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Compression Test

Uncle Wolfie
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Uncle Wolfie Enthusiast

Uncle Wolfie

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Hi Everyone I did a compression test on my 2002 gmc sierra It has a SL - V8 - 5.3L 5328cc 325ci I am looking for the compression spec and can't seem to find it on the web I am getting about 135 to 145 on each cylinder. Not sure what the spec is. Any help would be great.  The engine is running like crap so I am doing new plugs and wires and a compression test to start with.  When at idle the tail pipe has a very strong smell of gas. When I back up the truck bucks. After a few minutes when the truck warms up and I step on the gas there is a lot of smoke out the tail pipe. Rough starting as well. Those are the issues I am having. Her is a link address on you tube 

https://youtu.be/xkrSI28xHBc  This is a video I made of the truck on rough idle. Thank you for any help.

Mike GMC Enthusiast

Mike GMC

Posted
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The numbers you get from a compression test will vary somewhat depending on the altitude at your location.  In theory, a perfect test result is the compression ratio times the atmospheric pressure.  So for a 10.25 cr and an atmospheric pressure of 14.7 psi, a perfect test would be 150 psi.  Mostly what you should be looking for is a very small variance from cylinder to cylinder.  A total variance under 10%  is usually considered good.  You didn't mention the individual cylinder number, but if the low was 135 and the high was 145, it seems ok to me.

 

From your description of the issues, it seems you may be running rich.  I'd be looking at the O2 sensors and maybe the MAF and IAC valve.

Uncle Wolfie Enthusiast

Uncle Wolfie

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Very good ty I was thinking O2 myself. Where is the MAC and IAC located and do you have a pic for me to look at on those valves.  

Mike GMC Enthusiast

Mike GMC

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IAC is on the throttle body and the MAF is between the air filter box and the throttle body in the intake tube.

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Uncle Wolfie Enthusiast

Uncle Wolfie

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So here is an up date. I put in all new wires and plugs and did a compression test. The Compression test look good  all between 135 and 145 so that ruled out anything broke in the engine. But what I did find out is one of the plug had gas in it. I could dump the clean gas out of the plug on to my table. I repeated that process twice and both times when I took the plug out it had gas in it. Question could a dirty fuel injector cause that ? I also put a spark plug tester on the wire and started the truck and I had good spark coming off the test wire, I saw the light coming on and off.  I am thinking dirty fuel injectors. 

Her is a video on you tube of me stepping on the gas and the truck smoking. The smoke has condensation  dripping out the tail pipe and a very bad smell of gas.      https://youtu.be/Am_GreAFMsU  Please Help. TY Regards Wolfie

davester Senior Enthusiast

davester

Posted
Posted

Is it only that one spark plug that had lots of gas?

If so, maybe try swapping the injector for that cylinder with another cylinder, say, on the other side of the engine, and see if the gas on the spark plug problem follows it.  If it does, I would just replace the injector.

If you have a more advanced code reader that can do diagnostics, you could do an injector leakdown test.  That would tell you if one injector is putting out too much gas, but not if one is leaking.

Uncle Wolfie Enthusiast

Uncle Wolfie

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2 had gas in them by the way. I will move them to the other side of the motor Drivers side. That bank was clean no issues. I think they are leaking. I will work on that and see what happens. 

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