Harbor Freight Compression Tester: Two different gauges/two different readings!
 

I recently purchased a 1974 240D. 170k miles documented. New short block at 70k after original owner damanged the oil pan and lost oil pressure.

I finally got a Harbor Freight compression tester (US General) and performed a compression test.

Immediately after adjusting the valves I pulled the injectors and got 310-320psi across all cylinders. Smug and happy with that, I didn't bother with a wet test.

For the heck of it, I borrowed another gauge (same Harbor Freight) and repeated the test.

This time around I got only 270-290 psi at each cylinder and 320ish wet.

The car isn't using any oil (well, not in the 300 miles since I changed it) and fires up on the first or second turn after being glowed. On warm days I don't even have to glow. As a matter of fact, it almost tried to light off with all fuel injectors out after I added two teaspoons of oil for the wet test.

That being said, there is some smoke coming out of the oil fill at idle and also some oil leaking around the crankcase vent hose on the intake side. The lid will hover, but won't go flying across the engine compartment. It also idles rough until it's warmed up but then is smooth as butter.

Given the mileage, I thought that the 325 figure would be about right. But the symptoms listed above make me think the lower numbers are more accurate.

Which ones do I go with?

In any event, there seems to be a huge variance in US General Compression Gauges due to quality control issues. Some people out there might be fretting unnecessarily while others could be overly confident.

The variation between gauges is only a few %, for cheap gauges thats to be expected.
You dont have 1 cylinder way lower than the others.
Best you just drive the car & enjoy it.
A new seal on the oil filler & a clamp on the breather may be a good idea so as to keep things clean.

"Harbor Freight" and "quality" are pretty much mutually exclusive concepts.

Thanks!

Seen my goof about the smoke out the oil fill. Edited that.

I replaced the oil filler cap seal after doing the valves and will be ordering a new breather hose soon.

My frame of reference of diesels is very limited. When I see a level that's 25% below that of a new engine, I'm thinking it's shot....

I might call up the local Caterpillar shop tomorrow and see if they have something with a known and accurate compression reading and see how this Harbor Freight gauge compares. It could be useful for folks to know how far off these cheaper gauges might be.

I'll back up what the others said....the spread means even more than individual numbers. The closer they are to each other, the smoother running it will be. As the engine wears out....that spread tends to widen.

And yeah...Harbor freight stuff is known for being cheap....and you will pay for quality gauges (as in they aren't cheap). Cheap is good for quick and dirty readings for a shade tree mechanic....but a pro will get quality tools for when precision matters.

If it was blowing smoke out the oil filler like a steam train & had pressures around 150 , I would be worried.

Perhaps you need to do some injector work: Cleaning and pop pressure test.

Richard

The previous owner installed some rebuilt (Indian?) Bosch injectors about a year and 1/2 ago. Not that means all is well, mind you...

I did notice the lack of proper crush washers when I removed the fuel injectors. I also found some of the valves out of adjustment.

Lastly, I plan on changing the fuel filters this afternoon.

We'll see if all of that makes things any better.

If not, my next project was to have someone in the know to give the IP a good look over.

I shudder to think about replacing the injector nozzles already. I'm out enough money as it is...

OK, Gents. I found this rather authoritative post elsewhere and it seems convincing enough to me.

We've all (or most of us anyway) heard the term "compression testing you motor".

What many people don't realize (as I didn't either for a long time), is that simply going out and buying a compression tester and slapping it in the sparkplug hole may not give you the correct reading you think it might.

There are many things that need to be taken into consideration when compression testing a motor.

One of them is the length of the compression tester tip.
The tester tip should match the length of the sparkplug tip that is used in your motor as recommended by the manufacturer.

Another thing to try to avoid are attachments such as thread adapters that will effectively increase the volume of the head, thereby reducing the pressure read. (the actual pressure in the cylinder will be higher than indicated on the gauge).

Avoid the rubber style "push into the sparkplug hole and then hold it" compression testers. These are inaccurate at best, and often cannot seal the pressure of many high pressure set-ups.

Cheap compression testers (screw in type) will not give you true cylinder pressure readings, HOWEVER, they CAN be used for comparison purposes to determine if rings are getting worn AS LONG AS THE SAME TESTER AND FITTINGS WERE USED WHEN THE MOTOR WAS FRESH, AND READINGS WERE WRITTEN DOWN.

Just adding one adapter can reduce the reading by 20 lbs or more!!

The "best" compression tester to use, is the "Snap-on" line of testers.

They have the schrader valve inside the tip itself, and are very accurate when taking pressure readings.

So next time you hear someone say they have "X" amount of lbs of cylinder pressure, or "your rings are ok if you still have "X" amount of cylinder pressure, take it with a grain of salt.

Because unless they have used the above procedure, OR have spent lots of money on a truly accurate compression tester, chances are they don't REALLY know.
Nor do most of your "average Joe" riders want to buy the expensive set-up to really know what the "actual" cylinder pressure is, or should be."

http://www.allthingsmoto.com/forums/f-114/good-compression-gauge-11506/

Given the number of adapters used and the extra volume added by the tubing itself, I'm quite sure I've lost a few dozen psi that way.

As several of you have pointed it, it's the consistency that matters.

Stick a fork in this 'fread. It's done.

Thanks all!

Edit: further along in the thread I linked to is an even more in depth explanation.