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Old 03-09-2020, 04:08 AM #1
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Compression test Hot or Cold?

I've been contemplating a supercharger upgrade on the rig, and wanted to run a compression check on the motor before I get my hopes up. I did the following steps...

1. Unplug fuel pump
2. Pulled EFI
3. Removed plugs and wires
4. Put battery charger on battery
5. WOT while testing in each cylinder
6. Wrote down results
7. Drank a beer

I did this test while my rig was sitting 24 hours COLD. Results in each cylinder were...

1. 190
2. 170
3. 195
4. 190
5. 210
6. 190

Cylinder 2 got my attention because it's the lowest, but also the 20 psi difference between some. I started researching some answers and that's when it hit me. My idiot self forgot to test while the vehicle was at operating temp. Doh!

So i put everything back together, drove it around the block, and repeated the necessary steps. Results HOT were...

1. 210
2. 210
3. 210
4. 205
5. 210
6. 210

Now those are some sexy numbers. But now it got me thinking. Why such a difference in reults when its warm, especially for cylinder 2? I know the FSM says to do the test warm, but when you google this question, there are mixed reviews between the two. Some say hot, some say cold, some say it doesn't matter. Anyone care to share some knowledge?

Vehicle is a 96 with over 300k mi on original. Shes my hunting partner.

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Old 03-09-2020, 08:55 AM #2
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It's been a few decades since my engine theory class, but this is what I recall:

The numbers are mostly irrelevant so long as they're above the minimum needed for an ICE to function (about 110psi) all on their own. What you're looking for is the Difference between the numbers, with a variation of 10% being perfectly acceptable (20psi in this case).

As for the hot/cold, I've always noticed that cold were lower and have only done cold readings on non-operational engines. I really don't remember if there was info about doing the readings hot or cold, but it Must have indicated hot as that's what I've always done if it were an option. Best guess as to why is that A) that's the state of the engine while running so it's the best indicator of what you're working with (in other words, cold readings have no direct bearing on a functional/running engine). The variation can be attributed to ring/piston expansion and getting oil on the cylinder walls to form a better seal.

Another thing I remember is that the whole "add oil to the cylinder and check for compression increases" is bunk as you're decreasing the volume with a non-compressible liquid so you had Better see an increase in pressure. It just doesn't indicate what most people seem to think. Better to just move on to a leakdown tester and identify if the issue is valves or rings.
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Old 03-09-2020, 10:33 AM #3
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Iíve never known a person to add enough oil to make a volume difference.
I believe the theory is that a little oil will help seal the rings but not the valves. A shade tree way of trying to identify the source of low compression.
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Old 03-09-2020, 10:58 AM #4
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Cold engine - larger piston-wall and ring clearances, no oil on rings to achieve a proper seal. Beer consumption may have also affected the testing procedure
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Old 03-09-2020, 12:42 PM #5
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While it's been a while from my engine theory class. The point of using oil for the second test is covered here in these two links. While I have a leak down tester I rarely use it. If you do a wet and dry test you know where the leak is either rings or valves. The "leak down" will tell you if it's rings, valves intake or exhaust, or head gasket leak. Does it really matter because the heads are coming off any way unless you are going to just do a engine swap. So can some one educate me how doing a "leak down" will change what your going to do with the engine over doing a wet and compression. Where i have found the leak down to be help full is on a engine that I don't or can not spin with the starter for what ever reason. Those are great compression NUMBERS no rebuild in your future.

Cars 101: How to Do a Compression Test

How to Do a Compression Test in Under 30 Minutes

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Old 03-09-2020, 01:22 PM #6
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Great discussion guys! Thanks for taking the time to share your knowledge. I feel more confident in my numbers.

- Sean
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Old 03-09-2020, 01:51 PM #7
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If you don't mind me asking, what kit did you do?
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Old 03-09-2020, 02:54 PM #8
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Quote:
Originally Posted by jross20 View Post
If you don't mind me asking, what kit did you do?
I bought a kit off Amazon. OTC 5606. Just a step below the one Tim uses in his video.
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