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'95 993 / 2011 F150 Raptor SC / Land Cruiser D4D V8 / 500 SEC
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Discussion Starter #1
OK...wrap your head around this one :

Gas tank cap on : car needs about 5-7 revolutions when starting to catch and stumble to life.

Gas tank cap off : starts and runs as soon as you touch the key.

I'm thinking one of those two gizmo's under the air filter box. What's your take ?

Pete
 

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W124 400E, W126 500SE
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1) When it starts and you close the cap, does it run normally?
2) When you drive for a while and open the cap, is it pressurised or underpressurised?
3) do both fuel pumps work? it should start even with a single one
4) last fuel filter change?
 

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'95 993 / 2011 F150 Raptor SC / Land Cruiser D4D V8 / 500 SEC
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Discussion Starter #3
1) When it starts and you close the cap, does it run normally?
2) When you drive for a while and open the cap, is it pressurised or underpressurised?
3) do both fuel pumps work? it should start even with a single one
4) last fuel filter change?
1) Yes.
2) Pressurized.
3) I think so. I've done 120 mph and it didn't cut out.
4) 4 days ago. Same before and after.
 

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'82 Euro 500SEL, '85 Euro 500SEC AMG WB Cabriolet,'86 Euro 500SEC RUF
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When was the Accumulator last changed? If not recently that nmay be your culprit, but I wonder about the cap removal. Usually when the accumulator dies , the car is hard to start because the pressures has been lost.
 

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2012 CL550 4MATIC Coupe
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When was the Accumulator last changed? If not recently that nmay be your culprit, but I wonder about the cap removal. Usually when the accumulator dies , the car is hard to start because the pressures has been lost.
Yeah,

I agree with Charlie here. I have the exact same issue, except I haven't tried the cap off thing. But I just bought a new accumulator from AHAZ and it arrives today. I also bought some new crush washers and a new fuel strainer for the tank too.

Jon White was kind enough to get me a new tank because I thought that my tank may be full of gunk. I took off the old strainer from the new tank, and it clearly showed rust inside so I don't know if I'll use that one. I am going to pull out my tank and inspect it for debris, and I may reuse it.

One other symptom I have is when the car is low on fuel, under a quarter tank, and I make a hard right hand turn, like going around a right hand cloverleaf onto the freeway, the car will stall. As soon as it straightens out it restarts right away. But it's a PITA.

I'll take some pics and may do a DIY thread on it.

But yeah, I agree with Charlie about the hard start being the accumulator.
 

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1981 500SE AMG
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Yeah,

I agree with Charlie here. I have the exact same issue, except I haven't tried the cap off thing. But I just bought a new accumulator from AHAZ and it arrives today. I also bought some new crush washers and a new fuel strainer for the tank too.

Jon White was kind enough to get me a new tank because I thought that my tank may be full of gunk. I took off the old strainer from the new tank, and it clearly showed rust inside so I don't know if I'll use that one. I am going to pull out my tank and inspect it for debris, and I may reuse it.

One other symptom I have is when the car is low on fuel, under a quarter tank, and I make a hard right hand turn, like going around a right hand cloverleaf onto the freeway, the car will stall. As soon as it straightens out it restarts right away. But it's a PITA.

I'll take some pics and may do a DIY thread on it.

But yeah, I agree with Charlie about the hard start being the accumulator.
Please do a DIY on it. I wouldn't be surprised if my tank was nasty.

But I haven't had the cornering issue your stating, it happens in my Camaro but never ran into it in the Mercedes.
 

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Outstanding Contributor
1989 560SEC, 1989 560SEL, 1995 E420
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This sounds like it could be the evap system. There is some good info in the manual, sections 47-200 and -300 about function and testing. Easy to check and eliminate this possibility.
 

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'82 Euro 500SEL, '85 Euro 500SEC AMG WB Cabriolet,'86 Euro 500SEC RUF
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Hmmmm! Perhaps the tank vent valve clogged? Peter was having abnormal noise when he took the cap off IIRC.
 

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2012 CL550 4MATIC Coupe
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He should probably do what I am doing and take the tank out and inspect it and replace the strainer.

