Hunting Idle?

Hey guys, I've done a lot of research on this but I wanted an opinion on my personal situation before installing something else that has cost me a lot of money on my tight college budget. I know there are a lot of smart people on this forum, and I'm hoping someone has had experience with this.

The vehicle in question is a 2001 Ford F-150, with the Triton V8 5.4L engine and ~232,000 miles.

So here's my situation:

A few weeks ago, I replaced my serpentine belt and got my oil changed. Immediately after, I noticed my CEL had come on. I brought it to Autozone and was given codes PO171 and PO174 (running lean on banks 1 and 2).

About a week later, my engine developed a rough idle. It only seems to be an issue after the engine has warmed up (i.e. if I let the truck sit for a day it takes a little bit for the poor idle to begin). The truck will stall occasionally at stoplights, and the idle is usually hunting between ~500-1000 rpm. I have to put my truck in neutral at stoplights or in traffic because it is essentially lunging forward and I have to fight it with the brakes. More and more research led me to believe the issue was my IAC. I replaced that the other day, and mine was VERY dirty (since I have 232k miles on the engine, I figured it was worth replacing and not just cleaning). Unfortunately, this did absolutely NOTHING to fix the issue. This was last night.

Today, I took it back to Autozone, to get an updated reading of the codes, post-idle issue. Two new codes popped up, P1131 and P1151 (bank 1 and 2 upstream O2 sensor is indicating lean). I bought the oxygen sensors to replace (pricey for my budget), but am now hesitant to install them. What are the chances that the O2 sensors are actually bad? Isn't it rare that these go bad? And how about BOTH O2 sensors going bad at the same time?

Is there any possibility that the lean condition has caused my O2 sensors to go bad?

I know about the usual causes of these codes, such as a vacuum leak, dirty MAF sensor, leaky fuel injector, EGR, etc. And I am still investigating them. My biggest worry is that it will end up being leaky intake manifold gaskets. I am mostly looking for a second opinion from some people who are smarter than me, and I'm wondering the answers to the few questions I asked.

Thank you for your time and help.

Do you have a bottle of propane handy? used carefully (low flow, no flame (duh)) you can use that to find leaks. The engine will pick up where you are pointing at the leak.

I'd be more of the intake leak somewhere than anything else.

Oh, and instead of parts, get a good CDC MAF cleaner can. Cheaper- if that's the issue.

You may find something pretty obvious.

Hi Tmc22,

O2 sensors are actually one of the more common failure points. As you indicated, the odds of both failing at the same time independently are extremely low however, running lean results in excessive heat which could easily take both out simultaneously.

I always ask myself “what changed”…in your case, the answer is serpentine belt. So, what interacts with the serpentine belt, what is in close proximity to the serpentine belt, etc. I think the classic suspects of a wandering idle are sensors (O2, mass air flow, and crank angle) along with the vacuum hoses.

I’m a rotary guy so my opinion shouldn’t justify dropping coin on a game of parts roulette. Also, I’m older now which means I enjoy the luxury of being able to just wave my checkbook around and say “fix it”.

Good luck

I'm with Alfadriver, start looking for a vacuum leak. A can of carburetor cleaner will help find them too.

RX Reven' wrote: I always ask myself “what changed”…in your case, the answer is serpentine belt. So, what interacts with the serpentine belt, what is in close proximity to the serpentine belt, etc. I think the classic suspects of a wandering idle are sensors (O2, mass air flow, and crank angle) along with the vacuum hoses. Good luck

While you have a point about O2 sensors (and I'm pretty happy with how some age), the "what changed" is a good thing to think about- when you put a new belt on, what has to come off? I think some of the intake needs to be moved so that there's good access to the tensioner.

Anyway, what had to move to change the belt?

(and was the air filter changed? )

Did you clean the passages for the IAC valve? I had one car that would plug up the passages on either side. once a year I had to ream the holes with a small section of electrical fish wire. But with O2 codes your more likely looking for a leak / cracked hose that you bumped changing the belt

Thank you all so much for your replies. This place is great.

