Mercedes-Benz Forum banner

21 - 40 of 83 Posts

·
Registered
1991 560sec. 1969 280SL
Joined
·
987 Posts
Discussion Starter #21
There isn't a return spring in the setup of the mechanism is there?
 

·
Outstanding Contributor
1988 300CE
Joined
·
1,424 Posts
What's the procedure to check the plunger? Why would the plunger or sensor plate be unable to return to rest with cold engine?
What I have in mind primarily when I say “you might first take a closer look at the air flow meter’s & control plunger’s ease of movement when the engine is cold”, is the break-away force of the AFM (air flow meter) & the CP (control plunger), when the engine is cold.

There might be a mechanical problem requiring a higher force to break away the cold AFM from its base position, or there might be deposits or even corrosion in the FD requiring a higher force to break away the CP from its rest position when the FD is cold. … Btw, the more ethanol fuel contains the more it can lead to deposit forming in the FD & corrosion of the aluminium (aluminum :)) parts of the FD.

The CP not completely descending back to its rest position when the engine is switched off is what I have in mind secondarily, as one of the possible reasons for holding pressure loss, which, of course, can also be caused by contamination.

The reset force for the AFM is provided by a counterweight.

Here is how you can test the AFM’s & CP’s movability, when the car is in the condition in which it usually shows the starting problems:
  • Turn the key to "ignition on" two or three times to let the fuel pumps prime.
  • Push the AFM plate from its base position slowly down. It should have 1–2 mm play along which it should practically be movable without any resistance before you feel it reach & start to lift the CP.
  • Push the AFM plate slowly further down and feel the resistance. It should be slight & homogeneous along its complete path.
  • Let go of the AFM plate. It should quickly & freely move back to it’s base position leaving the slowly following CP behind.
  • Let the pumps prime two or three times again.
  • Push the AFM plate completely down again and this time let it slowly move back to it’s base position and feel if the CP follows the AFM plate (stays in contact with it) along its whole path, which it should.
Particularly pay attention to the first part of the CP’s stroke when you slowly push the AFM plate down, and on the last part of its stroke when you let the AFM plate slowly rise back to its base position.

H.D.
 

·
Registered
1991 560sec. 1969 280SL
Joined
·
987 Posts
Discussion Starter #23
Ok I shall do that in the morning. Thanks for the detailed response. Much appreciated.
 

·
Registered
1991 560sec. 1969 280SL
Joined
·
987 Posts
Discussion Starter #24
Hello
So today I checked the system as instructed and the sensor plate was not moving up to its rest position when pushed all the way down. I sprayed intake cleaner continuously with engine running raising RPM as necessary, then switched to a foaming type cleaner/ lubricant in the same manner. Checked again with cold engine as instructed everything checked normal with the sensor plate and control plunger but, there is still pressure loss after about twenty to thirty minutes, which is not acceptable and the engine still has the very same starting problem. I will check the F/ Accumulater first, if that checks good then hopefully I will be able to perform a fuel pressure test tomorrow and take from there.

I was out and about and the engine was hesitating when moving off from a traffic light but, once it reaches full operating temperature there is absolutely no hesitation at all. Sorry I failed to mention that at the beginning of the thread.
Comments would be very welcome.
 

·
Outstanding Contributor
1988 300CE
Joined
·
1,424 Posts
... the engine was hesitating when moving off from a traffic light but, once it reaches full operating temperature there is absolutely no hesitation at all. Sorry I failed to mention that at the beginning of the thread ...
That’s indeed an important piece of information … it indicates a problem either with the CTS, the connection between CTS & ECU, the ECU, the connection between ECU & EHA, or the EHA ... with effect on both cold start and acceleration during the warm-up phase.

Fortunately it’s usually the CTS … I suggest to check its resistance at the disconnected ECU connector when the engine is cold. Your car, presumably not being a California version, should be equipped with a 2-pin CTS in front of the FD. In that case check the resistance between ECU connector terminal 21 & ground. … Btw, disconnect & reconnect the ECU connector always with ignition switched off !

