Compression Test Questions

[TonyL]

I am going to attempt a compression test on my '83 GMC with a 6.2L. I have never done done this on a diesel and was told by one person i know that you should disconect both batteries before you unplug any of the glow plug wires because it causes a short in the system and that is when they swell. I am wondering if this is true and if so is it ok to diconnect the battery remove all the plugs and then test each sylinder or will having the rest of the glow plugs removed screw up the readings. Any other tips would be appreiated.

Thanks,

Tony

[aloharovers]

Originally posted by TonyL:
I <snip>was told by one person that you should disconect both batteries before you unplug any of the glow plug wires because it causes a short in the system and that is when they swell. I am wondering if this is true and if so is it ok to diconnect the battery remove all the plugs and then test each sylinder or will having the rest of the glow plugs removed screw up the readings.

I have never heard that.
I unplug all 8 glow plugs and then remove all of them.
I also disconnect the power to the fuel cutoff solonoid at the pump and the timing advance and idle advance.
My compression gauge uses an adapter that goes into the glow plug hole to get the reading.
Make sure to use a compression gauge specifically designed for diesel engines. My gauge goes upto 1000psi.
Having all of the glowplugs out helps the engine spin easier with out running down the battery.
Pete

[britannic]

Originally posted by TonyL:
I am going to attempt a compression test on my '83 GMC with a 6.2L. I have never done done this on a diesel and was told by one person i know that you should disconect both batteries before you unplug any of the glow plug wires because it causes a short in the system and that is when they swell. I am wondering if this is true and if so is it ok to diconnect the battery remove all the plugs and then test each sylinder or will having the rest of the glow plugs removed screw up the readings. Any other tips would be appreiated.

Thanks,

Tony

If some glow plugs were left installed and connected when the engine is cranked, its possible for them to receive more amperage than normal which might shorten their life - it shouldn't cause a short though.

It's good practice to remove all of the glow plugs for the compression test to prevent the engine starting (even if the shutoff solenoid is disabled, it's still possible) and disable the glow plug controller.

[ 10-20-2003, 06:58 AM: Message edited by: britannic ]

[TimK]

I agree with Aloha Rovers. I you don't have a guage already, you can order one on-line from Matco tools. I think they are about $65 or $70 and they are designed for a diesel engine.

The glow plugs on the 6.2L our wired in parallel so the terminal of each plug sees approximately the battery voltage when the circuit is activated. Each plug will independently draw current based on it's own resistance, regardless of any of the other plugs whether they are connected or not. In other diesels that may have the glow plugs wired in series with each other this would be different. (I had an old Land Rover 2.25L diesel that was wired in series.)

Definitely disconnect the terminal on fuel solenoid on the pump and remove all the glow plugs at once.

Since the plugs are out, this is a good time to inspect them. With an ohm meter place one probe on the terminal and the other on the base near the threads. An analog meter will show a resistance looking almost like a dead short and a digital meter would show something around 2 to 5 ohms.

Good luck.

TimK

[TonyL]

Thanks for the help. Does 2.5 five ohms indicate a good plug or a defective plug?

Tony