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1995 GMC C3500 SLE
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Discussion Starter #1
Background: 1995 GMC C3500 Crew Cab Dually 6.5L TB with the 4L80E with 248,xxx miles on it

I have a question about blow by, if the truck has a lot of blow by (i dont know what deams as alot of blow by but its kind of puffing out of the filler neck and the oil dipstick tube) does that mean that I have a possible cracked piston or bad rings? or could it be my valve seals? I have tried researching this but everyone has either said Cummins swap it (I like my 6.5 and want to stay 6.5) or just run it to it blows and get another truck.

I know i have to do a compression test on it and a leak down test, if it is my rings then would i need to rebuild the whole motor (machine shop and whole 9 yards) or could i get a rebuild kit and put it together with new rings, bearings, pistons, etc. or if it is my heads could i just do a head job and not mess with the bottom end, either way im fine with but the truck has to be ready to drive out of Alaska in 2 years and would like to get it done sooner than later.

Also what would be my compression numbers ( i have gotten everything from 18.1:1 to 23.2:1 ratios) for a stock motor in good health?

Thank you in advance
 

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Some Blowby on our old engines is expected. Blowby is usually worn piston rings allowing combustion pressures to pass, getting into the crankcase.
If your engine is chugging like a steam locomotive, pushing the oil stick out of the tube then you need to rebuild/retire the engine..
Like this example:
 

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Your compression test would be in psi, like 185, or 225, etc., however, the number really dose not matter as much as that, ideally, the numbers are all within 5% of each other. 10% is OK, too. You will know you have a problem when you see one or two cylinders with very low or no pressure at all.

The numbers you stated, i.e., 18.1:1 is a compression ratio, which is a static number from the parts used to build the engine. And is calculated without consideration for wear. An 11:1 327 chebbie is still a 11:1 chebbie when it has 100,000 miles on it, blow up for that matter.

Just test each cylinder the same as the others. I never get as high numbers as I see on forum's or advertised, but there are too many variables on how the tests are done to really matter. Just so the reading are all similar.

CB1
 

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1995 GMC C3500 SLE
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Discussion Starter #4
So Is this normal for these motors? Also when I get on it heavy I’m blowing grayish white smoke and going through about a quart of oil a week
635905
 

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Again some blowby is normal and expected with our old engines.. If you have enough blowby pressure pushing the oil stick out of the tube and it's looking like a steam locomotive with the oil cap off ( like the example in the video I posted) then you got issues.
Otherwise, just drive and enjoy.
Is the grayish white smoke constant? Checked your glow plug system? Checked for any headgasket issues?
 

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1995 GMC C3500 SLE
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Discussion Starter #6
Again some blowby is normal and expected with our old engines.. If you have enough blowby pressure pushing the oil stick out of the tube and it's looking like a steam locomotive with the oil cap off ( like the example in the video I posted) then you got issues.
Otherwise, just drive and enjoy.
Is the grayish white smoke constant? Checked your glow plug system? Checked for any headgasket issues?
I replaced the glow plugs about 6 months ago with ACDelco OEM’s and as far as Head gasket issues I have not I’m thinking about doing head gaskets and head studs on here just for the sake of it but also I’m wondering if my valve seals are the reason for the oil consumption so might order a new set of heads as well and put them on
 

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I replaced the glow plugs about 6 months ago with ACDelco OEM’s and as far as Head gasket issues I have not I’m thinking about doing head gaskets and head studs on here just for the sake of it but also I’m wondering if my valve seals are the reason for the oil consumption so might order a new set of heads as well and put them on
If the smoke smells sweet that is a sign of a head gasket issue.
Bubbles in the coolant tank or a hard upper rad hose is another sign.

Heads on the 6.5L are known to crack between the valves. If you remove them have them checked professionally.
 

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Discussion Starter #8
Yeah probably will but would I possibly cause damage if I just put new heads on?
 

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Yeah probably will but would I possibly cause damage if I just put new heads on?
New heads will be fine. I would install ARP studs though
 

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Discussion Starter #10
Sweet thanks for all the help I’m still learning about this motor
 

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White smoke in experience pointed to injectors leaking.


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