Runs worse at idle

Ray Diagnostician Ontario Posted   Latest   Edited  
Unsolved
Driveability
2015 Mercedes-Benz C300 4Matic 2.0L (274.920) 7-spd (722.9)
P0300 — Random/Multiple Cylinder Misfire Detected
Rough Idle

This car was at a dealer and they replaced the COPs and spark plugs. The engine still runs bad and the dealer says the chain is loose. But I think that if the chain is loose, the PCM would set a code. All 4 spark plugs are black but cylinder 1 spark plug looks the worst. I could not find a known good cmp, ckp waveform. Does anyone have a good cmp, ckp waveform? If you hold the rpms at near 2500 rpms, it feels like 1 cylinder misfiring, but at idle, several cylinders are misfiring badly.

In the first GIF and PSD files, CH A is COP 1, CH B is a WPS500 on the dipstick tube, CH C is a FLS in a tailpipe with one repeating higher pressure pulse, CH D is the starter amps and cyl 1 shows repeating lower relative compression. 

The repeating high pressure pulse from the FLS in the tailpipe tells me there is a valve problem in cylinder 1!!

In the 2nd GIF, with the PSD file, CH B is a WPS in cyl 1 cranking at 164 psi, CH C is a FLS in a tailpipe and CH D is the starter amps.

In the 3rd GIF, with the PSD file, CH B is a WPS in cyl 1 cranking at 200 psi? Then the engine runs at idle. The cylinder vacuum measures 20 inches of vacuum and there is a 4.7 psi pressure pulse near the end of the exhaust stroke. I don't know if that pressure pulse should be there?

Looking at the wiring diagram, the engine has an "Intake Camshaft Valve Lift Switching Actuator" I don' know if the actuator comes on cranking or at idle?

In the 4th GIF, with the PSD file, CH B is a WPS in cyl 2 cranking and the engine is trying to start. In this section, we see some 15 psi pulses near the end of the exhaust stroke?

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Nick Diagnostician
Missouri
Nick Default
 

That huge pressure spike could possibly be the exhuast valve closing to early? FLS is intake might shed some light on that.

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Ray Diagnostician
Ontario
Ray Default
 

It looks like the exhaust valve is closing too early. I could not access an intake vacuum hose to connect the FLS and there is a mechanical vacuum pump for the brake vacuum booster.

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Nick Diagnostician
Missouri
Nick Default
 

What about purge valve? For a single cylinder to have a valve closing early you looking at worn cam or lifter/lash adjuster issue maybe

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Ray Diagnostician
Ontario
Ray Default
 

The purge valve could be stuck partially open, I'll ck that. I agree with the possible worn cam lobe or a lash adjuster issue.

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Nick Diagnostician
Missouri
Nick Default
 

I met could you tap into purge port for FLS. Does it sound like it has valve train noise?

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Jordan Technician
Nevada
Jordan Default
 

I don't know if this will help at all, but this is a 2016 Metris with the 2.0L (274.920) engine. It had these codes stored. P001685 position of intake cam is implausible related to the crank, P001177 position of intake camshaft deviates from range. I don't have the known good because the van went back to the dealer under warranty and I never saw it again.

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Jordan Technician
Nevada
Jordan Default
 

This document has some waveforms captures of the cam and crank.

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Ray Diagnostician
Ontario
Ray Default
 

Thanks Jordan

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Stephen Technician
Tennessee
Stephen Default
 

The WPS on cyl’s 1 and2 show that the exhaust is retarded. The earliest that I found is about 20*BBDC. ( Non VVT engines usually base line at about 40*BBDC. ) I also found it as late as 15*ABDC. It moves around, due to VVT, I’m guessing. The FLS in the cranking shows that the exhaust pulses are all late. The running and trying to run shows that only 1 cylinder is actually firing and the “active”…

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Ray Diagnostician
Ontario
Ray Default
   

Thank you all for your help! I am closing this post for now. Today I first erased the codes, then I started the cold engine. I held the rpms near 1500 and the engine ran well, with no misfires. I scrolled thru the scan data and the FTs were ok. As long as I held the rpms near 1,000 rpms, the engine ran well, but if I lowered the rpm below 1,000 rpm, the FTs would subtract fuel to negative 25%…

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Dmitriy Analyst
Ontario
Dmitriy Default
 

Ray, these are fascinating captures! I know I‘m late to the party, but would you mind me asking why dipstick pressures (I assume WPS is set up to read positive pressures) are lowest when cylinders are on compression (thus, maximum blow-by)? Is it special for this example or you’ve seen it before and it is normal? Thank you.

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Ray Diagnostician
Ontario
Ray Default
 

The WPS is set to positive AND negative pressures. I use the WPS on the dipstick tube to measure the amount of piston ring blow by gases and the amount of pcv vacuum in the crankcase running or cranking. In this example, some of the cylinder's piston rings are fuel washed and then some of the compression is pushed by the piston rings, into the crankcase. I suggest that you should connect your…

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Dmitriy Analyst
Ontario
Dmitriy Default
 

Ray, what I meant by "set up to read positive pressures" is that values are above zero for positive pressures, as opposed to vacuum transducers where values are above zero for vacuum (negative pressure). Sometimes people use the custom probe feature in Picoscope to make pressure transducers act as vacuum transducers, which is why I asked for a clarification.

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Nick Diagnostician
Missouri
Nick Default
 

So are you suspecting cylinder wash in #1 causing the lower compression. After looking at the FLS on the dipstick it does look like compression is making it past the rings

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Solaman Owner/Technician
Trinidad and Tobago
Solaman Default
 

Hi Ray, The injectors on these engines are a common failure,we have seen it so many times as we normally replace all 4 of them and code,If you have another car just swap them,there is also a vent valve under the intake manifold normally gums up a lot,may need to replace also.

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Ray Diagnostician
Ontario
Ray Default
 

Thank you for the info, I guess the vent valve under the intake manifold that gums up, is a crankcase ventilation valve?

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Pedro Mechanic
Washington
Pedro Default
 

On your file of running compression i notice that the exaust closing you have a nose i heard in class that usually if the car is turbo is supposed to have a nose if its not turbo it usually a mechanical problem was wondering if some one has heard that and if they know why?

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Caleb Technician
Indiana
Caleb Default
 

Hey Ray I see you said you are done. Curious if you still want the known good cam crank waveforms. I have a 16 Metris here I can get them off of. Its just that Im busy and it will take me some time. So if you dont need them now Im probably not going to bother.

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Ray Diagnostician
Ontario
Ray Default
 

Hi Caleb, the shop is not going to repair the engine issues and THANK YOU for offering to get the cmp, ckp waveform!

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