Crank, No Start

Rick Technician New York Posted   Latest   Edited  
Resolved
Driveability
2016 Volkswagen Passat Comfortline 1.8L (CPRA) 6-spd (09G) — 1VWAT7A35GC031781
Crank / No Start

Hi all, this passat is a crank no start. fuel pressure is on spec. compression is 170-180 on all 4 cyls. has good spark. I’m thinking something in the valve timing realm, but don’t want to pull the cover only to find nothing. It’s not a huge job, but I hate to waste time, plus disturb the cover gasket unnecessarily. I have a snapon scope and a WPS 500, and I’m very new to pressure transducer diags. The in cylinder waveform confirms good compression, but the waveform looks bad. I’m hoping someone will see something obvious to pursue here. I don’t see evenness in the compression humps, plus no vacuum pockets. To me, it doesn’t appear to have excessive exhaust back pressure on the waveform, but it’s only cranking so maybe it’s just my lack of experience. any insights will be MUCH appreciated. Also, would anyone have a good cam / crank waveform for this 1.8L CPRA? Thank you in advance!

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Laird Owner/Technician
Nova Scotia
Laird
 

Use a second channel to see the ignition sync for that cylinder. It looks like you have a problem with the valve timing. I think the exhaust is way advanced

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Caleb Diagnostician
Indiana
Caleb
 

Put a sync on #1 coil and redo the pressure transducer test. You will see that the low peaks are probably extreme exhaust pressure. Looks like the exhaust cam is severely advanced to me. An ignition sync on the same cylinder as the pressure transducer will tell you this immediately.

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Dmitriy Analyst
Ontario
Dmitriy
 

This type of waveform happens when the exhaust cam is way advanced. diag​.​net/file/f48zelv57… ​ For those interested in how to obtain such a picture: go to …​.​com/software/AWSM , sign in with Google, then navigate to Signal Generation, Generate Cranking In-Cylinder Waveforms, and play with the numbers.

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Interesting
Stephen Technician
California
Stephen
 

Recall similar post approximately 9 months back with CPRA engine code. diag​.​net/msg/m692xbbnub…

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Tamiyal Educator
Florida
Tamiyal
   

Possible timing issue. But try removing the upstream O2 sensor. See if it will start up.

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Ray Diagnostician
Ontario
Ray
 

The exhaust cam is advanced and the exhaust valves are closing too early. When the piston is rising during the exhaust stroke with the exhaust valve closed, the cylinder will build near compression pressure. The capture is from a 2017 Golf 1.8 cmp ckp by Hans Jorgenson.

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Rick Technician
New York
Rick Resolution
 

Hello again, and thank you for the immediate responses! Based on the replies, I did another capture using the ignition coil from the same cylinder being tested - The result was inconclusive on it's own because it has multiple ignition strikes, but none of them are in the center of the compression hump. The tallest one is to the left of center, and the next one is to the right of center. Together…

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Israel Technician
Florida
Israel
 

I would have assumed if the cam timing was off by 7 teeth, you would have low compression across the board. Whats the compression reading normally on these things?

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Rick Technician
New York
Rick
 

The compression actually does seem a bit high. I didn't look up specs, but it was higher than I expected. I think it has to do with the exhaust valve opening and allowing too much air into the cylinder, hence the high compression. Brandon Stekler's syringe illustration makes sense here. The more air to be compressed, the higher the compression. I'm with you though, a little nervous, thinking…

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Israel Technician
Florida
Israel
 

If you do a vacuum test it should be lower than normal if the timing is that far off. I was thinking just lower compression due to incorrect valve timing and cyl not sealing due to valves being open at incorrect time. Does anyone know what the vacuum is at idle on these motors?

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Glenn Owner/Technician
Texas
Glenn
 

Hi Israel, In reality the compression is low for a 4 cylinder. I would expect to see around 200-220 PSIG. On a V-8, 170-180 would be considered acceptable.

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Joseph Technician
South Carolina
Joseph
 

Isreal, Generally speaking compression is more a function of intake valve timing ie when the intake valve closes. Exhaust can be way off and still have little effect on compression numbers. With that being said shooting from the hip it looks like if his exhaust was off 1 or 2 more teeth it would have moved the exhaust valve opening into the compression stroke and that would definitely lower it.

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Glenn Owner/Technician
Texas
Glenn
 

Hi Joseph, I have to disagree with the exhaust being way off having little effect. First, if the exhaust is late opening, compression would be too high. Second, if exhaust opens too early compression would be low. All cam engines have a built-in cam dynamic known as valve overlap. When this is affected, the engine will run poorly or not at all. With a seven-tooth difference, this engine lost…

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Matthew Diagnostician
Kentucky
Matthew
 

Intake is at least closing well enough to build pressure however exhaust is way early and closing early and building pressure again on the exhaust stroke exhaust can be way out up to 100 degrees and engine should still try and start and even may be run but very poorly

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Michael Technician
Indiana
Michael
 

Rick, When I wonder “is my exhaust plugged to the point of no start?”. Removing a spark plug,will generally give the exhaust enough of a passage, that the vehicle will run well enough to get it into the shop. not that I think that is what you have, just trowing that out there. Sounds like you have lots of great advice. Mike

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Robert Technician
Michigan
Robert
 

Does this VW have PCV and breather problems sludging the VVT with no codes .. I agree with Caleb sync it .

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Matthew Diagnostician
Kentucky
Matthew
 

because the exhaust is opening so early tdc point of reference is not accurate FWIW

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