Gunson Gas Tester - Emissions...

[bacardincoke 42 1 Cars]

Picked up one of those Gunson Gas Testers (digital type - G4125)

 

It's not quite plug'n'play... you need to be very particular about how you set it up and use it, plus it's got an advertised accuracy of +/- 0.5% meaning if you're hovering just below the 3.5% threshold it could be a problem.

Anyone else used one these / have longer experience of them?

Initially I was consistently getting between 2.4 to 2.8% with it (hence the accuracy being a worry) but having gone over everything with a fine tooth comb several times it's now down to a regular 1.5 - 1.8%.

Do you find them accurate / your readings match the MOT testers readings?

Separately... any one use the emission reducing additives, some even advertise as being specifically for pre-MOT use?

[Biggles 65]

I've got an older model but looking at it, it's functionally the same.  You need to be patient calibrating it but I've always had good agreement with the MOT station.

[bacardincoke 42 1 Cars]

Good to hear, thanks. I'd nothing to compare it to and was just wondering.

 

They're a bit fragile feeling construction wise... I wasn't expecting it to be so light weight, but it seems to be pretty reliable if the consistent readings are anything to go by.

[Richie-Van-GTi 72 2 Cars]

I have the older version of that as well, not been used in a lot of years and I have lost the instructions. Anyone care to remind me how to set it up and calibrate, I have a feeling I will need it when I get this XS engined car running.

 

[bacardincoke 42 1 Cars]

This is a PDF copy I'd downloaded before getting one, just to have an idea of how it worked etc.

 

As it turned out it's the same as the printed one that came with it, I'd image it's a similarset up etc. for older ones.

 

https://images-eu.ssl-images-amazon.com/images/I/91Ihi6jU31S.pdf

 

Just found this one... seems to go into more detail than the one above -

 

https://www.yumpu.com/en/document/read/22122814/gastester-digital-gunson

Edited October 15, 2020 by bacardincoke

[Richie-Van-GTi 72 2 Cars]

That second link is perfect, thank you

[bacardincoke 42 1 Cars]

Have used the Gunson quite a few times since originally posting and think I've finally got the technique sussed... if you've recently got one or are thinking about it I definitely recommend them, but be prepared to stick at it.

 

Despite what I wrote originally about getting the CO down to 1.5 - 1.8% the car didn't like it that low and I finally settled at 2.4% as being the optimal smooth idle / running / emissions trade off.

 

Coincidently the last MOT certificate shows it as testing at 2.4% (+ Hydrocarbons 140 ppm)

 

Covered all the recommended areas when trying to get it down further but short of changing the AFM it seemed to be stuck there.

 

Lots of articles out theret (some very conflicting) talking about cleaning and / or adjusting the internals of the Bosch L-Jet AFM so bought a cheapy one to use as a guinea pig.

 

Using that, the cleaning alone improved things and adjusting the wiper arm arc to run on fresh conductive material helped further - after the initial 2.4% of the replacement I could regularly get 1.5% CO with it in.

 

Similarly cleaning / arc adjustment of the original AFM could only get the CO down (from the same 2.4% of the guinea pig) to 2.0%... decided as I'd a working AFM to fall back on there was nothing to lose by seeing what adjusting its spring tension did.

 

2 x notches clockwise was all it took, that's a tiny amount when you see the number of teeth / rotations possible but it got things down to a regular 1.3%.

 

I had tried 3 x notches and got 0.9% CO but was worried that the Gunson error of + / - 0.5% could have potentially taken it too low if my machine turned out to be less than accurate.

 

Car did it's MOT today on its original (now 1.3%) AFM and the test centers own reading was 1.320%, so it's good to know the Gunson was pretty much spot on.

 

The Hydrocarbons came in at 153.0 ppm (previous MOT was 140.0)... it's well within the MOT limits of 1200 ppm but I've read that would be excessive at the level... are mine a high / low or expected level?

 

Car is running better than ever, I thought it was good before but it's like a new car. Absolutely no trade offs elsewhere in the rev. range, starts on the button cold or hot, rock steady idle no matter what and probably pulls even harder in all gears.

 

 

 

 

 

[Leslie green 111]

Nice work and at 140 ppm compares well with a modern car , I have used a gunson before and found it reasonably accurate against an mot machine , more of use for carb cars as injection cars shouldn't really need it unless something isn't right to pass mot.

[bacardincoke 42 1 Cars]

Cheers... I'm turning into a bit of a maintenance anorak and getting more out of this side of having an old car, no bad thing I suppose.

 

There's a very old post on the forum suggesting 100ppm for the HC's is the ideal, wasn't sure if that's still the current view and true or not.

