Compression testing with conventional gauge

When working on a drive by wire car to do a compression test using a compression gauge screwed into spark plug hole do you block open the throttle plate as i have heard different things.Some cars will not open the throttle plate when gas peddle is fully depressed ,what do you do with these systems to get an accurate reading on the gauge?
Thanks.

I find that most drive-by-wire setups will open the throttle during cranking. On the systems that won't? Honestly, I don't worry about it. :lol:

Holding the throttle plate open seems too risky, IMO. Just not worth the trouble.

Thanks , i have always done it with foot to floor and just cranked it .Just wondered that's all .

scopeman wrote: Thanks , i have always done it with foot to floor and just cranked it .Just wondered that's all .

S'all good! I actually hesitated to answer, 'cause I'm sure that 'by the book' I should rig some way to open the throttle on those uncooperative systems. :blush: Or at least create a big vacuum leak. Maybe I spent too long on flat rate? :lol:

Tyler wrote:

scopeman wrote: Thanks , i have always done it with foot to floor and just cranked it .Just wondered that's all .

S'all good! I actually hesitated to answer, 'cause I'm sure that 'by the book' I should rig some way to open the throttle on those uncooperative systems. :blush: Or at least create a big vacuum leak. Maybe I spent too long on flat rate? :lol:

As above but personally these days I tend to go straight for a leakdown test if any cylinder is suspect, I recently bought a very low cost Bergen Tools branded leakdown tester to replace my home made one. It works at only 1 bar pressure, my old one and most older factory made ones work at 100psi. I have made a coupling up to connect it to a little 12v compressor but have yet to try it out on a bad cylinder.
Matt (SchrodingersBox) did this with an 100psi type but from his YouTube Video although worked his little compressor struggled to make enough air at 100psi.


Link I found this one on eBay UK I am pretty sure it is identical to my Bergen one but at an even cooler price got to be a bargain.

Hey Andy, Keep us informed how it works with a little ol compressor as i am very tempted.Cheers.

When most engine now say something like this for the compression specification.
"The minimum compression in any 1 cylinder should not be less than 70 percent of the highest cylinder. No cylinder should read less than 690 kPa (100 psi). For example, if the highest pressure in any 1 cylinder is 1 035 kPa (150 psi), the lowest allowable pressure for any other cylinder would be 725 kPa (105 psi). (1 035 x 70% = 725) (150 x 70% = 105)."

To me performing the test in a consistent manner is more important than whether the throttle is open or not.
I usually do a relative compression check and then jump to cylinder leak down.
For example this one falls into the 70% range



This one on the other hand not so much.

Ok here is the update on the leak downtest with a 12v tyre pump compressor.
I tried running from the car battery and battery jump start pack (the type with a lead acid motorcycle battery) without problem,
My little compressor generated enough pressure and flow for the test on a known good cylinder but the pump died in the process before I could get a picture or test it on a bad cylinder. I should say the 12v compressor was over 10 years old and was both tiny and the cheapest of the cheap. I am pretty sure one of those 2 cylinder versions advertised for 4x4 use would have been fine One thing I did discover was with the little compressor you really have to be 100% sure the air connections on the supply side are leak free.
For normal use I will stick to using it with the little mains voltage compressor.

The design of the compression tester has a big effect on compresion test result but it is quite normal on european and asian multivalve engines to see compression test results over 200 psi and GDI engines will be higher. For example on a Mazda Mx5(Miata/Enos) 195 to205 psi is not uncommon.
On a Ford Zetec or Rover K in good shape I expect to see 165 to 185 psi.
Back in the days of push rod cast iron engines 135 to 155 psi for say an MGB engine in good shape.