Ranger Idles fine but RPM gauge fluctuates while driving

[BeeTeeBee]

Hi all,
I have a 1999 Ford Ranger 4.0L V6. My truck’s RPM gauge will fluctuate and if I don’t give it some gas it will stall. While this happens the truck feels like its chugging. It idles just fine (1000 RPMS). I took it to an auto shop and they couldn’t get the problem to repeat. They said the diag showed that in the past the IAC valve had triggered and the ECT sensor had also triggered but were both functioning fine. They recommended a tune up. I replaced plugs (they were really bad), wires, gas filter, PCV Valve, and IAC valve.

After all this, the trucks RPM’s would still fluctuate and still chugs, although not as bad. I have an Engine Coolant Temp sensor to install yet but with my plugs being so bad I was thinking maybe I need to buy a new ignition coil? The diag just registered a P0320 (Ignition/Distributor Engine Speed Input). Any help? Sorry if this was a little long. Any help would be much appreciated.

 

[TwoTon]

Is it giving any codes for oxygen sensors? I know that when my oxygen sensor came unplugged my motor would hesitate and stall, never fluctuated though. Probably not the problem but it wouldn't hurt to trace your oxygen sensor wires and make sure everything is snug.... the code leans more on a crankshaft position sensor, I'd look into that as well

 

[BeeTeeBee]

Thanks for the reply. I just hooked up the diag again and it gave two codes this time. P0320, which I knew and then P0153, which is the O2 sensor... Bank 2, Sensor 1. I am trying to figure out what O2 this one is but am failing. I have a New O2 and Crankshaft sensor arriving at NAPA tomorrow. I will check the wiring of the O2 before installing a new one. Poor Ranger, but so reliable.

 

[TwoTon]

That code corresponds to an upstream oxygen sensor(pre cat, or in cat) I am not familiar with 99 exhaust, I know my 96 ranger has two sensors, one before cat (upstrream) and one after cat(downstream). I know in newer models(idk about rangers) they even put O2s in the cat housing itself, check your catalytic converter for one, and check the one before your cat. Hope you can get it fixed, the 4.0 is one hell of a motor, especially the SOHC one you have

 

[BeeTeeBee]

Problem appears to have been solved (Fingers crossed).

Since I got codes for the O2 sensor (Bank 2 sensor 1) and the crankshaft position sensor I changed them both today. I fired the truck up and drove 15 miles home from work without one chug or stall.

As for the installs:

I found that the best way to get to this O2 sensor was to take the driver side tire off and remove the flap. My arm has a lot of scratches but I got it changed!

The crankshaft sensor was easiest (still really hard for someone with Popeye forearms) to get to from underneath. The old sensor didnt have a space to see the top bolt, so that was hard to get a socket on, but the new one had a window and made it a lot easier. Thanks for the help!