I have been reading for the last couple of days, on how to do an engine comression test. During this research, I have noticed alot of mention that people replace thier plugs, wires, cap, rotor, fuel injector's, ect in vien, trying to correct a rough idle or bad gas milage. That in truth, most times it is a worn out cam, or maybe a bent vavle ( or a valve not seating properly ). Most of the articles I have read stated that doing a compression test is the very first thing a person should do, to trouble shoot such problems. If this comes up roses, then you can 80% eliminate internal engine problems and start with the other items.

I am no mechanic, so dont take my word for it, do some research and read for yourself. Just thought I'd pass along something that might give some of you another avenue to persue.

 

Compression test are quite time consumning and often require 2 people, plus is the least likely scenario. 9 times out of 10 it is something simple. But when all else fails then start looking into the less common and more expensive stuff. Just my 2 cents. Plus you have to pull each plug individually to do the test anyway. So why not spring for the $16 (Bosch platniums) and replace them since they are such a pain in the **** to get to anyway. Unless your like me and don't have plugs....

 

I cant count how many times I have done a tune-up in the last 30 years, to some old wreck I was driving, trying to smooth a rough idle or get a little better gas milage. Only to have the same results after doing the tune up. Not being a mechanic, I never thought about worn out cam's and the such, so it really made sense to me, when I started to think about it. Then I read here on these forums, guys who have exhaused all thier avenues and still baffled as to why thier 100K mile motor is still running rough.

And a compression test doesnt really need 2 persons. A cheap tool called a remote starter, works wonders. And if a person's taking the time to replace the spark plugs, its only another hours worth of ( easy ) labor to do the compression test.

And I think the moderators should make you edit your Sig... I cant stand not clicking on that pic, damnit.

 

Right... But thats not what he's saying he's saying if you have an engine that is missing then do a compression test first. As well as saying that most misses are mechanical malfuntion. When this is false. Yes occasionaly mechanical malfuntions happen especially in higher mileage engines, but it is ussally something simple.
If there is a mechanical malfuntion there is ussally other indications too. I can't ever see myself assuming a miss is a mechanical problem especially if there is no other symptoms or indication that it is.

DO NOT CLICK ON THAT PIC!!! It is haunted.. I'm warning you!!! It follows me around to every post! Like a creepy shadow!!!

Edited to add:
Compression test does not diagnosis what the problem is you can have low compression on some cylinders and it might be a worn timning chain too. Valves are slighty open during compresion stroke you have low compression on that cylinder. Next stroke the slack is out of the chain and gives the next cylinder good compression. Honeslty the only time I use a compression gauge is right after a rebuild an engine to make sure that I hit my projected blueprints and make sure what fuel I can run in it.

 

Also on the older engines that still have spark plugs you can tell more of whats going on by looking at the spark plugs than you can a compression test.

(easy)???? SHHeaaa., obviosley you have not changed the spark plugs on the 97 and up Tritons. Much less tried to get a compression gauge fitting down there... I did my brother 97 4.3 Lariat When he got back from basic training in December. It took me 3.5 hours thats after reading all of the cheats and hints that I could find on here to get to them. My back hurt pretty bad after being bent over that long..
For reference my wifes 96 Explorer = 30mins, My 81 Trans Am = 20mins. My 87 Jeep Cherokee = 10 mins (or less).