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Old 10-07-2004, 03:03 PM
SL Owner
Join Date: Sep 2004
Location: East Coast U.S.
Posts: 131
What would you do? 380SL CO@1.8 and after much work...

I'm interested in opinions/viewpoints on this. Here's the deal:

- 1984 380 SL, owned the car for 10 years
- The car is only driven about 3000 miles a year maximum. It's a second car.
- Current emissions test good for 2 years from now.
- Just spent thousands on various repairs done by myself and a mechanic including new rebuilt fuel distributor (mechanic), injectors (mechanic), a new idle control valve, new CORRECT plugs and Bosch wires, new rotor, new distributor cap, electrical fully investigaged (me and mechanic), for cooling a new radiator and fan clutch, new fuel pump, and new fuel filter.
- Car now has a lean idle miss for any CO idle adjustment below 1.8%. The Mercedes shop manual for this car suggests a CO value in the range of 0.5 to 1.5%. For 10 other years of its life with me the car always sailed through emissions HOWEVER apparently one of the two precats was clogged and the car lacked power-- for 10 years I apparently had no idea how much power this car could really have-- even now if I set the CO value low and live with a lean idle miss, the car has tremendously more power. The rest of the emissions values fall into place and are what you would expect at 1.8% (NOX around 1500, HC around 200, CO around 14%, O varies but usually zero since we are clearly running rich).
- I don't care if the catalytic converter hates this 1.8% value, I'll eat it later if I have to on a new cat.

When set at 1.8%, the car drives perfectly in every respect-- smooth-as-silk idle, incredible power, instant startup (always had a good startup). About 20% reduction in fuel economy as-expected with such a CO setting.

I presented this to several mechanics, here is what they say:

Mechanic #1 who, who has made errors with this car charging me thousands for repair work that resulted in nothing, states it's a manifold vacuum leak (without him doing a full vacuum check on the car by the way). He wants more dollars to do the job after sucking an embarrasing amount already (he's the one who has now spurred me to do as much work as I can myself on the car after my experiences with him). OK, it certainly is possible the car has a vacuum leak-- indeed likely, however it cropped up during the 2 weeks he messed around with the car. His rebuilt fuel distributor and fuel injectors may be the suspect here, I sprayed around with carb cleaner in those areas though and couldn't find a leak.

Mechanic #2 says "leave the car alone. If we can get these 20 year old cars to be under 2% CO we're OK with it. "When emissions time comes, we'll deal with that however we need to deal with it (translation: adjust as-needed even if it rides like crap to get it through).

Mechanic #3 who runs an emissions repair facility also agrees with mechanic #2-- he states the emission values for the car really aren't that bad, leave it alone.

I've been ALL OVER the car myself looking for vacuum leaks around injectors, the fuel distributor, cracked hoses, etc. The car is very clean inside, you don't find much in the way of old cracked hoses, etc. I've sprayed carb cleaner where I could safely, etc. I'm the one who found this 1.8% sweet spot for the car by driving and adjusting to just a the smallest increment my hand can make on that screw. I know how sensitve that idle adjustment CO screw is so I turn it back to lean then turn it very slightly, put the car in park and rev, drive if needed, turn more as-needed, until I get to the sweet spot.

Everyone around me thinks I should, at this point, leave the car alone-- the idea that me and this car have had enough for now and we're just a bit over what MB has stated as max. Clearly we are working around something here but maybe it's best to quit while we are ahead (or behind depending on your perspective). Other than harm to the cat converter, I can't see much risk in running the car this way for the next two years. It's fun to drive and the engine sounds as though it's exceptionally happy-- never experienced it as smooth and "calm" at rest or at acceleration.

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Old 10-07-2004, 07:11 PM
Super Moderator
Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,316
What is your on-off ratio doing when you tweak the mixture? In theory, your small adjustments should cause the lambda system to compensate and the CO would stay constant.

It sounds like you may have an O2 sensor or other emissions component problem.

BTW, a 1.8% CO is high - you need 1.2% to pass in Virginia.

What vacuum reading are you getting at idle? If the idle is smooth, I would discount the claim that you need to seal the intake.
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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Old 10-07-2004, 07:57 PM
Registered User
Join Date: Sep 2004
Location: Seattle
Posts: 1,932
I agree with Mr. Taylor. I had a bad O2 sensor on my 87 Supra and the emissions readings went from great to what you have (real bad), but the car ran perfectly. Of course my car was an L-Jetronic based system and I think you have KE or some other mechanical based system it sounds like. But I would definitely check out the O2 sensor (or just replace it if it isn't too expensive), unless you know it is fairly new. IMHO.

1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra
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Old 10-08-2004, 01:19 AM
Registered User
Join Date: Aug 2001
Location: Evansville, Indiana
Posts: 8,150
You will have a slightly rougher idle below 1.8% CO, this is normal. To get perfect idle you must be richer than that anyway.

I would check out the mixture control system -- you need to set the mixture with a duty cycle meter AND a mA meter at the EHA -- set the mixture to get 0 mA at the EHA (that is, no correction) and duty cycle near 50%.

If you have not done so, I would replace the O2 sensor -- if it's flaky, you won't get good mixture control at idle.

Another thing to check is injectors. If you have one dripping or not spraying well at low fuel delivery, the mixture will be wrong (leaking injector adds too much fuel, so the others are actually too lean).

