[ColoradoBroker]

2000 SL500 - Having trouble with consistent starting. Fault code was for a bad Crankshaft Position (hall) Sensor which I replaced with a new Bosch part. Noted that the old sensor had some engine oil around the pins. Cleaned the female connector on the harness plug, installed the new CPS, and cleared the fault code. This seemed to eliminate the hard start issue for several days, but it came back and presented the same CPS fault code. New CPS checks out fine so it would seem to be in the wiring between the CPS and ECU. Any recommendations on how to check for continuity, ground fault or other issue? Thanks!

 

[RESTO SL]

Highly unlikely. CPS failure doesn't present as a hard start condition.

Need the back story, exactly what behavior is the car exhibiting and what codes are present and historic?

 

[Teutone]

2000 SL500 - Having trouble with consistent starting. Fault code was for a bad Crankshaft Position (hall) Sensor which I replaced with a new Bosch part. Noted that the old sensor had some engine oil around the pins. Cleaned the female connector on the harness plug, installed the new CPS, and cleared the fault code. This seemed to eliminate the hard start issue for several days, but it came back and presented the same CPS fault code. New CPS checks out fine so it would seem to be in the wiring between the CPS and ECU. Any recommendations on how to check for continuity, ground fault or other issue? Thanks!

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[ColoradoBroker]

Highly unlikely. CPS failure doesn't present as a hard start condition.

Need the back story, exactly what behavior is the car exhibiting and what codes are present and historic?

Thanks for your response/question. Sorry...it was the CAMSHAFT Position Sensor that I replaced based upon the fault code. The M113 engine appears to have only one Camshaft Position Sensor located on the front passenger-side bank. If I recall correctly, it was a P0340 fault code which I had erased after I installed the new Camshaft Position Sensor. If that fault code is incorrect, I will check it again in the morning and circle back. The behavior was/is that the engine will start on the first turn of the key in the ignition most times, but occasionally (intermittently) would simply crank for an extended time and sputter without actually starting. When this happens, it usually takes 3 or 4 turns of the key in the ignition before it will finally start. It seems that if I turn the key to position 2 and let it sit there for 10-15 seconds before attempting to start, it is more likely to start on the first time. The fuel pump and fuel filter are both new, so I don't think it is a lack of fuel or fuel line pressure issue, but I could be wrong. I did replace the CRANKSHAFT Position Sensor as well, but didn't pull a fault code for that...just did it based upon some informational threads that I read in this forum as well as some YouTube videos that I watched which described a very similar issue. Ambient temperature (cold/hot) or engine temperature (cold/medium/hot) don't seem to really make a difference. It is an intermittent issue without regard to ambient or engine temperature. I told my Mercedes mechanic about it when I had the car in his shop on Friday for an alignment. He said that he tested the new Camshaft Position Sensor and found that it was fine. He thinks it is likely the wiring between the sensor and the ECU that is the likely issue...either a broken wire or a ground fault...or possibly the little bit of motor oil contamination that I found in the connection pins when I removed the old Camshaft position sensor. I did use an electrical connection cleaner on the female wiring harness connector before installing the new Camshaft Position Sensor. I hope that all of the foregoing makes sense.
So I think that I need to check for continuity or ground fault in the wiring between the Camshaft Position Sensor and the ECU, but I am at a loss as how to do that easily. Anything that you can offer up on this would be greatly appreciated. Thanks!

 

[RESTO SL]

After a hard start, how does it run?

 

[Photodude]

I highly doubt Camshaft position sensor has anything to do with hard starting. Engine should run perfectly fine with no Camshaft sensor signal. ME controller will “guess” if signal is on the compression or exhaust stroke… and be right 50% of the time.

Code P0340 would indicate issue with Cam sensor electrical circuit. So, you might want to test continuity between sensor connector and corresponding pins on the engine control module connector but, this would be side issue IMO.

To diagnose hard starting, I would focus on more likely suspects… fuel pressure, ignition components…

 

[Ken S]

I agree with Photodude, you may have two separate issues at the same time.
How many miles are on the car? Did it have the same problem with the old fuel pump and filter?

My first thought is to check fuel pressure in a variety of typical circumstances to see if it varies:
Check fuel pressure with cold engine not running but fuel pump running, then with cold engine running, then with hot engine running, then hot engine but sitting for 5 minutes with engine (and fuel pump) not running and then hot engine but sitting for 30 minutes with engine (and fuel pump) not running to see how quickly pressure is bleeding out of the system. As always be carefull with gasoline, but the fuel pressure access port is pretty simple and tidy with the correct type of tester or you could have a shop do the tests if needed.

If fuel pressure drops too low, it could be leaky injectors (causing hard starting when hot because of too much fuel), or a bad fuel pressure regulator or fuel pump check valve (causing hard starting because not enough fuel).

Another test you could possibly do is to have someone else start the car (in a typical hard-start situation) and stand behind the car see if any black smoke comes out of the tailpipe during a hard-start indicating too much fuel.

 

[ColoradoBroker]

After a hard start, how does it run?

Seems to run great after a hard start.

 

[ColoradoBroker]

I highly doubt Camshaft position sensor has anything to do with hard starting. Engine should run perfectly fine with no Camshaft sensor signal. ME controller will “guess” if signal is on the compression or exhaust stroke… and be right 50% of the time.

