High CO at MOT

[zcacogp]

Chaps,

It's MOT-time. Actually, it's a couple of weeks until MOT time, which is quite good news. But I took my car to the MOT place today to have an emissions test done - it always struggles with the CO emissions. I took it the long route to the test place to get the engine good and hot and it was running very well when it arrived.

They did the emissions test first .... and it failed. CO was 3.8, above the maximum of 3.5. It's always high, but I changed the AFM in May for a reconditioned one so I was hoping things would be different this year. Clearly not - the CO was pretty much the same as it was at MOT time last year.

So, what can cause high CO at MOT time? I know it means the car is running too rich - too much fuel or not enough air. However the AFM was new in May, about 3000 miles ago, and the Fuel Pressure Regulator was new about two years ago. The engine temperature sensor was replaced at the same time as the FPR, and the car was serviced around 2.500 miles ago (new oil, plugs and cleaned the air filter.)

I've just fished the plugs and air filter out to have a look at them and the air filter is clean (not obviously blocked, and you can see the light through it if you hold it up to the sun). The plugs look a little sooty, as you'd expect if it was running a little rich.

So what else could it be? What else could cause high CO at MOT time? The options seem to be injectors being grubby with a poor spray pattern, or some kind of fuel pressure issue (rail pressure is too high.) I can get the injectors cleaned for about £50, but am not 100% confident that this will solve the problem. Is there any way of testing this at home, with a good DIY (but not professional) workshop? Or is there any other diagnostic test I should be looking at doing before sending the injectors off?


Oli.

 

[Christopher I Ralph]

Hi Oli,

Distributor cap or rotor?

 

[Richair]

Hi Oli,

I have the same issue as you: I have fixed all the leaks and junked the Venturi system, rebuilt the head, all ignition parts have been replaced as has the coolant temp sender and I've tested the AFM and TPS and both are within spec according to Clarkes garage. Oh and I had my injectors cleaned. The engine is all in very good order so I'm at a bit of a loss and I think my CO results are similar to yours, so I'll be keeping an eye on this with interest but sorry I can't be of any help!

Rich

1986 951

 

[zcacogp]

Christopher,

Hey, good to hear from you. You are a rare face around these parts now so thanks for your post. How's that 993?

To answer your question; the distributor cap and rotor arm are both about 5 years old, but there aren't any other problems with the ignition system. I'm aware that that doesn't rule them out but I think they aren't the prime suspects (although I don't have any prime suspects at the moment ... !)

Rich, thanks. I'm hoping there will be some good, accurate and helpful answers appearing on this thread any moment now, that will be invaluable for both you and me .... come on chaps, don't let us down! []


Oli.

 

[GPF]

It's not just fueling that can cause high CO emissions, it can be coming from the crankcase.

Piston rings?

Blocked breather?

 

[mike220]

As Graham says, crankcase breathback can have a huge effect. Having had the issue on mine of a leaky injector putting a tiny amount of fuel down the bores into the crankcase caused me the same issue at mot time once. It's a nice easy check tho, get it back to the mot station and on the meter then pull the dipstick up an inch or two. If it's contaminated oil/crankcase contam your readings will drop like a stone, at which point you can explain it to your mot tester and with 'pass' certificate in hand promptly drive home for an oil change []

Let us know how you get on ? Fingers crossed it's something simple for you

Mike

 

[George Elliott]

Oli, if you clean the injectors, it can only make the problem worse, they let in fuel

I would still suspect the AFM. Have you reduced the allen screw adjuster on the AFM?

I did an adjustment on an old AFM with worn resistor track. Raised the wiper assy on the spindle, and adjusted the arm to lower the ecu signal relative to the flap in the AFM. I was happy to do it as the AFR Guage told me the setting was safe. (see youtube video of afm rework, not elegant but ok while I save for Augtronic)

Most 944t software run the engines rich because thats "safe". It also covers all the short-comings of the engine management system.

The best solution I have seen to date is Augtronic, (not on my car yet) and it still amazes me that such a state of the art kit is available for a car that went out of production over 20 years ago.

My MOT reading was : emissions at idle 0.420% vol against a limit of 3.5%

HC Reading 99 ppm against a Max Limit of 1200 ppm

Hope you find an easy solution.

George
944t

 

[zcacogp]

Graham; blocked breather? Interesting. The sort of breather that could cause intermittent high oil usage? This thread here may explain more .... http://www.porscheclubgbforum.com/tm.asp?m=837537&mpage=1&key=&#844034

Mike, thanks. Part of my problem is that I don't have a machine to test the car with. In fact, the whole of the emissions system is something I don't have the tools to work on (no gas tester, no fuel rail pressure gauge) so I'm entirely in the dark. I'm tempted to swallow my pride and take the thing off to someone like Jaz in Wembley and ask them to solve the problem and do me an MOT, and pay whatever it costs. That would be the simple solution. I don't want to run out of favours with my local MOT place by bothering them too much to use their emissions machine. Would a blocked crankcase breather explain slight sootiness on the plugs?

George, I think the point about injectors is that if you clean them, they cut off more cleanly (and don't dribble), and the spray pattern is finer meaning that fuel is atomised better and burned more evenly. No, I haven't adjusted the screw in the AFM as it's supposedly calibrated and hence fully set up out of the box. I don't want to invalidate my warranty, but it's possibly one of the next stages to try. If I can find someone to let me use their gas analyser .... Are those figures you give with the standard AFM (after you tweaked it)?

