Hi All,

First i'm new to this forum and a very novice when it comes to repairing cars and trucks, but i'm aiming to learn.

I've an 07 3.0 ranger with 165k on the clock. Cylinder 4 compression is down 6% and i've got the check engine light on.
I'm not sure what the compression should be and if 6% down is a really issue? (this is information given to me by the dealer when replacing the airbags), they immediately wanted to put a new engine in there.

I've been reading that it's worth changing the spark plugs and the ignition coils, would this be a recommended first step?

If indeed changing the heads needs to be done, is this something a total n00b can have a go at? I watch videos working on engines and don't understand any of the terminology, components etc, so it would be a huge learning curve, but certainly would love to use this an opportunity to learn some new skills and keep my beloved ranger on the road for as long as i can.

Advice about tools to buy, parts to purchase, videos to watch would be greatly appreciated.

Thanks in advance.

John

 

Welcome to the forum

6% is not out of line, 10% is when there is a concern

But still worth a look since you got the P0304 code

If you want to learn a bit about engines then a Compression test is as good a place to start as any
You need spark plug wrench and a compression gauge
Compression gauge is not expensive, it screws into a spark plug hole, MUST BE the screw in type
It will also have a Release valve to reset it to 0, so it MUST HOLD compression reading

Cold engine
remove ALL spark plugs first and keep them in order, 1 to 6
Ford numbers V6 this way
3 6
2 5
1 4
front

Install compression gauge in #1
Push gas pedal down to the floor and hold it down, this turns OFF fuel injectors and gives engine max. air
Crank engine over for 5 "hits", you will hear the "hits" as #1 cylinder hits it compression stroke, can't miss it
After 5 "hits" look at gauge, WRITE DOWN the number as #1
Remove compression gauge and test #2................ect

When you have the 6 numbers, don't use the lowest and highest numbers, add up the other 4 and divide by 4, this gives you the average for the test

Say you got
165
158
168
163
160
161

You would not use 158 or 168, lowest and highest
165+163+160+161 = 649 / 4 = 162.25 <<< average
8 is 5% of average 162
162 - 8 = 154 lowest
162 + 8 = 170 highest

So that's the range of no real concern, 6% is noticed but shouldn't cause a misfire

The actual numbers are not the point of this because compression gauges are not calibrated, battery condition can slow crank speed so lower numbers, warmer engine, 80degF Vs 32degF, will change numbers
Also elevation, numbers will be higher at sea level and lower as you get above 2,000ft

So what "should it be" is not really answerable
What you can "expect" is, general rule of thumb is 18 X (compression ratio) = expected compression PSI
3.0l engine in 2007 used 9.3:1 ratio, so 18 X 9.3 = 167.4 << expected when brand NEW at sea level 70degF outside/engine temp


For a misfire you can pull #4 spark plug and give it a look
Google: spark plug tip images

Spark plug tips tell you whats happen inside the cylinder
maybe swap #4 and #5 spark plugs, see if code changes to P0305, if so spark plug is bad

Compression test it alot of work, but compression is either good or bad, can't be intermittent like spark or fuel.
So once you do the test, compression is either off the table as the problem, or it is the problem, so you don't waste money and time on non-fixes.

 

Thank you Ron, I really appreciate you taking the time to give me such an excellent write up.

I'll take these steps this week and see how it goes. I'm sure ill follow up with more questions.

Thanks

John