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Discussion Starter · #1 ·
i have no compression in cyl. 6. Any ideas? Whats most common cause on these engines with a cyl with no compression?
 

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Broken piston rings
 

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You poppey a holey in the peeston. Broken rings will still make some compression like the others stated above.
 

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Did the gauge not try to move at all, or did it flutter a little? Or is your gauge reading 0 and not even trying to climb.

I know you said no compression, but some say no compression, but they really have a little:D
 

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Ok so its a hole in the piston. What happened up till then I'm just curious, drivin like you stole it or normal driving what's the story?
 

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i would imagine that the cup isnt sealed, since you just had it apart.
 

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Discussion Starter · #9 ·
heres the story, i dont drive it very hard, i commute 110 miles a day to work. truck started smoking heavy greyish and was knocking around 2000 rpm at cruising speed, when the engine was warm. with the engine cold it wouldn't knock and ran ok other then smoking. i took it in to one dealer to have injectors tested and they said it had blown head gaskets and the injectors were "fine". I did NOT believe it had a blown head gasket because it had no symptoms of that, went to another dealer and had injectors tested and 7 of the 8 had high balance rates and after they cleared the codes and did whatever they did to test it made it run rough all the time but no knocking on the way home. so i bought 8 reman injectors and installed them, got it back together fired up and still runs rough, like running on 7, with light smoke. I scratched my head for a bit and went out into the shop and noticed that 7 of the 8 injectors i took out had the washers still on them and cyl 6 injector does not, (which is the cyl with no compression) and i do not remember the washer falling off the injector on the way to the bench nor did i hear it drop off. I know for a fact i installed 4 new washers on the drivers side so i may have 2 on cyl 6 injector but i dont think this would be my problem since the injector should still be seated on the washers to seal, maybe im wrong. so i took the truck to a shop that the owner/mechanic use to work for GM for 20 years or longer and he plugger in the tech 2 and cyl 6 balance rate was +15, cyl 3 was 0.3, and the rest were -7. he pulled cyl 6 glow plug which the tip was like melted off he said and did compression check and nothing, and this is where i am at.
 

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Discussion Starter · #10 · (Edited)
also, i had no cups pull out of the 8 but that cyl. 6 cup when i touched the edge i noticed would kind of 'rock' side to side but it did not pull up at all.
 

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is there a lot of blowby if you remove the oil filler cap when its idling?

First of all I would pull the #6 injector and make sure the injector is seated properly. An injector that is not seated absolutely perfectly will cause the exact issues you are describing.

It could be a cracked piston I suppose, but I do not think that is the case here.

Personally I have never heard of rings breaking/going bad on a dmax....

ben
 

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Discussion Starter · #13 ·
i checked for blow by before i did the injectors and nothing. im hoping its something to do with the injector. why would i get a +15 balance rate on that injector though?
 

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i checked for blow by before i did the injectors and nothing. im hoping its something to do with the injector. why would i get a +15 balance rate on that injector though?
because there is no compression on that cylinder.

Remember. Balance rates are merely an indicator of how weak/strong each cylinder is in relation to one another. Balance rates are NOT just "whether the injector is good or bad", which is what everyone thinks they are.

After each cylinder fires the ECM makes a note of what the crankshaft speed does. If the crank speeds up, the ECM interperates that as that cylinder being stronger/making more power than the cylinder that fired before it, and to compensate, it pulls back the fuel slightly in that 'more powerful' cylinder (negative balance rate). If the crank slows down after a cylinder fires, than that means that cylinder is weaker, and to keep a steady crank speed and smooth idle, the ECM needs to inject more fuel into that cylinder to make it stronger and compensate for lower compression (or anything else mechanical that can make a cylinder have a lower power output than the other cylinders).

If a cylinder has no compression (which in your case I think is due to a poorly seated injector), then its obviously not making any power...the ECM recognizes this and tries to compensate by injecting a lot more fuel (which is why you see the huge + balance rate) into that cylinder in effort to smooth out the engine idle/cylinder power balance.

ben
 

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Discussion Starter · #15 ·
thanks ben, good info:rockin
 

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Ben hit it on the head! Also that cup that rocked back and forth should not have done that. The cups are pressed in " and Locktited at the base of the cup to keep the compression in the cylinder where it belongs.

Time to pull it back down and get that cup out. You are going to want to clean the surface that the cup sits against real good to get the old locktite off so you have a good sealing surface. Also once you get the new cup in let wait at least a day "24hrs" for the locktite to harden before you put coolant back in the system.

Good luck and keep us posted:drink
 

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Discussion Starter · #17 ·
what do i need to do a leak down test on that cyl. What thread size is the glow plug hole? i want to get a compression gauge also.
 

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what do i need to do a leak down test on that cyl. What thread size is the glow plug hole? i want to get a compression gauge also.
The thread is I believe a 10X1.25 THRD, and you would also need a leak-down tester with that thread size adapter. So since you do not have a compression tester, you are just assuming it did not have compression, when you said you tested it:confused:

All the reply's you received in the 2 posts you have were under the assumption you already did a compression test with a TESTER:rolleyes:

I really do not think it is a holed piston, after you said one of the cups wiggled. If they are in correctly they will not move at all.
 

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i would look into doing a leak down test also but i would look at your injector first as was stated above. He said he had the compression checked at a shop but now hes wanting to do it on his own.
 

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He said he had the compression checked at a shop but now hes wanting to do it on his own.
My bad, I missed that part:banghead
 
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