Hi everyone, how are U doing ? I am new to this message borad, please bear with me.

I have a 86' 420 SEL with 173K miles on it. I bought it used about 9 months ago with a slightly rough idle problem (a discout price.) -- no matter hot or cold.

This car has -- new Timing Chain (double role),
new BOSCH 4-ground Platinum spark plugs,
new Valve cover gaskets.
new Ignition Coils, wires, Cap & rotors.
new Over Voltage Protection Relay
new Idle Speed Valve
new Fuel Pump Relay and Fuel Filter

After a private Mercedes mechanic replaced with all 8 new Bosch Fuel Injectors, he reset the car to original timing and fixed the vacumn hose wrong routing by previous mechanic..... but the car still has a rough idle problems --- though the gas mileage has been improved to about 17 MPG in city driving.

Now he told me that the center intake manifold's 8 vacumn plenums needs tobe replaced -- if I want to fix this rough idle problem......Well, I think he should tackle the vacumn problem first before doing this fuel injector jobs, so I don't really trust his Mercedes skills ... Questions : Can I replace this plenums myself and if yes, can anybody give me any problem areas to watch for if I proceed to buy the parts from the Fast Lane ???

Thanks for reading this message. Regards.

Well I am going off of other posts on this site, but everything I have read on here about the Bosch +4 plugs is that they suck on MBs. You may do a search (small blue bars above second from right) on this topic and see what you find. If I was you, I would replace the plugs with OE plugs. I'm not saying that this is the cure, but it is a good start. Also, the rough idle is a common problem, do a search on that too and see what you find (you'll find a lot!). Good luck.

__________________
'86 420SE Euro
904 Midnight Blue, Gray Velour
Dad bought it new, now I own it.

"A Mercedes-Benz is like a fine wine, it only gets better with age."

One thing other than pulling the intake and replacing the O rings to watch for on this engine is guide wear that causes rough idle. With this milage I wouldn't be suprised to see it needing a valve job, not to mention the potential for cams, rockers or a fuel head, unfortunatly the guides are noly seen with the heads pulled and stripped, (springs, seals etc...) you might be in for a long haul getting the idle straight but, every one of these I've seen get the top ends done ran super afterwards.


Joe

__________________
Project Smoker, '87 603 powered wagon
Hauler, 96 CTD can you say torque?
Toy 73 Cougar xr7 convertible
Acme Automotive Inc.
Raleigh NC
919-881-0364

Trash the Platinum 4s. They can cause a lean burn misfire. With the age of your car a few minutes on an engine scope should reveal what is causing the rough idle.

__________________
Scott Diener
89 300E
93 300E
92 Volvo 740 Wagon aka "Mutt mover"

HI, mbtjc, joep and Scott, thanks for the reply.

After reviewing the replys, all the Bosch 4-ground plugs were being replaced with the original single normal ground electrode Bosch Platinum plugs. But rough idle conditions still around, but with a better throttle response. I

Well, my mechanic had told me that he will expect several broken bolts will happen if decided to do it......I just hesitate to proceed.

As Scott recommended, why pull so many 'ITEMS' apart for just those 8 vacumn O-Rings --- if a complete Head job MAY solve these rough idle and guid seal problems.

Scott, the MB mechanic did scope the first time before did the fuel injector job --- here's the story.......

After the fuel injectors' replacement and timing back to normal, the idle seems gone 90 % at the moment we test drive and the first 3 days of driving.

The 2nd time he scoped the car and said two injectors were bad and replaced with warranty -- but this time, no matter what he had done to 'tweak' the car -- rough idle still there. I forgot to mentioned that two rubber intake hoses - R & L - replaced. Also, 8 new injector holders.

That's why he 'recommended' the center intake manifold plenum O-Ring job ? It's very obvious that he just 'guessing' the next solution AT MY EXPENSES.... He didn't even mentioned those easily wear out items -- like vlave guid wear, cam conditions good or bad on my car ..... because I ask him what did he found out other bad parts needed to consider before I O.K. the job. The parts and labor caused me around $670.