 

200ppm I think is the present day pass limit for post 1992 / catalyst cars? If so, on the surface of it 153 seems good for an older, high mileage engine - hope so!

 

If you do read a few of the articles on-line about the Bosch L-Jet AFM (mainly BMW, Alfa sites, but there's a particularly good one about Morgans) there's some who say don't touch the spring tension at all costs while others will say there's nothing to worry about... that's why I tried it first on a cheap second one just incase someone was right.

 

You can tell immediately the guinea pig AFM had been opened before... still not sure about my original though. If it had been it was re-sealed very well plus the insides were like new.

 

As a last resort, if your suffering problems and all else fails it might be worth at looking into what I did. Like I said before just cleaning alone made a noticible difference with even bigger gains possible if you're feeling confident.

 

One or more previous owners of my car had done quite a few of the old school period mods, all trendy in the 1980's and 90's so there's a good chance someone may have fiddled with the AFM.

 

If so, it could even be that I was putting things back to their original settings?

 

 

 

 

 

 

 

[Leslie green 111]

I think its 2002 before you get down to 200 ppm , I just checked an mot for out 2010 1.8 focus when it had only 33k on it and the result was 8 ppm ! but with a good cat and pipping hot all the HC which is just unburnt fuel should be sorted by the cat as even at that low mileage this engines uses oil .

By comparison my ohv 1.6 xr2, a carb  engine redesigned in 1971 from an even older engine was about 600 ppm a good few years ago and has gradually increased year on year and now is at or just under  1200 ppm and struggling to mot  and is due a rebuild its first in nearly 40 years but runs fine.

Id say most afm have been opened at some time now and they were cheap enough to get one and have a play and they will possibly become a rare and desirable/expensive  part in years to come when supplies dry up.

My engine block on the 205  was cracked so although i got it to run ( came as a non runner partially stripped) and it seemed ok never got to drive it before stripping it for resto.

[bacardincoke 42 1 Cars]

All pretty relevant at the moment what with BoJo announcing UK new petrol / diesel car sales will be ending in a decade.

 

It's a brave person who'd try to second guess how that'll affect things for our 'polluting' classics come that time.

 

Think you're right about AFMs... if what I found when trying to get a decent old 1.9 one is anything to go by there's not exactly a glut of them about.

 

Didn't look into it, but I think there's new repro 'Chinese' Bosch copies available for some models of AFM? Be interesting to hear more if I've understood that correctly.

 

The conductive / resistive strip (whatever it's called) seems to be the main part that physically deteriorates / wears out through long term use... if someone was able to offer a service to re apply it they'd be onto a winner.

 

The full AFM refurbs you see advertised are likely spot on, but unfortunately aren't cheap.

 

 

 

 

[DamirGTI 342]

There is some new Chinese knock off AFM's on the market .. pretty sure it's the same "operation" as with the "new" ignition distributors = crap ! 

 

Right , the resistor plate is the brains of the unit and the main part which is problematic/wears out .. there's also Chinese made replacement plates available , but people who bought and tried them said that the carbon track on them went bust within a week or two of use .

No doubt the same plates will be in the above "new" AFM units .

 

Resistor plate/carbon track refurbish (if wanted to do properly) is i guess pretty expensive setup for only single or just a few units and kinda complicated process , the material is acid etched onto the plate , then dried and possibly even themp. treated for it to fully harden .. that's why it lasts so long +20 years . "Bourns" and "Sfernice" made them , as written on the plates .

 

There's been some tryouts repairing the worn carbon tracks with conductive ink and paint , but neither didn't last for longer period of use .. the ink or paint layer was simply too soft to withstand the wiping action across the surface .

 

 

Most of them AFM's has been opened up during the years , guess you'll have to dig deep to find second hand OE sealed unit .. at the mechanics we used to open them in the 90's and tweak the spring tension a bit as sometimes it was the only way to fix the car . 

Kinda same as with the ignition timing is with the fueling also - factory preset spring tension wont be ideal for today's unleaded fuel , and mainly some adjustments are needed to compensate for the wear and tear during the years .

 

Nothing wrong with adjusting the spring tension , if you know what you're doing and if sealed up properly afterwards .. it's impossible to adjust it properly by feel/ear , best done with an wideband sensor plugged in the exhaust to monitor air-fuel mixture as you perform the adjustment .

 

 

D

 

 

[GTIJulian 0]

Hi all,

I had very similar situation and experience to the original posts from Martin - my 205Gti 1.9 87D 8V failed the MOT last year on CO emissions. I ended up turning the AFM mixture right down to the minimum so less air could bypass that measured by the sprung flap and resistive track. I also moved the wheel two notches.

 

The results given by the gas tester never quite convinced me that they were consistent - lots of time spent on the drive with the engine idling, and yes lots were near 3% so uncertain.

