01 Silverado Compression Test Complete

Discussion in 'Chevy Silverado Forum (GMC Sierra)' started by OE812, Oct 17, 2010.

  1. OE812

    OE812 Rockstar

    I have completed the compression test on my 5.3L. The engine will soon turn over 170,000 miles. The values are Cylinnder, 1st crank, and max:


    I don't know what is a "good" value. The Chilton's book states that the minimum allowed is 100psi. Does this mean on the 1st crank or the max:rules:? What is an average for our 5.3L?
  2. stephan

    stephan Rockstar 4 Years 5000 Posts

    It is generally accepted that the engine compression should be as high as possible after 2-4 complete revolutions, but getting high numbers after the first or second revolution is great as it indicates very good ring seal. It is also imperative you check comp. with engine at normal operating temperature so everything has expanded to normal size (pistons, rings, cylinder walls) and you get a true reading of ring to cyl wall seal. Compression readings that increase in small steps with every revolution denote poor ring sealing. When in doubt do a leak down test. Leak down should also be done with engine hot)
  3. OE812

    OE812 Rockstar


    Thanks for the information but I don't know how to do a leak down test. I did have carbon on all of the plugs (not on the tip or insulator but on the heel) which flaked off when I scraped it with my fingernail. Could this indicate a fuel or other problem? Could you explain how to do a leak down test and what tools I will need?
  4. stephan

    stephan Rockstar 4 Years 5000 Posts

    I was just getting ready to edit my post when I saw your reply.
    I wanted to know how many revolutions did it take to get to your 190-200 readings? The first revolution readings are good, but 190-200 seems kind of high and I'm thinking the guage is inaccurate to read that high, or your eng. has a lot of carbon in the combustion chambers causing high compression readings. This could also be caused by an old c. guage that is weak, or a cheap guage that just reads abnormally high.
    Carbon on your plugs indicates oil being burned in the cylinder. It could be caused from rings, valve guides, valve stem seals. Other junk on the plugs can be caused by a lot of stop and go driving, fuel deposits, short trips etc.
    Does your engine use a lot of oil? Does it smoke out the exhaust? When you park your truck hot & let it sit for an hour, then go back out & start it, do you get smoke from the exhaust?

    You would need a leakdown tester ($80.-$125.) for a good one, not worth buying unless you plan on doing a lot of vehicles or are in the biz. A leak down test is done with engine hot, & shut off, each sucessive cylinder on TDC, and leak down hose screwed into spark plug hole. You pump air into the cylinder and the guage reads amount that is getting past the rings & valves.
  5. OE812

    OE812 Rockstar

    I just bought the compression gauge from Autozone yesterday for $40 so I hope it is accurate. Anyway, the truck doesn't use any oil over a 3-4000 mile period. To answer some of the other questions. I do not get any smoke either in the am cold start or after the truck sits for an hour then starting. I do have the dreaded lifter clacking when temp is cool outside which lasts for about a minute.

    Getting back to the compression test, I followed the gauge operating instructions which stated to crank it 7 times to see where it leveled out. yAfter 4 cranks it leveled out but I went to 7 anyway.

    Regarding carbon in the chamber, I did a removal of the throttle body yesterday and decarboned the throttle, plate, IAC, and TPS. I also performed the TSB change on the PCV and throttle and adjusted the idle air screw to 500-600 rpm idle. The truck idles without a miss. While the throttle body was off of the truck, I poured a half can of seafoam down the intake and let it set for about an hour. Large quantities of smoke emitted once everything was reassembled and truck started.

    Thanks for the information and help on this.
  6. stephan

    stephan Rockstar 4 Years 5000 Posts

    OK, $40. for a guage, is a good one then. Since you don't use any oil & you never see any smoke, the carbon is probably just from the 170,000 miles accumulation in the comb. chambers. It sounds like this is a good runner for that many miles. :)

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