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post #1 of 19 (permalink) Old 01-26-2016, 10:55 PM Thread Starter
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Compression Test Results

Hey guys I've been trying to figure out why my gas mileage has been so bad lately (10-15mpg).

I ruled out all the obvious.....alignment, tire pressure, aggressive driving, brakes, etc.

So I decided to do a compression test. I only did a dry test and the results are as follows:

Cylinder PSI
1 90
2 70
3 70
4 75
5 90
6 85

These values seem extremely low to me. I find it unlikely that every valve or piston ring for every cylinder is shot. Any ideas?

2001 RT
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post #2 of 19 (permalink) Old 01-27-2016, 02:25 AM
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I would wonder how it would be running with compression readings that low. Are you sure that the tester was making a good seal and that the gauge is accurate?

I know from experience that cam timing jumping 3 teeth will make the readings on the affected side drop to that range, but for both sides to be evenly low like that then it would have had to been the belt jumping at the crank sprocket, BUT !!!!! if it had jumped there, then it wouldn't even attempt to start unless you disconnected the cam sensor.

My only other thought is if you have a severely plugged up air filter, but I'm not sure that I'd trust your gauge since the readings on all 6 are so low. When I had the timing jump on my first M, it was pretty much only running on the drivers side bank and running rougher than heck. I'd be amazed if your readings are accurate and it still runs halfway decent aside from gas mileage, but either a restricted air filter or a smashed spot in the exhaust pipe would be the two most likely things I could think of that would affect your compression on all 6 without it being an unlikely internal issue.
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post #3 of 19 (permalink) Old 01-27-2016, 06:16 AM
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Tested with or without WOT?
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post #4 of 19 (permalink) Old 01-27-2016, 06:19 AM Thread Starter
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Quote:
Originally Posted by 98IntrepidTan View Post
Tested with or without WOT?
With WOT
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post #5 of 19 (permalink) Old 01-27-2016, 06:20 AM Thread Starter
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Maybe doing another compression test with a different gauge to confirm the reading would be a good place to start next?
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post #6 of 19 (permalink) Old 01-27-2016, 10:03 AM
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I belleve the FSM suggests warming up the engine to operating temps before the test.

Copied from the FSM


The results of a cylinder compression pressure test
can be utilized to diagnose several engine malfunc-
tions.
Ensure the battery is completely charged and the
engine starter motor is in good operating condition.
Otherwise the indicated compression pressures may
not be valid for diagnosis purposes.
(1) Check engine oil level and add oil if necessary.

(2) Drive the vehicle until engine reaches normal
operating temperature. Select a route free from traf-
fic and other forms of congestion, observe all traffic
laws, and accelerate through the gears several times
briskly.

(3) Disconnect and remove all ignition coils from
spark plugs.

(4) Remove all spark plugs from engine. As spark
plugs are being removed, check electrodes for abnor-
mal firing indicators fouled, hot, oily, etc. Record cyl-
inder number of spark plug for future reference.

(5) Be sure throttle blade is fully open during the
compression check.

(6) Insert compression gauge adaptor Special Tool
8116 or the equivalent, into the #1 spark plug hole in
cylinder head. Connect the 0–500 psi (Blue) pressure
transducer (Special Tool CH7059) with cable adap-
tors to the DRBIII

(7) Crank engine until maximum pressure is
reached on gage. Record this pressure as #1 cylinder
pressure.

(8) Repeat the previous step for all remaining cyl-
inders.

(9) Compression should not be less than 689 kPa
(100 psi) and not vary more than 25 percent from cyl-
inder to cylinder

Last edited by tgs; 01-27-2016 at 10:10 AM.
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post #7 of 19 (permalink) Old 01-27-2016, 12:10 PM Thread Starter
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I followed all of those steps
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post #8 of 19 (permalink) Old 01-27-2016, 08:21 PM
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All of your readings are under 100 PSI. However, all of your readings are within 25% of one another (per FSM).

Bad compression gauge?

Do you have a bad idle or vacuum leak?
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post #9 of 19 (permalink) Old 01-27-2016, 08:25 PM Thread Starter
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Idle is fine. When I coast around 50mph the rpms jump up and down a little so I was thinking a vacuum leak somewhere. I just replaced the vent valve in hopes of fixing an ongoing evap leak
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post #10 of 19 (permalink) Old 01-27-2016, 09:09 PM Thread Starter
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So I redid the test with a brand new gauge and the results are:

200
185
150
195
190
190
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post #11 of 19 (permalink) Old 01-27-2016, 10:24 PM
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Quote:
Originally Posted by afifield94 View Post
So I redid the test with a brand new gauge and the results are:

200
185
150
195
190
190
Those look relatively normal.

The 150 reading, I would suggest re-testing...you are all within the 25% of the lowest reading, but close to the bottom. If it is still low, it could be a valve seat leaking...not common, but it happens...

What is your mileage now?

If you are getting poor gas mileage, it could be a high number of things. First, when was the last time the spark plugs were changed? What kind of shape is the air filter in? Check engine light on?

Last edited by Daytrepper; 01-27-2016 at 10:26 PM.
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post #12 of 19 (permalink) Old 01-27-2016, 10:28 PM Thread Starter
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160k miles. Spark plugs were changed last month. Air filter was changed last month as wel. CEL was on for evap leak. I just replaced the vent valve after doing a smoke test and finding smoke pouring out of it. The car is still going through the drive cycle
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post #13 of 19 (permalink) Old 01-27-2016, 10:31 PM
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How about changing the O2 sensors with OEM or NTK/NGK sensors?
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post #14 of 19 (permalink) Old 01-27-2016, 10:44 PM
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Quote:
Originally Posted by afifield94 View Post
160k miles. Spark plugs were changed last month. Air filter was changed last month as wel. CEL was on for evap leak. I just replaced the vent valve after doing a smoke test and finding smoke pouring out of it. The car is still going through the drive cycle
So what kind of gas mileage are you getting?
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post #15 of 19 (permalink) Old 01-27-2016, 10:48 PM Thread Starter
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Quote:
Originally Posted by Daytrepper View Post
So what kind of gas mileage are you getting?
On the last fill up I was getting 12mpg
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