 

Can I just check to confirm (I got it correct) that clockwise is referring to the wheel which will put a little more tension in the spring so it resists movement slightly more resulting in a tiny bit less movement across the resistive strip so the signal to the ECU results in less fuel in response to any given demand?

 

The car has been running well since, although I'm not sure what the CO actually is atm (the MOT emission print out didn't have a CO value, but HC quoted was at 21ppm!). It might be that my gas tester is duff. I'm going to get it tested pre MOT at a friendly garage so I can cross check my tester. After which I can do all the usual things again - re-seat Sensor/ECU connectors, clean/reposition the AFM track/arm ETC if necessary...

 

The AFM (my second) is quite new but remanufactured, and a decent one I think - not much wear on the track.

 

Historically, yes the car is much more drivable with a slightly rich setting - 6.5 turns on the mixture screw away from the weakest setting resulting in a CO value of approx 2.5%.

So, now, it isn't in its usual state and I don't immediately suspect the AFM - what other factors could cause the mixture to be too rich?  Are all the sensors on the back of the engine, or elsewhere just for instrumentation (gauges and warning lights) or do any send temp signals to the ECU which could affect carburetion? Any other suggestions?

 

I haver the original AFM - anyone know of a good place to send it for restoration?

 

[An aside - anyone know where I could get my original SAD reconditioned? My second (OEM I think), on the car now turns the supply of extra air off too quickly before the engine is properly warmed so revs drop to 700 - normal tick over 4 me is 1100. Maybe its water temp sensor is faulty - but isn't there just a bi-metalic strip inside? Another possible related symptom is a large sensitivity to damp at startup. The engine won't run properly if it's previously rained hard until it's all warmed up. Yes I will clean the HT leads and coil surface again but it's always been a bit of an issue. Is there any advice out there re damp prevention, besides getting a garage!? I used to put a rubber glove over the coil with lots of silica gel bags inside :-)].

 

 

Thanks in advance for any help

 

Cheers

 

J

 

 

 

 

[Anthony 1,003]

3 hours ago, GTIJulian said:

Hi all,

So, now, it isn't in its usual state and I don't immediately suspect the AFM - what other factors could cause the mixture to be too rich?  Are all the sensors on the back of the engine, or elsewhere just for instrumentation (gauges and warning lights) or do any send temp signals to the ECU which could affect carburetion? Any other suggestions?

 

 

Obvious one to check is the ECU coolant temperature sensor - blue 2 pin sensor, either underneath the dizzy (earlier cars) or on the back of the thermostat housing (late cars).

 

They tend to fail reading cold in my experience leading to the ECU giving constant warm-up enrichment, giving dreadful fuel economy and (presumably) high CO numbers on an emissions test.

 

Easy enough to test with a multimeter - should be somewhere around 2500-3000 ohm ambient, dropping as the engine warms up to something around 250-300 ohm with the engine at normal operating temperature.

 

One thing though - you said that you screwed the mixture adjustment right down to minimise the air bypassing the flap.... this would have raised, not reduced, the CO value.  Clockwise to turn the adjustment screw inwards increases the CO, anti-clockwise to wind the adjustment screw outwards decreases the CO.

[GTIJulian 0]

Thanks - a new ECU temp sensor is in the post from Pug Racing in case it's faulty after I've tested it.

 

Yes, absolutely, my bad, the AFM mixture screw is wound all the way out to allow maximum bypass air flow atm. I also discovered that if you press sideways on the allen key the adjuster bolt is removable.

 

J

[GTIJulian 0]

I've compared the resistance changes with temperature of new and old ecu temp sensors.

Old room temp (20C) 1800, just boiled water 250 ohms

New room temp (20C) 2300, just boiled water 290 ohms.

The contacts were dirty as there is a small oil leak from the end of the engine - a small amount gets in the distributor, or drips down. So cleaning them may have removed some contact resistance which wouldn't help...

Distributor sent away for testing too.

 

So I put the old one back, as presumably the ecu might enrich the mixture less, but then again they might both be below a (hot) threshold.

 

Next trying to find somewhere to refurb my AFM - apparently parts hard to find - any recommended?

 

What about the two sensors the other side of the SAD on the back of the engine?

 

 

Cheers

 

J

[Anthony 1,003]

4 minutes ago, GTIJulian said:

What about the two sensors the other side of the SAD on the back of the engine?

 

They're for the dashboard - one is the coolant temperature gauge sender, the other is the switch for the high coolant temperature light.  Neither has any impact on the ECU or engine running.

[welshpug 1,661]

Atp electronics are the only place i have ever seen doing remanufactured/refurbished airflow meters.