My 6.2 has developed a slight miss fire and a knocking sound also a noticable drop in power, I have trraced the miss and knock to the rear cylinder on the drivers side. I swapped the injector over from a different cylinder hoping it was an injector problem but the knock and miss have stayed at the same cylinder.
About 4 weeks back I had a real bad miss fire and a knocking sound, it sounded like a lifter had colapsed but it stopped knocking and ran fine again before I could find out where it was coming from, this knocking sounded totaly different from the one I have now, any ideas?

The plastic rocker arm tabs are known to break and sometimes they can allow the rocker to slide out of position.I would pull the valve cover and have a look under there.Occasionally valve springs are also known to break.

Just taken the rocker cover off, the springs are all good and it doesn't have the pressed steel kind of rockers, they are the cast type, I can't see anything wrong in there at all. Thanks for the input

So is the knock still heard when that particular injector line is loosened to that cylinder?If so,you could have a bad wrist pin.Piston inertia is what causes the noise which is why the noise does not go away when the fitting is loosened.When you loosen the injector fitting and the noise goes away,its linked to a rod knock providing everything else is A-OK.Let us know how that goes.

The nocking noise does go away when i loosen the the injector line to that cylinder but a rod knock wouldn't cause a missfire would it? I thought it was a bad injector with having the missfire and the knocking but the knock did not move with the injector when i switched it for another.

How old are the injectors?Have they been replaced before?When an injector sticks open,the noise does ressemble a rod knock.Water contaminated fuel will also cause the injector to stick open.

the injectors are only about 1000 miles old. the knock stayed with the same cylinder so it cant be the injector. the injector that came out of the rear cylinder is now in the front and it's not knocking

Unless you've got two injectors failing for some reason...

The injector i removed from the missfiring knocking cylinder is fine in the cylinder i swapped over.

Any more ideas anyone?

What should the cylinder compressions be on a 6.2? just checked mine, i'm getting just short of 300 psi on a cold engine, should they be checked at opperating temp?

I don't know the spec but I do know that the important thing is that all the cylinders have close to the same readings...

Don't overlook the obvious things. I had the same thing with mine when I first purchased it. The guy I bought it from couldn't figure it out. I figured it was an injector, it wasn't even though the knock would lessen when the injector was loosened. It happened to be a loose glow plug. I put in a new glow plug and running great since.

About 4 weeks back I had a real bad miss fire and a knocking sound, it sounded like a lifter had colapsed but it stopped knocking and ran fine again before I could find out where it was coming from, this knocking sounded totaly different from the one I have now, any ideas?



Hi-
based on your story, I got nearly the same some years ago...
It was a broken glow-plug that gotten "compressed" between piston and vents- finally it went out the exaust...
Never heard this terrible sound later- and I sold the whole truck two years after that... Now it owns a 350 gasser with LPG- the 6.2 was sold, too



mfG
Sonne

What should the cylinder compressions be on a 6.2? just checked mine, i'm getting just short of 300 psi on a cold engine, should they be checked at opperating temp?



the manual says:




-DO NOT add oil to any cylinder for compression testing. Failure to follow this caution may result in extensive damage to engine.

-Disconnect pink lead from fuel injector pump.

-Install compression adapter and compression gage in glow plug hole of cylinder to be tested.
CAUTION: Covering should be removed from air Intake opening before cranking engine. Failure to follow this caution may result in damage to engine. Record compression gage reading while assistant cranks engine. Allow six puffs per cylinder.
Normal compression should be 380-400 psi (2620-2758 kPa). Repeat steps 2 and 3 for each cylinder. Lowest cylinder reading should not be less than 80% of highest cylinder reading, and no cylinder reading should be less than 380 psi (2620 kPa). Remove and inspect valves and piston rings for any cylinder that fails test.


I guess this test should be performed at cold temp. not to burn anything;)


mfG
Sonne

Hi all, My compression tester only goes to 300psi so i will have to look out for another one.
It can't be the heater plug as i've removed this to check the compressions and i know it's tight now.
I've just done a 70 mile trip in the truck, took it easy the first half and the return journey i was doing 70 - 80 mph the knock is NO better or NO worse.
I have been given a set of the pressed steel rockers as a afriend says it might be the bushings in the cast rockers, any thoughts on this?

Well it wasn't the rockers, any one want to see a 1975 C20 Dually burn????????

Ok now i have had a developement with the truck. I started it up for the 1st time in a week or so and there is a very loud pop pop pop coming from the intake, so it's looking like a bent valve, would explain the slight missfire and the knocking sound. All the top end of this engine is re built due to head gasket failiure, i had the valves, guides and stem seals done and the heads skimmed, the shop found 1 bent valve then. What could cause a bent valve? I installed the thicker head gaskets due to the heads having been skimmed

The camshaft sprocket keyway on these engines are also know to shear off occasionally.You could have a partially sheared keyway therefore the valve timing is off just a little and that could explain your bent valve.This is just a guess however,but its worth looking into.

Hi. The cam shaft keyway is fine, i had it off not long back to check it as the front IP bearing had gone bad, i swapped the pump drive gears and checked the cam keyway while i was in there, thanks for the input :)

I've just been offered a pair of 6.5 heads at very cheap money plus I have a 6.5 turbo set up, thinking of tearing the motor down yet again finding the cause of the bent valve and building it up with the 6.5 heads and turbo. I want to get some copper head gaskets made as the standard gaskets are a fortune over here in the UK plus I could go thicker with them and lower the compression slightly, how thick would be ok? what torque would I need on the head bolts? I priced the parts this morning in the UK, for the head bolts and head gaskets they want $976 dollars not uk pounds :o