I have a 99 3.0 flex that I is giving me a p301 misfire. Very rough idle.

The plugs were bad so I replaced the wire and plugs as they were due. Coil is good as well, used a friends coil and the problem persisted.


I rented an Autozone compression tester but it took me forever to thread into cylinders 1. Not even sure if it was seated properly. I gave up. I want to rule out bad valves so I wanted to see if there was a trick to doing a compression test at home.

Access to cylinder 1 is a lot easier than cylinder 3. If I can’t get to 1 how do I get a reading on 3?

In the event there are no tricks to do it myself, Any idea of what an independent shop would charge for a compression test and leak down?

Thanks in advance.

 

Compression tester kit should come with the gauge that has a release valve, and a length of flexible hose with spark plug threads on one end with rubber washer/band.
Some also come with longer metal tube with spark plug threads to reach spark plug holes that are deep inside engines.

On your 3.0l you just need the hose, remove it from the gauge, then line it up with the hole and twist it until it threads in and is tight, you do NOT need to wrench it tight.

Compression is a "momentary" pressure
Metal valves on metal seats, and metal rings on metal cylinder walls do NOT seal in air very well.
Only takes a few seconds for all compression pressure to disappear, leak away
This is why you don't need to tighten compression gauge hose with a wrench, just tighten it bu hand, the small amount of possible leakage won't matter over all

A "Leak Down" test shows how fast a perfect engine losses pressure.
Leak down is when you put piston at TDC with both valve closed.
Then you apply a known pressure at the spark plug hole and see how much pressure stays.
Say you apply 100psi of air pressure, and gauge on hose shows 95psi, that means 5% of the air is leaking out in real time, and thats a VERY good leak down, most would be 90psi so 10% leakage.
If you cut off the 100psi air pressure gauge would drop to 0psi within a few seconds, leakage past rings and valves.

And thats OK, momentary compression is all that is needed in engines


For compression test you must remove all spark plugs first
Then prop open throttle for best air flow IN
Start with #1
You must get at least 5 "hits" of compression stroke, you WILL hear it with all spark plugs removed as engine slows each time compression stoke happens in any cylinder.
Always write down results

You must test all 6 cylinders, on a V6, to get overall readings.
There is no "right" numbers
Compression gauges are not calibrated before each use, so WILL vary over time, i.e. months and years, not minutes, lol.
Battery condition also matters, starter motors speed is based on battery voltage/amps, and the speed the engine turns effects compression numbers

So there are no exact numbers to expect
You want all 6 numbers to compare them, if they are within 10% of average then all is well.
With all 6 numbers in hand, don't use lowest or highest numbers, add up the other 4 numbers and divide by 4, and thats the Average, say average was 155psi, 10% is 15.5psi
If lowest or highest is more than 15psi away from 155 then that cylinder has a problem


Shop would charge about $200 to test a V6
They can do an electronic test that usually costs less, it is done with a laptop computer, and is OK but if it shows a problem then you do the hands on compression gauge test, so..........
Might as well do that first


If a cylinder does show lower compression you do a WET test
Use a straw, dip it into a bottle of motor oil, get it about 1/2-3/4 full, put finger over the end and then transfer it to the spark plug hole, and let it go into that cylinder
Then retest compression
Compression will always go up on WET test, how much it goes up can tell you if Rings or Valves are leaking the most
The oil seals the rings more than the valves

 

Thank you for a very detailed response.

The testing kit came with a metal connector that I threaded in first and attempted to thread the rubber hose to that metal connector. Nearly imposssible one cylinder 1. I will try again using just the rubber hose.

When you say “prop open the throttle “ I’m assuming you mean to depress the gas pedal to the floor.

I will remove the fuel pump fuse and let it die before testing.

Based on my research the only possible causes for my misfire is either a bad head or injector issue.