MD7A Compression

[weephee]

I recently purchased a used condition unknown md7a to replace my seized md6b. Before I install it, I want to check the compression. Specs call for 284 to 355 lbf/in squared at starter motor rpm. First question is when I do the test do I have the throttle wide open or closed or do I need to concern myself with throttle position for the test. 2nd question is what would be the minimum acceptable compression before I would need to check valves or rings. I assume that when I do the test, I would remove one injector then test then reinstall and remove the 2nd injector. Is that correct. Thx

[a64pilot]

This is a Diesel, so no throttle plate. Leave the throttle off. You are correct for a spark ignition engine to have it at full throttle.
I would pull all injectors so it will spin as fast as possible.
Now depending on how long it's sat, it's normal to be down on compression. If you have a borescope, that can tell a lot, and there are pretty decent ones that are USB powered and use a laptop as the display that are pretty cheap.
If it's down on compression, squirting a little oil in the cylinder can help determine if it's valves or rings.

[a64pilot]

Too much oil in a cylinder can hydro lock it, and that can bend a rod if spun with the starter, but if you spin it through a time or two by hand to check for a hydro lock, you'll be fine.

[Ex-Calif]

If you find a low cylinder then you have to figure out where the loss is.

Differential compression testers are nice. You enter a reference pressure, say 80 psi, send that across a fixed orifice. With a gauge on the downstream side and "perfect" seal you read 80 psi.

Using a dummy plug and air firing you apply the downstream supply to the cylinder, rotate the piston to TDC compression stroke and read the difference. Number like 80/78 are common.

The best thing about this is with a low cylinder you can hear the leak through the breather - pistons, exhaust - exhaust valve, intake - intake valve.

[weephee]

Thanks for the replies. I will let you know what results I come up with in a few days

[barnakiel]

Open the decomp lever, turn it over a couple of times. Then same, with the starter. Then close the decomp.

Now grab the wheel and try to turn it over.

You cannot, good compression.

You can, bad compression.

MD7 will start way beyond compression is BAD. With bad comp, it will smoke: a lot when cold, less when hot. Even when hot, it will smear the cooling water with oil.

They have no jackets. Fixing compression problem can be lengthy and may cost $$$.

PS If you have bad ring(s) you may find it next to impossible to buy new ones. If you can, they will be outrageously expensive. TIP they are said to be Nissan ...

Do not ask me how I know ;-)

Cheers,
b.

[hpeer]

I do t know if my compression is good or bad BUT..

She starts real easy, REAL easy
Does NOT smoke
Leaves no oil on the water
But...
I can spin her through by hand IF I go slow and easy.

So, just for clarity, I think you mean to say "grab the wheel and turn it over with some determination."

I'm asking as much as saying.

[Ex-Calif]

I don't like to recommend this but if you are brave put the palm of your hand over the intake and crank.

If it doesn't feel like your hand is gonna get sucked in the intake you likely have low compression. On mine when the valve seats finally gave out there was practically no suction at all.