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1991 560sec. 1969 280SL
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Discussion Starter #1
Hello.
How would a 91 560 act if ecu was faulty or even disconnected. Specificly when starting cold. Have checked power to cold start injector with noid lamp its good, set lambda at operating temperature to 50% the only other things I haven't done is check the integrity of the fuel accumilator also haven't been able to check fuel pressure. If I can remove the hose from the nipple on the back side of the F/A and plug it, put a clear hose on there and cycle the I/Switch to prime the system and see if any fuel is leaking out of the nipple past the diaphragm would that save me having to check the fuel pressure.
Any and all thoughts on the matter would be welcome. The car is very hard to start in the AM even after cycling the key several times. Once started runs like a top everything normal.
Any and all thoughts on the matter would be welcome.
Thanks.
 

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1983-2007 MB's
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8,013 Posts
Accumulator.

Does the car stay in high idle or smell rich even if the motor warms up? I would also check the coolant thermoswitch - it is the 4 pin unit

 

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1986/1990 W126
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13,379 Posts
If you mean the injection ECU, mine ran fine once warmed up but until then was like a car with a carb that didnt have choke applied on a cold morning.
 

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Outstanding Contributor
1988 300CE
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1,414 Posts
... How would a 91 560 act if ecu was faulty or even disconnected. Specificly when starting cold. Have checked power to cold start injector with noid lamp its good ...
If it’s faulty, it depends.

If it’s disconnected and everything else is in order, it depends on the temperature if & how it would start, because the CSV would be deactivated. The CSV receives it’s voltage from the FPR, which receives an edited CTS signal from the CIS-ECU. … If it starts it will run as described by ianrandom, because warm-up & acceleration enrichment via EHA control would not take place.

If it’s disconnected and not everything else is in order, it depends.

... If I can remove the hose from the nipple on the back side of the F/A and plug it, put a clear hose on there and cycle the I/Switch to prime the system and see if any fuel is leaking out of the nipple past the diaphragm would that save me having to check the fuel pressure. ...
No, the fuel accumulator is only one of several possible culprits for loss of holding pressure.

H.D.
 

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1991 560sec. 1969 280SL
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971 Posts
Discussion Starter #5
Accumulator.

Does the car stay in high idle or smell rich even if the motor warms up? I would also check the coolant thermoswitch - it is the 4 pin unit

No once started the engine goes into warm up cycle and is ready to drive as normal.

If you mean the injection ECU, mine ran fine once warmed up but until then was like a car with a carb that didnt have choke applied on a cold morning.
Starts for a split second and invariably quits within a seconds but once started runs flawlessly.

HD,
Apart from the F/A and the check valves in both the fuel pumps where else could I be losing residual pressure but, what is confusing to me is; how come cycling the ignition switch is not helping start right away if indeed I'm losing fuel pressure at rest?
I don't know if it's getting too much fuel and not letting it start but depressing the accelerator certainly doesn't help. Waiting five minutes seems to help. Need guidance from you good people.

I really appreciate all of you taking time to address my query. Thank you.
 

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Outstanding Contributor
1988 300CE
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1,414 Posts
... Apart from the F/A and the check valves in both the fuel pumps where else could I be losing residual pressure ...
Residual pressure could also be lost in the pressure regulator, via CSV, via (at least one of the) injectors, or because of a problem with the axial seal ring for the fuel distributor’s control plunger (worn, or not reached by the control plunger with the engine switched off, for instance because the Lambda adjustment screw has been turned too far cw. Or there could be an external leakage somewhere between the pumps and the pressure regulator, which would come along with a smell of fuel though.

... how come cycling the ignition switch is not helping start right away if indeed I'm losing fuel pressure at rest? ...
If the fuel pressure loss is related to the control plunger seal ring, cycling the ignition switch might even be counterproductive. … But there could also be other reasons for the starting problem. There could be deposits on the backside of the intake valves and in the last part of intake ports. In that case the first injected fuel is (partly) absorbed by these (in the morning dry) deposits, resulting in a poorly atomized air/fuel mixture. … Both cases could be a possible reason why waiting five minutes seems to help.

H.D.
 

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Mercedes-Benz 190E 2.6, 300E, Volkswagen Jetta 1.4T
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909 Posts
No once started the engine goes into warm up cycle and is ready to drive as normal.

Starts for a split second and invariably quits within a seconds but once started runs flawlessly.
My fuel pump relay was causing this in my 420SEL, hasn't happened since the URO replay melt to socket debacle I had one morning. Make sure you get one with the word "Kickd" or you're not gonna get first gear when you sink your foot into the carpet. Also avoid Kaehler relays as they will keep the car from taking 2nd gear after kickdown and cause the car to 'redline' at about 4500 RPM (which is very scary when you are trying to overtake someone doing 15 MPH on an onramp to a highway where traffic is doing 60+).

I'm sure someone could chime in on FPR testing, but really this should be one of your starting points for troubleshooting, not the Jetronic computer. Start simple, then escalate. Also, I assume that you replaced your fuel filter, if not then what are you waiting for?
 

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1991 560sec. 1969 280SL
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971 Posts
Discussion Starter #8
The ICV was checked at operating temp. to make sure rpm's went up when disconnected tested ok. Engine has no idling problems at any temp.
The fuel filter was replaced a thousand miles ago and no problems before or after installation. This just started suddenly a few weeks ago. After this started happening I changed the FP Relay as I had a known good mercedes brand one from a 1989 560.
If it was carbon build up in the intake plenum or on the inlet valves wouldn't the symptoms show themselves gradually?
With the engine off, cold or hot In a normal operating CIS fuel system when the sensor plate is pushed in a downward direction and released will it spring back to rest position.