Later, I will drive the car to either MBI inc. in Portland or local MB dealer at downtown Portland to take a look and get an estimate and posted the results on this message board --- so others knows the pros and cons. Thanks again for these great helps from all MB friends.

Now, any more suggestions besides a complete valve job or should I replaced with a new or used rebuilt head ????


Regards.

You might check out www.mercedesengines.net. I think they redo heads both on an exchange basis and on a recondition your head basis. I think for a M116/117, it is around 390 per head, excluding R&R costs obviously. From what I hear this place is a really good company that knows what they are doing. Keep us informed on what you decide to do and how it turns out. My idle isn't exactly smooth either!

__________________
'86 420SE Euro
904 Midnight Blue, Gray Velour
Dad bought it new, now I own it.

"A Mercedes-Benz is like a fine wine, it only gets better with age."

First when dealing with a rough idle you must look at everything . How is the c/o? Did they perform a compression test, cylinder leakage test? Did they perform a fuel distributor test. 50% of rough idles end up being ignition related, 20% fuel and the other 30% is mechanically related due to burned valves or even c/o not in spec. (lean mixture) . I have dealt with some rough idles that will make you cry, but performing the proper procedures for diagnosis is essential. It saves you (the customer ) money and me (the technician ) from looking like a fool for throwing parts at you car.. Just my 2 cents not meaning to offend anyone.

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euro 287
Mercedes Technician 7 Years (retired to Hyundai)
2000 Dodge Durango
98 Mazda truck

Hi, mbtjc, euro 287 and everybody who reply to this Help msg ...

Referring to euro 287 -- I don't know whether he had done the compression check. But I check my spark plugs, the white insulators were all pure white, no oil traces, engine has NO smoke at cold and at any speed. Oil consumption was about 1 quart for every month for city driving (around 700 miles I guess). The cams new or not, I don't know. But from I've seen on them when I replaced the valve cover gaskets, they don't have wear, burnt or discolorig markings on both of them. The timing chain still brand new with grey color on it. Guess the previous owner had done a cam & timing chain job.

Forgot to mentioned that my mechanic had put new engine mounts, engine shocks, auto-trans mount and differiential mount.

For c/o reading, the mechanic did using the MB tool connected to the scoket from my car and adjust it accrodingly. My car had gone through DMV emmission test and passed right away even before I had done anything to it. For the ignition side, he used
a SNAP-ON engine diagnostic scope to check the ignition -- shown by all eight dancing bar graph -- from what I've seen before replacing the fuel injectors -- all 8 bar graph were not even. After replacing all injectors, holders and 2 air intake rubber hoses connected to the injector holder, the car ran much faster and much better gas mileage. But after 2 days, the car suddently feel a drop of power and rough idle back for haunting.

That's the second time he did a scope and replaced 2 injectors.
The car restore the power -- But FAILED 2 brand new BOSCH -- Made in Germany fuel injectors at the SAME TIME -- that's much to say about German engineering and QUALITY CONTROL !!! He told me the rest of the injectors has ONE year warranty.

Well, for the compression, cylinder leakage and fuel distributor test -- things like that I don't know he did it or not. But he had spend 4.5 hours woking for the whole job including re-route all vacunm hoses. The rough idle was 90 % gone -- not completely -- when we did a short test drive and the next several days....

I know engine like mine with more than 172K miles need a complete check -- from top to bottom. Then I will know the root cause and so for the MB community for answering this help msg.

Thanks for mbtjc, joep, chowpit and euro 287 for taking their time and combined MB mechanic experience for answering my message. This place is great !!! Any more suggestions are very welcome. Bye for now.


Regards.

I think the advice is to NOT use platinum plugs of ANY kind in your engine.

Sixto
91 300SE
87 300SDL

I second sixto's note. The plugs of choice are Bosch Copper units and they are not readily available in auto parts stores. I bought them from Fastlane though, for my car. Sounds odd but they are the only plugs for these cars. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

in a pinch I'll use NGK's, with the understanding that they oughtta be pulled and replaced with the bosch supers in short order. Beru's work well too but, are tough to find.