Code P0340 would indicate issue with Cam sensor electrical circuit. So, you might want to test continuity between sensor connector and corresponding pins on the engine control module connector but, this would be side issue IMO.

To diagnose hard starting, I would focus on more likely suspects… fuel pressure, ignition components…

Thanks for your questions/input. Prior owner installed a new fuel pump and fuel filter within the last 6 months...also new spark plugs. However, I will try obtaining a fuel pressure gauge and checking fuel pressure at the engine. I'm also looking for recommendations on how to check continuity between the Camshaft position sensor and the ECU. I don't know what 3 wires/terminals at the ECU are coming from the Camshaft positions sensor.

Seems to run great after a hard start.

A lot of miles on the car...176,500. Other than the intermittent hard start issue, it really runs great. I'm personally impressed that it runs so well with this many miles on it. Everything that I have read about the M113 Ri29 says that this car was very well engineered and will run a lot of miles with proper maintenance.

 

[ColoradoBroker]

I agree with Photodude, you may have two separate issues at the same time.
How many miles are on the car? Did it have the same problem with the old fuel pump and filter?

My first thought is to check fuel pressure in a variety of typical circumstances to see if it varies:
Check fuel pressure with cold engine not running but fuel pump running, then with cold engine running, then with hot engine running, then hot engine but sitting for 5 minutes with engine (and fuel pump) not running and then hot engine but sitting for 30 minutes with engine (and fuel pump) not running to see how quickly pressure is bleeding out of the system. As always be carefull with gasoline, but the fuel pressure access port is pretty simple and tidy with the correct type of tester or you could have a shop do the tests if needed.

If fuel pressure drops too low, it could be leaky injectors (causing hard starting when hot because of too much fuel), or a bad fuel pressure regulator or fuel pump check valve (causing hard starting because not enough fuel).

Another test you could possibly do is to have someone else start the car (in a typical hard-start situation) and stand behind the car see if any black smoke comes out of the tailpipe during a hard-start indicating too much fuel.

Thanks for your reply. A lot of miles on the car...176,500. Other than the intermittent hard start issue, it really runs great. I'm personally impressed that it runs so well with this many miles on it...once it gets started! Everything that I have read about the M113 R129 says that this car was very well engineered and will run a lot of miles with proper maintenance. I don't know how the car acted prior to the new fuel pump and fuel filter...those parts were replaced by the previous owner. I verified that he also replaced all of the spark plugs, the Camshaft and Crankshaft position sensors (which I have also replaced)...so I am assuming that he was trying to correct the hard start issue as well. I will check the fuel pressure as you have recommended.


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[Ken S]

When you have a chance, perhaps remove the cover shield for the fuel pump and take a picture of the pump and post it here (assuming the PO didn't give you the receipt with the part number & brand) to rule out an incorrect pump.

 

[Photodude]

I'm also looking for recommendations on how to check continuity between the Camshaft position sensor and the ECU.

See attachment below for Camshaft Position Sensor electrical testing.

As for “hard starting”… what does this actually mean? Can you post the video what it looks/sounds like?

Did you know that your car has one touch starter function? Meaning you just turn the key into the start position and release… letting the ECU control the starter time out.

 

[ColoradoBroker]

See attachment below for Camshaft Position Sensor electrical testing.

As for “hard starting”… what does this actually mean? Can you post the video what it looks/sounds like?

Did you know that your car has one touch starter function? Meaning you just turn the key into the start position and release… letting the ECU control the starter time out.

Thanks for the test chart attachment...I've printed it and will see if I can decipher it.
As I had posted earlier in this thread...The (hard starting) behavior was/is that the engine will start on the first turn of the key in the ignition most times, but occasionally (intermittently) would simply crank for an extended time (letting the ECU control the starter time out) and the engine will sputter a few times without actually starting. When this happens, it usually takes 3 or 4 turns of the key in the ignition before it will finally start. It seems that if I turn the key to position 2 and let it sit there for 10-15 seconds before attempting to start, it is more likely to start on the first time. I hope this makes sense.

 

[Photodude]

I know you’ve mentioned that PO replaced fuel pump, filter etc. However I wouldn’t assume anything. What you’re describing sounds a lot like fuel problem so I would check the basics at the minimum. Like checking the fuel pressure at the fuel rail test terminal, right front, there is black cap covering the Schrader valve where you connect fuel pressure gauge.

The pressure should build up immediately when you turn the key ON, before you even start the engine. At idle the pressure should be 3.7-4.2 bar (54-61 psi). Snap at the accelerator (or throttle cable inside the engine compartment) few times, to ~4000 rpm, the pressure should never drop below 3.7 bar. When you turn off the engine the pressure should still hold for several minutes at least.

You could also test the fuel delivery rate by disconnecting fuel rail return line and measure the volume. It needs to be 1liter in > 35 seconds. But this method is more mess and less information. Pressure test is the way to go, IMO.

You also mentioned new spark plugs being installed by PO. Again, I wouldn’t trust without verifying. Remove one or two and check if they are correct type, at least. How do the spark plug wires look? I don’t necessarily suspect ignition components here but, spark plugs and wires are pretty much impossible to test… when in doubt replace them. Ignition coils are either good or bad… when everything else is OK… bad coils can be isolated by swapping them around.