Inside the box: V8 Idle Speed Control & Fuel Pump Relay - PeachParts Mercedes-Benz Forum

JamesDean · #1 05-25-2013, 08:35 PM

Evening everyone,

I was working on the 420SEL today and decided I would take some pics of a few things for future reference.

I was having an intermittent stalling problem, where all of a sudden the engine would die. My mom was the daily driver of this car so naturally I was unable to reproduce the stall. Well, finally it happened to me. I think it was the fuel pump relay so I took it out and found a few cold solder joints.

Fuel Pump Relay:
420SEL Fuel Pump Relay - Imgur

Im also trying to tackle a high idle problem. I've replaced the main air lines going about the injectors, IACV lines and a hard line to the EZL. That did well, it fixed a stuttery acceleration but the high idle remains.

I found that by the bypass line that goes between the two IACV hoses had one end crimped closed, so I replaced it with a normal through pipe and that might have made things worse! So I figured I'd check the IACV and its controller.

The IACV is was receiving about 4.5-4.8 volts and it measured about 4.5 Ohms, so somewhere in the neighborhood of 1000mA or so. Thats within spec according to the FSM. So, I, evidently still have a leak somewhere. I wanted to check the controller to make sure it was OK so i pulled it and here we are:
420SEL Idle Speed Controller - Imgur

Dubyagee · #2 05-25-2013, 09:13 PM

Interesting.

pawoSD · #3 05-25-2013, 11:42 PM

High idle is all vacuum leaks. You have to replace ALL lines on the motor to fix it. I recently did this, you even have to replace the injector bushings (the plastic piece the injectors mount into the engine with) as they have o-rings that can leak air.

When I did it I took the whole thing apart right down to the throttle plate. Replaced the main air boot on the bottom of the Air meter assembly too, old one was hard as a rock.

Before all the work mine idled around 1200 in park, and was loading it up in drive/wasting gas. Now it idles around 600 in park, 550 in drive and it much more calm. MPG went from an average of 14 city 21 hwy to 16-17 city and 23 hwy.

pawoSD · #4 05-23-2014, 11:11 AM

Almost one year to the day I posted....idle now rough when cold in drive. I am going to try swapping idle controllers with my dad's 420 and also suspect I have a vac leak at the trans modulator or elsewhere. (possibly the tube running under the fuel distributor) sigh.

pawoSD · #5 05-24-2014, 10:01 PM

FIXED. Issue was a combination of a few small vac leaks (connectors for trans modulator and the 3 way tee at the climate/econ gauge by the firewall) as well as the air flap potentiometer being toasted, installed new potentiometer, cleaned the contacts on the distributor cap and rotor, and adjusted the CIS mixture a bit to fine tune. Runs smooth now.