Hey guys, I'm new to the forum, and ford ranger trucks.
The truck has a problem. Its a 99 2.5l 4L. It rumbles pretty hard when I am at low RPMs, such as when I start to back up and pull out of my driveway, and when I am coming to a stop sign or stoplight. It rumbles pretty rough and can most times sputter and die, if I don't give it gas.
I changed all the spark plugs along with the plugs and wires. Changed the oil and filter.
I checked the compression through the left spark plug holes, and it is as follows:
4 left - 130psi
3 left - 125psi
2 left - 90 psi
1 left - 150 psi
Yesterday, I added a tablespoon of oil to #2, and it brought the pressure back up to 110.
I'm thinking I may have bad piston rings.
I've also changed the alternator, since it was having serious charging issues.
What about you guys, any ideas?
thanks for the help!

 

Welcome to FTE.
Maybe stuck/sticking rings from varnish, or carbon deposits, so consider running the engine to warm it up a little, then remove the spark plugs & add a table spoon or two of Marvel Oil to each cylinder, screw the plugs back in to keep debris out & let it sit at least overnight. Then remove the pugs, disable the fuel pump so the injectors wont wash the cylinders down & crank the engine to blow out any remaining excessive Marvel Oil, so you wont hydro-lock the engine & bend something up.
Then reinstall the plugs, re-enable the fuel pump, start & fully warm up the engine & go for a spirited high rpm drive, with rpm run ups at 3600 or so for a mile or two, to blow out loosened carbon deposits.
Then perform another compression test, or if you have a scan tool that'll perform a "Cylinder Balance Test", have it do that & see if it'll show any improvement.
If it does, maybe you can get some more improvement by running a high ester base engine oil to finish cleaning up ring land deposits & see if compression will improve some more.
This won't fix worn rings or cylinder bore, but may be able to remove ring land deposits that are causing the rings to stick & maybe improve & even out compression some & that can help idle & power that'll likely be more noticeable on a 4banger.
Some thoughts for pondering, let us know how it goes.

 

I would definitely consider that you may have a head gasket on the way out. The third cylinder is not as good as #1 & #4, and adjacent cylinders with a shared head gasket may show compression loss.
Try taking the readings with #2 & #3 plugs removed and see if that changes things.
If you don't go overboard with the MMO, adding a Tbsp to each cylinder, rolling the crankshaft a bit to distribute the MMO, and letting sit overnight could have some positive effect. I would not be concerned for hydrolock unless you add a 1/4 cup or so of MMO. The combustion chambers are larger than a Tbsp in volume.
Not too sure what a 'rumble' is as far as an engine running. If a cylinder is misfiring, it normally causes the engine to shake back and forth, not make a rumble noise. (rumble to me is the noise you generate when your tires go over the multiple strips spaced ~5-6" apart when approaching stop signs, etc, or the noise generated if you drift off to the right and your tires encounter noise makers).
tom