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  #1  
Old 03-05-2007, 12:40 PM
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Join Date: Jun 2002
Location: Charlotte, NC
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77 350 SL - Hard starting when warm

I have a problem I have been dealing with for about a month.

I have a 77 350 Euro SL with 58k miles and have been working on a "Hard Starting When Warm and has set for more than 10 mintes" problem. I have checked and tested everything I can figure out to test - vacuum lines, air sensor plate setting, etc. I have the MB service manual, and even bought a used "Automotive Engineering" book that goes into pretty good detail on the CIS system, and still can't seem to nail down the problem.

I have a real good friend who is an "old" mechanic (not CIS trained though) and he says it sounds like a "classic leaking injector" problem.

Can anyone help me?

Other than this hard starting problem, the car runs fantastic!!!!! What a ball to drive. The Euro model doesn't have all the emissions crud on it, and talk about acceleration. This is my last "issue" since purchasing the car.

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  #2  
Old 03-05-2007, 01:24 PM
david s poole
 
Join Date: Mar 2005
Location: dallas
Posts: 1,822
check the fuel pump accumulator[with the fuel pump and filter].it has a small hose going back to the inlet side of the pump.use some needle nose visegrips to clamp off the line between the accumulator and the front side of fuel pump and then undo the small clamp at the accumulator and remove that end of the hose.now start car and see if you have fuel continuously running out of that small pipe[which indicates a blown diapham in the accumulator].
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David S Poole
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1987 Mercedes Benz 420SEL
1988 Mercedes Benz 300TE (With new evaporator)
2000 Mercedes Benz C280
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  #3  
Old 03-05-2007, 01:53 PM
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Location: Des Moines, WA
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My '76 450SEL with CIS has a hot start solenoid on the back of the fuel distributor. The temp switch is at the rear of the drivers side head. I've seen a write up of how it works but can't find it right now. Mines not connected but since it's not that warm in Seattle, I doubt I'll ever miss it.

If it's not a leaking accumulator as David says, you might check to see if you also have the hot start solenoid. It's a bit bigger than a "D" cell and screwed into the fuel distributor.

Michael
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  #4  
Old 03-05-2007, 05:32 PM
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Thanks guys. I'll definitely try the accumulator leak test. One thing that puzzles me about that theory is - Why does the car start almost instantaneously when cold? I realize the cold start injector puts fuel directly into the port (I've checked it, and it's working as it should), so wouldn't the system be fuel starved after sitting for hours and was cold?

I do not have the hot start solenoid on this engine.
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92 300E - 116K miles - Sold
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94 Explorer 147k miles
2009 Hyundai Genesis - 65k miles
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  #5  
Old 03-05-2007, 09:49 PM
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The under hood heat of an SL can cause something like vapor lock. It starves the engine of fuel and you have to crank until you get correct pressure in the fuel distributor. The fuel pump only runs in the crank position or if the air flow plate moves off of the safety switch with the ignition on.

The hot start solenoid encouraged fuel flow but appears to have been dropped on the 450SE/SEL after S/N 47090 so that’s why you may not have it on your SL

Michael
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  #6  
Old 03-06-2007, 11:58 PM
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If the accumulator is bad, you will get plain old common garden variety vapor lock -- instead of liquid gasoline in the fuel distributor and fuel lines, you have gasoline vapor, and the resultant mixture will be much to lean to burn. YOu have to crank a while (with the fuel pump running as the starter does) to push the vapor out and get liquid gasoline to the injectors.

Best way to check is a fuel pressure test -- should be around 4 bars running, I think, and hold 3 indefinitely with the engine off. Requires the correct gauge, but thats better than the alternatives. As a cheapo (but dangerous) method, crack the fuel line fitting going to the distributor after the engine has been sitting hot for a few minutes. You should get a significant amount of fuel coming out under pressure. If not, the accumulator or check valve on the pump is bad -- check valve is much cheaper.

Also check for vacuum leaks -- a significant leak will cause too lean a mixture for the car to start hot -- cold is no problem because you have extra fuel then. Look at the idle control valve, the boot between the fuel distributor and throttle body, and brake booster line.

Peter
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  #7  
Old 03-07-2007, 01:01 PM
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Guys:

Excellent - I love it when someone explains how things work. Your explanation makes perfect sense. Now to test. I'll let you know what the problem turns out to be (soon I hope).

thanks
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92 300E - 116K miles - Sold
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94 Explorer 147k miles
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  #8  
Old 03-10-2007, 04:49 PM
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David Poole - After sitting for just a couple of minutes after the car has started, there is zero pressure at the fuel distributor. That's not good, so I'm under the car looking at the fuel pump and a bunch of other stuff. It appears the fuel filter sits directly above the pump, and I'm assuming the filter is on the output side of the pump. Sitting to the left of the pump (looking from the rear of the car) is a small "something" that is about 3 inches in diameter. It has a 1/2" diameter hose about 1 inch long between it and the pump, and a 1/4" hose going up to "something". Is the 1" long hose the one I need to remove and clamp, or is it the 1/4" hose leading to the "something" that I need to clamp? Also, you mentioned a check valve - is it in the fuel pump, or is it something else (maybe the 3" diameter thing?). I apologize for not knowing what these parts are. I've spent about an hour looking in the MB CD and just can't seem to find a diagram of the fuel pump assembly components.

Thanks for any help you can give. I've been up fixing my roof all day long and have just now had a chance to check this out. So, I'm a little late checking out your diagnosis technique.

David Lee
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92 300E - 116K miles - Sold
77 Euro 350SL - 67k miles
94 Explorer 147k miles
2009 Hyundai Genesis - 65k miles
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  #9  
Old 03-10-2007, 08:07 PM
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David - I finally found the diagram of the fuel pump assembly on the CD's. Turns out it appears I don't have a check valve (at least not an external one). The CD's say that early on, the check valve was part of the fuel pump. The item I was not sure of in my last post is called the resevoir on the CD set, and I'm assuming that is the item you refer to as the accumulator.

The clamps are so old, that I could not even unscrew them in order to take them off. Therefore, I coulnd't perform the test you mentioned. It got dark, so I'll pursue this tomorrow (maybe).

But, I have what is probably going to sound like a dumb question; why is the check valve not between the accumulator (resevoir) and the fuel distributor? That seems like a logical place for it. That way, the accumulator can be as broken as it wants to be, and I still wouldn't lose fuel pressure.

Anyway, tomorrow's another day.
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92 300E - 116K miles - Sold
77 Euro 350SL - 67k miles
94 Explorer 147k miles
2009 Hyundai Genesis - 65k miles
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  #10  
Old 03-21-2007, 09:02 PM
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Well, it turned out to be the accumulator, and you're correct - it is pricey. I went ahead and changed some of the "original" hoses as well. I ordered a check valve as well, since there wasn't one attached to the fuel pump. There may be one internal to the pump, but I don't really know how to determine that. But, the threads were not the same, so I'm not sure if I should work with Phil and get one installed.

Thanks for everyone's help. I never would have known to check the accumulator.

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92 300E - 116K miles - Sold
77 Euro 350SL - 67k miles
94 Explorer 147k miles
2009 Hyundai Genesis - 65k miles
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