Major missfire at 2000 revs. Desperate for help now - Mercedes Benz SLK Forum

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#1 Old 03-11-2014
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Major missfire at 2000 revs. Desperate for help now

Firstly, I apologise to anyone who has seen this issue in the introductions section, Im just hoping someone may have the answer if I open it up in the general section. Its a long one, sorry!!

The car starts first time everytime. It ticks over perfectly & will rev upto just before 2000 revs both when parked & when being driven. Then its like two cylinders shut down. It runs VERY rough, engine shakes etc. You can turn off the engine & immediately start it & all is well until the revs hit around 2000 revs again. No warning lights lit on the dash. No gearbox or ABS fault codes etc. Just the following codes - P0300, P0303, P0302, P0341, P1525.

This is how it started......

A couple of months ago now, I decided to sort the slight weep of oil from the front of the engine, I replaced the cam sensor magnet with a genuine one & fitted it with the genuine Merc sealant. The electrical plug that fits onto it was slightly damaged, so this was replaced & the breather pipe on top of the engine was also split, this was replaced too.
While the plastic cap on the front of the engine was off I thought Id paint it along with the cam & spark plug covers......so far so good!

Once the magnet & breather pipes were fitted, I took the car for a short run, only about a mile but it felt ok, I put the car back in the garage.
I went back to the car a few days later & the battery was totally dead. I fitted a brand new battery & the car started first time as usual. Ticking over nicely I reversed it out of the garage, no problems. I set off down the road, as soon as the car approaches 2000 revs the car runs really rough, engine shaking etc.
I didn't have time to do anything so put it back in the garage. Checked again a few days later & it started fine, reversed out fine, set off & same thing happened.

I had the codes read & it showed a missfire on cylinders 3 & 2 so I replaced the plugs, coil packs & plug leads. I also cleaned the MAF with electrical solvant cleaner. The codes were cleared & tried again. The car starts up perfect, ticks over fine but as soon as the car hits a certain amount of revs the missfire happens again.

I took it to a local garage who said the catalytic converter may be blocked, they disconnected the exhaust before the cat, no change!
They then thought the other cam sensor, the one on the side of the engine was at fault, this was replaced too, no change.
They now want to start taking the engine to bits but I have said no because the mechanic openly admits he has no idea what the fault is & opening up the engine is the next part in his investigation. I dont want to do this until we can at least confirm if the fault is electrical or mechanical. This garage has had the car a few weeks, he says his diagnostic equipment & his friends STAR equipment show the same faults. He also says the MAF is OK.

I dont have a local Indipendant near me that I know of & I cant afford a main dealer at the moment. Tax & MOT have now expired too.

Thank you
Stu

Last edited by Slippy; 03-11-2014 at 05:01 PM.
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#2 Old 03-12-2014
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This problem is way beyond my pay grade, but the code P1525 is for the Camshaft Position actuator (CMP) being defective or inoperable. You haven't mentioned checking that item yet. This is not the CPS sensor if I understand the jargon correctly.

Sorry I can't be of more help, but I'm stuck back in the late 30's to early 70's with British cars. Never had a reason to play with the computerized cars until this one (my 2001 SLK 230)
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#3 Old 03-12-2014
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Quote:
Originally Posted by JiminFL View Post
This problem is way beyond my pay grade, but the code P1525 is for the Camshaft Position actuator (CMP) being defective or inoperable. You haven't mentioned checking that item yet. This is not the CPS sensor if I understand the jargon correctly.

Sorry I can't be of more help, but I'm stuck back in the late 30's to early 70's with British cars. Never had a reason to play with the computerized cars until this one (my 2001 SLK 230)
Thank you very much for trying to help, ANY hints or advice are very welcome.

That may explain something as all this started after I changed the cam sensor magnet. I didnt have a clue what a camshaft position actuator (CMP) was, until I read your post & looked it up.....Its another name, probably the correct name, for the cam sensor magnet as I`ve called it??

I wonder if this fails, it knocks out the timing at about 2000 rpm & causes the missfire?? At least I can do some searching for info on a CMP now.

Thanks very much!!!
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#4 Old 03-12-2014
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I'm just gonna take a shot in the dark here as I'm a slk newb too. have you checked the K40 relay
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#5 Old 03-13-2014
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Ive had the k40 board out & had a look at the soldered joints etc, all looks well but not checked the relay. Ive just ordered a new cam sensor magnet. I'll look into the k 40 in more detail if the cam magnet doesnt work. Thank you for your input.
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#6 Old 05-23-2014
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Not been able to do much recently due to going on holiday but a brief update just incase anyone else comes across this problem & wonders what the cure is, well so far, for me, there isnt one yet -

2nd new cam sensor magnet tried......no change.
New MAF tried......no change
ECU removed and sent for testing......ECU is fine, no issues.

Ill report back if / when anything changes.
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#7 Old 05-30-2014
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Another auto electrician / diagnostics guy walking away scratching his head.

This one thinks because the missfire sometimes starts at 1500 rpm, sometimes 2000 rpm plus, sometimes it does not missfire at all especially on tick over or if its revved really hard........faulty / clogged injectors?

Or timing chain slipped ?

Other than that, no more ideas.

Is timing an expensive / lengthy job?
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#8 Old 05-30-2014
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Has anyone checked the fuel pressure ? Could be a failing pump
http://www.pelicanparts.com/techarti...eplacement.htm
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#9 Old 05-31-2014
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I considered a failing pump or blocked fuel lines but when he had his laptop plugged into the car & he had somesort of output graph on the screen, no matter what rpm he had it at, from tickove to 3000 rpm, cylinders 1 & 4 were 100% constant, never faultered but 2 & 3 were all over the place when the missfires were happening. He thinks thats why its injectors as if it was fuel supply, there would be missfires on all 4 cylinders. He also says He does not think its a timing issue fo the same reason?
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#10 Old 05-31-2014
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A pressure test on the valve would rule out the fuel pump. Injector replacement here http://www.pelicanparts.com/techarti...eplacement.htm
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#11 Old 05-31-2014
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On my engine the cam sensor and can actuator are separate components. Do you have a wiring diagram?
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#12 Old 06-01-2014
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Thank you for the info on removing the injectors, thats great, very clear!

I dont have a wiring diagram but there is the Camshaft actuator (also known as Cam sensor magnet), this is the round one on the front of the engine under the plastic cover facing the radiator. The other cam sensor (maybe called cam hall sensor ??) is also at the front of the engine but facing to the right (looking at the car from the front.
Both of these have been replaced. The magnet sensor I have replaced twice.
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