Hi all,

I have a 98 F150, 4.2 V-6, 183,000 miles..

Lately, she's been loping at idle. Especially while in gear... almost stalls.
Once revved up, everything seems normal. Still goes up hills fine if I get on the gas.

The light was on, and I got codes for lean condition on both banks.. (P0174, P0175).

I already cleaned all the sensors in front of the intake.
The MAF was real dirty, and the IAC was full of soot too.
They both cleaned up nicely, but it didn't help.

I borrowed a fuel pressure gage yesterday, and it showed 12 psi with the "key on, engine off".
It showed 29-30 @ idle, and only went up to 35 when I revved it a little.
The guy at the store said the range is supposed to be "30-45 psi with the ignition on", but I'm not sure if that means engine running or not..?

Either way, it looks like the pressure is on the low side, so I'm going to change the fuel filter next (and hope for the best).

If the fuel filter doesn't fix it, what should I go after next?

And, is there a way to test the fuel pressure regulator before buying a new one?

Any suggestions are greatly appreciated, thanks in advance,

Jim

 

Those pressures are confirmed here. http://www.justanswer.com/ford/2lhyd...f-150-4-2.html click the first click here in the article, it's a print out from All Data.

With the age and mileage, on the truck I would look at a weak/failing fuel pump, if the filter does not fix it.

 

Thanks for the link, that clears up my confusion.

... 12 psi with the "key on, engine off" is way too low..

I just did the fuel filter and, for a minute, I thought it was fixed... but then the loping came back.

I was hoping to avoid going after the fuel pump.. I've done it on a few other vehicles, and it's no fun. I guess I'll run the gas out for a couple days before going after that one...

When I did the fuel pump on an '88 Ranger, I removed the bed, and it was pretty easy after that. The F150 bed looks quite a bit heavier though, so I guess I'll have to drop the tank this time.

In the meanwhile, if there are any other simple things I can check, I'm all ears!

Thanks again,

Jim

 

I think I made a mistake when I said "I got codes for lean condition on both banks.. (P0174, P0175)." .. sorry bout that.. I think it should've said "P0171 and P0174".

I disconnected the battery since then, and when the codes came back, I got 171 and 174.

Over the last couple days, I did a vacuum test and a more thorough fuel pressure test.

The vacuum test looked pretty good. It held steady @ 20 during smooth idle, and bounced between 17-20 while loping (the loping is intermittent). The RPMs go from about 600 to 800 when loping.

I went ahead and bought a new fuel pressure gage, and this time the pressure showed 39-40 with the "key on engine off".
15 minutes later, it was down to 36 psi.
40 minutes later, it was 16-18 psi.
An hour later, it was 14 psi.
So, I guess that rules out the fuel pump and pressure regulator? ... or should it hold pressure longer than that?

I'm running out of things to check.. what else could it be?

 

The pressure should not bleed off that much, that fast.

I would put a long hose on the gauge and temporarily mount it so you can see it while your driving, so you can observe the "at load fuel pressure". Just make sure that the hose is secured to the test port and the gauge so that it does not blow off while your driving. You can leave your hood partly open on the first lock, put your truck in second gear and just drive around the neighborhood, nail the throttle a couple of times, you will see what the pressure is doing. The reason for second gear is that when you nail it it will put a full load on the injectors at a safe speed and should max out the fuel pump. If the pump hits the 45psi and holds the pump is fine.

I think your pump is fine because it did hit the at rest pressure mark, you can do the test if you want just to be sure.

If its not the pump, you have three possible problems based on the rapped pressure drop, two are easy to check, the third not so much.

The first would be a pin hole in the fuel line, that should be easy to find, pressurize the system, get flashlight and trace the lines out.

The second would be get a vacuum tester and hook it to your fuel pressure regulator, I would pull 5 inches of vacuum and see if it holds, if it bleeds off the regulator is bad.

The third, if you found no pin hole in the lines and the regulator is good, the only thing left that would let the fuel pressure bleed off that quick is a sticking fuel injector. The only way to find it, is take the truck to a shop that can track down a single bad injector or maybe multiple injectors or you can pull all of your injectors and replace them with new ones or you can pull them and send them to a repair shop and have them rebuilt. If you can afford a couple of days of down time that would be my choice. This is the cheapest place I have found. $16 per injector to rebuild, vice $45 or more per injector to buy new.
Mr Injector - Home

Good luck

 

Thanks again! I really appreciate the detailed answer, I just learned a couple things

Today, I cancelled the codes with a reader, and took a 30 mile ride.
When I got there, I stopped at the store, and the light came back on as I pulled out.
It's PO171 and PO174 again..

