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Compression test!

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Old 04-11-2011, 04:08 PM
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Compression test!

So I have decided to do a comp test to see if I'm having issues I've overlooked. I assumed with only having 125,000 miles on the truck I would have a good motor, now I question my theory! I'm getting a gauge from a guy at work but I need to know what size the fitting and what style to use? Also where should my pressures be? I'm almost scared of what I'll find! Advice and encouragement will be greatly appreciated!!
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Old 04-11-2011, 04:36 PM
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I have never done a compression test on a 7.3l but everything I have read on here and other places indicates ~350 PSI+ to be good compression, 320 is acceptable as long as you are balanced.

Wish I knew the size fitting to use, if you have an old glow plug lying around use that to find the right fitting......


go David go, you can do it!!

Last edited by clem1226; 04-11-2011 at 04:38 PM. Reason: adding encouragement
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Old 04-11-2011, 06:57 PM
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Dumb ? But I just found out the guys gauge only goes up to 250# so thats out. Where do I find one? Is it a special gauge?
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Old 04-11-2011, 06:57 PM
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Dumb ? But I just found out the guys gauge only goes up to 250# so thats out. Where do I find one? Is it a special gauge? Still don't know the fitting size.
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Old 04-11-2011, 07:06 PM
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Not sure what the numbers are supposed to be but I have the correct fitting that screws in to the glow plug hole. If you want I can send it to you to use. Unfortunately I do not have the gauge to go with it.
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Old 04-11-2011, 07:33 PM
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The fitting is a 10MMX1MM O-ring boss thread, I bought a diesel comp gauge from harbor freight then used an 1/8" grease gun whip to connect to the adapter I bought from my parker hydraulic store, which goes from the 10mm to 1/8" female. FYI my #'s were all 375PSI except for my bad cylinder which only had 75
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Old 04-11-2011, 07:43 PM
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Big Al has got you covered. Here's a link to an adapter also.
Mityvac , MVA5605 Diesel Glow Plug Adapter - Ford Navistar 7.3L Power Stroke
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Old 04-11-2011, 07:45 PM
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Here's the information I used when I made mine...

These tools are a necessity for testing compression on the 7.3L Powerstroke diesel. The reasoning for that is there is insufficient clearance between the rocker arm for the angle that the glow plug port goes in for a standard straight adapter to work. It will hit the rocker arm. No fun.

The nice thing about this particular setup is that it can be used on ALL 8 cylinders. You have to mess with the hose a bit on the back cylinders, and you have to noodle past the A/C Compressor for cylinder #1, but it will fit. At least it did on my 99.5 Super Duty.

As a bonus! I've seen a few people on here have TDI's- The 7.3L Powerstroke and the early TDI's (1Z, AHU, and ALH engines) have the same glow plug thread pitch- This tool can ease getting a reading as you don't have to snake the compression tool down amongst the injection lines. Bonus!

Your starting goodies-
An economy diesel compression tester. I know for a fact that the Harbor Freight tool #93644 ( 12 Piece Diesel Engine Compression Tester ) has the 1/8" NPT adapter we will need. So I recommend it.

You'll need a 1/8" NPT female-to-female connector (I call them barrels)
You can get the just about anywhere that has a brass fitting selection. I got mine from Parker Store as it was easier to get it at the same time as our next piece.

You'll need an adapter to fit the actual glow plug port. Parker Store part #M10-1/8F8OHGS accomplishes this. It is our glow plug port thread pitch, M10-1.0, in an O-Ring boss male fitting on one side, but on the other adapts to 1/8"NPT female threads.

You can see if there is a Parker Store near you by searching at McCoy Sales Corporation - Manufacturers Representative and Distributor

If not, here is an internet alternative- ( M10-1/8F8OHGS MROstop.com )

You will need a grease gun hose whip. Not the kind with a fitted grease fitting on one end, but the kind with 1/8"NPT male fittings on both ends. I got mine from Ace, but noone seems to have the 12" whips online.
You can get them pretty much any auto supply store. Autozone should sell a 12" whip for about $4.99. I think I paid $6 at Ace. Either way, it doesn't matter.

You will also either need pipe dope or teflon tape. I won't link that as it should be obvious to anyone who got this far in the writeup.

At this point, lets see....
I paid roughly-
$30 for the compression tester.
$6 for the grease whip.
$1.50 for the barrel adapter
$12 for the M10-1.0 to 1/8" NPT adapter
And I have a healthy supply of teflon tape on hand.

So, thats roughly $50 into this venture, about $20 into the adapter. Beats the price of just the adapter from Snap-On.
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Old 04-11-2011, 07:55 PM
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Here's some pics of the setup.

The adapter


Adapter attached to hose whip


The 1/8" NPT quick disconnect and barrel attached to the other end of the whip.


My offer from an earlier thread still stands though if you can wait the time it will take to ship I will send you mine to use?
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Old 04-11-2011, 08:09 PM
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Thanks guys, just what I was looking for. Allen did you remove all the gp's before
You started the test? Reason being I just had all the injectors out on the ds so I need to remove them to clear the cylinder anyways. Id like to leave them out if possible!
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Old 04-11-2011, 08:12 PM
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Yes leave all GP's out while testing.
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Old 04-11-2011, 10:17 PM
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Here's a old but good thread on compression numbers.
I haven't seen Mike(Tenn01PSD350) in a while?

http://www.ford-trucks.com/forums/86...n-numbers.html
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Old 04-11-2011, 10:55 PM
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Humm, I haven't tried the starting fluid bit yet. That must of been exciting!
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Old 04-12-2011, 11:18 AM
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Quote:
Originally Posted by BigAlsPSD View Post
And I have a healthy supply of teflon tape on hand.
Just for clarity, use teflon tape on the adapter that screws into the GP hole?
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Old 04-12-2011, 03:15 PM
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Quote:
Originally Posted by Mike F250 View Post
Just for clarity, use teflon tape on the adapter that screws into the GP hole?
I think the teflon tape is for everything else that screws together. I think the glow plug deal has an o ring on it to seal it.
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