98 4.0 Ranger Full rebuild. Lean and Rich code issue.

[NoDangerIfItsARanger98]

I have a 98 Ford Ranger that I just recently rebuilt from inside out.

Long block, all new internals all tested ready to roll.

Almost everything on this motor is brand new. For the first 500 mile break in I ran it at different speeds under different loads with no issue. Change oil, Repeat.

At 1,000 I changed oil again and repeat.

It has a little over 2,000 on it now and I just changed the oil to what I'm going to run with to the usual 3,000 oil change.

When I got the truck out of Texas I put new plugs and wires and fuel filter and what not in it and had no issue. I put 10,000 miles on it before a rod bearing went out (long story) as it had 224,000 when I got it.

Now even before I did a rebuild I had issues keeping plugs in the thing I was changing them all the time maybe 2,000 - 2,500 miles in a set. It always had O2 codes and no matter what I did I could not fix it. It would run just fine without any missing or any fuel smell even with the codes. Then it would start out of no where and I'd change them and repeat.

The new motor has been fine until now it started missing a bit and on deceleration it would rev up and down between 1500-2000 rpms. It runs perfect until it's at operating temp and the O2s kick on and then it feels restricted like it's pulling tons of air but there's no power behind it unless you floor it.

I pulled the codes on my scanner and they are reading rich passenger and lean driver.

Two of the sensors are reading between 1 and 3 milivolts and one is reading up on the 900 + milivolts.
Is that normal or is something shorted or what?

So this time I bought new plugs, new coil pack, 3 new sensors, and new cat. Still same codes and still same issue

The old cats the first was half melted and the second was plugged with soot.

My question is where do I look next? Is it something stupid I'm over looking? Anybody else have this issue? I need to drive it quite a ways this coming Monday and I can't with it running this way.

 

[alwaysFlOoReD]

There was a thread within the last couple days that talked about having both lean and rich codes. Try switching the front o2 sensor wires from one side to the other.

 

[RonD]

Yes, you have the 2 upstream O2 sensor connectors reversed, O2 connectors are universal, so easy to reverse sides

1998 4.0l
Grey/light blue wire is on Passenger side
Red/black drivers side


One heads up, O2 sensors are not used when engine is cold, OR at WOT(wide open throttle) cold or warm

So if engine runs rough AFTER a few minutes but better cold, and then also runs better at WOT then its most likely an O2 sensor issue

If none of that matters, engine is just running rough, then probably not O2 sensor related

 

[NoDangerIfItsARanger98]

UPDATE: I checked the O2 plugs and I have them going to the proper sides.

I traced the issue to the O2 sensor after the cat. After I put the new cat on and the new sensor I monkeyed with it last night and I don't know if It wasn't plugged in all the way or what but all seems well now.

I was aware that the O2 sensors only come on when warm but thanks for the heads up.

It ran a little rough right at first but smoothed right out. Same thing this morning but to a lesser extent.

The only remaining issue is between 1500-2000 RPMs it jumps up and down wether my foot is on the pedal or not. Has tons of power on takeoff and higher rpms but flutters when maintaining 2000+ Is this a vacuum issue? or maybe the TPS?

I was also told by my local mechanic and a veteran Ford tech that the GEMs in these rangers are painfully slow learners, could the thing still be trying to figure everything out?

 

[RonD]

GEM = generic electronic module, and is the Body computer in 1995 to 2003 Rangers, replaced by SJB in 2004

It runs cab electrics, it doesn't have a 'learning' aspect, it can be reprogrammed

PCM= power train control module, the engine/transmission computer, it does 'learn' some aspects of air/fuel ratios, like adjusting to changing fuel pressure over the years via long term fuel trims, also driving habits with an automatic, it changes shift points

 

[NoDangerIfItsARanger98]

Sorry! I should have specified between gem and pcm I'm just used to calling all computers where im at gem modules!

It has a reprogrammed GEM in it because the 4x4 in mine was bad.

I forgot to mention that if it's in park and you let it idle for a little bit it almost tries to stall itself out and will stall itself out if you give it throttle.

Once you let off the idle drops down to maybe 450-500 then slowly regains a normal idle. It does the same thing in reverse after running for a while and I've had it stall a few times.

Also as I just discovered a little bit ago my Ranger does not have the PCV ventilation valve on the valve cover just a little hose that goes from the cover to the plenum. I didn't realize it was supposed to have one until I was reading thru some posts on here.

That have any negative effects or causing any of this?

Thanks for the tips and info I really appreciate it as this is my first Ranger and first project. I should have joined long ago!

 

[RonD]

Driver side valve cover towards the rear?

Yes..................get a PCV Valve, thats a major vacuum leak without it!!!!

Surprised it runs at all

 

[NoDangerIfItsARanger98]

Seriously! I didn't realize it was a necessary part as it didn't have one when I got it, just the hose connected to the plenum and it all looked factory and correct so I just assumed that it was. It ran fine for the 10,000 miles I put on it without it and has been running without it!

I knew something stupid or simple was probably wrong with it! Darn it all now I feel silly but I guess when you don't know you can't help it! Like I said first Ranger and first project live and learn!

Will replace and update ASAP

 

[NoDangerIfItsARanger98]

Update:
The issue has returned so the downstream o2 sensor must not have been it.

I put a pcv valve in it and checked all vacuum lines and hoses. This of course had vacuum hubs from the factory which I replaced with manual hubs and blocked off the vacuum lines. I looked over everything and removed the blocked off lines and refitted everything.

Retorqued the plenum as I had an issue with an intake leak back when I first got it. No movement all torqued to spec.

No vacuum leak to be found.

I can't say it made a major difference but the idle feels smoother and when it drops down to maintain idle rpm it's not as violent of a drop.

Now just for giggles I decided to switch the O2 plug connectors around even tho I have them correct according to the wiring color code you gave me.

When i did this the idle now maintains a solid idle right close to 1000. I ran it in park up to various RPMs including the range where it would seemingly miss or jump.

No miss, no lope, no jumping. Zip zero nada. Ran like it was fresh off the line. So I thought maybe I'm just colorblind and had them wrong and decided to test drive it.

Ran like a dream no issue felt like it had more power wasn't sluggish and didn't feel restricted. I drove it maybe 5 minutes like this thinking it was finally resolved.

I was almost home and all of a sudden it was like someone put a ton of blocks in the bed all at once. This happened right around the RPMs where it would normally get jumpy and sluggish 2000-2500ish. It started jumping and missing and felt really restricted again.

I'm stumped, dooped and flabbergasted bamboozled even. What else could possibly be causing this?

I checked everything with the scanner (snapon Solus Ultra) The injectors all test positive. Fuel pressure is %100. Cylinders all test the same. EVAP systems are all good. The only thing I get is a misfire moniter fault and the O2 sensor codes.

Again I appreciate the help and input.