85 380SE Idle problems - Page 3 - PeachParts Mercedes-Benz Forum

Chadahar · #31 02-02-2008, 01:07 AM

I had an idle problem with my 380SE. It would not idle at all and then it would idle high. I accidently smacked the ICU on the firewall with a screwdriver handle and it died. Thinking it might be a coincidence, I started the car and smakcked the ICu agin...same result. I went on Ebay and bought a good used one and everything has been fine since. No high idle and it idles where it should in park or in gear.

The cross over vacuum line does go between the "Bumps on the hoses that are not drilled. I got the smallest ID vac line from the auto store. However I didn't install the line yet, because my problem was fixed by the new ICU.

elolson · #32 02-02-2008, 08:23 AM

Thanks guys. Honestly, I'd hold off on installing that vacuum line. I'll keep the forum posted on how mine is working out, but like I said, I have seen no idle difference with it installed.

Good tips on the starting. My car has always instantly fired right up. My measured system pressure is perfect as is the cold control pressure and the warmed up control pressure.

The lambda control unit replacement seems to have fixed the rough idle, but I can not declare victory yet, I need to drive it a few more days.

That is interesting about the ICU. I just had a rough idle, it did appear to idle at the right speeds at the right times, so I have never suspected the ICU or the ICV except early on when I was just starting out, that is the reference to the misleading post. I removed the ICV, cleaned it out and reinstalled and had claimed that fixed the problem. That was not true, I was just not patient enough to realize the problem was most definitely still there.

elolson · #33 02-02-2008, 12:02 PM

I warmed up the car good with a short drive to the store, got i back home. I am going to declare victory on the rough idle. I did one last fine tune of the on-off ratio. It still oscillates just a small bit, but the value is in the exact same range as at 2500 rpms. I can also trim it out anywhere by turning the screw, it is neat to see the control system adjust the ratio to compensate for the richer or leaner mixture. So now I know it is doing what it is supposed to.

Many causes for a rough idle. I replaced:
1) O2 sensor (just because)
2) Warm-up compensator (failed cold pressure test, not by a little bit either, it was clearly off), new one was right in the middle of the acceptable band for temp I took the reading at.
3) Replaced the lambda control unit. The old one was malfunctioning with a wide swing hunting for a value.
4) Added the shunt line around the ICV, I don't think this was necessary and I noticed no difference in idle.

Interestingly, the car seems a touch more sluggish now. I guess without that rich charge getting blasted into the engine with no compensation, it is just not as peppy.

Summary VICTORY over the rough idle.

raymr · #34 02-02-2008, 12:30 PM

I thought the ICV bypass was to help the random stalling problem that some people, including me, were having. I put it on mine, but it doesn't seem like enough air would go through to affect anything. All the same, mine hasn't stalled at all lately but I'm not sure if it's strictly due to the bypass.

elolson · #35 02-02-2008, 12:59 PM

You know that is a good point. The shunt was to correct for stalling after breaking, etc.. and had nothing to do with rough idling. I have had a stall or two on occasion just like yourself on others, we'll see if it ever happens again now that the shunt is installed.

kchemers · #36 02-02-2008, 01:09 PM

Sometimes a high idle can be related to the temp sensor not being read right. The engine will adjust fuel trim and then make the engine run at higher RPM. It could also be a bad fuel pressure regulator. Check all of these they are cheap to replace.