Compression test results
Thread Starter
Burning Brakes
Joined: Mar 2017
Posts: 876
Likes: 295
From: Asheville NC
Compression test results
I’m looking at buying a used 1999 3.4 engine with about 45k miles. I asked the owner for compression and leak down test results. Here is what I received:
1. 135 / 4%
2. 130 / 4%
3. 130 / 6%
4. 150 / 2%
5. 140 / 6%
6. 150 / 4%
Leakdown numbers look ok to me, but compression values look low, particularly in bank 1. I was expecting numbers in the 175 range.
Comments welcome.
1. 135 / 4%
2. 130 / 4%
3. 130 / 6%
4. 150 / 2%
5. 140 / 6%
6. 150 / 4%
Leakdown numbers look ok to me, but compression values look low, particularly in bank 1. I was expecting numbers in the 175 range.
Comments welcome.
Rennlist Member
Joined: Aug 2006
Posts: 9,213
Likes: 1,907
From: Chicago, IL
From what I’ve read, compression PSI figurescan have a lot of factors that cause variables and are not necessarily accurate or always provide much value, unless you see something like 50psi. I would be way more comcerned if leakdown was bad.
Former Vendor
Joined: Nov 2010
Posts: 6,968
Likes: 2,341
From: Cleveland Georgia
The shop didn't lock the throttle plate open during the compression test. The test was crippled by a lack of chamber filling due to this.
I can look at the values, and see that the highs and lows are classic examples of the intake characteristics, and how they effect chamber filling with a closed throttle plate. Cylinder 1 always has the lowest compression on these tests when the throttle plate is closed.
Air that doesn't enter the chamber on the intake stroke, can't be compressed.
It amazes me what shops don't know about the basics of engine operation. (well, actually it doesn't these days)
I can look at the values, and see that the highs and lows are classic examples of the intake characteristics, and how they effect chamber filling with a closed throttle plate. Cylinder 1 always has the lowest compression on these tests when the throttle plate is closed.
Air that doesn't enter the chamber on the intake stroke, can't be compressed.
It amazes me what shops don't know about the basics of engine operation. (well, actually it doesn't these days)
Rennlist Member
Joined: Apr 2016
Posts: 1,612
Likes: 354
From: SF Bay Area
The shop didn't lock the throttle plate open during the compression test. The test was crippled by a lack of chamber filling due to this.
I can look at the values, and see that the highs and lows are classic examples of the intake characteristics, and how they effect chamber filling with a closed throttle plate. Cylinder 1 always has the lowest compression on these tests when the throttle plate is closed.
Air that doesn't enter the chamber on the intake stroke, can't be compressed.
It amazes me what shops don't know about the basics of engine operation. (well, actually it doesn't these days)
I can look at the values, and see that the highs and lows are classic examples of the intake characteristics, and how they effect chamber filling with a closed throttle plate. Cylinder 1 always has the lowest compression on these tests when the throttle plate is closed.
Air that doesn't enter the chamber on the intake stroke, can't be compressed.
It amazes me what shops don't know about the basics of engine operation. (well, actually it doesn't these days)
