400E 1600rpm idle on 400E without ASR

[Big E]

I'd like to think I'm nearing the end of my 9-month-long thrash on the ol' 400E, most of which is captured in this thread. New fuel pumps are in and I just finished up replacing the rubber fuel lines from the tank to the pump and between pumps, as well as the tank vent valve. The stumble below 2000rpm seems to be fixed now, at least on my last test drives it didn't happen at all. So, yay!

The final remaining issue (until the next one appears) is an abnormally high idle. When first started, it idles around 1100 in P/N, but once fully warm, it's more like 1600. It is rock solid, just way too high. As far as I can tell the throttle linkage is adjusted correctly in that the throttle plate is fully closed when there's no pressure on the gas pedal, and there's no slack in the cable either, but maybe I'm missing something here. I'm also confused on the presence (or lack thereof) of an idle control valve. I was under the impression the ETA was handling those duties, but the AutohausAZ site shows a part number 000141242581, Idle Air/Speed Control Valve, shown for the E500, 500SL, E420, 400E, 500E. Mercedes dealer sites show same. I don't see how I would have missed seeing an idle valve while I was doing all the harness and ETA stuff, and a cursory check under the air box 5 min ago didn't show me anything I'd missed. So I'm confused on that.

I pulled the codes and pin 7 (cruise/idle control) had DTC 6, "starter lock-out / backup lamp switch" which I believe is the neutral safety switch. No codes on pin 4 (FI) and pin 17 (EZL) showed DTC 8, "transmission overload switch does not close". I did not clear the codes and then drive it for ~30 min before I pulled those codes, so it's possible those are red herrings. I think the starter lock-out one is real because that seems to always come back.

So, any suggestions?

 

[gsxr]

The final remaining issue (until the next one appears) is an abnormally high idle. When first started, it idles around 1100 in P/N, but once fully warm, it's more like 1600. It is rock solid, just way too high. As far as I can tell the throttle linkage is adjusted correctly in that the throttle plate is fully closed when there's no pressure on the gas pedal, and there's no slack in the cable either, but maybe I'm missing something here.

Make sure there is a tiny amount of slack (not quite zero) at the throttle cable, but otherwise it sounds ok. Follow the factory adjustment procedure just to confirm.

I'm also confused on the presence (or lack thereof) of an idle control valve. I was under the impression the ETA was handling those duties....

Your impression is correct. The T/LLR module controls idle speed by controlling the ETA throttle position.

... but the AutohausAZ site shows a part number 000141242581, Idle Air/Speed Control Valve, shown for the E500, 500SL, E420, 400E, 500E. Mercedes dealer sites show same. ... So I'm confused on that.

As usual, the interwebs are wrong. Idle control valve 000-141-24-25 is ONLY used on the early M119.960 engine with mechanical CIS-E injection, which was only in the 1990-92 500SL. This was never on any year of 400E/500E. Don't trust what what you see on part catalogs, the EPC is the only accurate source of information.

I pulled the codes and pin 7 (cruise/idle control) had DTC 6, "starter lock-out / backup lamp switch" which I believe is the neutral safety switch.

This is also correct, although the neutral safety switch (NSS) is multi-function... it also is a starter lockout switch, and on the 400E/500E, it sends a shifter position signal to the T/LLR module. While it would be nice to replace this ($100) it isn't likely to cure a 1600rpm hot idle. The NSS is adjustable, so it wouldn't hurt to check the adjustment, but again don't expect any improvement.

No codes on pin 4 (FI) and pin 17 (EZL) showed DTC 8, "transmission overload switch does not close". I did not clear the codes and then drive it for ~30 min before I pulled those codes, so it's possible those are red herrings. I think the starter lock-out one is real because that seems to always come back.

EZL code 8 is indicating a faulty O/L switch at the trans, p/n 002-545-45-14 (click here). This code generally only appears after a full-throttle upshift, and causes a funky WOT upshift feel. I'd replace this, but again it has no effect on your high idle.

So, any suggestions?

1) What happens when you unplug the vacuum line from the EZL, and re-connect it? Is there vacuum present, and does the RPM change briefly when the hose is placed on or off the EZL?

2) Repeat the smoke test for the intake manifold. The high RPM sure sounds like a large vacuum leak somewhere.

 

[Big E]

1) What happens when you unplug the vacuum line from the EZL, and re-connect it? Is there vacuum present, and does the RPM change briefly when the hose is placed on or off the EZL?

I got it fully warmed up, drove for about 20 min, and had no issues other than the high idle. When I got home, I let it idle and pulled the vacuum line from the EZL, and the idle went down from 1700 to about 1350. Reconnect and it went right back to 1700. There was definitely vacuum on the line - when I put it by my palm, it sucked my skin to the hose.

So, I'll admit, I'm not sure what that means...

Haven't repeated the smoke test yet.

 

[gsxr]

OK - that means the vacuum signal is present, and generally indicates the EZL vacuum sensor is working.

I'm back to repeating the smoke test.

 

[Big E]

On the smoke test, is there a suggested place (or places) to connect it up? I'm trying to remember where I connected when I did this the first time, 6 months ago.