Perfect mixture? Bad valves? - PeachParts Mercedes-Benz Forum

Tomguy · #1 04-18-2004, 03:20 PM

Finally, I think I got it right! No oxygen or CO sensors or anything handy, just going by smell and how she runs...

I pulled a plug out (cyl #5, one of the easiest to get) and the tip of it was white/grey, and the bottom black - it looked just like the plugs in the Bosch pdf file Litton posted in another thread, as opposed to black on the tip! It seems to run a lot smoother and quieter at a cold idle as well, although it still smells quite rich in cold idle if, say, I pull out of the parking garage, go a block, and then stop at the first light. I think that may have to do with the valve stem seals though, there's a little white smoke that only appears under 2 conditions (accel from a cold startup, or decel after a "hot" run at higher speeds).

I feel like I accomplished at least something! Now I hope I can best my record of ~10MPG, with my last fillup being a dismal 8.2! (It was when I was having the ignition problems and it was horribly misfiring, so it was wasting a lot of gas to go nowhere).

Now my issue is somewhat of a hissing sound most noticeable going up a steeper incline under load at lower RPMs, or decelleration on a hill with the same RPM range. (Don't know how else to describe it other than hissing). I would like to recheck my valve clearances - but does this sound like a possible burnt exhaust valve (or several)? It isn't from the exhaust leak, that sounds more like a tapping.

psfred · #2 04-18-2004, 03:48 PM

Sounds more like a vac leak to me. Also, if the base of the insulator is black, you are too rich at load.

These things run so rich on cold start that you get a black spot if you park with the exhaust up against a wall, they will then lean out quite a bit as the temp comes up. I'd check the resisitance of the coolant sensor both cold and at operating temp, and the resistance and wire condition at the air temp sensor. If either one reads high, it will run so rich you cannot compensate with the MAP.

Exhaust will smell rich at idle or low load cold, since it will be running rich even when set up correctly at speed.

I would definitely recommend using a CO meter to set the mixture. Unless you have a bad exhaust, the "average" mixture will be better than if you use spark plug color on a single plug -- you really need to look at more of them, since an air leak will make that one one plug look lean, and a drippy injector will make it look rich when in fact all the other may be right.

Peter

Tomguy · #3 04-18-2004, 09:11 PM

Peter: The sound I hear is perfectly in tune with the engine and sounds like it comes on the top of the compression stroke (before firing). Definetly not a vac leak, there's nowhere left for it to leak!
Vac leak sources, all working and some fixed:
- Door lock system
- Brake booster (works fine, retains vac)
- Transmission (shift aid?)
- Injectors (new seals)
- Manifold (new plenum gaskets)
- Vac retard on dizzy

Every line connected to the throttle body and/or manifold has been fixed.
The temp sensor is one I grabbed out of the one in the junkyard - it looked pretty new so I put it on in exchange for my old one (A while ago) but didnt notice any difference at all. I actually replaced them all but the switch (for the fan).

The base of the insulator isn't black, it's black inside the plug (presumably where the air can't get to the fuel). They look exactly like #1 in this pdf Litton posted:
http://www.boschusa.com/images/SparkPlugFaces.pdf

psfred · #4 04-18-2004, 10:56 PM

Leaking spark plug?

Peter

Tomguy · #5 04-19-2004, 01:04 AM

But it sounds like it comes from several cylinders, not just 1...

I'm thinking that it may be related to either sitting a very long time with a few valves open (and getting some minor corrosion which causes mis-seating now), and/or possibly when I was having issues with the mixture and the "Newer" cams from the smogged 450 when the headers would get cherry-red. That was probably more than hot enough to singe a few valves, no?

cth350 · #6 04-19-2004, 07:14 AM

There will be a tremendous "hiss" from the intake if you have the air cleaner off or if the hose that connects the throttle to the idle air screw is damaged. But you would have heard those sounds from the comfort of your driveway.

NY emissions stations all have dyno's. Doesn't your end of PA? -CTH

Tomguy · #7 04-19-2004, 07:22 AM

cth: Last time it went to the station was for inspection a while ago

It isn't the hiss from the idle air system, I know what that sounds like. It's also more pronounced going downhill (decent incline) at, say, 60MPH or so (low RPMs in 3rd gear) when the engine holds it back. Maybe I'm just hearing things!

250 Coupe · #8 04-21-2004, 07:05 PM

To find out if you have leaking valves or rings, you can build a leak down tester as show at this link.
http://www.xs11.com/tips/misc/misc3.shtml

Is the hissing sound pulsing like a loose spark plug or continous like a vacuum leak?

Hope this helps,
Michael
1970 250C

Tomguy · #9 04-30-2004, 09:10 PM

I don't feel the need for a leak tester or compression check at this point because I have 2 "new" heads for it which I plan on putting on in a few months.

When I was double-checking valve clearances (I got paranoid because of the hissing sound I said I heard), when I cranked the engine with the starter, when I stopped it with cyl 5 on TDC I could hear it hiss - quite loud (definetly noticeable), and it sounded like it was the intake but I'm not 100% sure on that (Would this result in a lot of carbon buildup and/or "high" manifold pressure? I pull 13-14" of vac at idle). It stopped after 2-3 seconds, Indicating to me that it lost all the compression. Is this normal? It did the same when I tried it on cyl #8 as well. I didn't try the others, the clearances were all OK.

Tomguy · #10 05-02-2004, 09:15 PM

Oh, and also: I pulled out all the plugs. They seemed to look good, and were uniform - except that the plugs on the RH side, specifically #'s 2-4, had a "purple" tint to them on the ground electrode and center electrode insulator. Any ideas?

psfred · #11 05-23-2004, 12:57 PM

Bluish color is normal from the heat. Purplish may be fuel additives, but I'd have to see them myself to form more of an opinion. So long as the insulators aren't stark white with blisters, you're OK.

I'm planning to start on the manifold on the 280 this weekend -- a chunk blew out a couple weeks ago, making it FAR too noisy to drive.

Should be a huge PITA -- the nuts are so erroded I can't get a wrench to fit, so I'm going to use my Dremel too cuts slots in them and use a chisel to break them off.

Peter