On the head subject, I've seen 2 117 motors with weird idle problems that are isolated to 1 or 2 cly's that were not the result of burnt vavles but, the guides walking around in the heads, both were in the 150K to 200K area. Try doing a idle drop test and see if you have one or 2 cly's that drop somewhat less, this combined with a comp test and a leakdown test should give you some insite into the mechanical health of the engine.

Cam's, look for scaring at the top of the lobe, it'll look like its been wiped with antisieze. also look for raised edges on either side of the lobe past where the rocker rides.

Got a smoke machine handy? They are wonderful for getting to the bottem of intake leaks. Other items include the rubber boot under the a/f sensor, mismatched flow rates from the fuel head, dist. shaft/bushing wear (rare) and vacum leaks from inside the car or the resivior. (ok, so I cant spell)


Joe

__________________
Project Smoker, '87 603 powered wagon
Hauler, 96 CTD can you say torque?
Toy 73 Cougar xr7 convertible
Acme Automotive Inc.
Raleigh NC
919-881-0364

Ok so you had the basics checked , now i take it he did not do a fuel distributor test, as for injectors you have to clean them in the injector tester before installation in the vehicle due to they are coated from the factory. I would stress on doing the cylinder leakage test and the fuel distributor test. This is only if you really want this car perfect. It is not uncommon for a fuel distributor to be partially clogged. The easy way to test this is if you have the car on the scope and do the cylinder balance test and one comes up low, lets say #3 is low well take the line from #4 and put on #3 and vise versa. If the cylinder changes to #4 well you know your problem and you had better check for contamination in fuel system. I have one in the shop right now I am replacing everything from tank to injectors . Diagnosis on these cars is fairly easy its just knowing where to go. Weirdest one I have had yet is a rough idle and it ended up being fault of rubber in brand new air rail wasnt punched out all the way. Good luck

__________________
euro 287
Mercedes Technician 7 Years (retired to Hyundai)
2000 Dodge Durango
98 Mazda truck

Hi, sixto, Jim Smith, joe p and euro 287, all others thanks for the reply.

Those OEM super copper core Bosch plugs were not carried by the auto stores close to my house. All they carried were either Bosch Platinums, AC Delco ...etc. Will go to buy Bosch copper tomorrow and put them on to see what happen next.

As for the cam's lobe surface conditions, they all look like mirror surfaces, no raise edges, not a single scratch markings on both of them. I remembered my mechanic kept spraying WD 40 around the valve cover and center fuel distributor areas --- he was looking for vacumn leak area.

For the compression and cylinder leak down test, I will let the pros to do those test as they have the experience, tools and correct procedures for doing it.

mbtjc mentioned the mercedesengine.net to me and I check it out today. Their head rebuilt price seems affortable -- $390 per head plus exchange core and shipping. It is worth to condition this option -- if those cylinder test come out OK.

Well, thanks for all the reply and I have to go. Good night and I will post what's the outcome next time.

FWIW

If you get worn out, and want to quit guessing, I highly recommend my indy in Bellevue, WA

Star Auto Service
13622 Northeast 20th Street, Bellevue, WA 98005
(425) 641-3389

He was trained in Germany on these cars, and now the W126 is what he works on most (there are lots of them up there).

Anyway, you could drive up there, see a Mariner's baseball game, and get it fixed right. No guessing on his part, he knows all the diagnostic methods and was more than fair on labor.

Best of luck

Hi old blue and many those who reply to this msg :-

Thanks for the detailed info of the qualified MB mechanic in our area. Will keep that address and phone number -- these days qualified and HONEST auto mechanics are like endangered spices.

Well, today I replaced all Bosch Platinum plugs with NGK copper core plugs (one of the major auto shops, again, doesn't carried any Bosch copper core plugs).

Suprise, those rough idle has gone almost 95 % !!! Only some very minor shake a little at stop, it's a high mileage car and I can live with that.

So that's it. Thanks for all who reply and give me this GREAT ADVICE. Especially sixto, joe p and euro 287.


Regards.