1990 2.9 Compression - Ranger-Forums - The Ultimate Ford Ranger Resource

2.9L & 3.0L V6 Tech General discussion of 2.9L and 3.0L V6 Ford Ranger engines.

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Old 07-09-2016
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1990 2.9 Compression

What should the compression be for a 2.9 with less than 100K miles.
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Old 07-09-2016
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Easy to figure that out for any engine

2.9l has compression ratio of 9 to 1, 9.0:1

Multiple that by 18
9 X 18 = 162psi

18 comes from 15psi air pressure at sea level, so if you are above 2,000ft elevation it will be lower
3psi comes from mechanical compression of the air by the piston going up to TDC

15 + 3 = 18

18.5 is actually the number to use but 18 is just fine for "government work" lol

reason being is that testing methods and equipment varies.
18.5 x 9 = 166.5psi, so only 4psi difference which won't tell you much in a compression test

Best method is warm engine all spark plugs removed, freshly charged battery, good starter motor and calibrated gauge.
Fuel pressure released, fuel pump disabled
Throttle opened to full
You do the test dry first
Then add a teaspoon of oil and repeat test.

But only an **** retentive OCD personality would do that, joking but with a grain of salt.

Cold engine is fine
All spark plugs removed needs to be done
Fuel can stay on, you are not going to wash the cylinders with gas
And if you prop the throttle open all the way then injectors will be shut off anyway

No spark plugs allows fast crank speed which will show higher compression
Metal cylinder walls with metal pistons and rings DO NOT provide air tight seals
Same as metal valves on metal valve seats DO NOT provide air tight seals
They are not suppose to
They are temporary seals so the faster the crank turns the less time the air has to leak out so the higher the compression numbers.

Compression gauges are rarely correct, a good one will be within 5psi, medium 5 to 10psi

The actual number you get is less important than the way all the numbers compare to each other

So write down the numbers for each cylinder as you get them

My '94 4.0l with 350k, which is same compression ratio as your 2.9l, has a spread of 152-162psi on my gauge, sea level, cold engine

Wet test is done to see if rings are worn in a low cylinder or it is a valve issue

If dry test got an average of 155psi, but one cylinder was 130psi, then you add oil to that cylinder and test again, if it came up to 150psi then yes bad rings, if it only came up to 135-140 then valve issue or blown gasket.
It will ALWAYS go up when you add oil, simply a better seal than metal on metal, but how much it goes up is what you are after

Last edited by RonD; 07-09-2016 at 07:59 PM.
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Old 07-11-2016
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Join Date: Jul 2016
Location: Las Vegas, NV
Posts: 6
Thank Ron, That was very informative. I will run my test this week.
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