Does anyone know what the most common causes are for this particular V8 engine (obviously it's not the 3.0 liter version) to lose compression in one cylinder (mine has ZERO compression in Cylinder #8)? Is there Variable Valve Timing that can be the problem, or perhaps a bad injector seal? I am hoping it's not a burned valve. The compression test was fine in all cylinders except for #8 which has 0.0 compression as I wrote, and the OBDII error code is "Random Misfire in Cylinder #8."

Thanks for any and all help.

 

"Random misfire" is really OBD2-speak for a firing impulse that is significantly less powerfull than all the rest... You already have the answer as to why this is the case from your compression test.

 

But "random" in the OBD2-speak (OBDII) you refer to makes it sounds as if firing is not consistent within that one cylinder, and therefore that cylinder "sometimes" fires and sometimes does not, perhaps because a VVT (variable valve timing) computer is not functioning properly, or there is something other than a valve that could be causing the problem. Would you agree that is a possibility? If indeed it is not random, then why add that unnecessary word? I understand there has been tremendous thought and resources expended at the Society of Automotive Engineers (SAE) regarding standardization of the nomenclature toward the OBD system, and it has been evolving for decades with various iterations of codes and languages so by this point it is quite refined. Therefore it would seem inconsistent that the word "random" would be randomly and unnecessarily placed in the code description.

Regarding my compression test providing the answer as to why the firing impulse is triggering the onboard diagnostic computer error, it does tell me of course that there is insufficient compression and obviously there needs to be sufficient compression to fire and therefore of course that is the reason it is not firing. But it does not tell me WHY there is no compression. And that question is the impetus for my question posed to the group: Is this particular 4.2 liter V8 Jaguar engine which is different than your 6 cylinder 3.0 liter S-type -- and I believe more commonly found on the XJ8 and XK8 -- known for burning valves when the engine has just 100,000 miles? Or is there some other common problem -- perhaps a computer problem -- which causes loss of compression?

 

What is the actual code? P0308? Or something different? The “random” nomenclature may be the generic definition as supplied by your scanner or code reader, and not the official definition that Jaguar uses.

In OBD speak, remember that misfire simply means low power output from that cylinder.

The VVT system isn’t the root cause for two reasons. First, there is feedback to monitor if the VVT position matches the command. You’d get a separate code if there was a mismatch. Second, a VVT fault would affect all cylinders on that bank, not just one. Nor would a VVT problem reduce compression all the way to zero.

Sorry i don’t have a better answer for you. There’s no magic compression reset. You’ll have to do a compression leakdown test to find the source of this major leak. Could be a valve, hole in the piston, etc.

 

Who cares about the wording when there's NO compression????

Move on about it...

 

quick answer- no compression is not a bad injector.

It may be:
bad rings
broken pistion
broken connecting rod
blown head gasket
burnt valve
bad valve seat
bent valve
cracked cylinder
stuck valve
cracked head

 

On your injector seal question.....yes, if the injector is directly in the combustion chamber, a blown seal will cause poor compression.... I don't know how bad....but you should here it escaping while doing the test.....there is another member having compression issues too....maybe check that thread and get some insight...random misfire means it is intermittent with no pattern and not on every cycle....could be some carbon broke loose and stuck to a valve even....the v8s are pretty stout

 

but injectors are not in the cylinder. they are in the intake manifold that bolts to the head before the valve.

If it were in the cylinder it would melt and get shot out under pressure.

 

That is why i said if.....to cover my butt....i did not know....my background is diesel, and they are in the head on most ....thanks for clarifying my friend, saved me some grief

 

I will have to get the precise code number to the person who asked. But I believe it is a burned valve. My instinct tells me so. I was hoping someone would say something along the lines of, "Oh god...that engine...they burn valves like there's no tomorrow." Or, "...I've never heard of someone burning a valve in the 4.2, but the valves do get stuck occasionally."

In any case, assuming that it is not going to be possible for me under the circumstances to fix it -- parts and labor resources are very slim in my current locale and so I would have to bring everything needed in a suitcase -- and I can deal with the very slight shakiness caused by the imbalance, I want to try not to waste fuel despite the misfiring. My mechanic said it is absolutely not true that the misfire will clog the catalytic converter, and if it does he can clean it out. So that's not a concern. Can I disconnect the electric power to the fuel injector or do something else so the engine stops pumping fuel into that bad cylinder #8?

 

If the valve is stuck or burnt the cylinderwill still fill with air if its the intake valve thenit will push it backout on the compression stroke. Probably not good

use the compression tester to fill the cylinderwith air whenboth valves are supposedly closed. The listen for the air leak. it will tell you if a valve is bad a d which one

 

Unplugging the injector will stop the fueling for that cylinder....there will probably be codes set and possible reduced performance but drivable i think....the big problem is oil fouling of the piston rings because they will not seal properly over time with no combustion....but, better no fuel than raw fuel washing the bores and pistons and ending up in the cat converter

 

The OP, or his mechanic, can remove the valve cover on the cylinder bank in question and visibly see of there is a stuck valve. A reasonably easy way to tick one more item off of the troubleshooting list...

 

A leak-down test, as suggested by Aarcuda above is the best method to eliminate a valve issue.

What is the history of this vehicle, has the engine been overheated?

 

History as requested: It has 150,000km. It overheated prior to my possession, which I am now told preceded the misfiring state. Of note, there are no exhaust gas traces in the coolant, no abnormal pressures in cooling system, no coolant contamination in oil, no water vapor from exhaust, and no high or abnormally variable engine operating temperatures in any livable Western Earth atmospheric/weather conditions. To be safe I replaced the radiator, water pump, thermostat, leaking cross-over cooling hose which which caused the overheat, 8 spark plugs, 8 ignition coils; all with new OEM parts. It doesn't overheat but I still have the misfire on cylinder #8. As I say I am trying to figure out my next step.

What is the best way to disconnect the injector? Is there a simple electric connector I can squeeze and pull to disconnect?

 

Do I understand correctly that you are not planning to repair the compression problem, but just disable the dead cylinder?

Can't say I'd recommend that as any sort of a long-term fix. Maybe if you were trapped somewhere in a war zone, and Russian tanks were heading your way, disabling the injector would get you to safety. But as a long-term "fix"? Ooh, sounds kinda spooky. No idea what would happen over time with one dead cylinder like that. The compression is still going somewhere, possibly into the crankcase (if a hole in the piston) or into the intake or exhaust (if a valve stuck open).

Strange things may happen down the road as the computer tries to compensate, totally unaware of the root cause. Right now, the computer is telling you everything it knows, that the power output is low from that cylinder. The computer is NOT smart enough to know exactly why. The computer is programmed on the basis that you will investigate and correct the root cause, not keep driving it like that forever.

 

@LoveUnreliableOverpriced, if you have access to leak test equipment, perform that test and determine the source of the compression loss. If the engine has as stuck/burnt valve, dropped valve seat, or other cause, the engine should be disassembled and repaired, or a replacement engine sourced.

The AJ-V8 gets very unhappy once overheated with head gasket failure and dropped valve seats are not an uncommon result.

 

Disclaimer....you should heed the advice in post 17 and the others regarding troubleshooting.....and yes you can unplug the injector and it will not fuel by squeezing and pulling up on the connector

 

How do you justify "Handling on the 4 cylinder is better!"?

 

Ignorance?

Think it's a dead thread anyway.