1991 300E Cold Start Issue - Mercedes-Benz Forum
 
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#1 (permalink) Old 05-23-2012, 02:52 PM
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1991 300E Cold Start Issue

Hey guys, I know the title is posted all over this site and others, but nobody's descriptions quite matched my symptoms. So here goes:

My 1991 300E saloon has about 256,000 miles on her. I bought the car last december and all the owner mentioned is that it was "a bitch to start cold" Tell me about it. But I like a challenge and i like to tinker on my days off, so i paid him $400 (i later found out the original owner GAVE it to him, but thats another story) and drove the car home. it had a terrible misfire, and if you popped the hood at nighttime there was a spectacular fireworks display on the right side of the motor. I decided to give the old girl a tune up, all OEM Bosch plugs, wires, dizzy cap and rotor. Whoever the last mechanic was must have had a hell of a time with the plastic vacuum lines because they are all pieced together with 1 inch long sections of rubber tubing. Most alarmingly is the one running from the airbox nipple, the part that connects also to the breather on the valve cover, to the ______ (i dont know what this is, but appearetly it must have a rubber connection that is now replaced by a crudely fit rubber hose) I will post a pic. The car will not start when cold....until it decides its ready. Cranks over fine, courtesy of an optima yellow top battery, but I have to try multiple tries, sometimes cranking for over 30 seconds at a time, for anywhere from 3 to 20 minutes! then TADA! she roars to life and settles into a smooth idle, no running issues like stalling or sputtering, no lack of power while driving. I can turn the key off immediately after starting, and she will fire back up as if theres nothing wrong. Until you park it. the cranking time seems to vary directly with however long it sat. I work 10 hour days, so its just as bad leaving work, as it is when im in a rush to try to get to work. But it never fails to start eventually, and again, never runs rough. so heres a list of a few things Ive tried not in order:

Cold Start Valve
It will eventually start with the cold start valve unplugged, just the same as if it were plugged in. cold start valve sprays for about 6-9 seconds when cranking. I did swap it out with one i picked up at the local pick a part, no change

OVP Relay
The fuse was fine, accidentally got a dual fuse model from the yard, still fit, no change in initial start up, but no change in driveability either. Bought a new one from the stealer anyways ($85):

Fuel Pump relay
junkyard model and new model, same results with both

Fuel Pump
Just changed these last night...wound up getting 2 Bosch pumps for 80 bucks, couldnt turn it down. I had read about check valve problems causing low fuel pressure, I dont have access to an adapter to make my generic fuel pressure tester work, and I figured with the age on the new pumps, and the price a fuel pressure tester would run me, It would be smarter to just spend the 80 on the pumps and rule out fuel pressure as the culprit. (im still having problems justifying the expense now, since nothing changed) And Im sure I still need a guage for further diagnosis, but for all intents and purposes, theres great pressure going into the front of the fuel dizzy, and on the outlet facing the passenger compartment.

I cant justify throwing another expensive part on the car, only for it to be wrong again. On top of that, I cannot justify fixing the 2 broken subframe bushings and overhauling the suspension on a car that fails to start when I need it to. Has anyone ever run into this particular problem? I love my car, its my first Benz so Im new to all of the KE Jetronic components. Pardon my ignorance please, I will be posting a pic of the mystery part in a few. Thanks!

Last edited by BenzBaby202; 05-23-2012 at 02:54 PM.
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#2 (permalink) Old 05-23-2012, 08:03 PM
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okay, so my Android tablet wont let me post a pic, but to the best of my knowlege the vacuum line in question runs from the hose on the airbox (that also runs to the valve cover breather) runs to the (a) fuel pressure regulator in front of the fuel distributor. however i thought i saw one connected to the fuel pump assembly... funny thing is, when you cap off the vacuum line it seems to have a shorter cranking time before it fires right up.. wont be able to tell until tomorrow morning whether its significantly shorter or not. its very frustrating trying to diagnose a problem that only shows up after 4-10 hours.
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#3 (permalink) Old 05-23-2012, 08:34 PM
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Is the Cold Start Injector Plug in good condition? Mine was so corroded that the terminals crumbled. I ordered a replacement plug from Autohausaz and scraped the corrosion from the pins in the Cold Start Injector. it fires right up now.
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#4 (permalink) Old 05-23-2012, 08:57 PM
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That "vacuum line" that runs from the breather hose to the fuel regulator is not vacuum, it is just a fuel runoff line in the event of failure of the fuel regulator. If the regulator diaphram fails, leaking fuel will be sucked into the intake manifold instead of all over the engine. If the line is disconnected, it should not affect manifold vacuum, as it enters the intake system from the top of the air cleaner.
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#5 (permalink) Old 05-24-2012, 05:24 AM
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Good to know, so that line is superfluous to my issue. As far as the CSV plug, its squeaky clean, i had taken some emory cloth to the terminals bak when i swapped out valves.

On a side note, i forgot to mention that i did also replace the coolant temp sensor and the 4 prong plug that goes to it, because when i pulled the plug off to test it, 2 of the terminals stayed stuck in the sensor
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#6 (permalink) Old 05-24-2012, 08:04 AM
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The fuel pressure accumulator helps keep the fuel pressure up when the car is not running. The valve keeps the fuel from returning into the pumps and filter. When it fails the pumps must work to pressurize the fuel to the entire system prior to the engine starting.


http://www.benzworld.org/forums/w124...ml#post2940644

http://www.benzworld.org/forums/w124...ml#post1375098

http://www.benzworld.org/forums/w124...ml#post1789406

http://www.benzworld.org/forums/w124...ml#post1457926
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#7 (permalink) Old 05-24-2012, 01:20 PM
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Fantasitic info in the posted links, thanks! Right now I have the line plugged off at the breather end, she fired up first try this morning, and first try when I went on my lunch break. Obviously I won't keep it that way, as I noticed a small lack of throttle response when driving to work, especially when accelerating from a stop and out of corners. I will scour the yards for a spare Accumulator, but does anyone know where to find a decent quantity of the hard plastic vacuum lines, all 3 or my local parts stores can't even get it, but Id like to go ahead and replace all of the hideously fragile lines in the bay. Along with that hose running from my coolant resevoir thru the firewall. Thanks for the help guys!
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