So, in your case there may be a problem with the holding pressure and the CTS signal.

H.D.
 

·
Premium Member
1991 560 SEC 1994 E500 2014 E350 Cab
Joined
·
7,029 Posts
HD. a hi-jack for gen2 USA with 4 pin CTS, what should the ohms be on the criss/cross pin reading with the key in the nbr 2 position?
 

·
Registered
1991 560sec. 1969 280SL
Joined
·
987 Posts
Discussion Starter #27
That’s indeed an important piece of information … it indicates a problem either with the CTS, the connection between CTS & ECU, the ECU, the connection between ECU & EHA, or the EHA ... with effect on both cold start and acceleration during the warm-up phase.

Fortunately it’s usually the CTS … I suggest to check its resistance at the disconnected ECU connector when the engine is cold. Your car, presumably not being a California version, should be equipped with a 2-pin CTS in front of the FD. In that case check the resistance between ECU connector terminal 21 & ground. … Btw, disconnect & reconnect the ECU connector always with ignition switched off !

So, in your case there may be a problem with the holding pressure and the CTS signal.

H.D.
Unfortunately the CTS in front of the engine next to the sensor with one pin connector was replaced last year.
I want to tackle the holding pressure issue first then after address the hesitation.

Would please explain the function of the four pin CTS that 281 is inquiring about http://i.ebayimg.com/images/g/iB0AAOSwZVlXmfvx/s-l300.jpg and how to test it
Thanks.
 

·
Premium Member
1991 560 SEC 1994 E500 2014 E350 Cab
Joined
·
7,029 Posts
Gogi that 1 pin is a reading sent to the instrument cluster. The 4 pin in your pic is the reading for the systems of cold start, enrichment, idle control valve, ECU, relay and lambda. It's all considered... and as an edit, the wires are very brittle from the harness.
 

·
Outstanding Contributor
1988 300CE
Joined
·
1,424 Posts
The CTS that has an effect i.a. on cold starts and acceleration during the warm-up phase is either a 2-pin or a 4-pin CTS. In case of a 4-pin CTS its resistance can be checked between the ECU connector terminals 21 & 7.

Here's a list with target values in kΩ:
0°C (32°F): 5.8 – 6.8
5°C (41°F): 4.4 – 5.4
10°C (50°F): 3.6 – 4.3
15°C (59°F): 2.9 – 3.6
20°C (68°F): 2.3 – 2.8
25°C (77°F): 1.9 – 2.3
30°C (86°F): 1.7 – 1.9
35°C (95°F): 1.4 – 1.7
40°C (104°F): 1.1 – 1.4
 

·
Premium Member
1991 560 SEC 1994 E500 2014 E350 Cab
Joined
·
7,029 Posts
Hd trying to send you a private message. It the box is full. Send me a pm when you create space.
 

·
Outstanding Contributor
1988 300CE
Joined
·
1,424 Posts
Hd trying to send you a private message. It the box is full. Send me a pm when you create space.
Yes, the box was full. Now there’s space for new messages again.

Shouldn’t there be an email notification in a case like this ?

Btw, for quite a while now I also haven’t received email notifications as I used to receive when there was a new post in a thread I participated in.
 

·
Registered
1991 560sec. 1969 280SL
Joined
·
987 Posts
Discussion Starter #32
I'm changing the four pin sensor regardless. I removed the connector and checked wiring it looked ok. The wiring did not feel brittle. I don't feel like digging out the ECU to run checks.

So I removed the fuel hose from nipple on the back of the F/ accumulator and plugged it off. After letting the gas drain out of the nipple, I had my friend Cycle the ignition switch three or for times, there was no leakage whatsoever from the diaphragm in the accumulator. Then I performed a fuel pressure test but was unsuccessful as the fitting I had on hand was incorrect and was dripping fuel. With the pumps primed I was seeing 82psi but as soon as th pumps switched off the fuel pressure dropped to zero. I'm not sure if it was all due to the dripping.
So will go to a part store and try to find a bubble fitting to make sure the setup doesn't leak and run another FP test.
Thoughts please. Thanks.
 

