Diesel engine compression testing

[Steve Nelson]

I find a lot of information on lower PSI limits for diesels engines that have been warmed up before testing. I have a 4 cyl. skidsteer engine that is not running. I would like to know what to expect when I test this engine cold. Right now I'm getting 220 PSI. Not good.

Thanks

 

[Glen D Anderson]

Not sure what make & model you have but 220 psi does sound low............even with a lower compression engine I would think you should be at 300 - 350 psi???

Do you have adequate cranking speed?

Massive intake restriction?

Are there any decompression devices that are partially engaged?

Are all the cylinders even at 220 psi?

Did you do a dry compression test followed by the "oil in the cylinder" trick to determine if the low compression is a result of worn rings/cylinders?

Just a couple of things to rule out first.........

 

[Steve Nelson]

Thanks, Glen
I get close to 200 on each Cyl. By the way it is a Bobcat with a Isuzu in it at about 40HP.

There are no restrictions.

I did not try the oil test yet. Each cyl had wet glow plugs from previous attemps to start.

I have wondered about the turning speed being low. I had the starter rebuilt by a very reliable rebuilder but it still seems slow.

 

[Bob]

Aren't the glow plugs working... if they were working and red-hot during your starting attempts, they would be burned clean. Yet, you say they are wet.

 

[Dave in Tx]

If I remember correctly, that may be a pre-combustion chamber engine that has to have glow plugs to start. On the kubotas with pre-combustion, 220 psi is not bad, but the glow plugs have to work and the engine must turn over well to start. Even on an 80 degree day, the glow plugs must be used to start the first time. A freind has an Isuzu in a Luv pickup that has to have glow plugs to start when cold. Once started it will usually start the rest of the day without glow plugs. I think the compression ratio is like 13.5 to 14 to 1. On a DI engine with 18 to 1 ratio, they will usually start if they will turn thru 2 or 3 compression strokes.
HtH

Dave

 

[Steve Nelson]

Thanks Bob
The engine would always start without using the glow plugs. I did however use the glow plug system in very resent attempts . I did not want to get into a thread on trouble shooting the engine. I just wondered if anybody had experience doing any cold compression testing.
Steve

 

[JDemaris]

Sounds low to me but check Kubota specs. I've got several IDI diesels and they are all higher compression ratios than most DI engines. My Isuzu four-bangers with glow plugs and precombustion chambers are supposed to test at 440 PSI at cranking speed of 200 RPM. 21 to 1 compresion ratio. My IDI Volkswagen, GM, and Ford are all near the same.

 

[John B.]

You can remove your glowplugs and test them on a good battery by using jumper cables. A glow plug must glow red at the tip and work it's redness up the probe. If it glows red in the middle or upper portions of the probe it's bad and needs to be replaced.

 

[Bob]

If you do that BE CAREFUL!

A defective or overheated glowplug will literally blow open in your face, with a blast of hot metal and ceramic pieces.

 

[ca.hess]

For comparison Case 504btd engine calls for400psi at 200cranking rpm. No glo plugs used.Seems Case 301 cu in were to have 450 -500 psi an that would increase temp to 350degres an ignite fuel

 

[ca.hess]

For comparison Case 504btd engine calls for400psi at 200cranking rpm. No glo plugs used.Seems Case 301 cu in were to have 450 -500 psi an that would increase temp to 350degres an ignite fuel