I have 97 Ranger 4.0 with the check engine light on. The codes say that both banks are running lean. It also tends to idle somewhat rough and also stalls when coming to a stop. Hmmm, vacuum leak I say to myself. I had a extra O2 sensor, so I replaced one sensor just to make sure it wasn't as pair of bad sensors, but both codes came back up. I've went looking for a vacuum leak, but haven't been able to find one anywhere, and I put a gauge on it and it pulls a solid 20-21 inches on manifold vacuum.

So my question to you learned folks is: how much of a vacuum leak does it take to lean out the engine enough to through those lean burn codes? Should you be able to find it pretty easy? Should a gauge show a low reading? I think 20-21 is pretty good - is it?

TIA,,.

- jf

 

I would start by cleaning the MAF. If it is dirty, it will make the engine run lean. Also, check the PCV hose for cracks, especially if it has any sharp bends in it.

 

Thanks for the tip LxMan - could a dirty MAF sensor also cause lousy idle and stalling when coming to a stop?

I'm still wondering if anyone out there can answer my question about a vacuum leak - would a vacuum leak that is bad enough to cause a pair of lean codes drop the manifold vacuum enough so that it would read low on a gauge? Or could a small leak, one that might not be noticed when testing with a gauge, be enough to lean out the engine?

TIA...

 

It doesn't take a big leak. The PCM uses the MAF to measure incoming air into the engine. With a vacuum leak, you are getting more air into the engine but the PCM doesn't see it so it doesn't add the extra fuel needed to run properly, which leans out the mixture. If it is dirty, it will also not see all of the air entering the engine.

 

You should also not rule out the fact that the engine may actually be running lean. A fuel pressure check would rule out the pump, disconnecting vacuum at the regulator will cut down the return flow and smooth the engine somewhat if supply is the problem. I believe I would check the fuel filter for blockage before all else if you Follow LXMan's advice and the problem persists. He's quite correct about a MAF causing a lean condition if correct airflow is not sensed. In essence the PCM could be seeing a MAF reading which would locate the vehicle at oh,say, 7,000 to 8,000 ft. elevation which is still within the PCM allowable parameters for the sensor output so it determines that fuel must be cut to achieve 14.7 / 1 mixture to appease the emissions gods. Hey, good luck.

 

Thanks for the reply. I do believe the engine is running lean - while both sensors could have gone bad, I'm still getting lean codes from both banks after replacing one sensor. The fuel pump checks out ok - right in spec. I cleaned the MAF, no change by doing that - I guess there's a chance it's bad and cleaning won't help. I'll read up a bit and see if I can figure if there's a way to test that. I think my next attempt will be to snug down the intake manifold, although I'm still thinking that the manifold vacuum reading is too good to have that be the reason it's idling poorly and stalling. I haven't ruled out that there might be two problems - a small vacuum leak might cause the lean codes, and the rough idle and stalling may be something else - TPS maybe? Oh, and the fuel filter has been replaced within the last 5000 miles...

As a curiousity question, I also wonder why the PCM (since it has info from both the MAF and the O2 sensors) wouldn't richen up the mixer when it gets lean O2 sensor readings? Seems that it could compensate for low fuel pressure, a small vacuum leak, faulty MAF and probably a dozen other causes for lean running. It could still throw error codes saying there was inconsistencies bewteen the MAF and O2 sensors, but why not make the engine run right? The PCM could actually test the O2 sensors by leaning out the mixture, then enriching it to get the sensor to swing full range. Seems that they're only doing half the job they could do once they have all these sensors and computing power...

 

Well, lets see. What the PCM does is to interpret incoming sensor readings and apply those to operational formulas programmed into its memory (Flash reprogramming makes changes to this info). If you have O2 sensors giving a lean value you would expect the PCM to increase long term fuel trim to compensate, however once the PCM has seen that the O2 readings and other sensor inputs do not agree with its formulas it must put itself into a failsafe. At this point the fuel control system is locked at a predetermined setting. Your MLP will be illuminated indicating a concern or Hard Fault has been recognized. When addressing this sort of situation I would recommend careful scrutiny of all drivability PIDS. If you have access to a Snap-On scan tool (speculation due to your ability to retrieve OBD2 codes) you can note any codes, clear the codes, use the troubleshooting section to access Fast-Track data scan which when followed will show you your readings and give you average values to compare them to. A fuel pressure test, measured at the fuel rail, would be my course of action for a multiple lean code, when vacuum appears to be normal. MAF values and altitude tables are also located in the Fast-Track section of the scan tool. Good luck with her.