This thing has been missing at idle for a couple weeks. It will stall once or twice on start up if I don't feather the pedal. It runs but is sluggish. Same situation on either fuel tank.

I sucked a can and a half of SeaFoam into the brake booster line, then poured a bottle of Oreily brand concentrated fuel injector cleaner into a nearly full tank. No difference yet.

I checked my plugs and found a cracked one so I replaced my plugs and wires, and cap and rotor. I thought that would do it, but no.

I put a fuel pressure gauge on and got 42PSI at idle which is just below spec (45 - 60PSI) so I replaced the Fuel Pressure Regulator. No change.

I checked codes. It's showing 172 System always lean.

I changed the fuel filter within a year or two. Air filter looked okay but I beat it on the curb before putting it back in.

It feels like a bad wire or plug, but also seems like a low fuel pressure issue.

Any thoughts?

 

Forgot the transition year from MAP to MAF. If MAP, check that the vacuum tubing is intact with no leaks. Haven't heard of any problems with the sensor failing, but I suppose it's a possibility. If MAF, has the sensor and screen been cleaned lately - be sure to use the special MAF cleaner, carb cleaner will eat it up. Also a possibility of failure, but again, haven't heard of any problems. This would be the #1 suspect, given the code, IMO.
Faulty O2 sensor is a possibility, but with the code, would put to the backburner for now til other things are investigated.
Hopefully good quality ignition parts were used - leery of anything with Duralast or Woods or Pronto, would encourage Motorcraft and Standard and such.
Has the coil been checked? There are resistance specs in manuals, also the spark should be bluish, not orangish.
Could be the injectors aren't misting fine enough - but would think after the SeaFoam and fuel injector treatments - hopefully not a problem. Don't see a problem with your reported fuel pressure - IMO, close enough.
Using good fuel? Concerned about the unbranded stuff, prefer for the name brand top-tier (e.g. Exxon, Mobil, Phillips, etc.) since it's priced the same around here.
The stalling may be related to your IAC being dirty, a known problem with this vintage - a little carb cleaner and patience to get the piston moving freely usually does the trick the first few times.
Good luck, and keep us posted with findings - any information helps. Gotta lotta miles left in that motor. :-)

 

Stop replacing parts blindly. Code definitions are NOT diagnosis, and the fact that a part is named or suggested in a definition does NOT mean it has anything to do with the cause of that fault.

The 4.9L is known to be sensitive to ethanol in unleaded gas, so it's possible the miss is from a sticking valve. You could try an engine flush, but only if you follow the instructions precisely. The engine probably needs a flush at this age, but doing it wrong can cause a LOT of damage quickly. It should only be done ~20~50 miles before an oil change, because the sediment that it breaks loose can block up the oil filter. Especially if it hasn't been flushed in a decade.

If that doesn't help, it will probably take some real diagnosis to identify which cylinder is misfiring (it would have been easier if you HADN'T changed the plugs), and then to diagnose WHY.

 

Thanks. This one is MAP. I checked all the vacuum lines especially going to the MAP sensor and everything seems good. I was measuring about 22 on my gauge.

I pulled off the IAC and it seemed okay but I sprayed some carb cleaner in it to see if anything would come out but it seemed to be moving freely. I put it back on.

The ignition parts were not Mopar or top shelf but they were the "best quality" lifetime warrantee store brand ones.

I haven't checked the coil yet, but I will. I replaced it about two years ago when it did test out of range.

 

Hey now, Steve83 I don't think replacing plugs, wires, cap and rotor is blindly replacing parts. That's just a tune up. I replaced the FPR because it tested out of range (but not by much.) Code 172 refers to the HEOG sensor but no one is talking about replacing the o2 sensor.

I did a head gasket on this about two years ago and replaced the valves at that time. Would that change your opinion on doing a flush? I don't think I have ever had problems with ethanol in this motor, but I'm not sure if I would know if I did.

Should I consider a compression test to start tracking down the miss?

Thanks

 

EFIs don't need tune-ups; they re-tune themselves a few hundred times per second. And none of those parts is on the maintenance schedule, so replacing them without specific reason is "throwing part$ at it". There is no range for the FPR. The only tests Ford specifies for the FPR are that fuel rail pressure is inversely proportional to vacuum at the FPR nipple, and that no gasoline comes out the nipple. What a code refers to is irrelevant - what its definition SAYS is all that matters, so read & post the COMPLETE definition of each code your truck has. No, a head gasket replacement is not the same as an engine flush. If you do it correctly, a compression test won't hurt anything, and can provide useful information about the engine's condition.

(click this text)

 

"..."best quality" lifetime warrantee store brand ones."
Not to be persnickety, but went through this with AutoShack parts (the former branding of AutoZone). Sounded great, heck yeah, but the warranty lost its luster on the 2nd and 3rd and Nth replacements. Parts were indeed replaced free, but my labor and frustrations were priceless.

 

I hear ya here. I should probably get some decent wires. I figured for troubleshooting it would make sense to put new wires and plugs just to be able to rule them out. Truth be told, one of the wires was bad out of the box and I had to swap it already anyway. So, I'm not even convinced my "tuneup" has excluded a problem with wires, plugs, cap or rotor. Dang.

I tested my ignition coil per the Haynes book and it is in range.

Haven't done anything else yet except stare at connectors and wiggle them around a little bit.

 

Feel your pain here - want things to run right, but can't seem to find the problem. Aaaargh!
Another perspective on Steve83's comment - if you happen to replace a good part with a bad part - now you're chasing two problems. If you replace a good part with another good part, just wasted money. Not above just replacing a part because nothing else makes sense, but have to appreciate the risks of doing so.
Hesitancy on replacing the O2 sensor lies with why hasn't the control system richened the mixture up, and if it has - why hasn't the miss disappeared or things smell like they're running rich?
Off the wall thought is whether the EGR system may be stuck closed, or has been bypassed?

 

I get it. I have definitely replaced good parts with bad parts before - and bad parts with bad parts. Both can cause real headaches.

I replaced the Fuel Pressure Regulator because it did measure out of spec according to the Haynes manual and seemed logical that I could have a low fuel pressure issue. It seems to me now though, that if the FPR fails it is more likely to have a *high* fuel pressure problem though. Oh well. I still have the old one.

The EGR system has never been bypassed. It seems to be working right. I did some testing and cleaning with the EGR valve in the past and it has been apparently working okay since then. It might be worth me checking again though.

I don't feel like swapping a O2 sensor would tell me much. It seems like the O2 sensor is telling me that the system is lean, not that there is a problem with the O2 sensor.

A question I have is what would tell the O2 sensor the system is always lean besides being fuel starved?