When you remove the tank, take care to remove the vent line on the drivers side of the tank bottom. It's a $90.00 hose if you break it.
 

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'95 993 / 2011 F150 Raptor SC / Land Cruiser D4D V8 / 500 SEC
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Discussion Starter #10
I drained the tank through the pump feed line when I replaced the fuel filter, and I did not have the impression anything is wrong upstream of the pump.

What is, however, also a symptom, is that when I switch the contact to start (but I don't actually start), I hear the fuel pumps run for a few seconds every 30 seconds to 1 minute or so. So my guess is I'm losing pressure somewhere. Which points to the pressure reguator on the spider.

But what is really going on here ?

When the pumps prime the system, the pressure in the fuel line is increased. There must be a pressure switch somewhere that tell the pumps to stop. I haven't found it yet.
The pressure regulator under the hood subsequently reduces the pressure to the injection pressure. Since there is no fuel consumption at this point, and the pressure regulator looks at the downstream pressure, it will close when this pressure is reached.

At this point, snce you don't start, the pressure in the system should remain constant. If this regulator is leaking, however, the pressure will drop causing the regulator to open, and the pressure in the feed line will drop since the pump is not running. The pressure switch would detect this and tell the pumps to start up.

But there is no consumption of fuel, so all of it is going straight to the return line and the tank, pressurizing it over time.This will increase the back pressure on the regulator, forcing it shut. except for the leak.

When you proceed to start, you would actually inject at too high a pressure, causing the system to overfuel.

This is all conjecture, however, but I'm not going to start ripping out and replacing parts without having a good reason to suspect said part.

Keep thinking guys. We'll get there.
 

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2012 CL550 4MATIC Coupe
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When the pumps prime the system, the pressure in the fuel line is increased. There must be a pressure switch somewhere that tell the pumps to stop. I haven't found it yet.
The pressure regulator under the hood subsequently reduces the pressure to the injection pressure. Since there is no fuel consumption at this point, and the pressure regulator looks at the downstream pressure, it will close when this pressure is reached.
What tells the pumps to stop is not pressure, but the FPR (Fuel Pump Relay) It engages for two seconds to "prine the pump' as you say and then it stops and waits for a signal from the EZL that the engine is running. They they turn back on.

What regulates too much pressure is also the FPR, but a different FPR, the Fuel Pressure Regulator. That is the barrel like device on the lower side of the Fuel Distributor. The vacuum port tells it that the engine is running and how fast and that regulates the overpressure from the line.

The "Fuel Accumulator" is basically a check valve. It has a rubber diaphragm in it and allows the fuel to flow to the pumps. Since it's an open loop system, if there is a pinhole in the diaphragm, then air leaks in and the line goes empty forcing you to turn the key a lot. The line is long, about 8 feet on a coupe and thick and needs to fill with fuel, and since the FP's only come on for 2 seconds at a time, that's not enough to completely fill with fuel in that time, so it takes a few turns of the key to cycle the fuel pumps to get enough to start.

I did a whole write-up on this a few years ago.

Got my fuel accumulator and strainer yesterday, going to try to do the R&R today. See what happens!
 

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Discussion Starter #12
What tells the pumps to stop is not pressure, but the FPR (Fuel Pump Relay) It engages for two seconds to "prine the pump' as you say and then it stops and waits for a signal from the EZL that the engine is running. They they turn back on.

What regulates too much pressure is also the FPR, but a different FPR, the Fuel Pressure Regulator. That is the barrel like device on the lower side of the Fuel Distributor. The vacuum port tells it that the engine is running and how fast and that regulates the overpressure from the line.

The "Fuel Accumulator" is basically a check valve. It has a rubber diaphragm in it and allows the fuel to flow to the pumps. Since it's an open loop system, if there is a pinhole in the diaphragm, then air leaks in and the line goes empty forcing you to turn the key a lot. The line is long, about 8 feet on a coupe and thick and needs to fill with fuel, and since the FP's only come on for 2 seconds at a time, that's not enough to completely fill with fuel in that time, so it takes a few turns of the key to cycle the fuel pumps to get enough to start.

