bad emissions + rough idle @ 750 = no suck or bad engine? - Mercedes-Benz Forum
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post #1 of 31 (permalink) Old 08-25-2006, 09:25 AM Thread Starter
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bad emissions + rough idle @ 750 = no suck or bad engine?

This is my first post regarding problems with my 1990 560SEL, 302k miles. Just went to get my emissions tested... came out 3x over the hydrocarbons limit and 4x over the carbon monoxide limit... in addition, there's a moderate side-to-side jiggle when stopped at idle (warm and cold). Idle stays steady at 750 but the motion it creates reminds me of a car trying to stall (as if a clutch was 1/2 way out). The only other noticeable symptom is a 'sometimes' reluctance to get-up-and-go; it wants to take a big gulp before doing so. Most recent service is motor mounts and brakes. There's no burning that I can smell and it does not leave any messes on the floor (for sure, since it's missing the undercarriage rock guard). No lights blaring from the dash. Air filter recently (1k miles ago) replaced, checked the bulbs in the dash to make sure they're working and they are. Haven't played with the O2 sensors. Haven't gotten it restested. Any ideas or plans of attack?

My first guess would be a vacuum leak somewhere. Of course, any help or links regarding that system would be a big help to me since I've never worked with one. I've read some of the other posts regarding the idle control valve / relay and I haven't replaced or cleaned those, either. My idle problem doesn't seem to be the erratic idle that others are having. Of course, a vacuum leak would explain both the 'gulp' and the emissions problem... please, someone!
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post #2 of 31 (permalink) Old 08-25-2006, 09:47 AM
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As others on this board have advised me when I had an idle problem, you first need to do a complete tune-up. I would replace rotor, cap, SP wires, plugs and clean the injectors with cleaner. Then through the process of elimination you can better understand whats going on and go from there if needed. Otherwise its a wild guess... too many variables. The vacuum leak is a very good possibility as well.

I'm impressed you have over 300k on the engine! That's encouraging since I have a 90 560 sel as well. Did you rebuild the engine and if so at what point?
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post #3 of 31 (permalink) Old 08-25-2006, 09:54 AM
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I forgot to mention all filters as well....
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post #4 of 31 (permalink) Old 08-25-2006, 09:55 AM Thread Starter
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First engine, no rebuilds. (I fear the worst but with 300k+ I can't complain.) I've spent a small fortune keeping the A/C working, however. New tranny around 260k. You might be impressed by this: original radiator! Woot! (My secret? I'll never tell... because I don't know how that happened... something about the stars aligning just right.) Okay, I'll see if I can scare up the tune-up parts locally and do it to it tonight.
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post #5 of 31 (permalink) Old 08-25-2006, 09:57 AM Thread Starter
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I'm pretty good about filters and oil changes. I'm pretty lazy/forgetful about spark plugs and cleaners. It's been a good year+ so that's a great idear.
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post #6 of 31 (permalink) Old 08-25-2006, 10:18 AM Thread Starter
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Can't find the parts locally... I'm autohausaz.com-ing it. I'll post again when I'm done. Also, one more symptom I forgot about (because it doesn't happen much). Restart when warm is difficult. Once turned off the car won't restart for a good 5-10 minutes.
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post #7 of 31 (permalink) Old 08-25-2006, 10:40 AM
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Check the fuel pressure. Check the basics and look for obvious vac leaks. Replacing the basic tune-up parts is fine but won't cause the CO% to be 4X over. Did the smog place try to adjust the CO% to see if it would? Just curious.
Fuel pressure, accumulator, cold start valve are some areas that can dump too much fuel and go super rich. Vac leaks will make the HC go up. O2 sensors are a maintenance item and should have been changed a few times by now. Also the convertors do go bad over time and with it running this rich it's probably history. It might be wise to visit a reputable shop that understands these cars and CIS systems and have them check pressures and stick the emissions probe in the pipe, rather than guess. Throwing some of these parts at it can add up real fast.

Last edited by Dave D; 08-25-2006 at 10:48 AM.
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post #8 of 31 (permalink) Old 08-25-2006, 10:49 AM Thread Starter
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convertor... you mean the cats? All of the things you mentioned are at least 100k miles old. Whew boy, I've got my hands full and my pockets potentially empty.
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post #9 of 31 (permalink) Old 08-25-2006, 10:51 AM Thread Starter
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...and to answer your question: no, the county emissions test place doesn't adjust CO%. I haven't taken it to a shop, yet. I'm a vacuum system dummy--what are the obvious vac leak areas? Can you point me to a procedure to check the fuel pressure?
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post #10 of 31 (permalink) Old 08-25-2006, 02:01 PM
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Originally Posted by veryslowbenz
...and to answer your question: no, the county emissions test place doesn't adjust CO%. I haven't taken it to a shop, yet. I'm a vacuum system dummy--what are the obvious vac leak areas? Can you point me to a procedure to check the fuel pressure?
First, listen for anything that hisses. Odds are, the hissing sound is a vacuum leak.

Next, hook a volt meter up to one of your O2 sensors. Check to see that the voltage is switching between two different levels (usually around 0.1 volts at the low side and 0.9 volts at the high side). It should be going back and forth. The quicker it changes, the better the O2 sensor is.

Now, with the volt meter on the O2 sensor, spray a little bit of "throttle body cleaner" down the intake at the air flap (with the air cleaner removed). Watch what happens to the voltage at the O2 sensor. It should go up and stay at 0.9 volts or so. That's an indication that it's running really rich (the extra "fuel" from the cleaner spray).

Start spraying cleaner around the vacuum hoses, and especially around the base of the fuel injectors. Watch your volt meter and listen to the idle. If the volt meter shoots up to 0.9 volts and stays there or the idle speed increases when you spray cleaner on something, there's a leak at that point.

The O-rings at the injectors are a notorious leak spot. Vacuum hoses get old and crack. The hard plastic vacuum lines (the really thin ones) get brittle and break. Intake manifold gaskets can also crack and start leaking on a car this old. You'd see that if you spray where the manifold and head meet.

Oh, and since you have a V8, you might have a left and right O2 sensor (I don't remember on these cars, and I don't have V8's in mine). If that's the case, hook the volt meter to the right bank O2 sensor when spraying the cleaner around the right side of the engine, and hook up to the left bank O2 sensor when spraying around the left side.


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