compression numbers

[Yngdmax92]

Going to do a compression test tomorrow. Have researched up on this a lot but not on temps. I am going to do cold temp which it is around 45 degrees and then operating temp. What numbers will I see at cold... Or should be hoping for? Warm I know 380 to 410 are safe and with in 15 percent of each other.

 

[OregonDMAX]

Gm spec is within 10% anything above 350 is decent.

I have 275-305 across all my cylinders my motor is shot :thumb:

 

[duratothemax]

350 is low IMO.

A healthy high-mile duramax should still be decently above 400psi.

 

[Yngdmax92]

What is epexcted out of a cold engine? I read I. the past that I will get reading of 80 to 90 psi lower when cold. Just wondering.

 

[hondarider552]

I really don't think it makes a difference.

 

[ty_LBZ]

The pistons are sloppy when cold, the tolerances are forgiving than a gasser, it will certainly be lower when cold.

 

[Yngdmax92]

Well I'm doing both any ways. I'll let you guys know what the outcome is.

 

[S Phinney]

All stock ones that I have checked have been right at 450.

 

[duratothemax]

Ive never bothered checking compression with the engine hot because......well, its hot. Its a pain to work on.

With modern engine designs and tolerances, I would be surprised if you really saw THAT much of a difference testing cold vs hot. I wouldnt be hell bent on doing a "hot" test. A cold test will tell you basically everything you need to know, IMO.

Ben

 

[Yngdmax92]

symptoms of truck. start a little hard, one cylinder was farting, popping and going in an out until warm. idle fine after warm, no smoke or nothing. rev up truck to 1800-2500 rpms....truck would start to produce a blue type of smoke and had a little vibration that was not normal. I shut off number 8 injector, no more blue smoke and still have the same vibration and very minimal change in engine sound.

Before i started the truck engine was reading 35 degrees

cylinder 1- 400
cylinder 2- 400
cylinder 3 -400
cylinder 4- 400
cylinder 5 -400
cylinder 6- 400
cylinder 7- 390
cylinder 8- 375


Warm with engine temp at 161 degrees. cylinders 1-7 ranged from 430-450 and number 8 measured at 400. I tested first bump on 6 and it first bumped to 225, then retested 8 with a first bump of 200.

Very good note to you guys, again i broke glow plug off in number 8 before, i had to drill out the plug and long story short i screwed up the head so ended up drilling a bigger whole to fit a 1/4inch pipe plug in it. So to properly get it sealed and what not i had to use a 1/4" air fitting and install a small port into the fitting to hook my pressure gauge to it. So number 8 is reading so different because of the space that is open that the glow plug usually plugs up, i was pretty much getting a compression reading with it piped off with the fitting out side of the head, so i am certain the cylinder is still good to go.

 

[Yngdmax92]

Now back to the injectors thing, Injectors have literally 10,000 miles on them and have no idea what is going on with them but number 8 is defiantly not working properly and will be calling merchant automotive tomorrow to ask if they can do anything about it. I have a water serperator and cat filter on my truck, no reason it should fail at 10,000 miles and still dont understand what could possibly be going on with it to create such a blue haze at higher rpms.

 

[Mpsoldier]

Pull the intake tube off going into the motor and see of there is oil in the tubes.

 

[Yngdmax92]

I will be pulling the engine in the next couple of weeks anyways. I'll be sure to check that out but just wanted to make sure no bent rods/ weak compression were causing my issue. Doing it since my head needs replacing anyways and its a lot easier to do so while out of the truck