[W126 Coupe] idles rough when properly lean

Fri Mar 4 20:10:54 EST 2005

Sulaiman,
    I don't think compression is my problem since now that I've got new plugs in the car pulls strong (even with the old plugs it pulled strong, but with the new unfouled plugs it pulls really strong, if I didn't know it had 155hp I'd honestly think it had upwards of 240, and this comes from someone who drives an e39 bmw 540 with fair frequency so it's not like a 380 has lowered my expectations) and even with the old plugs it'd do 125mph with a full trunk no problem (I know that as high as 60 my speedo is as accurate as one of those "your speed is signs", so I don't think it would be very off at 125 either). This makes me think it's more likely the vacuum leak. As an aside, for the time being the needle has gone back to pegging at idle, but still idles rough. I wonder why the mechanic's guess was the air flow sensor? I'm not positive if the idle is worse when cold and gets better as it warms up. Back when I had the mixture set way rich (now it's just somewhat rich) I did notice that when the coolant temp hit 62 it was like a switch went on to smooth out the idle, but now it seems like the idle is consistently rough.
Thanks for the help,
Richard

________________________________

From: mbcoupes-bounces at mbcoupes.com on behalf of Sulaiman Abbasi
Sent: Fri 3/4/2005 1:03 PM
To: mbcoupes at mbcoupes.com
Subject: RE: [W126 Coupe] idles rough when properly lean

Richard,

It is not necessary to force the A/C on to do the test since you have already confirmed that you have low vacuum when the transmission is in "D" while engine is cold. From what you have written I would deduce that the unevenness in idle will be more pronounced when cold but improves as the car warms up. My asking you to turn on the A/C was a way of adding more load on the engine while the car was stationary. If there is no Freon in the A/C the pressure switch will not allow the compressor to kick in. This is a safety feature to protect the compressor and you should not force it to come on as you may ruin it in the attempt. 

In the absence of sophisticated testing gear, you could use the vacuum gauge as an indicator of the health of your engine. If everything is spot on, the vacuum needle should be pegged hot or cold in "D" with A/C running. Two main reasons for poor vacuum are vacuum leaks and improper seating of the inlet valves.

Yes, an engine with poor compression will suffer from performance issues. These are mainly, poor acceleration, high fuel consumption and also uneven idle if you have a case of wide variance in the compression from cylinder to cylinder. Factors contributing to low compression are poor inlet and/or exhaust valve seating and worn out compression rings. You could have a compression test carried out to find out how healthy your engine is. By my experience poor idle may not necessarily mean poor compression but I could not say for certain in your case. If I was you, I'd zero in on why you have low vacuum. 

Sulaiman
'89 500SEC Euro 
-----Original message-----
Date: Thu, 3 Mar 2005 16:35:38 -0500
From: "Shayegan, Richard" <rishayegan at davidson.edu <mailto:rishayegan at davidson.edu> >
Subject: RE: [W126 Coupe] idles rough when properly lean
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Is there a way to force the A/C compressor on even though there is no freon? A/C has a leak and hasn't had freon in it for years. Though sometimes when the engines cold in gear at idle the economy needle is off the peg. Does that still indicate possible poor inlet valve sealing? Wouldn't an engine with low compression suffer from performance issues?
Thanks,
Richard

________________________________

From: mbcoupes-bounces at mbcoupes.com <mailto:mbcoupes-bounces at mbcoupes.com>  on behalf of Sulaiman Abbasi
Sent: Thu 3/3/2005 4:26 PM
To: mbcoupes at mbcoupes.com <mailto:mbcoupes at mbcoupes.com> 
Subject: [W126 Coupe] idles rough when properly lean

Richard,

Now that the mixture is set "correctly", does the vacuum needle now move off the peg when the car is standing still in "D" with the A/C running. This could be indicative of poor inlet valve sealing. You could try a compression test to make sure.

Sulaiman
'89 500SEC Euro

-----Original Message-----
Date: Wed, 2 Mar 2005 17:05:51 -0500
From: "Shayegan, Richard" <rishayegan at davidson.edu <mailto:rishayegan at davidson.edu>  <mailto:rishayegan at davidson.edu <mailto:rishayegan at davidson.edu> > >
Subject: [W126 Coupe] idles rough when properly lean
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While my mechanic had the car in to replace the ball joint, I had him look into why my plugs were so fouled up. Turns out the fuel mixture as I had set to get a smooth idle was rich, but setting it lean makes the car idle rough. Any ideas? He said the ignition and fuel systems check out ok, so he was thinking air flow sensor. If it is air flow sensor, anything I can do short of replacing it to get things running right? I have a can of BG 44K lying around, even though the fuel system checks out throwing that in couldn't hurt might help, right?
Richard

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