Idle speed on '87 300e - PeachParts Mercedes-Benz Forum

Sopicki · #1 02-25-2007, 09:15 PM

Hi folks,

I'm finally getting over the hump on the repairs to our "New" 300e. I don't think anyone would be real interested in what it has taken to save this car but let's just say I've sunk way more into this car than it's worth. I'm just a sucker for solid old cars.

The idle speed seems a little low on the car, about 600 rpm. This has 2 results; 1) The alternator does not put out enough voltage at that rpm so my rear defroster kicks off. 2) More importantly, it would seem that my transmission does not produce enough hydraulic pressure at that rpm to hold the clutches tight so when I step on the gas (and the clutches engage) I get a not so smooth start.

I just replaced: Cam, rockers, valve guide seals, O2 sensor, ALL vacuum hoses firewall forward, Idle air valve hoses, distributor cap, rotor, plug wires, plugs, air filter ...

I looked in my factory CD manual for an idle speed adjustment for the 103 engine but I found nothing, which I assume means the idle speed should not be adjusted. Anyone have any suggestions?

Thanks in advance,
Bob Kopicki
Massillon OH
'67 TR-4a
'76 GMC C-15
'85 XJ-6
'87 300e
'97 VW Cabrio

david s poole · #2 02-26-2007, 10:52 AM

there was a factory mod on the wiring at the f.i. computer.unplug harness and cut into the wiring,find the purple wire and shave insulation.find solid brown wire[ground],cut into it and use a spare pce of wire to join the solid brown to the purple.this will raise rpm's by about 75.[should be enough to overcome the issues]

richard28 · #3 02-26-2007, 11:01 AM

David, I have the same issue. I assume the f i computer you referenced is the larger of the two behind the battery. I recall reading about this mod before in the posts, and I thought it was suggested to find the purple wire beneath the fuses. Where is the best location to find and ground this purple wire, given that the amount of space usually is very tight. Thanks.

david s poole · #4 02-26-2007, 11:27 AM

the easiest place is to unplug from the computer and then cut into the wiring.the f.i. computer is black,the abs is silver.

richard28 · #5 02-26-2007, 11:47 AM

Thanks.

richard28 · #6 02-26-2007, 01:17 PM

David, to facilitate this procedure, is there any reason why a tap/splicer of the type sold at Home Depot couldn't be used? I'm referring to the small plastic thing with round slots or tunnels into which the two wires are inserted and, when closed, a metal "bridge" cuts the insulation of each wire and makes the connection between them.

david s poole · #7 02-26-2007, 01:19 PM

yes you can use this

richard28 · #8 02-26-2007, 01:28 PM

thanks again.

Sopicki · #9 02-26-2007, 03:26 PM

Dave,
Thanks for the info, I'll be giving it a try once I get home.

Richard,
Dave is correct that a splicer will provide the same results electrically as joining the wires using a piece of wire. However, having spent many years working on British cars (with dissimilar metals being a big problem in their notoriously unreliable electrical systems) I would caution against using a splicer. I can't tell you how many of these things I've pulled off where the wires where the splicer entered the insulation were corroded beyond conductivity. This does seem to be accelerated if exposed to the elements. My '76 pickup was full of them and I've been cutting them out of circuits for years. If longevity is important (sometimes it isn't) I would recommend soldering the connection, this is the fuel injection system we're messing with right, it would probably be worth the extra time to make a clean connection. If one must use a splicer, I would recommend a liberal dose of di-electric grease on the connection to slow down any corrosion. Just my 2 cents.
Good luck!
Bob

Sopicki · #10 04-19-2007, 06:56 PM

Hi all,

I've run into several problems that may or may not be related concerning the idle speed modification. I did perform the splicing of the purple wire to the thickest solid brown wire that I confirmed was a ground. (There were several solid brown wires in the bundle, but I confirmed that the thickest one did go to ground - I don't know where the other ones go.) This did seem to pick my idle up slightly and solved my 0 to anything smoothness. Shortly afterward I noticed that my mixture had gone off the deep end rich and my starter would not immediately disengage from the flywheel when starting leading to a dreaded grinding noise. I changed the starter with a rebuilt unit with no effect.

I started a thread on the mixture problem where we discussed EHA adjustment and coolant temp testing. I tried the EHA with no luck, the O2 sensor is new and the temp sensor tested fine. I don't have time right now to learn the whole fuel injection system so I broke down and took it to our local MB (non-stealer) shop. I told my mechanic about my purple to brown wire splice and my starter and mixture problems. After much testing he cut the splice and my starter problem went away but it had no effect on the mixture. He performed many tests but bottom line he was not getting enough fuel pressure (which seems backwards to me - low pressure seems like it would be lean - I'll admit ignorance here). He was getting the right mixture at idle but at anything above about 2500 RPM it went way too rich. His recommendation is to buy a new fuel distributor. He knows I'm capable of this so he left it to me to do to save money. Since I've got 8 hours of labor to get this far he said he'd adjust the mixture for free once I change the distributor.

Ok - questions:

My mechanic couldn't get the right fuel pressure, could the problem be the fuel pressure regulator? He said fuel pumps (2 on mine) tested fine.

Or does the distributor sound right?

I don't mind buying what I need, I just don't want to buy what I don't need.

Thanks as always,

Bob

'67 TR4a
'76 GMC C15
'85 XJ-6
'87 300e
'97 VW Cabrio