1978 280CE troubleshooting sag/stumble issue - PeachParts Mercedes-Benz Forum

Squabble · #1 04-25-2011, 08:25 PM

well, i had a thread here: http://www.peachparts.com/shopforum/showthread.php?t=297629 but i replaced the fuel pump and things did not get better, so i wanted to throw it all together in this thread. now i feel like this is all related so one thread makes it easier. let's start from the beginning:

here are the issues:

after getting her out of storage 4 weeks ago, she's been stumbling and sagging when i idle in gear. idling in park she sounds great, put her in D or R and she stumbles when you sit there and really stumbles/sags when you hit the gas. runs like a top at regular/highway speeds. had a bit of a power loss but i thought that was just me. 4 days ago, i'm driving in city traffic, not exactly start and stop but definitely slow then crawl and she just died on me. dash lights come on, everything cuts out. would crank and not start, power still worked, listened to the stereo waiting for the tow. i get towed home in the rain, about a half hour, park her in my driveway, and she starts right back up again.

here's what i've done:

before this issue got bad, i pulled out the B.S. cali vacuum system. cut it down to the basics. if you look at the vacuum diagram, i cut out everything except for #2, #9, and #39 and all the lines that run to them. also plumbed in my old style Unwired Tools servo upgrade that runs off the vacuum. everything else was torn out and/or capped off. car ran exactly the same with this job done, A/C worked great, so i figured, no harm done.

after my car broke down, i replaced the fuel pump and filter, along with the fuel line that runs between them and all the necessary washers. noticed definite increase in power and response BUT the stumbling in gear is still happening, possibly even worse. i also noticed when i was under there a couple old, weeping fuel lines under the car. can that throw the system pressure off?

i talked to my indy about this. he suggested, as i lose oil pressure when it sags/stumbles, that i'm not using the right oil. i've got 10w40 in there and i'm in upstate NY. he suggested 20w50 and said perhaps the fuel pump relay is cutting out based on low oil pressure. is there anything to this? should i just replace the fuel pump relay? anyone know where to get one and where it goes? this is a non-euro 280 so it doesn't have the large black relay, can't seem to find a regular relay for sale (or a diagram as to where it goes). i did my best to get under the car and listen for the fuel pump buzz while my wife put her in gear, idled, put her in park, etc. the buzzing seemed to be continuous.

my other friend and former mechanic suggested i've done something terrible to the vacuum system and now i'm paying for it. any truth to that? i swear it's all good, i used brand new tubes and connectors. it is possible i have a leak somewhere in the interior, but i'm almost positive there couldn't be one under the hood.

i'm scheduled to see my indy next week to replace all the injectors. could it just be my spray pattern? i can't believe it would be that simple. any ideas?

mbdoc · #2 04-26-2011, 08:23 AM

Fuel & fuel pressure are the keys to a good running CIS fuel system MB.

Did you start by draining the old fuel?

Do you have any way to measure the fuel pressures, control and system pressure?
Have you tried changing the fuel mixture slightly?

Stretch · #3 04-26-2011, 08:47 AM

Fix all the leaking fuel lines first and then check that the car idles "correctly" => as defined in the FSM / owner's hand book etc etc etc?

The stumbling sounds like not enough revs to me

Squabble · #4 04-26-2011, 01:17 PM

i didn't drain the "old" fuel as it was less than a quarter of a tank and only as old as last october when i took it off the road. i've since then filled it up once with Sunoco Ultra 93 gas. by now, it's cycled through all the old gas i assume.

i don't have a way to test the fuel pressure but i can get it to my indy next week. maybe he can do that? is that standard for all cars or just mercedes? if mercedes, he might not be able to do it.

what do you mean by changing the fuel mixture? how do you go about doing that?

as for the idling, the car idles great (if not high) when in park, no issues whatsoever. you'd never know there was a problem until it's in D or R. also runs great at highway speeds, it's just the initial take off and idling in gear at a stop sign.

the car, with the new fuel pump, is running better and responding better than before when going. it's just the initial take off and the in-gear idle. it'll even sag/stumble less if i really stomp on the pedal.

is it worth just replacing the relay to check it? planning on putting in new 20w50 oil tonight...

porkface · #5 04-26-2011, 02:53 PM

if your indy has the correct tool for checking pressure on a kjet system, then he can do the benz. kjet is not benz specific, but it was euro only for a long time in the 70s-audi, vw, saab, volvo, bmw mb, etc and into the 90s with benz and ferrari. if he can't, find someone who can. otherwise it will get real exspensive when things replaced don't fix the car. and kjet stuff is high enough already. spend the money on diagnosis, it'll be cheaper in the long run. good luck, chuck.

Squabble · #6 04-26-2011, 06:44 PM

well, this is good news as he works at a "volvo only" shop. pretty cool little set up: http://www.volvonly.com/index.php

i might just throw in the towel until i see him next week...

Quote:

Originally Posted by porkface

if your indy has the correct tool for checking pressure on a kjet system, then he can do the benz. kjet is not benz specific, but it was euro only for a long time in the 70s-audi, vw, saab, volvo, bmw mb, etc and into the 90s with benz and ferrari. if he can't, find someone who can. otherwise it will get real exspensive when things replaced don't fix the car. and kjet stuff is high enough already. spend the money on diagnosis, it'll be cheaper in the long run. good luck, chuck.

Squabble · #7 04-27-2011, 01:10 PM

well folks, i figured out one of the problems here. my unwired tools servo upgrade was over-taxing the vacuum line to the distributor. i had to pull it and plumb it to the intake on it's own dedicated line. after that, no stumble, no sag.

this, with the new fuel pump, might also be the solution to the stall and no re-start in the middle of the road last week but i'm still not positive about that. i feel like they were two separate issues. i hope the fuel pump solves the stalling issue. i drove her to work today (13 miles or so) without a problem. ran better than she ever has. granted, this was not city driving so it's not an equal test. that will happen after work today. we'll see i guess...