Thanks for your response.
 

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1988 300CE
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1,414 Posts
... If it was carbon build up in the intake plenum or on the inlet valves wouldn't the symptoms show themselves gradually? ...
Certainly.

... With the engine off, cold or hot In a normal operating CIS fuel system when the sensor plate is pushed in a downward direction and released will it spring back to rest position ...
Correct. … But that doesn’t necessarily mean that the control plunger follows the sensor plate’s lever all the way back to its rest possition (on top of the axial seal ring).
 

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1991 560sec. 1969 280SL
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Discussion Starter #10
Certainly.


Correct. … But that doesn’t necessarily mean that the control plunger follows the sensor plate’s lever all the way back to its rest possition (on top of the axial seal ring).
Ok when I checked the the sensor plate, I believe it was with cool engine the plate would not spring back up. Could that be due to loss of residual fuel pressure. If so what component would be the likely Culprit. Or could it be any of the ones previously discussed such as the FA, F.P.Regulator,the check valves and maybe even part of the fuel distributor. There's nothing better than an informed opinion.
I'm amazed by the quick responses and the knowledge being put forth to solve this problem of mine.
 

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1988 300CE
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1,414 Posts
... when I checked the the sensor plate, I believe it was with cool engine the plate would not spring back up. Could that be due to loss of residual fuel pressure ...
No, that is not fuel pressure related. That has solely to do with the air flow meter’s mechanism. If you push down the meter’s plate by hand … even with the fuel pumps running via jumper wire in the FPR socket (in other words, with full system pressure) … and you let go of the plate, it should imediately and freely jump back to its rest positon, and the control plunger would be left behind by the meter’s lever, more slowly moving back to its rest position on top of the axial seal ring, unless it gets stuck before it reaches the seal ring, for instance due to contamination in the fuel distributor.

... could it be any of the ones previously discussed such as the FA, F.P.Regulator,the check valves and maybe even part of the fuel distributor ...
Could be. ... But before wondering which component might be causing a holding pressure loss, I wonder if there is a holding pressure loss … and suggest to test it.

H.D.
 

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1991 560sec. 1969 280SL
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Discussion Starter #12
But before wondering which component might be causing a holding pressure loss, I wonder if there is a holding pressure loss … and suggest to test it.

H.D.[/QUOTE]

So I just went to the car and ran it for a few minuets shut off the engine and then immediately removed the air cleaner cover and pressed down on the sensor plate. The plate returned to its rest position. So, can we say, since the S. Plate did not return to rest position with completely cold engine and it does return shortly after running that it is not a FD problem?
 

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1988 300CE
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... So, can we say, since the S. Plate did not return to rest position with completely cold engine and it does return shortly after running that it is not a FD problem?
The air flow meter plate not returning to its rest position has nothing to do with the FD anyway, no matter whether the engine is cold or warm, or whether the fuel pumps are running or not. … If it doesn’t quickly & freely move back to its rest position and gets stuck somewhere, there’s a mechanical problem with the air flow meter ... which the weak force of the slowly descending control plunger, pushing on the other end of the meter's lever, can not overcome.
 

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1991 560sec. 1969 280SL
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Discussion Starter #14
The air flow meter plate not returning to its rest position has nothing to do with the FD anyway, no matter whether the engine is cold or warm, or whether the fuel pumps are running or not. … If it doesn’t quickly & freely move back to its rest position and gets stuck somewhere, there’s a mechanical problem with the air flow meter ... which the weak force of the slowly descending control plunger, pushing on the other end of the meter's lever, can not overcome.
So since the engine runs strong and has no drivability problems only cold start problems what are your conclusions. Should I check the F/ Accumulator and take it from there, or do I have to man up and do a FP test. That's if I can find the fitting.
 

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1988 300CE
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A test of the fuel system’s holding pressure covers all possible culprits for pressure loss, and is a lot more comfortable to do, IMHO. … On the other hand, if you find out that there is a lack of holding pressure, depending on how quickly the pressure drops, checking the accumulator may than be the next logical step anyway. … So, you decide. :)

H.D.
 

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1991 560sec. 1969 280SL
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Discussion Starter #16
I'm going to check the fuel accumulator as I had it in mind to do that anyway. I'll report back as soon as I do that. Thank you very much for your patience and great information.
 

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1988 300CE
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Okay … and if the accumulator is not leaky, and a holding pressure test isn’t very convenient for you, you might first take a closer look at the air flow meter’s & control plunger’s ease of movement when the engine is cold … takes only a few minutes.
 

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1991 560sec. 1969 280SL
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Discussion Starter #18
What's the procedure to check the plunger? Why would the plunger or sensor plate be unable to return to rest with cold engine?

Thanks
 

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What's the procedure to check the plunger? Why would the plunger or sensor plate be unable to return to rest with cold engine?

Thanks

Sludge. Years of gunk build up.

You have to remove it to clean the inside where the mechanism is located.
 

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1991 560sec. 1969 280SL
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Discussion Starter #20
Thanks I will tackle that as soon as I get a chance. The weather is beautiful here today so I'm going to enjoy it with the family.
Tanks again. ?
 
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