I did a whole write-up on this a few years ago.

Got my fuel accumulator and strainer yesterday, going to try to do the R&R today. See what happens!
It's the cap on/cap off conundrum that's got me scratching my head. I don't see the accumulator attributing othat. It must be back pressure related. Somehow.
 

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2012 CL550 4MATIC Coupe
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It's the cap on/cap off conundrum that's got me scratching my head. I don't see the accumulator attributing othat. It must be back pressure related. Somehow.
Okay,

Well I can confirm that my hard starting issue has been resolved finally! And it WAS the Accumulator. I did not change the strainer, or tank yet.

I did do the Accumulator, and I took a few pics, and I found that I had a couple of tiny leaks coming from the fuel pump fittings. I had bought a bunch of copper crush rings with the accumulator for just such an event, and they proved handy.

No more leaks, and no more hard starts.

It used to take three long cranks at least to start after it had been sitting for over an hour. Now, after 6 hours or so, it starts first key within a couple of cranks.

Problem solved! This is probably your problem too. It was an eay swap, about ten minutes under the car after the tank had been drained. That was a bitch! Had to get a couple of oil pans under there. I cracked the side that has the clamp on and broke the seal, then let the tank drain out. This was after I had siphoned out all of the gas that I could and transferred it to my other coupe.

After the tank drained out, I loosened the clamp holding it in place, and then there is a 12MM on the FA and a 17MM on the fuel line. |Loosen and swap the FA, then re-tighten everything and refill with gas and go!
 

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Hmmm. I just found part of my hard starting on Mosby. 5 out of 8 injectors are no more good. I left the car sit undisturbed for almost a week, then broke loose the injector lines at the injectors, expecting to find fuel in the lines and in the tops of the injectors. 3 lines were full and these 3 injectors had fuel at the top. 2 lines were empty and the injectors dry. 3 lines were dry, but the injector tips were wet with fuel.
I would expect to find fuel lines full with good injectors, fuel held at the top when disassembled and dry tips. OK 3 good injectors out of 8.

Fuel Distributor appears to be recently replaced. Bosch Numbers, very clean and new looking, etc. Hopefully not a Python rebuild.

So Mosby is out of commission until Monday when the new parts arrive. I am also changing out the Accumulator as when I pulled the inlet line I didn't get as much fuel from it as should have been there with the correct pressure. return line had the expected amount of fuel at the connection to the FD.
 

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2012 CL550 4MATIC Coupe
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AutohausAZ had the best price on the FA that I found. $110.00 with free shipping and no tax.

The car started right up this AM with a very short turn of the key and it only turned over 3 times before it caught. That's three revolutions, not three turns of the key with 10 or so rev's each turn like it used to be. So I think we can check that one off the 'To Do' list.
 

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Greek God of the R129
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AutohausAZ had the best price on the FA that I found. $110.00 with free shipping and no tax.

The car started right up this AM with a very short turn of the key and it only turned over 3 times before it caught. That's three revolutions, not three turns of the key with 10 or so rev's each turn like it used to be. So I think we can check that one off the 'To Do' list.
Yeap,
The fuel accumulator... from autohausaz. ;)
I replaced it about three months a go.
Now it starts right up.
Like Mike said three revolutions. :)

Before that I did replace the leaking injectors.....
It didn't help with the starting problem.
More power?
Yes.

Regards,
aam.
 

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I don't think the new injectors will fix the hard start either, but it will for sure fix the running rich and leaking of raw fuel into the cylinders. I replaced the Accumulator today and checked the fuel pump, filter and pump seem to be recently replaced as are the fuel lines and hoses all with MB star fittings....
 

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Greek God of the R129
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I don't think the new injectors will fix the hard start either, but it will for sure fix the running rich and leaking of raw fuel into the cylinders. I replaced the Accumulator today and checked the fuel pump, filter and pump seem to be recently replaced as are the fuel lines and hoses all with MB star fittings....
Charley,
You are correct on the starting problem.
But raw gas on a non running pistons is not something that I want/like. Lol.

Regards,
aam.
 
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