I noted on another thread that I was about to replace the CPS in my 08 W211 4matic for a no start issue. I've had to do this previously for other MB's I've had, specifically a W210 and a W211. I got the sensor, replaced it, and my issue was resolved for one night and the next morning. Now when I go to start the car, I get the same behavior again, with the car sounding like it wants to turn over, then eventually just giving up and not starting.

I've tried to pull obd codes, but none are getting stored (yet, at least). Does anyone have any thoughts where I should go from here from a troubleshooting standpoint?

 

a shot in the dark...my wifes Nissan had a recall on a CPS...of course Nissan wouldn't honor it...but I digress. So pulled old CPS put new one on...ran great...for a day. Then started stumbling again....sure enough new CPS was bad...so bought another one...fixed it...at least that particular problem. Part of the recall for the CPS was as well Cam Position Sensor too....very same part number and sensor. So changed that just to be safe and I was in the area anyways. Lo and behold after doing those I get fault codes for cylinder misfire. Move coil pack to different cylinder...problem code followed. Replaced coil. Sure enough...another misfire code...different cylinder. Replaced that coil too. Ran like a champ. I can't even imagine what the dealer woulda charged for all that. Either way....happened after she made her last car payment. Hope this helps...

 

Another shot in the dark. Did you replace it with a MB CPS? Some have had trouble using a aftermarket CPS.
Mike T.

 

Bosch cps is reliable. FCP Euro has them, or amazon. If that's not it, check your fuel pumps / relay / pressure.

 

Update - I replaced the CPS again, this time with one from the dealer (the first one was from advance Auto). Identical behavior. The car ran fine for two start/stop cycles, and this morning I tried to fire it back up and the car won't start, and no codes being thrown.

Something is causing the car to eat the CPS. Before I send to a mechanic, any last thoughts? I don't mind changing the cam sensor, but would hate to throw good money after bad.

 

When you say "No codes" is that with the OBD2 scanner or a C3/C4 STAR scanner? As to other issues it could be....to me...def. sounds electrical. Have you...disconnected main battery for 5 minutes and reattached and restart and let car idle for 10 minutes as it reprograms/relearns itself? Kinda sounds like a MAF.....not saying it is...I'd def. buy a C3/C4 machine if you don't already have one. That it starts and runs fine for 2 cycles tells me it's got air, fuel and ignition. The last one being where I think I'd start....again...C3/C4...I'm new to Mercedes so sorry I am not of more help

 

no codes means no OBD2 codes, although I can induce a P0335 code if I disconnect the CPS and try to start the car. I don't have a C3/C4 (yet) but have been strongly considering getting one from the interwebs, since I already have a Dell D630 sitting around gathering dust and the cost of the scanner would probably be less than the cost of a visit to a dealership or an indie with a Star system.

I have not tried disconnecting the battery, but I can try that. Given that I suspect the car is eating the CPS sensors, I suspect it will not make a difference, but I will try and report back.

 

I agree with starving.. check fuel pressures. I had a no start issue also along with CPS issue at the same time. This was with my GL450. I changed the CPS a couple times until I figured it was the fuel pump. There is a way to check the CPS so maybe yo should check it

 

Tried disconnecting the battery. No joy except now I get to set my clock to the right time for DST.

 

You don't need to go as far as a C3/C4 system. There are several other systems out there that will read MB specific codes. A few that come to mind are Carly, icarsoft, Autel, X431 etc. Carly and the X431 are probably the cheapest of the bunch, I think you get a free initial scan with Carly. I use the X431. If you search the forum for scanner, you'll probably find a few others.

 

fuel pump/relay, check. Or your "Me" could be faulty.... Voltage drop test would be my next step after you've ruled out fuel pump.

 

Not saying you're wrong, but if it is the fuel pump or relay, why would a fresh CPS get things going again? How are they related and how do they interact with each other?

When you say a "Me" error, I presume you're saying I'm making a mistake in process and overlooking it? Or an actual component problem?

