3.0 runs rough
 

Finally joined the forum because i just cant figure this out.

1999 ford ranger 3.0 manual

bought it real cheap with blown motor, swapped a new one in and she runs pretty bad.

sometimes it idles fine with fuel trims at +-5% othertimes it idles high with the trims in the +20s when driving it runs lean and stumbles wont rev past about 4k. when i unplug the MAF it drives good with good trim values but idles a little rough. on the reader i have the o2s bounce from .1-.7v at idle. is that normal?

new plug wires
new egr valve and tube
checked for vacuum leaks
cleaned maf
good fuel pressure
exhaust seems clear

any help would be greatly appreciated.

Welcome to the forum

What year is the replacement 3.0l?

O2 sensors read oxygen levels in exhaust
High oxygen is .1v, which is Lean
Low oxygen is .9v, which is Rich

If a cylinder misfires then no oxygen is burned/used so when it is dumped into the exhaust manifold O2 will drop to .1v.

So rough idle would show O2 voltage swinging around.

Computer normally runs idle rich, and ignores O2s at that time, which is why you see the .7 at times,fuel trim at idle will normally be +5 to +8

The +20 fuel trims can also come from misfires, same as above, but while driving the computer adds more fuel than at idle.

After engine is warmed up let it idle and unplug the IAC Valve, RPMs should drop to 500 or engine may even stall, either is good, it means no vacuum leak.
If RPMs stay 700+ then you have a vacuum leak.

Watch MAF voltage or set reader for gm/sec, Grams per second.
At idle you will usually see 3gm/sec because it is a 3.0l engine
Voltage at or just under 1volt

But what you are looking for is steady readings for either, no jumps or drops if RPM is steady.
Raise RPMs up and hold it steady at 1,500, 2,000, 2,500, 3,000
You are watch for steady reading, the actual number is not as important as the steadiness of it, if it jumps around replace MAF

motor is out of a 99 flexfuel just like the truck.

unplugged the iac and idle was was jumping around 650-750

the Maf reads about 4.5 g/s at 800 idle and goes up by ~3g/s every 500 rpm so that seems fine.

looks like a vacuum leak somewhere, ive checked most places anywhere hidden leaks could be?

With IAC Valve unplugged remove 1 vacuum hose from the intake at a time and plug port with your finger.
Do that for all the hoses.
Idle dropping would be a sign that hose has a leak.

Lower intake and throttle body are also spots for leaks
Cold engine
Loosen and pull off fan belt
Have a can of Quick start(ether) ready
Start engine, it can run a few minutes without water pump, battery light will be on.

Spray ether around lower intake and throttle body

You want fan belt off so fan isn't blowing stuff around.

so i one by one plugged all of the vacuum hoses and none made a difference. when i took the belt and ran it i could hear what sounded like a leak but i cant find it. next step pull the intake manifold and replace gasket? im not sure where it is but it sounds like its in the area where the brake booster line comes out of the manifold but its hard to tell.

You can use smoke to find a leak as well
You can rent a smoke machine made for this test, or get a cheap cigar

Pull off air tube on intake, seal opening with plastic wrap and rubber band or tape
Plug vacuum ports
Use brake booster hose, or PCV hose, to blow smoke into the intake manifold

Look for wisps of smoke from area you suspect