Slow air pressure build

hogi6123 · October 13, 2020, 08:59:35 AM

Two months ago I timed my air pressure build and I remember it being about 1:45 from 90 to 125 psi.

Since then I have driven about 5000 miles and changed my engine oil. It was really thick black but an oil analysis said it was consistent with having been driven 9100 miles. (that was the first oil change since I bought the bus) I put in NAPA 40W oil and filter.

Now I think it takes at least twice as long to build from 90 to 125, though I haven't actually timed it.

My bus holds pressure fine once it's fully aired up.

My first guess would be compressor seals. What likely causes are there that would affect the pressure build time? What should I do to investigate the cause?

Thanks.

chessie4905 · October 13, 2020, 09:29:30 AM

Check for oil at tank drains. Some have a small muffler tank off compressor with a drain petcock also. Check there for oil also if yours has one.

Jim Eh. · October 14, 2020, 03:27:07 PM

Also look at the exhaust port of your air dryer for signs of oil contamination. You could be due to service your air dryer.

luvrbus · October 14, 2020, 04:03:14 PM

Most MCI 5 have a separate air cleaner for the compressor check for the filter and replace it,check the intake hose if you have one for kinks if the compressor doesn't breath they don't air very fast

hogi6123 · June 27, 2021, 04:42:32 AM

My compressor has two unloader pistons. They recently both stuck open and the compressor stopped putting out any air. I am now confident that the longer air build time happened when the first unloader piston stuck open.

windtrader · June 27, 2021, 09:19:42 PM

wonderful, please confirm after checking out. sounds like you did get it though!

Friday1 · June 29, 2021, 07:23:14 PM

miles. (that was the first oil change since I bought the bus) I put in NAPA 40W oil and filter.

Not to start another oil thread but check to see if that oil meets the spec for 2 stroke Detroits.

hogi6123 · June 30, 2021, 05:08:25 AM

It's been a long time since I did the research but the NAPA item number is 75118. 40w conventional heavy duty diesel motor oil. Comes in 5 gallon pails.

Edit: The pails say "NP75118", and something like "Meets cf-2 standard... Approved for 2-stroke Detroit Diesel 71/83/92". The description on the NAPA website says something completely different.

hogi6123 · July 04, 2021, 07:58:45 PM

Quote from: hogi6123 on June 27, 2021, 04:42:32 AM
My compressor has two unloader pistons. They recently both stuck open and the compressor stopped putting out any air. I am now confident that the longer air build time happened when the first unloader piston stuck open.

I replaced the unloaders (I could have cleaned them but new ones came with the compressor gasket kit). Tested them and they moved correctly, so reinstalled the compressor. Fired up the bus and compressor works again. It took 1:40 to build from 90 to about 130 psi, with Fast idle on.

Jsrcaptain · July 04, 2021, 09:03:32 PM

Can you replace the unloader valves without removing the entire air compressor or does the head need to come off?

hogi6123 · July 05, 2021, 05:30:45 AM

Quote from: Jsrcaptain on July 04, 2021, 09:03:32 PM
Can you replace the unloader valves without removing the entire air compressor or does the head need to come off?

The head needs to come off to remove the unloaders. That means the coolant needs to be drained.

If the in-floor engine access hatch is usable, the compressor does not have to be removed from the engine.

Dave5Cs · July 05, 2021, 07:33:02 AM

Replaced mine on my 700 Compressor. Though the access hole in the rear floor by removing the rectangle cover on top of the compressor about 2" x 1" and taking the two unloaders out with needle nose pliers and then inserting the new one close it back up and done in my 5C

hogi6123 · July 05, 2021, 11:37:29 AM

Quote from: Dave5Cs on July 05, 2021, 07:33:02 AM
Replaced mine on my 700 Compressor. Trough the access hole in the rear floor by removing the retangle cover on top of the compressor about 2" x 1" and taking the two unlooaders out with needle nose pliers and then inserting the new one close it back up and done in my 5C

My access hatch is covered over, so I had to remove the compressor from below to see what was going on (unloaders are on top by the floor). I had the compressor head off already so I didn't try taking unloaders out the side. The manual doesn't mention the possibility either but it makes sense that the unloaders can be plucked out as you said without removing the head.

Jsrcaptain · July 05, 2021, 12:40:14 PM

After some investigation, the valve on one cylinder is stuck open. Not the unloader. I'm thinking there may be something stuck under the valve keeping it from sealing. I'd like to avoid taking the head off. Any ideas? A blast of compressed air?

hogi6123 · July 05, 2021, 01:59:24 PM

Edited

Can you watch both unloader pistons move up and down completely? A blast of air shouldn't hurt anything. Remove the unloaders. The valve stems should slide freely. There are light springs pushing down on the valve disks, you should be able to see/feel the force when you push up on the valve stems.

The input valve disks ride up and down in a small cage. The valve disk could could be hanging up on the cage. Can you remove the valve stems through theCo intake hole? Then you have direct access to poke a wire up in there and feel around.

Taking the head off would make the troubleshooting fast and easy LOL. Just have to figure out how to store the coolant and get it back in.