420 sel flooding - PeachParts Mercedes-Benz Forum

crusher · #1 09-01-2004, 08:05 PM

Hi all, newbie here. Hope somebody can help me cause i'm lost. I have a 1988 420 sel that had a very bad hesitation right off idle. I replaced the fuel filter and air mass meter and the hesitation is now gone. However, it's flooding out something terrible at idle now. Any help would be greatly appreciated.

psfred · #2 09-01-2004, 08:59 PM

Exactly what part are you referring to as the air mass meter? The fuel distributor?

Have you checked the fuel pressure and the differential pressure in the fuel distributor? They can go bad from corrosion in the diaphram between the upper and lower halves, causes the engine to run way rich.

Did you re-set the fuel mixture?

More details, please!

Peter

crusher · #3 09-02-2004, 08:00 PM

The air mass meter is the part the fuel distibutor is attached to. It's like the upper half of the intake. The actual meter is mounted on the right side of it but you have to buy the whole part. As far as resetting the fuel mixture, I dont know how to do that. If it is the fuel distributor can you repair them or do you just have to replace them?

psfred · #4 09-02-2004, 10:24 PM

You need to reset the mixture, I'm sure -- you have to adjust the flow sensor when replacing it if you don't replace the fuel distributor.

You will need to set the flap of the flow sensor to sit just very slightly below the venturi at idle, then adjust the mixture screw (3mm allen through the small hole between the venturi and the fuel distributor.

EHA needs to be plugged in (back side of the fuel distributor) and you need to check the fuel pressure, too, although unless the fuel distribultor went south while you were working on it, it should be fine.

Pin #3 of the diagnostics connector (round plug with a screw cover) has the duty cycle, get a digital voltmeter with duty cycle function, connect the red lead to #3 and the black lead to #2, set the mixture screw slowly, waiting 20 sec or more between changes, to get 50% or so on the duty cycle.

You need to engage the 3mm allen in the socket, then press down to engage the actual adjuster. Don't push down very hard, it goes rich when you do.

If the duty cycle won't change no matter what you set the mixture screw, something else is wrong.

Peter

crusher · #5 09-03-2004, 09:37 AM

ok I'll try that tonight and let you know how it works. Thank you very much.

crusher · #6 09-04-2004, 11:07 AM

Thanks bud, you da man. I guess if we're going to own this car I better find a manual for it. I'm almost ashamed to admit I'm a tech at a GM dealership! lol These Mecerdes are a whole new ballgame for me. Anyway, got the idle set and it runs like a top now.Thanks a bunch.

psfred · #7 09-06-2004, 10:28 AM

Yeah, Benz is a whole different world, for sure!

CD manuals are available from FastLane, and usually other sources, and paper manuals (which I prefer) are often on eBay in the $100 range. Worth every penny, as many things are distincly different than standard American practice!

Look at it this way - Benz has used aluminum heads on all their gasoline engines since the 50s, and the last pushrod OHV engine they made was a diesel before WWII -- everything has been OHC (including some with jack shaft drive rather than chain drive) since the 30's. EFI since 1970, etc.

The one weak point on your 420 is the timing chain -- you MUST check the stretch (ie, wear!) every 30,000 miles and replace when it gets to 6 degrees or so, else it slaps on startup, fractures the upper guide on the right side, and the broken off part drops into the cogs, breaks the chain, and bend all the valves on both heads. About $4000 worth, more or less.

Check the stretch by removing the right valve cover and rotating the engine by hand until the notch in the thrust washer behind the cam sproket is centered around the little protrusion on the cam tower (at about 1:30, I think). Read the degree wheel on the crank pulley -- new chain will be within 2 degrees of TDC, the more wear, the later the crank will be. By 8 degrees, pull in a new chain. Check the condition of hte chain guides, at the same time -- if dark brown, replace them, they are brittle.

Otherwise, the engine should run at least 250,000 miles between valve jobs, and you shouldn't be able to wear out the bottom end -- MB used the silicon impregnated aluminum block GM developed with Reynolds for the Vega, then gave up on about the time it was perfected. Unless overheated, it should run about 2.5 million miles before it wears out of spec!

Peter

crusher · #8 09-06-2004, 11:34 AM

Thanks for the info. I prefer paper manuals myself, but it seems that every publisher skipped on past 1988. I'm in the process of getting one directly from Mercedes for about 120 bucks. I guess if we're gonna have the car I better have some books.

psfred · #9 09-06-2004, 12:03 PM

A manual will save you big bucks! Some procedures aren't obvious, and MB seems to have more than the normal amount of "hidden" fasteners (bolts behind the balancer, recessed bolts on covers, etc) that will drive you nuts if you don't know they are there.

In good shape, these cars are a nearly perfect driving experience (although the 420 might be a bit low on power for some).

You should have a nearly silent, very supple ride with excellent handling (watch the speedo!!!). The car should track dead straight at 90 on the interstate with your hands off the wheel, should be no loud thumps or groans on bumps. Engine should be leak free, no oil useage between changes (if you get a puff of blue smoke on startup with a quart in 1500 miles usage of oil, you need valve guide seals), etc.

Check the coolant -- if green, you need to do a complete drain and flush pronto, use ONLY MB or Zerez G-05 coolant, not Dexcool or green antifreeze, else the radiator will crud up terribly and the hoses will go to perdition fast. No phosphates AT ALL in the coolant, it clogs the rad...

You will love this car!

Peter