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Old 04-30-2016, 03:36 AM
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squirt squirt is offline
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I've recently picked up a 2003 D2 to wrench on, and it's got a very frustrating misfire on cylinder #8. It only misfires at high RPM under heavy load. (kickdown while climbing an offramp, for example) I can drive around for a week or more, as long as I keep it to about half throttle at max, no CEL or misfire code.

The only code I ever see is P0308.

Naturally, I suspected the usual plug/wire issue, so I put in Champion copper plugs & Kingsborne wires. Good practice for a new purchase anyway. No change. Old plugs all looked similar, darkish grey color, basically what I would expect for a used plug. I also took the time to use a borescope and compare cylinders. Nothing special about cylinder #8, no contact marks on the piston, no evidence of coolant or oil leaking into cylinder.

Next step was to do a compression test, since I noticed that the headgaskets seem to be original (~140k). 155-150-150-160 down the passenger side head - cold, closed throttle. Looks pretty good.

Then I swapped the coils to see if the issue moved. No change. Then, for the hell of it, I swapped in a new spare coil I had. No change.

Then I swapped in a known good MAF I had. No change.

Then I pulled the injectors, cleaned them, installed new o-rings, and swapped all injectors from passenger to driver side. No change.

Next, I used a vacuum gauge to determine whether it could be an exhaust restriction or sticking valve. Cold idle registered 18mmHg, dipped and spiked appropriately with a snap of the throttle plate. Held steady at 2500rpm. Only questionable thing about vacuum reading was a continuous vibration of the needle (max 1mmHg fluctuation) at any speed, though I suspect this is because I'm using an unfiltered gauge directly off the manifold.

I'm starting to think it may be a valvetrain issue. Next on the agenda is to measure rocker arm lift and compare to cam specs/other cylinders to see if I've got a worn cam lobe. I'll also be looking for a broken rocker shaft or valve spring, if the rocker arm movement checks out. Barring that, what else could it be? Collapsed lifter? I feel like I'm starting to look for pretty obscure causes.

Is there anything else that I should be looking for while I've got the valve cover/intake off? The odd thing is that this engine is extremely quiet. It's the quietest Rover engine I've ever owned. No ticking/tapping/knocking, nothing.

Thanks in advance for any suggestions.
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Old 04-30-2016, 11:41 AM
discostew discostew is offline
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The exhaust valves have been known to stick on these engines. Usually the middle 2 cylinders and usually on older trucks than yours. I don't know if they finally made the guides a little looser towards the end of production but you never really can be to sure that you have 03 heads on it. Someone could have swapped them out a couple headgasket jobs ago.
It sure sounds like you have eliminated everything but an engine mechanical issue. So If it was me, I would be thinking sticking valve. The way to check that is to take the spring off that valve and rotate it in your fingers while opening and closing it. I like to put a plastic zip tie in the keeper groove just to be safe. If that exhaust valve is hanging up your gonna feel it.
I figure you know how to get the valve spring off while the head is still on the truck just based on what you posted already. Speak up if you don't. Also I guess you could smoke check it for a vacuum leak but based on your description of the problem I think I would go for the exhaust valve. Don't waste your time with the intake valve cause I've never seen one of those stick.
Also keep in mind that these are known to break the outside ring gear of the oil pump. The lifters at the rear of the engine being furthest away from the pump would be the first to loose oil and not open as far.

Last edited by discostew; 04-30-2016 at 12:00 PM.
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Old 10-21-2018, 07:49 PM
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squirt squirt is offline
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OK, 2 1/2 years(and 15k miles) later, I'm back to trying to figure this out. I can drive the thing for months without a hiccup as long as I keep it under 3k RPM, but as soon as I cross that threshold I get a flashing CEL for P0308.


I've taken the head off and observed the rocker arms while cranking. Cyl 8 rocker arms are moving just as much as the others. I didn't pull out the magnetic dial indicator, but my calibrated eyeball tells me that all cylinders looked the same. Per discostew's advice, I checked BOTH valves on cyl 8 - they spin freely. The injector is spraying a nice pattern. The spark jumps a 1"+ gap when tested. This all explains why the thing starts and runs like butter at low-mid RPM. No other trouble codes whatsoever.



Additional diagnostic steps -

- Removed front O2 sensors to free exhaust path in case the cats were clogged - no change of course.

- The same behavior is shown with a dead cold engine or toasty warm.

- I just re-checked compression, and it remains as stated in the OP.
- Since the last post, I've been running a Eurovan oil filter, which has given me great oil pressure numbers
- The valvetrain is brown with oil residue, but there is no sludge or buildup



I considered the factor of the engine twisting on its mounts at high load & RPM, but if I was stressing a ground it would affect all cylinders. In doing this, I noticed that it seems to be load- and RPM-dependent. (both need to be high) If I just sit and rev it in the driveway, it will not trigger a CEL, although I'll still get occasional misfires. If I am climbing a very steep grade at low RPM, no issue.



At this point, I'm going to record the ECU injector pulse output and coil spark delivery on a scope at low and high RPM. I've moved from the obscure possibilities to the snowball's-chance-in-hell of a weak driver in the ECU. I just can't explain why this would be happening only on one cylinder, outside of the common fuel/spark issues or valvetrain problem. There are lots of other possibilities for misfire, but none that I can think of that would indicate a single cylinder.
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Old 10-22-2018, 11:18 PM
JohnB JohnB is offline
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high energy misfire is almost always spark related.
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