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1999 S420; 2007 S600
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416 Posts
Discussion Starter · #1 ·
Greetings, oracles of W140...

Both of our W140s, '99s with relatively low mileage, have labored starting - one much more so than the other. The RPMs do not get much above 1200 and it takes 7-8 cranks to get it to stay running. Before I go chasing down every possible path to diagnose the problem, is there any NORMAL cause of labored starting OTHER than a weak battery? (The batteries on both cars are <1 year old and hold perfect voltage and CCA)

Is there, for example, a PCV valve on these that could get clogged? Fuel pressure regulator? Worn starter motor or loose/corroded starter motor ground cable? Etc. etc.

Just looking for common causes seen repeatedly in many W140s, as a "first thing to check". In terms of the whole universe of things that COULD cause it, I can wade my way through - I just don't know what the usual causes may be.

Thanks in advance!
 

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1989 560 SEL, 1993 400 SEL, 1986 420 SEL, 1977 450 SL, 1984 300D, 1981 380 SL, 1983 500 SL
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606 Posts
Do the cars run properly once they are started? If so, I would check the fuel pressure regulator. It is located on the fuel rail on the drivers side and looks like a little space ship. Pull the vaccum line off of it and see if there is the smell of gas. the bladder inside it can crack with age, especially so if the car is sitting for a long time. Let us know if the cars are running properly and if not, what the symtoms are if they are not.
 

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1999 S420; 2007 S600
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416 Posts
Discussion Starter · #3 ·
They are running properly after startup. Fuel pressure regulator would be a first thought but for the fact that they are running smoothly even into high revs, and are not getting abnormally high fuel economy. I may just do a fuel pressure test on the rail this weekend. Alldata probably has the correct pressure specs.
 

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2013 E350/41k, 1997 E320/290k, 1999 FJ100/256k
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314 Posts
Both have the same issue? - while statistically rare - first thought was the gas - are both cars filled at the same station??
 

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Premium Member
1998 140.070 CL500, 1966 406.121 U65
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1,489 Posts
I may just do a fuel pressure test on the rail this weekend. Alldata probably has the correct pressure specs.
While I do not have the correct ranges here, I made a video of the measurements
of my car, all within the correct range;
http://www.pvv.org/~syljua/merc/FuelPressureME.mov

It still could leak down causing vapor lock, but I guess your case is at all
conditions, warm, cold, and not just after standing for ~15min?

Have you tried to let the fuel pump run before you turn the starter on?
If it takes a while to build up pressure, I would suspect your fuel filters, or
fuel pump.

br,
syljua
 

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1999 S420; 2007 S600
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416 Posts
Discussion Starter · #6 ·
Thanks all -

Yes, both cars have a relatively sluggish startup but the S500 only takes about 4-5 cranks and the RPMs shoot up to 1800 or so, settle to 1500 until the startup cycle is complete and oil is circulating, then settles in to the normal idle speed. It may just be a perception; I had a '94 540i which also took a few extra cranks to fire up. My '96 Vanden Plas starts up after 2 cranks. Other I6 and V6 cars I've had also start up at 2-3 turns. It's possible that it's just a V8 thing.

The S420 takes 7-8 cranks and doesn't even hit 1500 after startup. It's just very slow, sluggish, labored. Once it's started, it runs normally. It is far more sluggish than the S500. It was a bit more sluggish since we bought it but it seems to be getting worse over the course of months.

The labored start is definitely in all conditions. Very consistent. Hot, warm, cold. Sitting for 10 minutes or 2 days.

I have tried cycling the key to 0-1 a couple of times before cranking it. The pump sends a burst of pressure to the rail to dump in more fuel at startup; if there's a small pressure problem like a bad check valve on the feed line or bad regulator that hasn't completely failed, this will build up enough pressure to have a quicker, sharper startup. In this case, it has no impact, which means either there is a major problem holding pressure at the rail (probably just a failed pressure regulator), or else, the problem is not with fuel delivery.

I have already replaced the fuel filter on both cars as standard preventative maintenance.

A failing pump would likely cause the weak startup - but it would also necessarily cause other symptoms, like misfires, OBD-II codes from the air-fuel mixture being too lean, lumpy idle, abnormally high fuel economy, white buildup on the plug tips from running too hot. None of this is the case. The S420's idle is much, much smoother than the S500, fuel economy is normal, plugs are normal.

It sounds like maybe there's nothing much different in these motors than any other - and a fuel pressure regulator is a likely culprit!

Thanks,
Andy
 
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