Compression test how to???????

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matty169

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Any particular procedure to follow during compression test? I did one and the numbers are crazy. I also cant find any figures for what I should see during cranking compression, if anyone knows. Thanks

Driver side is all about 240 psi
Passenger side is 185-215 psi.
 

Tach_tech

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To do it properly you should have one plug out of each cylinder,fuel disabled, and throttle body should be open. Install the compression gauge on one cylinder crank the engine over for about 5 revolutions. Technically speaking minimum is 100psi. Shouldn't have more than a 25% difference between cylinders.

If you're getting 240psi on a stock engine, you're doing something wrong or the gauge is messed up.
 

dodge dude94

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My question is why are you doing a comp test on a 3 year old engine in the first place unless you're trying to figure out a persistent miss on a certain cylinder or pair of cylinders.
 
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matty169

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Test

I know HOW to do a comp test, just wanted to see if there is a certain procedure for the MDS hemi.
Gauge is NOT screwed up. Tested 3 times and wrote down numbers. All cylinders on driver side are within 5 PSI, BUT all above 230 psi.
Pass side low is 180ish, high is 215ish.
Snapon and Matco gauges.

I have a odd miss that I feel on throttle but for some reason the misfire monitor is set to Global Disable in the tuning. Not sure why.
 
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matty169

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These are the instructions I followed. Not sure why Chrysler makes it so screwed up to do misfire monitoring.........

MISFIRE MONITOR DRIVE CYCLE INSTRUCTIONS

Do not turn off the ignition switch during the test procedure.
All
Onboard System Readiness codes are cleared if the battery is
disconnected or when the Powertrain Control Module (PCM) is cleared with
the scan tool.
This drive cycle can be used to determine if the
Readiness code for this Monitor will run and complete, and if the
Malfunction Indicator Lamp (MIL) will remain off after the vehicle is
driven.
Operating Ranges: Whenever a range of values is
provided, try to operate the vehicle in the middle of the range. For
example, if the vehicle speed range is from 45-65 mph, operate the
vehicle at as close as possible to 55 mph. If the Short Term fuel trim
range is -20% to +20%; try to operate the vehicle near 0%.
Scan
Tool Data: There are certain Parameter Identification (PID) values that
must be within range before this Monitor will "run". It can be helpful
to pull up those PID items on the scan tool so an assistant can view
them during the test.
The Readiness code for this Monitor
defaults to "continuous" or "monitoring" because this Monitor runs
continuously whenever the engine is running.
This information is a representation of a Drive Cycle, and may vary due to software revisions to the PCM.


MISFIRE MONITOR PRETEST CONDITIONS

MIL is off.
No emission related diagnostic trouble codes (DTC's) are present.
Adaptive Numerator "learn" function completed.
Decel Counts Learn from 20 to 65,535.
Disable timer value from -0.50 to +0.50.
Inhibit = -0.50 to +0.50 ('0' = OK)
Miles Force Learn from 4,997 to 65,535 miles.
MIN Start RPM between 448-8,000 RPM.
MIN RPM to Learn between 1,728 and 3,392 rpm.
MIN VSS to Learn between 60 and 100 MPH.
NUM Not Learned between -0.5 to +0.5.
Trips Force Learn between 500 and 65, 535 trips.


MISFIRE MONITOR ENABLING CONDITIONS

None


TEST PROCEDURE

Connect the scan tool to the data link connector (DLC).
Start the engine and allow it to run at idle speed for 1-2 minutes.
Accelerate
at part-throttle to Cruise speed and maintain that speed for several
minutes. Bring the vehicle to a stop, but do not turn off the ignition
key.
Check for any diagnostic trouble codes (DTC's) in memory.
If any DTC's are set, refer to the appropriate trouble code chart to
make the repair. If no DTC's are set, the test is done. Return to the
service bay and turn off the ignition key.


Notes on Misfire Monitor

Decel Counts To Learn:
The number of Full Fuel Shutoff Decel counts required before the PCM begins to monitor for engine misfires.
Disable Timer:
The amount of time (seconds) this Monitor is disabled after startup.
Inhibit OK:
If the Misfire number is XXXXX this number will be other than zero (0).
Miles Forced Learned:
If the vehicle has traveled more than the specified mileage, the PCM will attempt to relearn the Adaptive Numerator value.
MIN Start RPM:
Minimum engine speed before the PCM checks for a misfire.
Numerator Not Learned:

A data bit that indicates the current state of the Adaptive Numerator
Learn function in the PCM ("0" means not learned, "1" means learned).


MIN RPM to Learn:

The required engine speed to achieve before the PCM enters Full Fuel
Shutoff Decel in order to learn the correct Adaptive Numerator value.
MIN VSS to Learn:

The required vehicle speed to achieve before the PCM enters Full Fuel
Shutoff Decel in order to learn the correct Adaptive Numerator value.
Trips Forced Learn:

If the vehicle has had the minimum number of starts (with a Stable Run
Time recorded), the PCM will attempt to relearn the Adaptive Numerator.
Global Disable:

The Monitors on Next Generation Controller (NGC) vehicles can be
"globally" disabled when certain "run" conditions exit. Refer to the
examples below:

New Catalyst Bank 1
New Catalyst Bank 2
Very Low Battery (approximately 7.5v)
High Ethanol (approximately 25-30%)
PTO Engage
Low Fuel (less than 15%)
High Fuel (more than 85%)
High Altitude (above 8,000 ft)
Low Battery (less than 11v)
High Battery (more than 16v)
Low Ambient Temperature (less than 40ºF)
Very Low Ambient Temperature (less than 20ºF)
 
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matty169

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Huh

Nobody has any ideas on what could be causing these numbers?
 
