Compression test for 300Tdi

chris snell

chris snell

Administrator
Callsign: NW5W
Staff member
Can someone give me a run-down of doing a compression test on a 300Tdi? I assume that I would need to disconnect the fuel cut-off solenoid and pull the glow plugs but that's about all I know. What's the right tool and adapter? What's the pressure numbers for good/bad?

Thanks,
 
Napalm00

Napalm00

Technical Excellence Contributor
Remove wire from fuel stop solenoid. Remove glow plugs. M10x1 thread most adapters won't seal since the glow plug seals on the taper all the way down at the bottom. When I've done this in the past a few years ago I just wrapped the adapter with some thread tape until it was hard to thread in.

You're really looking for consistency across all of the cylinders. 5% deviation across all cylinders would be considered extremely healthy.

I couldn't tell you what a good number would be as it was so long ago last time I did this.
 
T

terryjm1

Well-known member
His videos are the best. There is next to nothing you can’t do following his videos. And, he is genuinely entertaining.
 
jymmiejamz

jymmiejamz

Founding Member
Callsign: KN4JHI
I’ve got a snap on adapter that works on 300 Tdi. I can give you the part number on Monday when I’m back at the shop.

To do it properly you should remove all of the glow plugs and make sure your battery is fully charged. I did mine without pulling the #1 glow plug because I have AC. The results were bad enough that I didn’t need to check #1 :(
 
Napalm00

Napalm00

Technical Excellence Contributor
When you guys are checking this and you're not using a full length glow plug you will get lower numbers than normal. You can use the numbers that you get to compare percentage from cylinder to cylinder but I wouldn't rely on that number to give you a compression reading if that's what you're looking for.

Without the body of the glow plug in place you are triple or quintupling the available volume to compress. So if you really want an accurate measurement need to use a plastic spacer or find the style of dummy glow plug that takes up the entire length of the hole in the head.
 
chris snell

chris snell

Administrator
Callsign: NW5W
Staff member
I don't understand. I thought that you threaded the hose of the tester into the glow plug hole. Why would you even need a dummy glow plug? You're using the glow plug hole for testing, right?
 
Napalm00

Napalm00

Technical Excellence Contributor
The glow plug is like 8 in long. It threads on the very top but seals on the very bottom.

Most m10x1 compression adapters are only about an inch long. And you have to use thread tape on the threads to keep the pressure in the cylinder because they're not sealing on the taper at the bottom of the missing glow plug.

Since you're now missing about 7 in of glow plug physical body you are trying to compress the air that is now in this space. This will give you a much lower than expected reading. Because the volume is higher that the piston has to compress now that the glow plug is missing.
 
chris snell

chris snell

Administrator
Callsign: NW5W
Staff member
I get that but I'm confused about the dummy glow plug. Is this an adapter for the compression gauge that mimics the size of the glow plug?
 
Napalm00

Napalm00

Technical Excellence Contributor
chris snell said:
I get that but I'm confused about the dummy glow plug. Is this an adapter for the compression gauge that mimics the size of the glow plug?
Click to expand...
Yes.

we would just call it a compression adapter but it's commonly called a dummy glow plug in the UK. I think laser tools used to actually make one that fits the TDI I know they make them that fits the later v6 and V8 TD engines.some Chinese kits also come with the full length adapters. I've never bothered to dig.
 
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