·
Premium Member
1991 560 SEC 1994 E500 2014 E350 Cab
Joined
·
7,029 Posts
I think it should be holding pressure and IF I read you right, you are losing pressure. Wait for others, but your accumulator seems suspect if I understand you right. The only time you get good pressure is from a hot engine, with consistent fuel flow/demand right.
 

·
Registered
1991 560sec. 1969 280SL
Joined
·
987 Posts
Discussion Starter #34
I think it should be holding pressure and IF I read you right, you are losing pressure. Wait for others, but your accumulator seems suspect if I understand you right. The only time you get good pressure is from a hot engine, with consistent fuel flow/demand right.
The thing is I checked the accumulator by removing the hose from the nipple, plugging it and energizing the fuel pumps. There was no fuel coming out of the nipple that I had removed the fuel hose from. That means the diaphragm in the accumulator is not ruptured and holding pressure, right?
am I not testing it the right way. From what I understand the FPRegulator also holds pressure in the system as well as the check valves in the fuel pumps.

Any thoughts.
 

·
Outstanding Contributor
1988 300CE
Joined
·
1,424 Posts
I'm changing the four pin sensor regardless. I removed the connector and checked wiring it looked ok. The wiring did not feel brittle. I don't feel like digging out the ECU to run checks ...
The CIS-ECU & the EZL both get input from a double CTS, either a 2-pin or a 4-pin version. In case of a 4-pin CTS one thermistor is interconnected between pins 1 & 3, and the other thermistor is interconnected between pins 2 & 4. By default pins 2 & 4 are connected to the ECU, and pins 1 & 3 are connected to the EZL. But since the pins for each thermistor are positioned diagonally to each other, it actually doesn’t matter how the connector is plugged onto the CTS.

Instead of checking the CTS’s resistance at the ECU connector, you can also measure it directly at the CTS, of course. To check the 4-pin CTS directly, just measure the resistance between pins 1 & 3 and between pins 2 & 4. The target values are the same as listed in post 29.
By measuring the resistance at the ECU or at the EZL, however, the integrity of the connectors and the wires is automatically checked at the same time. … What’s the use of an immaculate sensor if its signal doesn’t reach the unit that processes it?

... checked the accumulator by removing the hose from the nipple, plugging it and energizing the fuel pumps. There was no fuel coming out of the nipple that I had removed the fuel hose from. That means the diaphragm in the accumulator is not ruptured and holding pressure, right? ...
If no fuel was coming out of the nipple on the back side of the accumulator while the fuel pumps ran, it’s holding pressure … at least for 3 seperate seconds. The test would, however, be more realistic with the fuel pumps running for, let’s say, at least 10 consecutive seconds. With the disconnected hose securely blocked and the drain hose securely aiming into a collecting vessel you could bridge the FPR socket with a wire to let the pumps run, or start the engine and see if fuel comes out with the engine running at idle. If then no fuel comes out, you can say that the accumulator is not leaky.

... From what I understand the FPRegulator also holds pressure in the system as well as the check valves in the fuel pumps ...
Neither the fact that no fuel is coming out of the accumulator nipple, nor the fact that there's fuel pressure while the pumps run, means that the fuel pressure regulator and/or the check valves at the pressure port of the fuel pumps are holding the pressure after the pumps are switched off.

The usually underestimated piece of mechanical engineering, that the fuel pressure regulator IMHO is, not only regulates the system pressure (on your car to a range of 90-93 psi) while the fuel pumps run. It also lets the pressure drop to about 50 psi at the moment when the fuel pumps are switched off. These are 2 separate functions of the pressure regulator. It may perfectly ensure the correct system pressure while it can not ensure the correct or any holding pressure anymore.

The check valves are, of course, supposed to open with pressure coming from the pumps, so that there is pressure at the FD. But the question is if they immediately & properly close when the pumps are switched off.

Btw … the quicker the pressure drops to zero as soon as the fuel pumps are switched off, properly connected test equipment provided, the more likely the check valves on both pumps are bad.

