Crankcase Pressure Testing

Tanner Instructor Wellford, South Carolina Posted   Latest  

I recently saw a post showing the use of a manometer being used for crankcase pressure testing. I have seen several different ways through the years of doing this Including some that use a scope.

what is your preferred tool to use for this? Also how are you going about doing your tests step by step. Where are you finding your information for what is an acceptable amount? 

Keith Instructor
Collinsville, Oklahoma

I use a MAP sensor with a custom made 5 vdc steady power supply that houses the BNC port for connection to a scope. I have built custom Pico Probe files so I can view the measurement in mBar, PSI, and inHg... Along with the kit is a variation of PCV and oil cap adapters I have made to attach it to a variety of vehicles.

+2 Ð Bounty Awarded
Chris Diagnostician
Commack, New York

Funny you should ask Tanner. Amazon sells a really inexpensive digital manometer.

This manometer is cheap, portable and accurate. As for a spec, from what I've seen, a good rule of thumb spec is 4-7 inches of water. This is why a manometer is important because your regular vacuum gauge isn't accurate that low as you can see here.

and here

*images courtesy of amazon and Alldata

+4 Ð Bounty Awarded
Joel Technician
Belfast, Maine

Buying a digital manometer has been on my to-do-list form 6 months but I wasn't sure which one to buy. Thank you for this post. I just ordered one from Amazon.

0 Ð Bounty Awarded
Brady Owner/Technician
Yakima, Washington

I use a magnehelic gauge for testing crankcase pressure in inches of water column. I've been doing it that way for years and I couldn't say for sure why I settled on that unit of measurement. As far as what is acceptable, I take known good readings and record them. Most vehicle's fall between 2-8 inches of water column in my experience.

I also use my homeamde "first look" style sensor and pico to check crankcase differential pressure, mainly used to see if I have a piston ring sealing issue. 

0 Ð Bounty Awarded
Luke Instructor
Buellton, California

I use a handheld digital manometer similar to what Chris Martino showed. The particular model I use is the 478a made by Dwyer. I like this one because it has a range of -60in to +60in water. It also has some neat features like a min max recorder and the ability to switch between measurement units. I made a simple adapter for quick measurements by drilling a hole in a new oil cap and glueing a vacuum nipple onto it.

Personally I measure in mBar. But that is because I work on BMW and they use that unit of measurement for all their pressure values. On the back of the tool I have one of those stick on card holders you see being handed out as swag that is supposed to go on a cell phone. I keep a piece of paper in there with some nominal values written on it.

After several decades of maintaining a spec of 12 - 14 mBar of negative pressure differential in the crankcase, BMW has started holding a greater vacuum. Current engines have a spec of 38 mBar! This is done in an effort to both better seal the rings and reduce pumping losses in the crankcase.

+5 Ð Bounty Awarded
Ray Diagnostician
North York, Ontario

The first waveform is from an 07 Corolla.

I have a WPS500 pressure transducer on the dipstick tube at idle to measure the blow by pressures and the pcv vacuum for each cylinder in the crankcase.

The crankcase pressures for each cylinder measures positive 7 inches of water column.

The pcv vacuum for each cylinder in the crankcase measures negative 4.8 inches of water column.

The second wave form is from an 06 Escape 3.0 with some fuel flooded cylinders because of a defective PCM.

The red CH B is a First Look pressure transducer on the dipstick tube and the larger crankcase pressure pulses line up under the low compression cylinders.

The green CH C is the starter amps with normal and low compression cylinders.

The tan CH D is a WPS500 pressure transducer in cyl 2. The max compression measures 77 psi, the compression/expansion stroke has a leaning tower and there is a large vacuum pocket at the bottom of the expansion stoke which looks like a leaking exhaust valve.

When I put some oil in cyl 2, the compression came up to 200 psi.

The PCM was keeping some of the injectors on for up to 188ms.

A used PCm fixed it.


+2 Ð Bounty Awarded
Ronny Owner
Culver City, California

We use Magna with modified oil cap adapter.

+1 Ð Bounty Awarded
Matthew Owner/Technician
Grant Park, Illinois

Hi Tanner, I hope all is well!

I use my favorite tool, the Pico WPS 500. I test crankcase pressure for two reasons:

  • Testing excessive engine blow-by (usually diesel trucks)
  • Finding the reason for low compression (my experience here is with gas engines). 

To test diesel blow-by an orifice is usually required. Since most heavy truck engines simply have a filter system excessive blow-by would not be seen looking at dipstick pressure - they are vented to atmosphere through a filter. When an orifice is installed in the vent tube pressure can be tested. Not going to lie - I have yet to purchase an OEM orifice special tool. I usually combine some pipe fittings to fit in the vent with a tee that hooks up to my WPS. I have a long hose that I can run into the cab of the truck for test drives, since the specs are usually under full engine load. I then drill a hole in a pipe thread adapter to fit the orifice spec in the service info. OEMs usually recommend a manometer or some type of transducer (like a meter). The WPS in my opinion is far more accurate and durable. Check out this waveform of a International Prostar with a Cummins that was reaching the blow-by spec limit, it had excessive regen issues. Time for an in-chassis. 

If an engine has low compression I have started to experiment with pressure at the dipstick. The idea is to find where the pressure went - I have confirmed with a relative compression test that a cylinder is low. Moving my transducer around (intake, exhaust, and crankcase) I can try and find where the compression went. In this waveform you can see a pressure pulse in the crankcase where the relative compression is low. The helpful part here is determining the difference between recommending an engine or cylinder head work. I also made an adapter with a foster fitting that fits into my Lisle oil filler funnel adapters for more connection options. I still love my 8mm spark plug boot with a foster fitting over the dipstick tube though.

+4 Ð Bounty Awarded
Ben Technical Support Specialist
Eaton Socon, United Kingdom

Hi Matthew,

Great work on the above and it comes just as I'm looking at more HD work. I have an odd capture and I wonder if you could take a look? We know that there excessive crankcase pressure as the vent puffing away as much as the exhaust!! I wish I had put WPS500 on the crankcase breather as you have done above as I believe this would have made all the difference! 

My question is though have you ever seen the double peak that I've captured in the relative compression waveform? I'm trying to work out why this would happen, it's almost the pressure is lost but as the piston is still moving up the cylinder it regains some compression maybe from left over oil present in the bore but I really don't know. Have you or anyone else come across this before?

Thanks in advance and keep up the great work!

+1 Ð Bounty Awarded
Matthew Mobile Technician
Bartlett, Illinois

I have seen it, I guess it is fairly normal. Never given it much thought why, but looks normal to me. 

+1 Ð Bounty Awarded
Ben Technical Support Specialist
Eaton Socon, United Kingdom

Thanks Matthew,

Just an odd one as I've never seen it. The first capture I did with the engine a little colder, it wasn't there as frequently but as the engine warmed up it become every other.

I believe the engine is being stripped now so will feedback once I know more

Thank again


0 Ð Bounty Awarded
Darren Mechanic
West Allis, Wisconsin

Picoscope for sure with pico pressure sensor, checking blowby cylinder by cylinder using a sync of course, usually no. 1 ignition.

0 Ð Bounty Awarded