Else live with the rough idle or higher CO.

1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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Old 10-08-2004, 02:44 PM
Super Moderator
Join Date: Apr 2000
Location: Falls Church, VA
Posts: 5,316
Peter -

This engine is pre-EHA. The on/off ratio of the frequency valve is controlled by the O2 sensor via the lambda controller. You are supposed set the on-off ratio to 45% with the mixture adjustment and that should give you a good CO reading. If everything is working correctly, the only way to make the engine run richer or leaner is to adjust the mixture beyond the system's ability to correct (or disconnect it).
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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Old 10-08-2004, 04:04 PM
SL Owner
Join Date: Sep 2004
Location: East Coast U.S.
Posts: 131
appear to have had some success here (more info)

Thanks to everyone for their respones. On the O2 sensor, before I got into all of this I had put a new one in there, and to verify operation, swapped with the new and old (because the new one was a more recent Bosch revision).

Yesterday after my post, I went all over the car again looking for new vacuum leaks. I at last asked myself the dumb question-- did mechanic #1 put the fuel distributor on properly. That is, was it sealed properly? If I had 1 ounce of real experience and common sense on this, I would have asked myself that question immediately. Anyway, at last I asked it.

There is s an o-ring you place onto the bottom of the fuel distributor where it connects to the air housing. This o-ring MUST BE LUBRICATED before installation and needs to be carefully handled during installation of the fuel distributor. The distributor cannot be twisted around once put on the air housing.

So, first I leaned the car-out by adjusting the CO air/fuel mixture screw (the 3mm allen adjustment in the air sensor housing) so that the car would miss at idle. I sprayed some carborator cleaner in that seam (gap) between the fuel distributor the air flow housing. The car immediately stopped missing and then started again. I did this many times consistently. This confirmed a likely issue with the fuel distributor seal. I called mechanic #1 and told him what I did. To his credit, he agreed that he'd pull the distributor out in the morning (this was yesterday evening that I spoke with him) with me right there and we'd inspect the seal. One thing I will say for mechanic #1-- he is customer focused and very friendly. He has re-earned my respect. At any rate, he pulled-it out and I learned a few things while he did it and he showed me a few things. Note he did not install the fuel distributor, one of his other mechanics did. The seal was, without question, torn-up when he removed it. He acknowledged that. We also both agreed there is probably also a manifold leak. But we also agreed this fuel distributor seal was a big issue. I handed him a new gasket (I picked it up from MB yesterday) and he carefully installed it as I watched. He put it back-on. I took the car for a drive and it has massively improved gas economy. I did not test the CO rating while driving (on rollers, that would have required another run by the emissions station) and I have verified that the IDLE CO rating on these cars bears no relevance to the actual emissions rating because in my state they test with the car on rollers. I looked at graphs from my last emissions test results (run in a special test mode at the station) and the driving CO value is 1/5th of the idle value (the tests at various speeds). So for example if you are blowing 4% at idle, that has nothing to do with what you blow at 15 mph or 30 mph. In fact, my experience says you can divide by as much as 5. The car, already smooth, was now driving to an even better level (I didn't think it was possible) and the gas economy is now ** dramatically ** improved. I went for 80 miles gunning the engine whenever I could and the car used a fraction of the gas it used before. This improved gas mileage tells me that far less unburned fuel is now going out the exhaust while driving and hence the emissions are greatly improved. I will have it tested again at some point to verify, but the car is now different and I believe the issue can be put to rest. I will say that this new fuel distributor and the unclogged catalytic converter gave the car about 50% more power than I ever experienced owning it. Even if I lean-out the car at idle so that it misses at idle, its acceleration and driving is miss-free and clean. Only one final possibility remains with this car in my mind, which is that with its new power (unclogged cat and new properly tuned fuel distributor), it may now be more sensitive to any manifold leaks that may have existed before it. I will determine that during testing in the future and will share results. But again keep in-mind that prior to this fuel distributor/injector/cat work a few weeks ago, this car sailed through emissions for the past 10 years and only recently passed on everything except for NOX and there I failed only at one speed (15 mph) and only because they had made the NOX requirements more stringent than in previous years-- I also only failed by about 10% over the max allowable. All other numbers including CO were/always have been much lower than state requirements.

I've learned a ton working on all of this and have made nearly every mistake you can make but learning-- I learned by reading posts/reading manuals/watching and managing mechanics/working on it myself and owning this car for over 10 years now. I hope I have the energy and stamina to sum-up my recent experiences hoping they will be happy for others. Again, thanks to everyone for their help,.
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Old 10-08-2004, 08:04 PM
Registered User
Join Date: Jul 2000
Posts: 638
smooth idle, lean, low emissions?

Great Job in thorough detective work.

If you have a smooth idle now, with it lean, that means EVERYTHING is right on. It has to be or it will not run smooth. Intake leaks are not uncommon around th efuel injector seals, but since yours are new--should be 4-5 years before you have that problem again. Now run good fuel to keep the injectors and intake valves clean. That has a tremendous impact on idle quality. Even a little crud on the intake valves will cause a rough idle, especially at start-up.

Often you can improe idle quality and emissions level by increasing the spark-plug gap. It takes more energy to fire a lean mixture and a wider plug gap will help there. Of course plugs have to be sharp--not rounded off edges. You might get a miss at high speeds and loads if the gap is too wide for your coil and wire condition..

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