Compression tested the motor and here is what i found-

#4 160psi #8 155psi(plug looks fine but that is the low one.)
#3 165 #7 164psi
#2 168 #6 162
#1 170 #5 162

All done @ WOT same amount of cranks.
My problem is that I am having missfires at Idle and light light cruise off the gas most noticable.

What do you guys think about these findings??

Zach

 

155 to 170
That's about 10%
How many miles and were they 1/4 mile at a time?

Did you happen to squirt a little oil in that #8 and see if it came up?

If it does that is an indication of rings.
If it stays the same valves.

I like to go about 7 or 8 cranks with a warm engine when I test.

I don't know what the book calls out for compression but anything over 150 at our lower compression ratio sounds pretty good to me.

Those numbers will generally go down with altitude about a .97 per thousand feet at 60 degree temp.


Zach you are a patient person to compression test one of these puppies.

You get an "A+" for effort.
Thats what I have to say about your findings.

 

Ha Ha patience......It took me about 1 hour to pull all coils, coil packs and compression test each cylinder.(I have pulled the plugs way too many times). Patience is doing valve springs on Lightning for the first time. That took awhile along with putting in a new short block and cutting off JDMs caged pulley. I have alot of time and money in this truck for it not to run right.

I did not do a wet test however i will soon.

Zach

 

Was the #8 (155psi) the last cylinder checked? If so did you have a battery charger attached to the battery?

”A decreasing battery charge over the course of a compression check can erroneously indicate that the last cylinder checked has the lowest pressure.”

That comes from an excellent book written by Corky Bell, titled Supercharged (Design, Testing and Installation of Supercharger Systems page 220…

 

I have read that book it is very informative. Yes i did have a charger attached and even went back and did the #1 and 2 cylinders again after the #8 and then checked the #8 one last time. My question is will this cause my problems I am having.

Zach

 

as far as i'm concerned, they are only good enough to tell you the piston is moving up and down.

You wanna know what's going on, leak down test is the way to go.

 

Yeah i had better do a CLT tomarrow.

Zach

 

If you want a simple direct answer I would say "NO" the compression variation is not causing your misfire.

I would look at your injectors or coil packs.

I never did catch the mileage......

I have had normal plugs idle perfect but when you rev the engine or accelerate under load it would missfire. The plug usualy is darker.

Now if it were the old days I would hook up a good diagnostic tachometer and remove the wires one at a time while it was running and watch the RPM drop. The cylinder that you see no change from is the one with the problem.

These modern engines are in some ways simple but in other ways a royal pain for a shade tree mechanic like me.

 

Still a great way for diagnosing coil packs!!!! That's how I found mine when it blew. Engine felt like the timing chain blew apart just driving up and down my street. Pulled the coil packs one by one and listened and watched for engine performance differences.......finally wham.......i think it was number 6 showed no difference. Swapped it and and it's fine!!!