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S-Class (W126) 1979-1991: 300 SE, 300 SEL, 380 SE, 380 SEL, 420 SEL, 500 SEL, 560 SEL, 360 SEC, 500 SEC, 580 SEC, 300 SD TURBODIESEL, 300 SDL TURBO, 350 SD TURBO, 350 SDL TURBO


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Old 05-21-2009, 03:24 PM   #1
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Idle issue at start up

1991 560 SEC with 90K miles. When I start up the car in the morning - it starts rough - usualy will turn over and start running real rough but then dies. Starts on the second try but the idle goes from the normal rpm level 800or so and then bounces up to 1,200 then back down to normal. It revs like this every few seconds...for two minutes or so gradualy decreasing the rpm's each time till it eventualy evens out and runs smooth. Other than that the car runs great. Fuel filter? Fuel pump? Cold start valve? I plan on repalcing the fuel filter this weekend to see if that the culprit. How about the small smog filter in the air box? Could that be an issue too??? Any help would be greatly appreciated!

Jeremy
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Old 06-18-2009, 09:47 PM   #2
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Quote:
Originally Posted by jhusen View Post
1991 560 SEC with 90K miles. When I start up the car in the morning - it starts rough - usualy will turn over and start running real rough but then dies. Starts on the second try but the idle goes from the normal rpm level 800or so and then bounces up to 1,200 then back down to normal. It revs like this every few seconds...for two minutes or so gradualy decreasing the rpm's each time till it eventualy evens out and runs smooth. Other than that the car runs great. Fuel filter? Fuel pump? Cold start valve? I plan on repalcing the fuel filter this weekend to see if that the culprit. How about the small smog filter in the air box? Could that be an issue too??? Any help would be greatly appreciated!

Jeremy
Jeremy;

I don't have an answer, but can relate; here's a series of my posts.

Parts description / warm start & smooth idle issue

Some time back, I posted a problem with my 1987 420SEL regarding a starting issue when the engine is slightly warm, taking two tries for the engine to catch. The problem still exsists and now the engine idles a little rough until completely warmed up.

Listing the three parts below, could someone tell me what each part specifically does and how it could relate to starting or idle issues.


Okay, this morning I replaced the Idle Air/Speed Control Valve and a Engine Temperature Sensor. The replacement and installation went smooth with no surprises, though at this point it doesn't appear to have resolved my issue. Part of the problem is that I'm no mechanic and when I asked the parts supplier for help in identifying two parts based on the picture I sent, they were looking at a sensor, thinking that's what I meant, when I meant what I now know is the cold start valve, or cold start fuel injector. Because I didn't know the correct name, I ended up with a incorrect part. Oh well; on a 22 year old vehicle, any part I replace must be okay in the long run.

Drove the vehicle today and replacing those two parts certainly didn't solve the problem; in fact, it's a little worse. Had to order a used cold start injector off of eBay, as I couldn't find a single auto parts online source that either carries it or had one in stock. I didn't even hear back from my local M-B dealer. When I get the part installed, I'll post the outcome, as I sure hope this does it...

Okay, I replaced the cold start injector this evening with a new gasket. The engine didn't really cool down much, so it isn't a cold start, per se, but I feel at this point it is no better. I'm definitely feeling an engine 'miss' unless really warmed up.

So far, I have replaced the cold start injector (used), the idle air /speed control valve (new), one engine sensor near the cold start injector (new) and the EHA (used) some months back. When I replaced it, I don't remember having the cold start problem; it's kind of progressed over a period of time. I looked at all the vacuum lines and such, and all appear to be okay, though I can't necessarily tell one isn't bad. I don't readily see anything that is amiss and don't see any fuel leaks. I also checked the 10 amp fuse for the overload protection relay. I can buy a used one from eBay for about $32 with shipping or brand-new for $54 with shipping from Autohaus. I guess this would be the next part to replace. Looking at the service records, it looks the O2 sensor was replaced about a year and a half ago.

I could put the original idle air /speed control valve back on but since I replaced in trying to solve this issue, if I put it back on it probably won't solve it, though it does seem to me that this problem got more pronounced after changing it out.

Anyone have some additional thoughts on what to try next? Taking it to an indie mechanic certainly comes to mind but then that's admitting defeat...


Mmmm, interesting. An eBay seller has also suggested the same thing, replacing the overvoltage protection relay. I did check the fuse, which was good, but that doesn't mean the relay isn't bad.

At this point, I'm admitting defeat and have an appointment scheduled with an indie next week. I'll kick myself a few more times if it turns out to be that, since it's only a $55 part. I'll post whatever they come up, and the price of that bit of expertise.


Not sure if it's okay to post a link of the thread since it's a differernt forum. The entire thread shows the reponses and a picture of the parts I ended up replacing. But since I don't want to replace every part under the hood that could somehow be the culprit, I'm hoping the indie can narrow it down quickly.
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Old 06-19-2009, 12:41 AM   #3
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Poking around the forums and auto parts sites, I came up wondering two things: is the Electronic Control Unit (ECU) the same thing as a MAS Control Unit, or are they one and the same and two, if my MAS was acting up, would that be a likely part that could cause the poor cold starts and rough idle?

Thank you,


https://www.autohausaz.com/search/pr...ber=0115457932
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Old 06-23-2009, 09:30 PM   #4
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Took my 420SEL with a cold start and idle problem in to be checked out; came back running a lot smoother. Turned out that I had three vacuum leaks; one was at the idle speed control valve that I just replaced and there were two disconnected hoses, located behind the passenger headlight (switch-over valve for the air injection? Two light-blue, small diameter hoses, one with a red stripe and one with a blue stripe.)

Additionally, the tech reports that it had incorrect spark plugs (he replaced all eight) and the #3 spark plug connector was not on tight and had minor ‘corrosion’ from arcing. Definitely runs a lot smoother and started with the first key-turn running errands on the way home. I’ll find out tomorrow morning how she does on a cold start.

The tech also recommended that I replace the old style metal injectors and recommends replacing them with the newer copper style injectors, along with the seals and stands, whatever that might be. Is that a DIY project?
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Old 06-23-2009, 09:30 PM
 
 
 
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1200, 300se, 420sel, 560, cold, idle, mercedes, problem, rough, sec, starts, startup, super, w126, warm



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