In reply to alfadriver and Toyman01:

I did try spraying around the base of the throttle body and along some of the vacuum lines with carb cleaner, but I found it extremely hard to tell whether it was actually having an effect on the engine because the idle already fluctuates so much. I can't tell whether the engine picked up because of where I sprayed it or it was just the usual, random rpm hike. I've never seen anything like it; the truck is constantly revving itself like it's trying to just drive away. I changed the air filter about a year ago, and it doesn't look too dirty. I also looked at the MAF sensor and it appears to be really clean. Since it's only a few dollars for a can of cleaner, that may be my next step.

In reply to RX Reven' (and alfadriver):

This is my kind of thinking: the "what changed" attitude. That is where I initially started. I'm not a strong believer in coincidence. One thing I noted that could have been bumped when changing my belt is the crankshaft positioning sensor. However, when I crawled up under the car and looked at it, it looked fine. Didn't look like it got jarred loose or any connections were unplugged.

Nothing was really moved when I changed my belt. Aside from something that could have been bumped by accident (which would currently be unknown), all I did was push the belt down in the engine, crawl under the car and loop it over the bottom 3 pulleys, then used my breaker bar to loosen the tensioner so I could pull it over the top pulley.

I've ruled out the EGR and IAC valve so far. The vacuum hoses are near impossible to get to, so I guess I will plan on removing the throttle body this weekend and giving them a good visual inspection. The PCV valve and hose appear to be in good shape as well, which are a common culprits of a vacuum leak on this truck.

Does anyone know if the gasket between the throttle body and the intake manifold is re-usable (if I'm careful)?

Thanks again for the replies, guys. This whole thing is a headache. I'm tired of my truck stalling at every other stoplight.

In reply to 44Dwarf:

I did spray some carb cleaner into the passages on the intake manifold side of the IAC valve, but I bought an entirely new valve so I found no need to clean the passages on the actual IAC valve. I don't know how much of a "cleaning" that was.

The only vacuum hoses I know about are far behind the belt, either behind the throttle body or somewhere else near the back of the engine bay. The likelihood of me bumping them while changing my belt is slim. But it's definitely something I have to check more into. I just haven't had the time to take off the throttle body. This weekend should fix that.

Reach around back and follow the PCV tube to a rubber elbow.

And return the oxygen sensors. The reason they are showing lean is because the engine is running lean.

In reply to Tmc22:

If you didn't notice, you didn't find the leak. With propane or carb cleaner, the difference would be really obvious. That's why we recommended it.

Wandering idle = vacuum/intake leak 95% of the time in my experience. But that is on 1970's vintage V8s, so your results may vary.

In reply to Streetwiseguy:

I do know about that PCV tube elbow and plan on giving it a really good inspection tomorrow when I have light. As for the O2 sensors, that was my thought, and I'm glad to have it confirmed.

alfadriver:

Okay, that's good to hear. I wasn't sure how noticeable it would be. I will keep trying with the carb cleaner.

pinchvalve:

That is where I am currently leaning. I think when I pull off the throttle body this weekend I will be able to get a good look at all the vacuum hoses and hopefully find a leak.

Thanks again for all the replies guys. You have all been a great help.

Does anybody know about the gasket between the throttle body and the intake manifold? Is it reusable if I'm careful? Or should I go ahead and order a new one?

I would say order a new one. Also, I don't really know what your engine looks like, but if it has any kind of rubber intake pipe from the MAF to the throttle body, take it off and look for cracks in it, that seems to be a common place for leaks also.

Streetwiseguy wrote: Reach around back and follow the PCV tube to a rubber elbow. And return the oxygen sensors. The reason they are showing lean is because the engine is running lean.

That is the problem. DO NOT REUSE THAT HOSE! Replace it! See Fords TSB and Safety Recall.

I pulled the throttle body this weekend, cleaned it out, and found that PCV elbow that goes to the intake manifold. Sure enough, that was the problem. It had deteriorated and formed a hole. I taped it up with electrical tape for now, because from what I've read, the elbow is a dealer specific item. I won't be able to get a new one until tomorrow when they are open again, but now that I know exactly where the elbow is, it should be a 2 second changeout tomorrow.

Thanks for all the help, guys. I was able to return those O2 sensors and save about $100.

Awesome. This board rules.