MOT

Took my GTA atmo for a MOT at the weekend. It failed on hydrocarbon emissions (about 1300ppm) left bank. Right bank was approx 500ppm. So I thrashed it down the dual carriageway and back, and it then passed (just).

Car was presented for MOT after a 15 mile gentle run.

It nearly failed fgor the same reason a couple of years ago. Surely you can't say the emissions are from left or right bank by measuring out of that tailpipe as the gases mix in the silencer?

Has anyone had similar problems?

[mettersl]

Not sure how that happened, there are only two pipes from the last silencer, before that there is only one....so the mixture should be the same from both!
If it has no CAT, the burn up the road trick won't be very effective,

Hydrocarbons = Unburned fuel.

When was the last time your Spark Plugs, Ignition leads, Dist Cap, and Rotor arm were changed ?.

Mine was 1900ppm when i purchased it back in Jan this year. The exhaust gases absoultely stunk.
Now its 135ppm at idle, after a full service.

That sounds about right Simon. Leads replaced a couple of weeks ago and plugs checked at same time. Looks like the distributor then.

when was the last time the engine oil was changed? throw some new stuff in if long in the tooth...

Fresh Mobil 1 a couple of weeks ago.

Just out of interest, my GTA is on it's original exhaust. Wonder if anyone else's has lasted that long (since 1990)?

[mettersl]

I agree that distributor and the other ignition parts are a good start

Cheers
Lee

So most likely to be the over complicated carb set up possibly the cold start set up. My carb has been rebuilt but the cold start has not been fettled, it runs really rich until it warms up. What stat do you have fitted is it a genuine Renault item? If its not the stat is quite different not only in price but design and my car now has a Renault item fitted and the car warms up much quicker now so comes off choke much sooner.

If your full consumption is not good it may be worth setting up the carb up from start to finish, I still have the pleasure of this to come once I have solved the overheating problems.

Many thanks to john c for pointing put the differences in the genuine coolant stat not only does it open as a stat would do it also controls flow through the engine depending on temp. It does this when the stat opens it shuts off the flow and diverts flow through the pump to cool the heads, as I understand it.

The fact that you took it for a blast it appears the car may not have been complete warmed up or the cold start is possibly over enriching the mixture.

Si21

Hey Guys...Andy is talking about an Atmo here, My original silencer has two inputs, one from each bank of cylinders, and two outputs, I have never opened one up, but logic suggests there are two seperate silencers in one unit.....unless someone knows for sure. So in my view the ppm`s could well be different on each side
Turbos on the other hand, are different, as both cylinder banks feed the one turbo before going off to the silencer in one pipe.

Ok, so if all major ignition components have been replaced, and it has recent fresh oil. Then it must be over fueling to produce such a high HC PPM.

Would be interesting to see if the figure drops if you remove the Crank-Case breather. So it vents to air.
If it did suddenly drop, it would indicate the cyl walls are being washed in petrol and not being burnt. The vapours of which end up in the crank case and sucked back in.

High HC can also be caused by worn badly sealing valve in a high milage engine. But this would show up on a compression test.

Andy...have you tried setting the mixture adjustment on the carb. If it is out of adjustment then the ppm readings could be out of speck.
Also was the engin temp up to normal when it was tested, because the choke flap is only fully open when the coolant has reached running temp and has operated the heat expandable wax component in the choke unit. The fact that the readings were better after a decent run suggests that it may not have been up to temp when it was tested.
Another point to consider is when at the MOT garage, the engin should be run till the coolant stat opens before testing, and if it is warmed up at idle the readings will be inaccurate, the engin must be warmed up by running it at around 2000 to 2500rpm if the car is stationary.

I don't believe there are separate silences for each bank. Both headers feed into one silenced box I believe the silencer is split into two sections and one tube links them together with the tail pipes coming off the second silencer, one large split silencer acting as two.

You say It's the original silencer, it may be that there is more of a restriction on one side of the silencer so more gases taking the easier route. Is it possible this would cause a higher unburnt fuel reading, or would that relate to a percentage of the gas being emitted?

Si21

[mettersl]

Stupid me- missed the word ATMO!!

Obviously ignore all the injector stuff.

DOH
Lee

I don't work on the car myself. During the 8 years I have had it, it always goes to Roadspeed for services ( last a month ago). The car has always idled nicely, so the carbs have not been messed with.

JohnC's point about the MOT testing is possible as the young tester did not seem too sure about measuring left and right emissions

The idea of over-fulling seems possible as it seems to be consuming a lot of fuel. This would be worrying if excess fuel is washing around