my c55 been losing power for years, but every once in a while starts with much louder sound and drives like 6.2l n/a; very powerful and responsive, until couple mins later starts to warm up and rolls off again.

i have been trying to figure it out and recently discovered that by disconnecting what i assume is MAP sensor, or at very least something that controls vacuum action inside manifold, that car goes a lot better, also gets around 27mpg on highway, being more than it is supposed to.

i have ecu tune, larger airbox, mid length headers, no cats.

anybody have an idea what might be broken or offline then, if doing this seems to correct it?

 

That is indeed the MAP sensor but it doesn't control vacuum in the intake manifold. My guess is your car is running open loop off of a preset fixed fuel map with the MAP sensor disconnected, which would indicate that you do have a running issue that is affecting your closed loop operation. I believe I gave you a similar answer before when you had a bad MAF sensor. I don't think you mentioned then that you have a tune. Did your tune take into account that you are running headers and are catless?

 

wow, impressive memory, bru.

yea, scan tool monitors 'map kpa' while driving, and when plugged in, fluctuates constantly, lowest around 12 on hard decel, and apparently 27 is normal atmospheric pressure at sea level.

when unplugged, is stuck on 133, which is presumably max, and throws p0108 code.

unplugged, delivers extra sensitive throttle response, both accel and decel, and after long 800km trip, does not seem to be hurting anything.

i wonder if you would be willing to try same, and let me know if makes any difference your end...

sometimes, i also get 'esp unavail' on dash, which also seems to match when best throttle action happens.

tune is by v-tech, who describe its contents in file name, so does have headers and decat.

as to vacuum; when unplug the rubber line with blue band on it, next to the map sensor in pic, there is vacuum, so will try blocking to see what happens.

yea, i know this may all be waste of time, but basically gave up on this car, and bought faster amg's, so just mucking around trying to understand it.

 

There's supposed to be vacuum on the line to the secondary air switchover valve. That "blue band" is a check valve. By blocking that line, you are only going to introduce a problem into your secondary air system. Did your problems start before the v-tech tune?

Have you done any basic troubleshooting like looking for vacuum leaks? Are you able to revert back to your stock tune? Poking around unplugging this or that to see what happens isn't likely to help you find what your problem is.

Your v-tech tune throws a wrench into an already low probability situation of anyone being able to help you diagnose your problem over the internet. There's a possibility that the problem is in the tune and no amount of hardware trouble shooting would help.

 

I should qualify this statement by stating that without knowing how bad the "rolloff" in power/responsiveness is after you enter closed loop operation, it was premature for me to suggest that you have a running issue. To an extent, the feeling of rolling off of power/responsiveness after warmup can be normal depending on a variety of factors like your driving style, quality of your gas, etc.

 

doubt any vacuum leaks, but i could be wrong.

i have owned this car 6 years now, and was very lively at first.

not sure if cutting cats off, then adding tune and headers had anything to do with its decline.

per your suggestion, i have reached out to v-tech, for copy of stock ecu file.

basically, i am wondering if the intake manifold itself does not function properly, as i have read there are baffles inside, and that very difficult job to replace, OR, maybe fuel pressure sensor needs replacing...

you sound very knowledgeable, so wonder; do you think i have symptoms of either those ailments?

if manifold is toast, would be sign from above to throw kompressor and spares that came off my sl55 when modded that, on instead.

thx.

 

You have to be willing to go through the trouble shooting steps to eliminate problems. A vacuum test isn't difficult to do. It can be enjoyable if you like cigars.

Your intake manifold runner flaps may not function properly if you have a vacuum leak from the rubber vacuum line going to the flap vacuum actuator. To test that line, you have to remove the line to the actuator and introduce pressure/smoke from that end. There is an internal check valve that would prevent smoke/pressure from revealing itself at that line if introduced from anywhere else, as you otherwise would to check the rest of the system for vacuum leaks. Short of opening up the intake manifold (which is HIGHLY not recommended), the only other test you can do is apply vacuum from a hand vacuum pump to the vacuum actuator and see if the arm moves. Also, if you apply vacuum to the intake manifold vacuum line, it should hold vacuum. If it does not, you have a problem with the internal check valve or a leak in the line. Worst case scenario, replacing the intake manifold is not that hard, if you are handy.

Edit: Perhaps you can remove the throttle body and insert an inspection camera into the intake manifold to inspect the runner flaps while actuating the flaps with vacuum. I have not tried and am not certain the flaps are visible in this manner.

By fuel pressure sensor, do you mean fuel pressure regulator? The FPR is integrated into the fuel filter. How many miles/kms does your car have and when was the last time the fuel filter/pump was serviced?

What scan tool are you using to pull codes? Do you have a Mercedes specific scanner that can pull more than universal engine codes? What are you short terms and long term fuel trims?

Without further troubleshooting on your part, I can't guess whether your problem presents as a problem with the intake manifold or FPR.

 

love these fellow 55 Naturally Aspirated owners that cut off their cats, screw up the back pressure, make the car sound like the exhaust has rusted off, and then post about the engine not performing well.