On the way home, I floored it while going up some hills on the highway and, it does seem to be fluttering a little while at maximum load.

It's an automatic, so I can't just do second gear.. but I do understand your point about low rpms @ WOT though.. I used to have an old carbureted truck that fixed itself on a steep hill around here.
When the loping started up this time, I went right back to that same hill, but it didn't work this time..
I still have a heavy dose of injector cleaner in the gas right now.

The bottom of the truck is reasonably clean, and I already looked at the fuel lines (a little bit).
I didn't notice any wet spots... I guess I'll take a closer look soon.

I think the pressure regulator is good, but I'm not sure...
I put a mityvac on it the other day, gave it "8", and watched it for a minute or two.
It held steady, but I didn't check how long it would hold.

If it's the injectors, is there a way I can test them?
Or better yet, rebuild them myself?

Thanks again,
Jim

 

Just put your auto in 2nd, so it does not up shift.

Go to the Mr. Injector site, he sells the rebuild kits but look at the videos on his site, they are vary informative. There are a bunch of videos on U-Tube about diy injector testing.

 

their is a rubber elbow just in behind throttle body on the bottom side. Directly under the air idle control. Pull it off and check for leaks. Mine had a hole in it. Some days you could hear the vacuum leak and others you could not. took me for ever to find it but was pulling same codes and symptoms you have. All fixed now

 

Right on, thanks for the tip.

I just went and looked at it with a flashlight, but the engine is still hot.
You mean the one that goes to the cannister on the driver's side of the firewall, right?

I'll get a closer look at it tomorrow.. It "looks" ok right now, but I didn't remove it tonight. It's stuck on there pretty good, and I don't want to break it tonight because I gotta work in the morning. I'll post what I find, thanks again.

 

Oh, I guess I didn't understand your point the first time then. I assumed that you meant to "lug it" in second gear..

Thanks again for the link too. I'll take a look for those kits and vids

 

I'm not sure if I understand the "rubber elbow" part of that..

On mine, there's a metal elbow under the IAC, with a hose that goes back to the firewall. ('98, V-6, 4.2 L)

Yesterday after work, I pulled that hose off and tested it with a mityvac. It held 25 psi for a minute, no problem. So I put it back, and added a clamp for good measure.

This morning, the truck ran smooth!
I was thinking it might be "a win".. but it was a short ride. .

On the way home, she was idling a little rough again. so the hunt continues...

.....

.. I haven't heard "loping" in a few days now, but the idle is still a little rough sometimes.

It's intermittent. Sometimes she runs smooth, and sometimes the rpms drop to about 600 (and the dashboard vibrates).

I still suspect the injectors. I'm thinking (and hoping) that the heavy dose of injector cleaner is still working... maybe it's not done working yet..

How long do fuel injectors last? .. I'm at 184,000 miles.

 

May sound screwy but give the clutch on the fan a once over. Grab it and check for wobble and observe if there is silicone seeping from it.

 

Jim,
the varying idle indicates the IAC is attempting to control the idle but is being interfered with by some other air leak source.
Soft rubber hole that is reacting to engine vacuum can vary an air leak.
The IAC tries to counter it by shutting down it's air.
The fuel regulator also gets into the act of varying the fuel pressure because the intake vacuum is swinging.
The crank sensor sees the RPM change and tells the computer to work the IAC but it can't find a stable idle point.
The end result of all this is what you see happening.
Idle hunting.
A Scanner looking at the OX sensors outputs would show/verify all this going on by their switching swings over a wide range.
The fuel trims may also be out of limits. This sets the codes 171/174.
None of it is really a mystery if you consider all the actions going on and what can cause it to happen.
Good luck.

 

Thanks Dan, the fan clutch looks clean, and spins smooth -with slight resistance.

When I checked for wobble, it was tight on the first try..
Then I spun it 90 degrees, and tried again.. and there was a slight wobble the second time.

I'm not sure what that means.
.. It never felt really loose (like a loose when bearing), but there was a little bit of "side to side" on that second try..

[edit] "side to side" is not correct. The play was from "front to back".

 

lol, maybe not to you, Bluegrass.. but, to me, it's like magic. Thanks for the response.

I'm just a carpenter who works on his own truck. ... There's a lot I don't understand about engines, especially when there's a computer involved.

So, correct me if I'm wrong... it sounds like you're saying that the IAC and fuel pressure regulator are doing their job correctly..??

I have a 'borrowed' scanner on hand, but I don't know everything about how to use it. It's an "Innova 3030"..
I'm not sure if it can do the "OX sensor output" test that you mentioned... I'll have to learn more about how to work this thing.

Thanks again,

Jim