Failed Smog Test - Making Adjustments

[EZ Gruv]

So, for the first time since I bought my bus 8 years ago, it failed emissions testing.

My idle speed CO was 3.28% (bus idles at 910 rpm), when the limit is 2.50%. It was within specs at 2800 rpm.

I read in Bentley to adjust the CO by turning the adjusting screw on the AFM. I guess I need to turn it counter-clockwise to lean it out some.

My question is (searching hasn't found an answer yet) without having it hooked up to an analyzer, how far do you turn it to go down 0.78%ish? Any ideas?

2.0L Type IV FI

[bretski]

Dollars for donuts says all you need is some basic tuning...did you adjust valves, points, dwell, timing before your smog test?

Don't go messing with your AFM screws yet. If you've done the proper tune-up already, then might want to lean it out a bit. One other question: is your catalytic converter in place and in good working order?

[EZ Gruv]

My plugs are about 6 months old. Valve adjustment and oil change last week. Points, dwell, and timing all checked last week as well.

My 77 is a non-catalyst model.

[bretski]

Sorry, for some reason I thought you had a 78 with a cat...

I'd still double-check all of the basic tuning items, starting with the valves. You have solids?

[EZ Gruv]

Yep, solids.

[Amskeptic]

Yep, solids.

You are close if the cruise portion of the test passed.

Raise your idle speed with the idle speed screw to 980 rpm. Turn the mixture screw clockwise and see if you can get it to go up to 1,000 rpm. Then turn it back out and see if you can get it to drop to 950 rpm. If it runs smoothly at this MIXTURE DEFINED IDLE SPEED, then you can bring it back down to 900 with the speed screw. Fortunately, your HC readings were not a problem, right? We are trying to get your engine to run smoothly enough that we don't inflame the HC readings from misfires, but lean enough to pass the idle CO. .78 seems a bit lean for the specification. Did they input your information correctly? You are non-catalyst with no oxygen sensor for crying out loud.

When you bring it back in, make sure the engine is good and hot with a 30 minute freeway blast, this will help prevent crankcase hydrocarbons from affecting your CO and HC readings.
Colin

[EZ Gruv]

Raise your idle speed with the idle speed screw to 980 rpm. Turn the mixture screw clockwise and see if you can get it to go up to 1,000 rpm. Then turn it back out and see if you can get it to drop to 950 rpm. If it runs smoothly at this MIXTURE DEFINED IDLE SPEED, then you can bring it back down to 900 with the speed screw. Fortunately, your HC readings were not a problem, right? We are trying to get your engine to run smoothly enough that we don't inflame the HC readings from misfires, but lean enough to pass the idle CO. .78 seems a bit lean for the specification. Did they input your information correctly? You are non-catalyst with no oxygen sensor for crying out loud.

When you bring it back in, make sure the engine is good and hot with a 30 minute freeway blast, this will help prevent crankcase hydrocarbons from affecting your CO and HC readings.
Colin

Just checking for my own understanding:

1. Turn idle speed screw until idle is 980rpm
2. Turn mixture screw on AFM clockwise and see if idle increases to 1000rpm
3. Turn mixture screw on AFM counter-clockwise down to 950rpm
4. If running smoothly, turn idle speed screw back to 900rpm

No mention of HC problems. The printout only tells you why you failed.
The 0.78 ins't the limit - it is how much I missed the limit by. The limit is 2.50% - I registered 3.28%.

I had the engine warm as suggested today, and will again at retest.

Thanks

[Amskeptic]

Raise your idle speed with the idle speed screw to 980 rpm. Turn the mixture screw clockwise and see if you can get it to go up to 1,000 rpm. Then turn it back out and see if you can get it to drop to 950 rpm. If it runs smoothly at this MIXTURE DEFINED IDLE SPEED, then you can bring it back down to 900 with the speed screw. Fortunately, your HC readings were not a problem, right? We are trying to get your engine to run smoothly enough that we don't inflame the HC readings from misfires, but lean enough to pass the idle CO. .78 seems a bit lean for the specification. Did they input your information correctly? You are non-catalyst with no oxygen sensor for crying out loud.

When you bring it back in, make sure the engine is good and hot with a 30 minute freeway blast, this will help prevent crankcase hydrocarbons from affecting your CO and HC readings.
Colin

Just checking for my own understanding:

1. Turn idle speed screw until idle is 980rpm
2. Turn mixture screw on AFM clockwise and see if idle increases to 1000rpm
3. Turn mixture screw on AFM counter-clockwise down to 950rpm
4. If running smoothly, turn idle speed screw back to 900rpm

No mention of HC problems. The printout only tells you why you failed.
The 0.78 ins't the limit - it is how much I missed the limit by. The limit is 2.50% - I registered 3.28%.

I had the engine warm as suggested today, and will again at retest.

Thanks

You read correctly, I did not.
From your best mixture-defined idle speed (starting at 980) as in: if the mixture screw does give you a change in rpm, find the fastest idle that the mixture screw will give you, then lean it out by 35-50 rpm. Final step is the idle speed screw to put it within factory specs.
Good Luck!
Colin

[EZ Gruv]

So after looking at the valves again (all at spec), air filter (clean), dwell, timing, and idle...everything ok.
I went ahead and tried the mixture adjustment. The smog tech was cool and let me do it while connected to the machine. Still couldn't get it down enough.

I went ahead and took it to the one mechanic in town that I know really KNOWS FI VWs. He made the same adjustment until the adjustment screw came out completely. After putting it back in and positioning it about halfway within its home, he popped open the AFM itself. He made an adjustment to the spring tension. He said this adjustment will tell the system to use less gas.

Passed the test on the first try. I went from 3.28% CO to 0.72% CO.

Plus, my engine should be happier after the adjustment.

[Amskeptic]

So after looking at the valves again (all at spec), air filter (clean), dwell, timing, and idle...everything ok.
I went ahead and tried the mixture adjustment. The smog tech was cool and let me do it while connected to the machine. Still couldn't get it down enough.

I went ahead and took it to the one mechanic in town that I know really KNOWS FI VWs. He made the same adjustment until the adjustment screw came out completely. After putting it back in and positioning it about halfway within its home, he popped open the AFM itself. He made an adjustment to the spring tension. He said this adjustment will tell the system to use less gas.

Passed the test on the first try. I went from 3.28% CO to 0.72% CO.

Plus, my engine should be happier after the adjustment.

Good.
However, he pre-empted our next step. Now you need to know (because this is important) the high rpm mixture. The spring adjustment was one of two possible adjustments and, whether or not he knows this is immaterial, the one he chose actually has a greater effect upon the upper rpm range. Had I been there, I would have moved the wiper clockwise to lean the idle right there, and then I would have relaxed the spring as necessary to bring the high rpm back to where it needs to be.

You said your high rpm CO was within specs, what we need to know is if it leaned out with the spring adjustment. This could enter valve burning territory. An idle of .72% is lean, it sure is lean, but it doesn't have too much effect on exhaust valve tempereratures. Too lean at high rpm or under load is trouble.
Colin

[EZ Gruv]

At my next opportunity I'll get a full printout of the levels at idle and at speed. Unfortunately they don't print out any values on the state test unless you fail.