H.D.
 

·
Registered
1991 560sec. 1969 280SL
Joined
·
987 Posts
Discussion Starter #36
The usually underestimated piece of mechanical engineering, that the fuel pressure regulator IMHO is, not only regulates the system pressure (on your car to a range of 90-93 psi) while the fuel pumps run. It also lets the pressure drop to about 50 psi at the moment when the fuel pumps are switched off. These are 2 separate functions of the pressure regulator. It may perfectly ensure the correct system pressure while it can not ensure the correct or any holding pressure anymore.

So are you suggesting that we can rule out the FPR. Also I double checked the sensor plate, with cold engine as per previous instructions ignition off and it stays down when released. If I turn the ignition switch on it instantly pops right back to rest position. When the system is primed and ignition off I can push down on the S. Plate and it will return to rest once or twice before the system loses pressure.

The check valves are, of course, supposed to open with pressure coming from the pumps, so that there is pressure at the FD. But the question is if they immediately & properly close when the pumps are switched off.

Btw … the quicker the pressure drops to zero as soon as the fuel pumps are switched off, properly connected test equipment provided, the more likely the check valves on both pumps are bad.

H.D.[/QUOTE]

I just have a hard time accepting both check valves are bad at the same time.
Could not find the correct fitting to use for the cold start port of the FD that needs to be a bubble fitting. That's where the setup was dripping.

Comments and suggestions please:smile
Kind regards.
 

·
Outstanding Contributor
350SDL, '17 GLS450, "Grandpa's Roadster" Project Car
Joined
·
3,030 Posts
I just have a hard time accepting both check valves are bad at the same time.
The first one could have failed 10 years ago. You'd never know until the second one fails...

Had a customer tell me it couldn't be the bulbs because both headlights went out at the same time. Guess what? Both bulbs were burned out...
 

·
Outstanding Contributor
1988 300CE
Joined
·
1,424 Posts
... So are you suggesting that we can rule out the FPR ...
No ... but what you reported so far doesn't put the pressure regulator on top of my list of suspects.

If, when the fuel pumps are switched off, the pressure drop down to zero takes a few seconds, it could i.a. be the pressure regulator. But if, with a good accumulator and properly connected test equipment, the pressure drops to zero immediately when the pumps are switched off, it’s very probably both check valves.

... Also I double checked the sensor plate, with cold engine as per previous instructions ignition off and it stays down when released. If I turn the ignition switch on it instantly pops right back to rest position. When the system is primed and ignition off I can push down on the S. Plate and it will return to rest once or twice before the system loses pressure ...
The AFM plate should not get stuck at any position. It getting stuck indicates a mechanical problem with the AFM itself. But as long as there are no resistance jumps when the AFM plate is being pushed down !, that’s not causing the cold start problems. … And what’s making it pop back to its rest position when you switch on the fuel pumps, is the CP with full system pressure on its top.

... I just have a hard time accepting both check valves are bad at the same time ...
See what John350 said. … The fuel pumps (with their check valves) are hydraulically switched in series. :wink_2:

H.D.
 

·
Registered
1991 560sec. 1969 280SL
Joined
·
987 Posts
Discussion Starter #39
The first one could have failed 10 years ago. You'd never know until the second one fails...

Had a customer tell me it couldn't be the bulbs because both headlights went out at the same time. Guess what? Both bulbs were burned out...
Good point John. I ran the engine, Switched it off and, immediately loosened the CSV port at the FD although there was a little pressure, not much fuel came out, certainly not 85psi worth. As soon as I get a chance probably tomorrow I'll recheck the accumulator if that's good for sure then I will replace bothe the check valves.

Opinions please. Thanking you all.
 

·
Registered
w126
Joined
·
1,535 Posts
not much fuel came out, certainly not 85psi worth.
With everything working correctly, the pressure will drop to 40-50psi immediately without any help. You can't really equate pressure to volume.

You need a non leaking connection at the CSV port with your guage. That is the test point for residual pressure.
 
21 - 40 of 83 Posts
Top