I ran a compression test over the weekend on my 350 (2004.5, ~70k miles) and have a couple questions for the group at large.

- I did the test cold and all cylinders were in the 126-129 range with one cylinder coming back at 120 consistently (ran each cylinder multiple times then took the average). I also noticed on the 120 cylinder the spark plug was black all the way up the threads whereas the rest were dark about 1/2 way up. So something obviously is "different" about that cylinder, but based on the results is there anything to be really concerned about? If it was drastically different, say at 90, then I would say something was really wrong but for a few psi off the median is that a big deal?

- What makes the spark plug darker as opposed to the rest? I would assume its getting some oil in there and burning that off as well as the fuel resulting in more residue? I haven't noticed any blue smoke coming from the exhaust but maybe its not getting enough in there to be noticeable.

- I got the following errors on my first start up after putting everything back together. I assume since I pulled apart a ton of stuff this is just the ECU freaking out from bad polling while attempting to start the car. I cleared with Cipher and after ~50 miles all still seems well.

-------- ECU --------
Found: 3 codes!
**********
P1122,Electronic Throttle Control Actuator
**********
P0223,Throttle Position Sensor
**********
P0123,TP Sensor

-------- ABS --------
One code found!
**********
C1131

Thanks for any advice/help.

 

First off you should be doing the compression test warm as that is how most are rated. You can get erratic results from cold pistons and the metal contracts in the cold and expands when warm. Your funny readings on one cylinder could be as a result of doing them cold.

One plug being darker is not oil, more likely one cylinder running rich or experiencing misfire. I would replace all the plugs if that is the result.

The codes sound like the computer received power when stuff was unhooked. Reset the codes and see what happens. When taking apart electrical stuff always pull your negative terminal from the battery.

 

which compression tool did you use? im shopping for one right now they are fairly cheap just not sure which one to pick...

can anyone recommend a decent one?

 

I know that on my atv the acceptable compression range is between 70 - 100 psi. I know the z is different but I wouldn't be worrying about that 120 cylinder.

Try the test warm if possible. I would bet $ you get different numbers.

 

Curious about the tool used and maybe the steps of doing a compression test on are cars (DIY)