Augtronic (and the like) would be lovely but a bit beyond what I'd be happy to spend on the car.


Oli.

 

[scam75]

I had a similar problem. It turned out to be the connection on the loom between the afm plug and my maf. (It's a SciVision so plugs into existing wiring). That wiring has been bent back towards the afm for 25 years and is a common issue. However in my case, after suspecting the loom for a bit, it turned out to be a poor connection. Either that plug (most likely) or the bit on the maf it plugs in to. Anyway I got some DeOxit spray and applied it to both the plug and socket. Now my AFR's (and CO's) are rock solid.

Previously I had been running rich and idling rich. Tried all manner of things before finding the problem. The signal wire (or connector on the end more specifically) looses sensitivity. It reads between 0-5v and tells the ECU what's going on in the AFM. When it loses sensitivity it results in running rich, or very rich in my case!

Give it a bash, DeOxit was £15 delivered from ebay.

Stuart

 

[PSH]

Hi Oli



I'm sure you'd be welcome to pop into my son's garage to test the emissions and perhaps sort them out for you, he could also arrange an MOT for you if you wish....he's about 2 miles from Jaz so close by. He's away this week but I believe back on Monday..just don't mention Jaz to him..let's just say he wasn't impressed with their past work on my car...

Pete

 

[zcacogp]

Pete,

Thanks - that's a kind offer. There is a couple of weeks left to run on the old MOT so I am very tempted to take you up on that (proxy) offer.

I'll send you a PM. Thanks again.


Oli.

 

[bmnelsc]

Good thread here - lots of information. No mention of O2 sensor here...is that a possibility? e.g. O2 sensor signalling lean so fuel delivery increased to get to proper mixture (according to O2 sensor) resulting in rich condition?

My last check at the dealer (we don't have mandatory emission testing in Alberta Canada - come on over!) showed emissions at the tail pipe were good but they couldn't measure at the test port due to corrosion of the plug (they weren't about to damage it just for a pre-sensor reading). Based on this, it appears there may be some value in measuring on each side of the O2 sensor/catalytic converter as part of the diagnostic.

 

[Neil Haughey]

Happened on our 968. The O2 sensor was tired and cycling slowly which you could see on the CO test as it was cycling between rich and very rich. This didn't register as a fault in the ECU btw probably because its such a basic system.

These cars certainly shouldn't be setup rich, they will foul the plugs in no time and its something we have played with a little on the race car. Turbos are something different entirely.

 

[zcacogp]

Bruce,

Good idea, but the car has no cat and hence no O2 sensor.

Neil, I presume your 968 was a cat-equipped car?


Oli.

 

[Neil Haughey]

Yeah 968 had a cat, race car always a bypass pipe and jumper in the loom into the ECU to tell it to ignore O2 input then CO adjusted via the screw in the bottom of the AFM (allen key). I had it running at a couple of % before as made the most power that way on the rollers but even with 100% track driving the plugs ended up black in no time. For a road car the correct setting is something really low like 0.6% on these.

 

[George Elliott]

Oli, had not considered the cleaning may help injector cut off, good point. I achieved a significant improvement with the allen screw adjustment. Then I replaced a bad wiper track AFM with a supposed good one which had a siezed adjuster screw []. So then I just waded into the internal adjustment. It is super sensitive and not the correct approach, but it is effective []

Yes those readings are from a standard AFM.

I should add it has Klaus Hoffman chips and a ball bearing turbo, which you can interpret as you wish, but I think it translates as - a few crude tweaks some German thought may make it quicker 20 years ago, to which has been added a turbo with a different air delivery profile, bigger injectors, different FPR?, a better I/C, a 3" D/P & Exhaust, which the whole ECU is unaware of ........and its amazing it runs at all. []

I removed the square plastic cover to make adjustment inside the AFM easier, but I do have an AFR soI know if its going too lean which is important if you dont wanna melt her..[8|]

George
944t

 

[zcacogp]

Chaps,

Bit of a thread update here; many thanks to Pete's son Richard, who runs a garage in West London (http://www.rshperformance.co.uk/). I took my beloved S2 to him this morning who had solved the problem within minutes. His observation was that the car was running too well for anything much to be wrong with it, and adjusted the mix on the Air Flow Meter (AFM). The AFM was new in May and I had fitted it 'out of the box'; a twiddle on the adjustment screw made a CO reading of 3.8 come down to 2.8 and I was very happy.

So, thanks Richard. Thanks Pete. Thanks to everyone for their suggestions. I'm glad it was simple, and it's booked in for another MOT on Monday. Fingers crossed all will be well.

Richair, I know my diagnosis won't be of much help to you but I hope you gets yours fixed soon.


Oli.

 

[Richair]

Glad you've got it sorted Oli.

I've already fiddled with the CO2 screw on mine as I've have a DIY meter at home, that got it down to around 3.5% which is just about acceptable to me. I've stopped worrying about it to be honest, as having covered around 1300 miles in Europe on holiday with some sitting in heavy traffic for long periods in Brussels all was well and the plugs come out a nice colour. When I get round to it I'll test the pin out readings at the ecu's for the TPS and AFM, but as it runs perfectly I'm in no rush...

Rich

 

[PSH]

good to see he could help Oli....happy motoring..

Pete