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matty169

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***** it..............installing new motor.
 

Tach_tech

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If you use a messed up gauge, it doesn't matter how many times you do it, it doesn't make the numbers right. It wouldn't hurt to try another gauge. Reason I say that is 240psi is obscenely high for a stock motor, even 180 is kind of high in my experience for a hemi.On the side that's low, you could try puting a tiny bit of oil in the cylinder before doing the test. If it goes up its rings, if not it could be in the heads.

I wouldn't be pulling the engine yet.

As for the misfire monitor. That's not a Chrysler thing, a lot of those parameters are set by Industry standards. The PCM detects misfires by monitoring the crank sensor. As a cylinder fires the crank rotation speeds up. The PCM knows where #1 cylinder is on the crank tone wheel, so from there it knows which cylinder should fire next. If it doesn't see that acceleration that means a misfire has occurred. It's quite complicated, it needs to be able to adjust its readings over time in order to factor in engine wear.

If the misfire monitor is set to global disable its because one of the self tests either hasn't run or its failed. If you have aftermarket software that could also be causing issues.
 
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matty169

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If you use a messed up gauge, it doesn't matter how many times you do it, it doesn't make the numbers right. It wouldn't hurt to try another gauge. Reason I say that is 240psi is obscenely high for a stock motor, even 180 is kind of high in my experience for a hemi.On the side that's low, you could try puting a tiny bit of oil in the cylinder before doing the test. If it goes up its rings, if not it could be in the heads.

I wouldn't be pulling the engine yet.

As for the misfire monitor. That's not a Chrysler thing, a lot of those parameters are set by Industry standards. The PCM detects misfires by monitoring the crank sensor. As a cylinder fires the crank rotation speeds up. The PCM knows where #1 cylinder is on the crank tone wheel, so from there it knows which cylinder should fire next. If it doesn't see that acceleration that means a misfire has occurred. It's quite complicated, it needs to be able to adjust its readings over time in order to factor in engine wear.

If the misfire monitor is set to global disable its because one of the self tests either hasn't run or its failed. If you have aftermarket software that could also be causing issues.

I tried a snapon and a matco Guage. .......same thing.
Tuner has misfire monitor set to disable for some reason. Makes no sense why that would be disabled.
I am starting to think its injector related. AF is kinda wonky.....goes crazy rich at random....like 10;1 then goes lean for a second........
No vacuum leaks, intake leaks, etc.

I have probably comp tested more engines than 90 percent of you here. Been racing bikes, cars (late model and modifieds) and trucks since the age of 12. Build our own engines, etc.
I fully realize the numbers make no sense, hense the reason i asked if i needed to do anything out of the ordinary because of vvt or what have you.
 

john55

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....ya know, if you've done more compression tests than 90% of us....that leaves me out, go ask a mechanic, maybe they can help.
 

Tach_tech

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I tried a snapon and a matco Guage. .......same thing.
Tuner has misfire monitor set to disable for some reason. Makes no sense why that would be disabled.
I am starting to think its injector related. AF is kinda wonky.....goes crazy rich at random....like 10;1 then goes lean for a second........
No vacuum leaks, intake leaks, etc.

I have probably comp tested more engines than 90 percent of you here. Been racing bikes, cars (late model and modifieds) and trucks since the age of 12. Build our own engines, etc.
I fully realize the numbers make no sense, hense the reason i asked if i needed to do anything out of the ordinary because of vvt or what have you.

No need to be snippy. I can guarantee I've been inside more hemis than you can imagine. I've been a Chrysler dealer tech for several years and have all of the factory engine specific training they offer. I'm trying to help you, not sure why now.

That being said, as for your question about the variable can timing. That won't have an effect on your compression test results. Have you tried returning the PCM software to factory and see if the misfire monitor will return? At least then you could narrow down your possible misfire.

You could also try doing a running compression test on each cylinder and see if you get more consistent results bank to bank.
 
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matty169

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I wasnt being "snippy". I felt maybe you needed to understand that I used 2 different guages already, as I thought the first one was screwed up. It wasnt.
Also, the fact that I know "HOW" to do comp tests and this isnt the first time.
As for returning to stock...........thats not really an option as the truck would do about nothing with the stock tune from my understanding. I am also gonna use a WiTech instead of this POS Snap-on Verus trash I have here.
I am gonna build a forged motor, larger blower, bigger injectors regardless. This is UNBROKENS old truck that I bought........
2012 Ram R/T - Blurred Lines - Truckin Magazine
 

Tach_tech

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Ok ya, I was unaware it wasn't a stock truck. If you can get your hands on a witech and the software that would very helpful although I imagine it would be pretty hard to get unless you know someone.
 
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