Interpeting Emmision Test Readings

[Litton]

I just got back from getting my smog test and I thought I would share the numbers with the group and see if someone can tell me what they mean.

In AZ three measurements are made:

1) at idle with the car in gear
2) at 32 - 35 mph (don't know how much or if there is a load)
3) at idle again but in park

My car passed with the following readings:

1) 952 ppm HC, 4.27 ppm CO
2) 237 ppm HC, 0.20 ppm CO (standard 300 ppm HC, 3.00 ppm CO
3) 246 ppm HC, 0.49 ppm CO (standard 400 ppm HC, 5.00 ppm CO

Big differance between loaded idle and idle idle. I seem to recall from early posts that the relationship between the HC and CO readings was important in telling us how rich or lean the MAP is set. Being a CA car I suspect the the mech set things a bit on the lean side for smog purposes.

[Tomguy]

If your car passed I wouldn't worry too much about it but raising the idle speed with the idle speed screw should lean it out and really improve your idle (in drive) emissions. Then you could make it a bit richer since you would have more headway, for a bit more power.

Mine seems to have MORE power now and I LEANED it... of course it was absurdly rich (possibly to the point of being half-choked, I looked and my headers WERE RED!).

[Litton]

I'm not to concerned about the idle readings. In fact at the 800 rpm I have set now, it really pulls hard at a stop light. So much so that I have been kicking it into neutral. The book says 700 to 800 so I think I wll go down to 725 - 750. The silly thing actually is happy at 600 (that was an oversight).

If I am on the lean side I wouldn't mind going for more power. Economy in not a consideration as I doubt if I will put 5,000 miles a year on it.

[Tomguy]

Yeah, they do idle fine at 600, but it's just in the grey area because the fan runs so slow and coolant circulates less, so on a hot summer day... you get the idea.

I wouldn't kick it into neutral though. If it's really an issue with creeping at idle speeds, I'd check the brakes and the booster. I suppose it is possible that your vacuum to your booster isn't up to snuff as you have a leaking door lock system robbing you of vacuum power. When mine is at fast idle (and it has been lately - probably 20° out now) it does pull a bit if I release the brake, but it isn't overly hard to keep it still and the pedal has lots of room before the floor. You might need to bleed your lines too.

[Litton]

Brakes are still a small issue but getting better. The vac leak that I still have is really to small to bother the brake booster. I just seems the the trannys torque converter is really "tight".....the car just wants to move not sit still. Plus, you generate a lot of heat when the tranny is stalled.

Today will be a muck about day with the car so I will try a couple of things. Yesterday, it started to miss a bit after runing around town get smog and registration. It hasn't behaved quite right since I took he dizzy dust cover off. I must have distrubed something.

I'm still looking for some interpetation of those smog check readings.....am I on the rich or lean side. The CO readings are right at the minimums.

[Tomguy]

Well, I am not 100% sure but applying general chemistry...

CO and hydrocarbons are produced from incomplete combustion, which occurs when:
1) The fuel burns too quickly to allow complete combustion
2) There is not enough oxygen to allow complete combustion

There will ALWAYS be at least some HC's and CO due to #1. Looking at your results, it seems that #1 is the ONLY reason that you have any at all (other than idle under load - your results are ESPECIALLY good for almost ANY car, including modern cars, nevermind a 32 year old one!) That means that you can allow more fuel to be burned: Doing so will increase power to a point (to the point where adding more fuel just means it isn't burned properly, resulting in more CO and HC's). It would be great if you were allowed to set your mixture WHILE doing the tests. Mercedes says maximum CO is at 2.00 while maximum efficiency is 0.50 (I believe those are for idle in park). You're right on the max efficiency dot but adding more fuel can result in quite a power boost. Above 2.00, incomplete combustion due to #2 is in larger proportion (adding more fuel does increase CO and HC's because like #1 states, it just can't all be burned quickly enough - but after a point there isn't enough O2 to burn all of it). I would say 1.50 would be a great point to shoot for, and you'll find even more power in that old girl!

EDIT: I didn't outright state it but will just in case you need to hear it: You're VERY lean! (not to the point of burning valves per-se, but I would personally increase it a bit)

[Litton]

Went back and looked at previous posts on adjusting the MAP. I think I am to lean, especially with 0.20 ppm CO under load. So I will give the 'ol MAP a tweek......CCW for rich if I remember correctly.

Nice think about our inspection station is that I can go there anytime and have it checked for $11.15. Now that I have a base line to work from I can chart HC vs. CO vs. MAP turns.

So todays project is to track down the miss and play with the MAP.

[psfred]

Litton:

Fix the vac leak first -- the D-jet system uses manifold vac to determine fuel mixture, so vac leaks can cause mixture variations, depending on where they are. I'd replace the MAP hose (probably rock hard by now if not replaces), check the door lock system for leaks, and replace the rubber bits on the vacuum advance lines. Also check the booster line, I've seen the check valve snapped in half at the check valve.

Your idle mixture is way rich at no load-- go two or three clicks lean on the idle mixture control on the toothed wheel on the ECU. It should not change between load and no load conditions, you need to check the idle contact in the throttle valve switch for proper operation. Do you get fuel shutoff on coastdown? You should have no fuel between 2500 rpm and 1500 rpm with your foot off the pedal. Adjust position of the throttle switch if not.

Loaded mixture is a little lean, you want to go 1/8 turn CCW on the MAP screw and re-test.

Best running conditions are about 1-1.5% CO at both idle speed and load. Set the load condition first, then adjust the idle mix, as MAP setting affects the idle mix.

Peter

[Litton]

OK, I have gone 1/8 turn CCW on the MAP. Idle speed droped ~75 rpm and I reset to 700.

I found the ECU but no toothed wheel in plain view. Is it under the dust cover on top? Also, which way for lean?

My vacuum leak in the doors is really minimal. With the engine running there is not differance in vacuum with the line plugged or unplugged. Holds a steady 15". Doing a vac drop it take about 20 seconds for the vac to drop from 21" to 0" (tank is bypassed for the test)

I did a vac drop test on the MAP and hose and it held 20" for 5 minutes with a leak down.

[psfred]

CCW lean, CW rich. Wheel is under the rubber cover toward the radiator.

Peter