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w203 c320 avantgarde auto saloon
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Discussion Starter #1
Hi all

Need help again

Car. C320 2001 95k

Since I own car revs fluctuate on idle. I used help from this forum and I managed to smooth them out. However revs still bouncing up and down. I noticed that when tthat pump (I think it's vacuum pump)comes on revs star to fluctuate and I can feel vibrations. I thought it's cooling fan but I am sure now it's this pump. Also purge valve keeps ticking. When is nice and hot outside revs are almost smooth as should be only pump comes sometimes. When colder this vacuum pump comes more often causing revs jumping up and down and car gets vibrations.
Is it normal that this pump has to come on and off?

No misfires, no codes

I sealed air intake which was causing misfire, new filters, cleaned maf, throttle body etc etc <- improved a lot.

Can u help please?
 

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w203 c320 avantgarde auto saloon
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Discussion Starter #3
Well I think it's vacuum pump. I guessed it looking at vacuum diagram for m112 engine. It is located right on the top front of the engine under air box cover. As I said when this comes on revs start bouncing up and down for some time. It only kick in when I am in traffic. Also I worked out it usually is on for few seconds and makes noise like fan is working when coolant temp on displayer shows 60C and arround 85C-90C. I know it sound strange but knowing what is it for and why it comes on can give answer?
 

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If the air pump is coming on after initial startup, then something is wrong. It only operates at idle, and when the engine is cold. I'd check (i.e. replace) the vacuum hoses that feed the shut-off valve.
 

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w203 c320 avantgarde auto saloon
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Discussion Starter #7
Thanks guys for quick input


Oh I said vacuum pump sorry. Yes I meant electric air pump.

What size in mm is that vacuum hose rudeney? The one u saying to replace
 

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w203 c320 avantgarde auto saloon
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Discussion Starter #8
I looked at .pdf now

Yes air pump m33.

So. I have to check or replace vacuum hoses between these two secondary air injection shutoff valves no. 126 yes? Any others? Ones going to air pump or?

Thanks
 

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2002 C240
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If the air pump is running when it's not supposed to, I would be looking for an electrical issue here.
Let's say if coolant temp sensor is giving you erratic readings, it could explain both the rough idle and why ECU turns on the air pump. With engine cold, ECU will try to give it a richer air/flue mixture.
I would hook up the scanner and drive it like that until you get those revs again then look at the coolant temp reading.
Or maybe disconnect the air pump relay and see whether you're still getting the rough idle.
 

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w203 c320 avantgarde auto saloon
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Discussion Starter #10
If the air pump is running when it's not supposed to, I would be looking for an electrical issue here.
Let's say if coolant temp sensor is giving you erratic readings, it could explain both the rough idle and why ECU turns on the air pump. With engine cold, ECU will try to give it a richer air/flue mixture.
I would hook up the scanner and drive it like that until you get those revs again then look at the coolant temp reading.
Or maybe disconnect the air pump relay and see whether you're still getting the rough idle.
i drove with scanner like that. nothing happends, coolant temp is accurate to scanner as well.

so this is electrical fault as you said, not vacuum leak right?
if i will disconnect this pump and revs will stop to fluctuate what that will mean? new air pump or...?

thanks
 

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w203 c320 avantgarde auto saloon
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Discussion Starter #11
ok

just was surfing online.

i found that this pump comes on to drop emmission, feed cats,exhaust with fresh air. i don't understand that but relaying on informations from other members on this forum(found that in other models like e500, e55 amg etc)that means some kind of pump relay might be guilty, sometimes only fuse needs to replace in worst scenario pump itself.however when there is a problem check engine light or codes should come up. no codes in my car.
however nobody mentioned fluctuate revs

secondary air injection pump - MBWorld.org Forums


also found this
http://www.mbwholesaleparts.com/vcm/MBWholesaleParts/Files/Feb09StarTuned-AnotherChance2Burn.pdf

if this pump kicks on in my car to drop emmision what will that mean to me?can u correct and explain to me?thank you
 

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I's till check the vacuum lines. I have no clue what exact size they are, but I think most of them are generic hoses. And I'm talking about the hoses feeding the two vacuum switches - not the large hoses for intake and exhaust of the pump.
 

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Discussion Starter #13
I's till check the vacuum lines. I have no clue what exact size they are, but I think most of them are generic hoses. And I'm talking about the hoses feeding the two vacuum switches - not the large hoses for intake and exhaust of the pump.
do u mean no. 126 on diag right?
two, they look like mushrooms right? hoses say size of 3.2mm or 5mm whatever it is not the thicker hoses.
 

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2002 C240
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Good reading there. It did say that air pump may get activated on warm engine as well.
"The system is also activated during closed-loop
operation, but only to test the system's ability to
pump air into the exhaust -- the ME control unit
looks at the reaction of the O2 sensors."

I would look at O2 sensors as well. Normally they need to be changed every 100k miles.

P.S. I just don't see why vacuum lines would cause anything here. If vacuum line is bad then shut-off valve will not open to let air into exhaust manifold. No harm really.
 

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w203 c320 avantgarde auto saloon
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Discussion Starter #15
How about in your cars? Is this pump come on while engine is hot?or only on startup when cold to help cats to warm up?
Looks like it might come on and is normal regarding to this article but why is it causing revs to bounce up and down?

I checked O2, both banks giving correct reading from 0.2 to 0.7v and with engine off 0.460v which is good. Car passed MOT 6months ago with perfect emissions, then revs were even wore than now. I/m readlines shows all cats and o2 good.

One thing which makes me wonder is MAP. With engine idling 39-40kPa with engine off 0kPa(should be arround 100kPa).
No codes either
 

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Discussion Starter #16
Well I found very similar issue on internet. Fellow had that pump coming on so often causing battery drain.
Anyway he found cracked vacuum hose in his audi a4 4 cyl between combination valve(I have two right?) and solenoid valve. What is solenoid valve in my car? Looking at diagram pump feeds combination valves then from valves it goes to air pump valve. Did he mean that pump valve or something else? Thank you for your help so far
 

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Matt - yes, the vacuum hoses leading to the "mushroom" looking valves.
 

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Discussion Starter #18
I had bit time now. Checked these hoses. Nothing suspicious however the look old and dirty.
What it did was started the engine. Pump came on and I was pulling vac hose from each valve.
I pulled hose from one where other end was on.I could feel great suction via hose, like home vaccum cleaner, where that valve from which hose was removed did not suck.then I attachedit back and did same scenario with second valve and same result.I tried to see how it goes on hose feeding air pump valve but pump only works for 20-30 seconds no I had no chance to check it at this point.
Also I checked this air pump valve vac hose going from valve itself with blue rounded bit( think this is check valve) towards to manifold.hose shows great suction as well.

So if there is good suction throughout these hoses does it mean hoses are ok or it doersn't mean nothing regardless to their condition?

I did few pictures and I will upload them later.
 

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I checked O2, both banks giving correct reading from 0.2 to 0.7v and with engine off 0.460v which is good.
I will try some guesswork again... It's possible that your O2 sensors cannot react fast enough to the air coming in.

With O2 sensors what matters is how fast they cycle from low to high voltage. Bad sensors cycle slowly. Having said that, I've never diagnosed a bad sensor using OBD2.

P.S. I've never noticed my air pump coming on before either cold or hot. I don't drive C240 daily as it's my wife's car.
 

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Maybe it has to do with engine size, but there is no mistaking the air pump operating on my CLK550 - I can hear it running whenever I start the engine from cold. On the C240, I've never heard the pump.
 
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