Compression Testing

GordnadoCRD · **Posted:** Mon Sep 23, 2019 8:04 pm

thesameguy wrote:

GordnadoCRD wrote:

In the power distribution center (next to the battery) there is both a fuse and a relay labeled for the fuel injectors.

Pulling either one or both will allow you to crank the engine either normally, or with a normally-open, momentary-closed to jump the starter relay, with NO FUEL sprayed from the injectors.

The same effect can be achieved by just unplugging the fuel injectors.

I'm sorry if I'm being dense... but it's not just a matter of cranking with no fuel from the injectors, it's a matter of cranking with the injectors physically removed.

If the injectors aren't there, what stops diesel from spraying from the fuel rail? If the CP3 is timing belt driven, isn't it going to be pumping 2900psi of diesel out of the injector ports?

(The problem is in the past, but I'd still really like to understand the process!)

No, I guess I misunderstood how you are doing this.
Usually compression tests are done one cylinder at a time.
There is a cap that screws on the threaded port to cap the line that has been removed. Some fudge their own cap from crimping an old steel fuel line.
If you have problems sealing that, pull the electrical plug on the Fuel Pressure Solenoid screwed into the rear of the fuel rail. With no excitation, it cannot block the fuel from returning to the tank. There is no worry about the cp3, as it simply pumps the fuel back to the tank.
*WARNING: the CP3 is mechanically driven as long as the timing belt is in place. The CP3 MUST have fuel flowing through it, as that is the method of cleaning and lubrication. Without the presence of fuel flow, the internal parts will score, and may seize, destroying your pump!*

GordnadoCRD · **Posted:** Mon Sep 23, 2019 8:11 pm

RDBUL wrote:

thesameguy wrote:

I'm sorry if I'm being dense... but it's not just a matter of cranking with no fuel from the injectors, it's a matter of cranking with the injectors physically removed.

If the injectors aren't there, what stops diesel from spraying from the fuel rail? If the CP3 is timing belt driven, isn't it going to be pumping 2900psi of diesel out of the injector ports?

(The problem is in the past, but I'd still really like to understand the process!)

Remember, pressure is a byproduct of restricting the flow. With the system sealed (injectors connected), the de-energized injectors act as a "plug" in the system. When you crank on the engine, the fuel flows from the CP3 into the rail and "stops" when it gets to the injectors... thus allowing pressure to build. With the injectors hydraulically disconnected from the rail (like when your doing a compression test), the CP3 pump has no way to build pressure and fuel will barley dribble from the fuel rail connections.

As for disconnecting the regulator on the pump (aka: MPROP)... the regulator on the pump is a inlet volume control device that "springs" to the max volume position when de-energized (unplugged). So, unplugging it would would give you max volume/flow.

The regulator on the back of the rail (aka: DRV)... is a pressure control device that "springs" to the zero pressure position when de-energized (unplugged). So, unplugging it would "dump" any fuel in the rail back to the return circuit.

I hope this helps!

You got it!

GordnadoCRD · **Posted:** Mon Sep 23, 2019 8:20 pm

Here's my confusion.

If your compression tester works through the glow plug hole...
WHY are you removing the injectors?
They (or some replacement device) must be in place or no significant pressure will be built.
If your glow plug is removed
and if your injectors are disabled (fuse or relay)
Nothing else should be done regarding the fuel system.
There is no reason to loosen, disconnect, or remove any of the steel injector lines, and many reasons to not..

If your compression tester works through the injector hole...
WHY are you removing all the injectors?
They (or some replacement device) must be in place or no significant pressure will be built.
If your glow plug is removed
and if your injectors are disabled (fuse or relay or unplug the FPS at the rear of the rail)
Nothing else should be done regarding the fuel system.
There is no reason to loosen, disconnect, or remove any of the steel injector lines, and many reasons to not..

I'm in the dark as what journey you are embarking on with your engine.

Any compression related injury to the short block would be visible as soon as the head comes off.

TKB4 · **Posted:** Tue Sep 24, 2019 4:12 am

Gordon, the adapter he is using replaces an injector not a glow plug as I understand it.

Thesameguy
Doing the timing belt maintenance really isn't that difficult, its just different than all but a few other engines. Changing the head gasket is pretty straight forward just some tight places to get to etc . It is helpful to rent the VM tool to hold cam gears and the pins for intake and exhaust cams (which you can make fairly easily and use a counter hold tool) or get one of the alternative kits made to accomplish this such as this:
https://www.idparts.com/timing-belt-too ... -6235.html
There are also similar kits less expensive on eBay etc.

Personally, I can't see removing engine to do this work, seems it would be as much work to disconnect and reconnect everything especially if you remove front cap to remove engine as it would be to do the work. Guess I will know for sure how much work it is if I ever get around to removing engine with cracked cylinder to replace or repair it. But if you have oil leak etc and can't find or deal with source or want to upgrade torque converter anyway etc it might be worth it to me.

To each his own.

thesameguy · **Joined:** Sat Oct 01, 2016 8:24 pm **Posts:** 652

GordnadoCRD wrote:

Here's my confusion.
WHY are you removing the injectors?

The VM Motori tool for doing anything with the combustion chambers is VM.1072, which as you can see, replaces an injector:

This tool is used for leakdown tests, compression tests, or pressurizing the combustion chamber for valve stem seal replacement. It's referenced numerous times in this capacity in the FSM. There is no tool for doing this work through a glow plug hole, as is common on other diesels.

Quote:

Any compression related injury to the short block would be visible as soon as the head comes off.

That's simply not true. There is no way to accurately judge the condition of the piston rings by looking. If nothing else, whether they seal properly or not can only be determined when the engine is warm. A leakdown test can also provide information about the head gasket, valve guides, valve seals, and the valves themselves. It's a relatively quick test to learn a lot about an engine's health.

Here is a good article about leak down testing:

https://www.pegasusautoracing.com/docum ... =TECH00137

I'd personally never pull an engine out of a car without first doing at least a compression test followed by a leak down test if problems were indicated. Unless of course the intent is to fully rebuild it. In this instance, I really, really don't want to commit to that. Since the injectors must come out to use VM.1072 and I could not find a way to disable the pump, I skipped the compression test and moved straight to leak down testing all four cylinders.

Diesel Dan · **Posted:** Fri Sep 27, 2019 9:06 am

I do believe we made a compression adapter for our unimog from a old glow plug.
Drilled out the center and silver soldered a coupler to the body.

thesameguy · **Joined:** Sat Oct 01, 2016 8:24 pm **Posts:** 652

Yeah, I think that's the common approach. I suspect the reason that's not a thing on these Jeeps is because the glow plugs are very fine & fragile, and not something you want to handle more than absolutely necessary. Pulling glow plugs from these motors is like playing Operation. Also, access is very difficult, especially on the middle two plugs. I don't thing the glow plug solution would work at all in this situation. No doubt that's why they made a weird, expensive tool to test through an injector hole.

Compression Testing

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