Checking cranking compression on a diesel

ScottNC

Member
I want to make up a compression tester for my DB885. Is gutting and modifying an old injector the easiest solution to hooking the gauge hose to the engine? What is the cheapest gauge that'll work? This'll probably be only a one or two time use unit.

I am assuming the injectors seal compression via some sort of washer (Cat) or cup in the head (Cummins). By fitting a different injector or in this case a compression gauge adapter, aka old injector, is there a danger of creating a leak?


Also, with the injectors out can hoses simply be stuck on the fuel lines and directed into a container?
 
(quoted from post at 10:43:28 07/17/18) I want to make up a compression tester for my DB885. Is gutting and modifying an old injector the easiest solution to hooking the gauge hose to the engine? What is the cheapest gauge that'll work? This'll probably be only a one or two time use unit.

I am assuming the injectors seal compression via some sort of washer (Cat) or cup in the head (Cummins). By fitting a different injector or in this case a compression gauge adapter, aka old injector, is there a danger of creating a leak?


Also, with the injectors out can hoses simply be stuck on the fuel lines and directed into a container?

You will have to fabricate a check ball arrangement to trap the compression from leaking back into the cylinder, commercial built ones have a button to release the pressure, a gutted injector would be ok as a adapter but from personal experience I can tell you that the quality and length of the connecting hose will alter your readings. It is not the norm to do running compression tests but have done it myself. Let us know how it turns out
 

Just spotted one from Harbor Freight for $30. Heck if it has an adapter that fits the DB it'd not be worth the effort to cook-up a scratch built piece.
 
(quoted from post at 12:01:47 07/17/18)
Just spotted one from Harbor Freight for $30. Heck if it has an adapter that fits the DB it'd not be worth the effort to cook-up a scratch built piece.

It should have a minimum of 500 lb gauge, Harbour Freight has a dedicated diesel set but well over 150 if I remember correctly.
 
I bought this compression tester for cheap off the online auction, but it did not come with directions, so I could never figure out how to use it. I'm pretty sure that it is identical to the ones sold at HF. The problem I had with this set is that the check ball is inside that angled silver colored fitting. That is the only check ball in the whole set! Ideally, each one of those adapters should contain a check ball at its tip (to minimize falsely low compression readings).

I have attached another photo of the gauge hooked up to the David Brown adapter (with a real DB injector next to it). Notice there is no check ball in the system as it is hooked up. I could not figure out how to properly incorporate the ONE CHECK BALL into this circuit.

I'm dumb, I know. I tried to jury rig a connection with the check ball using several pipe fittings, but I kept on losing the accumulated compression through leaks (the schrader valve near the gauge kept leaking too). I finally gave up.

Let me know how you end up doing your compression test. Thanks
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The $30 Harbor freight unit was listed as being for diesel. I checked Amazon also where there were comparable looking sets for near the same price, if fact one was slightly cheaper. I read the reviews for the cheap one and about 100% of respondents had the leaking check valve issue. Harbor Freight reviews had maybe a 50% issue w/the check valve too. Since the H.F store is nearby, returning it if defective is painless.

I'm gonna give it a shot this weekend and will report back.

Any idea what kind of number a DB885 should make to eliminate low compression as the reason for hard starting?
 
(quoted from post at 09:03:32 07/18/18)
The $30 Harbor freight unit was listed as being for diesel. I checked Amazon also where there were comparable looking sets for near the same price, if fact one was slightly cheaper. I read the reviews for the cheap one and about 100% of respondents had the leaking check valve issue. Harbor Freight reviews had maybe a 50% issue w/the check valve too. Since the H.F store is nearby, returning it if defective is painless.

I'm gonna give it a shot this weekend and will report back.

Any idea what kind of number a DB885 should make to eliminate low compression as the reason for hard starting?

I have the more expensive HF unit and the check valve has been perfect on mine so far, I am going to guess with a little bit of knowledge that 350-400 lbs cranking should be sufficient, There are people more well versed on here who may have a more educated opinion on this but my 530-188 gives about that and starts very nicely.
 
The HF kit is fine, it has the schader valve built it.
A gutted injector is fine but you'll have to adapt a fitting to connect to it.. {very easy}
OR the kit may have something you can use right out of the box.??
The inj. seals w/ a copper gasket on the bottom.. just use one off your engine injectors..
just don't crush the h*ll outa it if you plan on reusing it..
Your looking for a "problem" I'm assuming.. so anything under 300psi is gonna be "it"..
If your looking for a skip or miss, the injectors are likely the culprit.
Good luck w/ your test & let us know how you make out.
 
(quoted from post at 07:53:32 07/18/18) The HF kit is fine, it has the schader valve built it.
A gutted injector is fine but you'll have to adapt a fitting to connect to it.. {very easy}
OR the kit may have something you can use right out of the box.??
The inj. seals w/ a copper gasket on the bottom.. just use one off your engine injectors..
just don't crush the h*ll outa it if you plan on reusing it..
Your looking for a "problem" I'm assuming.. so anything under 300psi is gonna be "it"..
If your looking for a skip or miss, the injectors are likely the culprit.
Good luck w/ your test & let us know how you make out.


You nailed! A "problem". The drug of choice is ether. Figure to start with compression. It puffs smoke at the turn of the key but will not start without a snort. New filters, thorough bleeding, new battery, new 2-O cables fresh, clean connections and lastly, a new thermostart.
 
(quoted from post at 11:46:02 07/18/18)
Forgot to include... lashed the valves.

On my HF kit the adapter for the two bolt injectors was a bit to wide to go down over the injector studs, I used a die grinder and relieved the inside of the slots in next to the threaded adapter to get it to fit.
 

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