I have run into some problems with my 1999 Ranger with the 4.0 V6 OHV engine. This might be quite lengthy, given the specificity of the problems going on, so I apologize in advance. First off, about a month or two ago, I got 2 codes that came up, Cylinder 2 and Cylinder 3 misfire. I promptly checked ignition, replaced spark plugs, wires, distributor (given that the wires and spark plugs hadn't been changed in a long time, figured this might fix it; (got a distributor for good measure given that they are known to crack, etc.). Cleared codes and waited, then the codes lean bank 1 and 2 came up a week or so later. At this point I knew it could possibly be an intake leak so I sprayed carb cleaner around the intake and sure enough, it started choking up. With this info, I pulled the upper intake where I found a bad seal, but also decided to do the lower intake seal as well which didn't seem too bad. Since I was already this deep and had an oil leak around the timing chain/oil pan, I replaced water pump, thermostat, timing chain cover/timing chain + guides, tensioners (made sure timing on top dead center), Oil pan gasket, valve cover gaskets, belts, etc. Since I had already checked ignition, I wanted to check fuel as well and tested the fuel injectors and gave them a good clean to which they were spraying well. Got it all back together to have yet another check engine light come on which was, again, a misfire, but this time only on cylinder 2. I finally checked compression to find these numbers: (all done with warm engine, all plugs removed, same protocol)

Cylinder 1: 120psi
Cylinder 2: 115psi
Cylinder 3: 120psi
Cylinder 4: 130psi
Cylinder 5: 130psi
Cylinder 6: 125psi

I have seen from other forums posts that typically if one cylinder is over 10% lower than the others, it is to worry about. However, although 2 is lower, it is still within range. Are these numbers normal? I was hoping to find something out of the ordinary to point me in the right direction for possibly a bad lifter, rings, etc., but I don't see one. If there is something I am overlooking, any help would be greatly appreciated. I am young and don't have a ton of experience so I apologize if I am completely missing something here but totally open to any knowledge that anyone is willing to offer up. This whole thing has been quite frustrating after putting all this time into it. Thanks!

 

Pretty low compression numbers
4.0l OHV runs 9:1 ratio(CR) so should be 155-165psi

Rule of thumb is CR x 18 = psi, so 9 x 18 = 162psi, you can knock off 10psi or so after 150k miles for ring wear, so 145-155psi
18 is 15psi air pressure at sea level to 1,000ft, then 3psi for mechanical compression from piston
Also that is with all spark plugs removed so fastest possible cranking speed from starter motor, compression is temporary, metal on metal is NOT a good air seal, lol

But could just be the gauge is reading low, and yes if all cylinders are in the same ball park then valves are usually OK

You can unhook coil packs 4 wire connector, cold engine
Cycle key on and off 3 times
Crank the engine for 10seconds
Pull one spark plug at a time and check tip for WETNESS all should be equally WET with fuel, judgment call on if all injectors are equal

Misfires cause Lean codes, but only on the bank with the misfires
And Lean(or rich) codes are set by O2 sensor readings, and O2 sensors only last 12 years or 100k miles, so Lean codes can be set by old O2 sensors, and that can also cause misfires
So if you don't know the age or miles on the O2s change them
If lean codes come back then at least you will know they are "real" not from old O2s

 

Okay, that is good information. I don't remember the last time the O2's were replaced and the truck is currently at 204,000 miles, so that could be apart of the problem. In addition, I agree, I thought those compression numbers were quite low but I don't think the gauge is reading low. I am getting a lot of the symptoms of low compression such as bad gas mileage (~10/11mpg), low power, really rough idle but smooths out a bit when higher rpm, etc., so I'm thinking those numbers might be right. I can test the injectors once more using the method you mentioned, but from my external testing, they seemed to be all similar (however, could be a different story when in the engine). I am leaning very hard in the direction of bad rings due to the consistent, but bad compression that is present. Would it be bad to assume this given what I know? I'm not sure what else would cause all of those symptoms and rings seem the most likely to cause it, no?

 

You can do a WET test, Dip a straw in an oil bottle and put finger over the top of straw, lift straw out of bottle, then put straw in spark plug hole and release finger
You want about a tablespoon of oil in the cylinder

Test compression, the oil will seal the rings but not valves
If compression shoots up to 160psi then yes worn rings

 

Hi...Just finished pulling my 4.0 OHV (last night) to find the infamous crack in each head between the valves cyl #3 and #4. I've chased stupid high LTFT and the never ending P0171 & P0174 codes not bank specific never a misfire code. Was close to insane performing every test known to the internet....it's a vac leak... it's vac leak. After changing all intake gaskets a smoke test again showed a vac leak. Redid intakes again extra goop on china rails fixed that. In the heat of this past summer, I finally did a compression test almost getting dead from the extreme TX heat. Low comp 1-cylinder each bank. All plugs looked perfect. With those clues, I'm closer to knowing what could be wrong...rings or valves. Truck still ran so wife drove it until Christmas eve....white smoke gushing from tailpipe. The lesson here is test everything you can WITHOUT replacing parts then determine is the prob electrical sensor or mechanical. Never trust it could only be one single thing that has failed.

EDIT: For reference 101k miles...good cylinders were at 160psi 170psi with a squirt of oil.

 

+1 ^^^

I had the white smoke out tail pipe so knew it was coolant getting into a cylinder but compression numbers were fine, until.................
I drained coolant below head height and redid compression test then I saw the drop in one cylinder, wasn't much be enough to require pulling the heads
I was lucky just one head was cracked but had other pressure tested
Not saying this would have worked in your case, but it is another FREE test you can do on an engine, lol

 

Yes Ron....test test & test again. This time with a 10mm socket in your left sock standing on right leg with #4 plug wire on tip of the tongue as Mom always told you...."always have clean undies on dear" before hitting the starter. I should start my own thread to describe my whoas ....just realized this.