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What is the procedure to do a compression check

9K views 60 replies 10 participants last post by  clydesdale 
#1 ·
Do you put the tester in the GP hole, disconnect both injector harnessess and crank for a few seconds? That is a total guess and have never done this before, so fill me in. I am going to replace my LL8 and thought it might be good to check it while I am there. What compression is expected and where can I purchase the tool. The compression gauge that I have is good for lawn mowers and such, so I doubt it goes high enough. Thanks.
 
#2 ·
After searching the archives, it looks like I am supposed to pull all 8 GPs. But, I am only changing the one injector and I don't want to mess with things I don't need to. Can I pull just the one single GP and check the compression on the 8 cylinder. I am not really having any major symptoms. No smoke, no missing, no stumble, just an idle knock and failed CCT on my LL8. But, I thought I would check the compression while it was opened up.
 
#9 ·
From what I gather, the other cylinders might contribute to resistance of the spinning motor which is important to the compression. Most compression tests I have seen revolve around taking out all the other plugs and testing them all so you have comparison numbers between each cylinder.
 
#6 ·
I have to unplug the driver's side to replace the LL8. I also have to unplug the passenger side so that the truck won't fire when I turn it over to clear the fuel and oil from the #8 cylinder. So, can't I just leave the cps alone? Thanks.
 
#7 ·
You also have to unplug the 42 pin connector to remove the driver side valve cover. With that unplugged, the passenger side UVCH is disconnected, the CPS is disconnected, same with every other engine sensor.

So you are correct, you don't have to remove anything, you don't even have to unplug the passenger side. Just make sure the 42 pin connector is unplugged.
 
#8 ·
...and be sure to check compression BEFORE you pull the old injector.

Once you pull an injector, and replace it with the new one...you will get *some* fluids down into the cylinder which will skew the compression readings (higher than actual).

Excellent compression is 400-425psi
Good compression is 375-400
Marginal compression is 340-375
Poor compression is 290-330 or so...
dead cylinders are below 200psi
 
#11 ·
OK Golfer, that makes sense. Thanks for posting the compressions. If i do this, I will do it with the old injector in. It is not quite clear as to whether or not I can do this for just the one cylinder. Do you think it will be ok to do just the one cylinder? I don't think I have an issue anyway, so I really don't want to unplug all 8 GPs or even pull the driver's side cover. Can a compression tester that will go to 425 be bought in Advance or is that more like a specialty tool that needs to be found online?
Airsley, what and where is the 42 pin connector? I thought I was going to unplug the uvch on each of the two sides. Is there one connector that disconnects both? Thanks guys.
 
#12 ·
The 42 pin is the main engine harness that is attached to the drivers valve cover gasket.You pull that she ain't firing.

I thought that pulling all GP was the reason that you specified aklim but didn't want to mislead cause I wasn't 100% if that was true or not.

Any comp gauge that reads high enough will work.It is the damn GP adapter that you need to work properly.Make sure you get a 7.3 POWERSTROKE not 7.3 gp adapter.There is a difference.
 
#13 ·
you can buy and $80 dollar diesel compression testing kit at napa that has the right adapter, I got the kit last year and it works well.
Also, another way to keep it from firing is to just remove the idm relay, that way you don't have to diconnect any wires (unless you already have during valve cover removal)
 
#15 ·
Yeah, I was thinking harbor freight also. I will look around the net also. I was hoping for less than $80. Thanks.
 
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#21 ·
Well, I bought the cheaper one from harbor freight. If I need to remove rockers, it ain't happening. But it has a long flexible hose that attaches to adapters that look like they screw into the GP hole. The gauge has a long flexible hose. Does that sound like one that will work, or will that not have the same effect, in theory, as the adapter that Tom posted. Otherwise, I will order the adapter for the next time. But, hopefully I won't ever NEED one.
 
#23 ·
I will go slow and be extremely careful. If it doesn't line up, forget it. I has solid adapters that look like they screw into the GP and then the rubber looking hose that attaches to that and the gauge. Hopefully it will work.
 
#25 ·
I am curious if the quick connect on that adapter is pretty much universal to the other set ups like Harbor Freight. Kainers is right that flex adapter is easy to use in the truck.

Tom
 
#26 ·
I don't get the flex adapter deal. Won't that be the same as having the solid gp adapter and then a flexible hose? Will the flexible adapter that you guys are talking about, fit the harbor freight gauge?
Looks like it will be raining in NY again tomorrow so this project might get pushed off another weekend. If it does, I will order the flexible adapter on Monday to make sure that I have something that will work. I appreciate you guys walking me through this. Thanks.
 
#27 ·
I think you needed the flex adapter to be just like it is skinny with no connectors till you get past the top of the head. I also think the flex adapter had a slight bend in in when installed.
 
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