97 Explorer miss under load

[BrainofJ]

97 Explorer 4.0 OHV (x engine)

Vehicle symptoms: approx 1-2 months ago, vehicle would start as normal & idle fine when cold, but would immediately miss/stumble when put in reverse or drive. I would pull forward a bit before exiting driveway & put back in park. Engine would return to smooth idle. I would then return to drive & exit driveway & all was normal for the remainder of the drive. Now (for approx. a week), the miss/stumble condition persists on the drive, especially under load at low rpm. Condition appears to be somewhat intermittent while driving. It will appear to have gotten better while driving at times, but will reoccur when stopping at a light & restarting or slowing down & then climbing a steep grade. Check Engine Light has never flashed or been present during this period. I had parts store scan for codes: (PO171 & PO174 lean bank 1 & 2) were the only codes present. Any thoughts given my unique symptoms?

 

[MtrJunkie]

Those codes typically indicate a bad intake manifold gasket. Fel-pro sells a new gasket for upper and lower both together for about $16. I just had this issue, and replaced them.

You might also search on here for PCV cracked, especially if you are replacing the gaskets for upper and lower intakes.

 

[Flash]

Those symptoms are typical of bad spark plugs/wires.

Fix the gasket and see how it goes.

 

[ZWilson07]

If you are missing and running lean, you more than likely do have a vacuum leak. Like mentioned since it is affecting both banks, the intake is a good place to check but really it could be multiple things.

If your running lean especially enough to throw codes on both banks, no amount of spark plugs is going to fix the misses.The best way is with a smoke machine but you can get by with the propane trick, etc. The reason why they are sometimes intermittent is the computer is switching your long term fuel trims to try and compensate for the short term being lean but it can only do so much especially during quick speed/rpm/load changes.

I am actually in the process of this same issue on my 08 Jeep Grand Cherokee. I am about to build my own smoke machine or break down and buy an expensive one for future use.

 

[BrainofJ]

What is the most effective spray to use for leak detection that is also somewhat safe? Some of the flammables often suggested freak me out a bit...

 

[BrainofJ]

Does the fact that it misses under load & appears fine when at idle point to anything specific or eliminate anything? & surely what it was doing in the beginning (smooth start - miss upon gear engagement - smooth again at park - then fine from that point forward after drive engaged again) points in a more specific direction (fuel - spark - vacuum leak)? I've read so much with regards to the possible issue that I end up more confused than when I started.

 

[ZWilson07]

What is the most effective spray to use for leak detection that is also somewhat safe? Some of the flammables often suggested freak me out a bit...

I wouldn't recommend really spraying anything. At most I would get some propane with a hose attached to it and try and see if you can see a change in idle with it being sucked in a vacuum leak. But even that can be hit and miss especially without having a monitor next to you to show you the O2 and STFT/LTFT changes real time as you do it.

 

[ZWilson07]

Does the fact that it misses under load & appears fine when at idle point to anything specific or eliminate anything? & surely what it was doing in the beginning (smooth start - miss upon gear engagement - smooth again at park - then fine from that point forward after drive engaged again) points in a more specific direction (fuel - spark - vacuum leak)? I've read so much with regards to the possible issue that I end up more confused than when I started.

It pretty much all points to a vacuum leak of some kind/running lean for one reason or another. The codes only help to solidify that thought even more.

In that instance, the reason why it would buck more under load is because the lean condition is causing more negative affects when having to add fuel and the computer is trying to work to compensate for it with a lot of different readings and sensors. Versus just at idle, it is much easier for the computer to quickly switch LTFT to help compensate for the lean condition. You got to remember when driving, hitting the gas, letting off, more load, less load, there are tons of things that are being monitored and STFT/LTFT with O2 readings are going back and forth like crazy. At idle, it is easier for the computer to level it all out.

If you had some type of monitoring system or knew someone that does, you could really help to further narrow that down as you would be able to tell from the STFT/LTFT what is going on as well as using smoke. Nothing is ever for certain but the more and more you can use helps to further narrow things down. Like mentioned before, at this point in time and with the information given so far, a lean condition from some type of vacuum/exhaust leak is what I would be concentrating on or at least working to rule out.

 

[drdoom]

What's the fuel filter situation?

 

[BrainofJ]

Thanks so much for the input Zwilson. I'm going to attempt to locate a scanner with live data or a smoke machine this weekend. I've found a good tutorial that uses a live data scanner to narrow things down a bit. One more thing to add: I now notice a distinct sound coming from the radio speakers when struggling under load (like electrical firing). Does that indicate anything?

 

[ZWilson07]

Thanks so much for the input Zwilson. I'm going to attempt to locate a scanner with live data or a smoke machine this weekend. I've found a good tutorial that uses a live data scanner to narrow things down a bit. One more thing to add: I now notice a distinct sound coming from the radio speakers when struggling under load (like electrical firing). Does that indicate anything?

You're welcome.

I wouldn't really worry about that at this point in time. When you start having issues it is real easy to "start noticing" little things that you didn't really notice before. It could just be coming back through from the misfires, could be a million things, could be nothing. I'd wait on it and monitor some things with a scanner first.

I am not saying it has nothing to do with it but I have never really seen that actually happen from a problem associated with the motor and such. Like you already have done, study up or watch videos on the STFT/LTFT and O2 readings. Once you have a grasp on all that, it will be super obvious and readily apparent if you area having a vacuum leak/exhaust leak. Then if you have access to smoke it can easily show you.