune resistor is broken or disconnected causing the ecu to be running the wrong map. Slide your ecu out and check that the tune resistor (white, on the loom near the ecu) is still fitted. To check the tune resistor disconnect power from the battery, disconnect the multiplug from the ecu and check resistance between pins 5 and 27 of the multiplug, it should be giving a reading of around 3900 ohms on a multimeter.

If you find the tune resistor is faulty and you replace it you'll need to re-adjust the AFM CO base setting. Start the engine and let it warm up completely (take it for a 10 min run). Adjust the hex screw on the AFM until you get a reading of 1.7 volts between the red/black (ground) wire and the blue/red (signal) wires on the AFM.

I'd then take it back to your "friendly" mot tester and ask him to stick the probe back up the cars bum and check the co readings again for you.

It may not be the tune resistor at all but if the ecu is running the correct map (catalyst) it should be keeping the mixture correct automatically and would have (I think) ignored the adjustments made by your mot tester.

Agreed,one thing i do remember when i was having a play with the CUX system and a Sun 4 Gas that when you adjust the CO screw on the AFM the CO changes as you would expect on a car on the non-cat map,the thing is it sometimes take a while for the system to play catch up and realize its over/under fueling and needs to alter the fuel mixture(although the length of the MOT Cat test i doubt this)so maybe i am wondering this as what as happened to the OP,s car and once he as got back on the road it as just reset to the previous high CO values(original problem persists)chances are its as Mark/Blitz said running the non-cat map it was just a thought.

Edited by That Daddy on Tuesday 16th December 21:46

[supadik]

Hi Daddy.

If the screw sets the base CO reading I would think that the ecu will probably only "read it" during the initial 15 second start-up when the ecu sets it's base fuelling adjustments. So chances are the ecu will have compensated for the adjustments then next time the cars ignition was cycled provided the CO hasn't been adjusted out of range??? I'm only guessing tho' as so far the info i've found on the 14cux is "sketchy".

Probably find after all these "hi tech" recommendations that it's just the sodding air filter that's blocked

[That Daddy]

its a very odd system alright,as Mark/Blitz will confirm,it can never be under estimated though(idle valve behavior is a good example) and is quirky to say the least,it probably does not help the fact that the closed loop side of this ECU was added to the earlier non closed loop design,so more of a update that a revamp well from what i understand of it,though i may be corrected on some of that info.

Edited by That Daddy on Wednesday 17th December 10:06

[wrightp]

Thanks for the comments chaps.

Complicated issue I can see but the present situation is that the car got through the emissions test and it seems to run OK (I believe). If the tune resistor was broken (It has not been tampered with in my ownership), would the car not have some performance syptoms making a problem or wrong adjustment obvious?

The tester asked me to take the car for a good run after the successful emissions MOT test and he retested statically and the results were another pass - does this not suggest that the adjustment he made has remained in the system as the car was turned off more than once between the tests?

I am taking the car out for a run shortly and I will revisit the MOT garage as he offered, to rerun the static emissions test again. My thinking could well be flawed, but, if the test is OK again 4 days later then should I need to be concerned in any way?

I hope this makes sense and further views would be appreciated.
Cheers

Phil.

[blitzracing]

Interesting one. A past Mark Adams post siad that it did effect the amount of correction the ECU would have to make to the catalyst map. I dont have Marks hardware and software,and I cant read whats going on inside the ECU, so I removed the lambda inputs after an ecu reset, and tweeked the CO setting on the cat map to see the results. At this point the ECU cant correct the mixture as it has no feedback, and simply thinks the lambda's are still cold and not switching, so no fault condition is generated. Altering the CO value should then alter the long term fuel trim value as the ECU cant compensate, and I would have seen a mixture change in the exhaust. (Measured via the now open circuit lambda's) which I did not. I did ask Mark Adams why he specified a voltage for the setting on the cat map, and he simply said it was the factory set voltage. To add to this, if altering the value does alter the amount of correction the ECU has to make, it would alter the shunting problem some cars have at low speed, when they run lean and the ECU runs out of adjustment range on the short term lambda feedback, and Ive never found any effect at all. What I did try was switching to the non cat map, and then trim the CO until the lambda's switch. This would then imply virtually no futher compensation is needed in the long term (tickover adjustment time) when you switch back to the cat map. This does not work reliably however, as I guess the catalyst map must be more complex than simply be the fixed map, with additional trim adjustments. I do find it fustrating I cant get at the values inside the ECU however !

Mark

[blitzracing]

wrightp said:

Thanks for the comments chaps.

Complicated issue I can see but the present situation is that the car got through the emissions test and it seems to run OK (I believe). If the tune resistor was broken (It has not been tampered with in my ownership), would the car not have some performance syptoms making a problem or wrong adjustment obvious?

The tester asked me to take the car for a good run after the successful emissions MOT test and he retested statically and the results were another pass - does this not suggest that the adjustment he made has remained in the system as the car was turned off more than once between the tests?

I am taking the car out for a run shortly and I will revisit the MOT garage as he offered, to rerun the static emissions test again. My thinking could well be flawed, but, if the test is OK again 4 days later then should I need to be concerned in any way?

I hope this makes sense and further views would be appreciated.
Cheers

Phil.

Its possible as Superdik says that it just takes the reading on power on, and this means I would not see any alteration as I adjusted it with the engine running....?? UM . This however does not tie in with the tweeking you had done origionaly that appeared to alter things however, not that that made sense. As for poor running, these engines used to run carb's so are pretty tollerant to variances in mixture and the ECU can always run in fall back mode as well. I ran with a broken wire to one lambda probe for some time and never noticed a drop in performance and It only shows up with a fault code reader or a test meter on the probes. The really tight fuel control is just to keep the Catalysts working correctly. One thing of note is you have to let the engine tickover for about 25 seconds after new parts are fitted, so it can retrim the mixture. If you just get up and go for a test run this does not happen, and can lead to poor running.