So got an odd one here on a straight 6 '96 Jag.
It was a sudden crank no start, did a 300 mile trip then left for 3 weeks and crank no start.
I went straight to an RC soon as I heard it crank and sure enough ;
Went in cylinder on 3 a "good" cylinder, ran out of range on my 150psi transducer but extrapolating the curve it's about 165psi. Note spec is 150 (new).
Exhaust valve opens at 45 Before BDC Intake closes at 40ish after BDC, I took several measurements across compression events - all within a few degrees.
Then Cyl 6 a "bad" cylinder
First up peak compression is 88psi, this was identical on Cyl 1 (another Bad cylinder)
On this one EVO is at 24degs BBDC and IVC at 60 ABDC - I should also say that the spark synch was at about 10degs before compression peak too.
Also on Cyl 1 the IVC event was the same time as Cyl 6 at 60degs ABDC... but the EVO was at 42 degs BBDC. - compression value the same at 88 ish.
IVC spec is 51 or 55 degs After BDC depending on the engine number that I didn't get see attached bulletin
Screenshot from 2021-12-12 12-25-20.png (22.15 KiB) Viewed 4813 times
So first thought is timing - but then how can 2/6 cylinders in a straight 6 be OK ?
My theory was that as there are 2 timing chains with an intermediate sprocket that either that sprocket is missing teeth or has enlarged it's keyway allowing it to move back and forth altering the timing on individual cylinders.
I suggested rocking the crank with a bar and seeing if the cam sprockets had a dead spot (no actual cam position sensor on this car) However the garage went ahead and pulled the head off and found the valve seats badly deteriorated - so where have I gone wrong !? - Though it's not back together yet...
Any ideas on how I was wrong on this? If I have a major misunderstanding /misinterpretation of the data it's better to know now !
Still can't make bad valve sealing and the compression I got gel with "ran great until sat for a while"
Interesting case indeed and often one we may never conclusively diagnose now the engine is dismantled
Focusing on peak compression alone…………
The fact the vehicle drove fine for 300 miles and then returns the relative compression results in your fist image is odd!
Forgive me if I am wrong but I assume this vehicle does not utilize hydraulic lifters? If it does, this is an area for consideration re your individual valve timing results
Would you know if the car is used regularly or a Garage Queen?
During the 3-week stand, was the vehicle moved (shuffled around) My line of thinking here is bore washing from over-fueling after repeated cold stop starts
Regarding the measurements taken, I assume the engine was cold at WOT?
Could you boost compression by adding oil to the cylinder?
Did you manage to start and finally run this engine?
I would be very interested to see repeated compression test results with a hot engine
With regards to valve seats, these can often be detected with in cylinder pressure measurements during prolonged cranking whilst noting the compression tower peaks which should remain equal.
Below is an example from my colleague Randy in the US (Poor valve seating following by improved seating as the valve rotates)
Picture 1
Repeated deviation of peak compression values during cranking accompanied with simultaneous analysis of intake and exhaust pulsations will point towards valve seating
Looking at the captures and the individual valve timing results I have concerns surround a possible measurement error.
The resolution and response of the custom pressure transducer appears to introduce variables when trying to determine precise EVO and IVC events
Below I have EVO @ 30° approx. BBDC and IVC @ 42° approx. ABDC on cylinder 6 but when zooming you can see just how challenging this is to conclusively prove these events
TDC can not be taken from compression peaks on a leaky cylinder as the cylinder starts to lose compression faster than the piston can generate it due to a slowing piston speed as it approaches true TDC there fore advancing the compression peak.
In the image below I have aligned all three cylinders using ignition events. Makes much more sense now.
As a side note. The crank speed appears low 130rpm approx. That will worsen the issue by allowing more time for the gas to escape. Anything below 100psi and it won't fire. Flat battery from sitting for three weeks????
(sorry for the brief explanation, I lost my post twice)
Also may explain the high compression on a well sealed cylinder due to the longer duration the inlet valve is open for, allowing the cylinder to 'fill' more efficiently...........Possibly
@fat_freddy
Sorry for the late reply, really appreciate the input, what you say about the leaky compression moving the peaks makes absolute sense - won't get caught by that one again! I knew there had to be a reason and hoped to learn from it.
Still does not explain how it "drove perfectly 300 miles" before being CNS with this compression. Garage thought it was E10 fuel burning the valves - despite Jag saying it's OK for 92 onwards.
IMO your cranking speed is way to low (from sitting parked up or because of endless cranking ). But that would defo exacerbate the problem as more time for the cylinder to leak down.
Again, IMO, I wouldn't expect a cylinder to fire with less than 100psi during cranking, although at idle the compression will only reach about 70psi under normal conditions.