 

Thanks cetialpha5. Will look into those.

 

Check and see if your fuel rail has a Schrader valve on it. If so, just thread on a fuel pressure gauge and tape it to your windshield. I used this method to troubleshoot a fuel issue on my E55.

 

So I got a fuel pressure gauge from Autozone and hooked it up. when I turn the key to the on position, it does not appear the fuel rail is getting pressurized. Should the system be priming the fuel rail as soon as I turn the key on? I wondered if maybe I was just connecting the gauge wrong and thus not getting a reading, but after an (unsuccessful) attempted start, I manually pressed in the pin on the schrader valve, and just a few drops of gas dribbled out. It seems to me like the pump is not sending gas.

My understanding is the CPS tells the fuel pump to start sending gas via a signal sent to the ECU. If my CPS is faulty for whatever reason, would it also keep the pump from priming the fuel rail?

Is it possible a bad fuel pump can send bad info back to the CPS and make it go bad?

 

Things got "going again" because vehicle cooled down. If you have no pressure at rail you need to figure out why. Pumps are suppose to prime for a split second when key is turned on. You'll get a voltage reading of 11.5/12 volts for a second or so then it drops to zero, only with key on though, not during cranking. During cranking voltage is suppose to drop as well, but that's another story. My suggestion is find out if pumps are getting/sending voltage. Once you've figured out that they are doing what they are suppose to do you then can chase the "ME" motor electronic or as some call it ecm issue.
you have either a bad fuel pump, faulty relay, faulty rear sam or a fried ME.

 

Update - I took the battery to a parts store a couple of days ago to get it charged, and the battery is apparently failed (wouldn't accept a charge from O'reilly's machine).

I presume there is still something else downstream of the battery causing the no start condition, but have not had a chance to get the car into the mechanic yet.

Could a bad battery have damaged the ECM?

 

check to see if your intake manifold flap is broken. it's the trianglular plastic piece

 

NO. A bad ME is due to mechanical defect and or whoever has owned it before you.....

those are not going to cause a no start drive-ability issue

 

UPDATE!

TL;DR - core problem was the connector to the crankshaft position sensor, but it turns out I also had an intermittently failing fuel pump.

I finally gave up and took the car to a local independent that has a star diagnostic. After doing extensive troubleshooting on my own I had resigned myself to the idea that I would need a new ECM / ME, but I wanted to get confirmation before I dropped that much money on a part.

I had the car towed to the shop, and they called me back the next day with their findings. The problem was actually the connector to the crankshaft sensor. It was providing an intermittent connection to the crankshaft sensor. Thus why it would work when I replaced a sensor, but eventually would return to its original no start condition. They ordered a replacement connector, cut off the old one and soldered the new one on, and it started consistently with no problem.

But wait! There's more!

After they replaced the part, the car went back to occasionally not starting. The shop checked the pressure at the fuel rail and there was none. It turns out simultaneously, the fuel pump is intermittently seizing up. They recommended replacing both the pump and sender, which would have cost about $850ish for parts and labor. I pointed out Mercedes had extended the warranty on the tank and pumps, and the shop guy told me he would check in with his foreman and call me back.

I got a call back a couple hours later. It turns out the pumps are covered for the leaking fuel issue only. If the pump just randomly dies one day, it is not covered under the letter of the warranty extension. Lucky (?) me though, they took a look at the pump and there was fuel pooling on top of it. So not only was it seizing up (not covered) but it was exhibiting the failure for which Mercedes extended the warranty (covered).

I picked up the car and took it to the dealership. My service advisor did not seem at all concerned about the failure of the pump vis a vis leaking vs. seizing, but YMMV depending on your dealership if you run into this issue. Dealer replaced the tank assembly, which included the pump, sender, tank, and a bunch of other miscellaneous associated parts.

I'm back on the road again!

 

YUP! Thats a good look, that SA is smart, especially if he cared less about them failing. a leak is a leak. pooling is pooling.