At idle (engine warm fwiw), pressure indicates 31psi. This is on the low end, as specs say 28 - 40psi is the range for this vehicle ('99 F150, 5.4L. It has a return-type fuel system). Disconnecting the vacuum hose from the regulator (FPR) while idling brings the pressure up to about 42psi, as expected.

If I shut off the engine with the vacuum hose connected to the FPR, the pressure immediately drops like a rock to zero; if I shut off the engine with the vacuum hose DISconnected, the pressure remains at about 40psi for at least 5 minutes.

The Haynes manual is unclear as to under which condition the reading should be taken.

Which is the correct way to check for internal leakage?

Thanks!

 

Wow, that makes zero sense. Both conditions should hold pressure after shutting off the engine and the FPR shouldn't be able to affect it. Can you reliably duplicate the results? When the FPR leaks internally, disconnecting the hose with the engine running will result in fuel leaking out of the vacuum fitting.

What is the condition you are trying to diagnose?

 

Yes happens every time. Except. . . today, I replaced the fuel filter to see if it made any diff in the running pressure (didn't). When I restarted it after that, it actually did hold pressure with the vac hose connected, twice. Then, on the third and subsequent tries, it went back to dropping to zero.

I've had no evidence of fuel leaking from the FPR vac hose connection - doesn't even smell of gas.

Whoa, long story! Crappy idle after hot soak, but only on very warm Central TX winter days (75F-85F). Hot summer weather or colder days in winter, no problem. Figured it was winter blend of fuel giving the truck fits, for some reason. No codes stored. Had no driveability issues after the idle clears up. . . until this past week. On an 81F day, had the typical hot soak idle issue, but it then morphed into serious driveability problems - surging badly, like turning a switch off and on. Triggered the MIL with P0171 & 0174 codes (only), but the next AM at 40F, it ran just fine.

Went through the engine compartment looking for a vacuum leak - replaced the PCV hose assembly + some other nasty-looking hoses, but found no actual leaks. Also ran tests on suspect components: IAC, TPS, air charge temp sensor, EGR solenoid, and all passed. So, now looking into fuel delivery issues. Fuel pump is only 2 years old, BTW. Truck only gets driven 1 - 3 times a week, so it has 90K on it, thus the fuel pump should have very few hours use.

Any thoughts on this?

 

I'm just guessing here. I know very little about injectors. But, could your injectors be leaking down flooding the engine when you shut it off?

 

There are only 3 places the fuel can go to drop the pressure after shutting off. It can go back through the fuel pump because the check valve has failed, it can go out a stuck open injector, or leak out of the lines, rail, o-rings etc, but you would smell that.

The P0171\P0174 is classic PCV elbow on the back of the throttle body. You said you did some PCV hoses, was that done?

FYI, my original V8 was hydrolocked from a failed leaking injector at 148k, so Roadie may be right.

 

Thanks much for the replies!

Yes, the PCV connection at the back of the intake was replaced - it all comes as one assembly. (Old one was just fine, still flexible, but after going to all the work to get to it, I wasn't gonna put old parts on.)

I'm certainly (and obviously!) no expert, either, but according to the research I've done, if there was an injector leaking down I would see at least some smoke on start-up. Doesn't happen. Also if the system was leaking down thru the check valve, I believe I'd experience extended cranking required to start a cold engine. Again, that doesn't happen. It can stand for a week and start like I'd just shut it off.

And, if there was leaking thru the check valve (or some other place) why does it hold pressure just fine with the vacuum line DIS connected?
Also note that the pressure drop (with vac line connected) is pretty much instantaneous, not a slow drop.

Not arguing, just sharing the thought process I've been going thru this past week.

 

Just happened to think: With the vac line disconnected from the FPR, does the regulator fully close and thus prevent fuel from returning to the tank? If that is true, then the check valve could be leaking down, and badly. Not completely clear on if that's how the regulator works.

Still, I should have some extended cranking to start it from what I've read elsewhere - that's one symptom of a bad check valve.

 

Your thought process is flawless. Yeah, you should see some black smoke on startup if an injector leaks, and if the check valve has failed, you will get extended cranking time.

I have no explanation on your fuel pressure leak down - which is ironic because I am fighting the SAME issue on my Viper right now and I have replaced the fuel pump. It only throws a P0171, but it has dual throttle bodies. Maybe we can help each other.

 

RE: your P0171 code, since it is only one bank, wouldn't the O2 sensor be a good suspect? I imagine you've already dismissed that. Are you using a K&N filter? I understand the oil from those can coat the MAF and eventually lead to a 0171 or 0174 code.

Another above-normal-temp day here, and once the ambient hit about 72F, I started noticing the same hot-soak issue. Thankfully this day it cleared itself up in about 15 seconds each time it happened, so there was no surging while driving. Took it by a shop that's supposed to be pretty good, just to have someone smarter than I am witness the condition. The guy seemed knowledgeable, but really had no further suggestions w/o putting it on their scanner, which he didn't have time to do. He did not know for sure about the leakdown but agreed that if the check valve were really leaking I'd have extended crank times. He felt there was still a vacuum leak somewhere. (Maybe, but it will have to be very temperature-sensitive if so.)

He also mentioned that the MAF sensor sometimes goes bad on these, so I tested it when I got home. At idle it is supposed to read between 0.5 and 1.5V signal-to-ground (check). As the rpm increases it is supposed to get to +/- 2.0 V, but the best I could achieve was about 1.6V. Also the Haynes manual sez that there should be resistance across the signal and ground terminals of the MAF, but my tester shows an open circuit across those two sockets (hmmm).

A bad MAF would explain a lot of the weird behavior, but is it even possible for the engine to run and not throw a code if the MAF wire is burned thru or broken?

 

The reason I suspected leaking injectors is that you stated you had hot soak idle issues. Starting it cold leaky injectors may help it start quickly but not so if the engine is hot.

 

That's possibly true, however the hot-soak idle issues don't start immediately after the engine fires up. It takes 5 sec. or so for it to go funky.

Also, as mentioned above, no smoke from the tailpipe either hot- or cold-start, so it doesn't seem as tho there is any additional fuel in the cylinders.

Thanks!

 

There appears to be at least two issues present at the same time.
It takes some additional testing to get some leads.
First, the regulator may be suspect. It can stick open and allow the pressure to bled back to the tank through the return line.
Or stuck closed leading to excessive fuel injector pressures that cause an over rich condition.
Testing with a hand vac pump might show this.
The average pressure difference at idle with and without vacuum applied to the regulator is about 8 psi.
The reason the regulator has vacuum applied it is to reduce the fuel pressure at idle when the motor doesn't need as much fuel through the injectors.
Next, the live data should be looked at for Long Term Fuel Trims to see how much they might be shifted due to an intake air leak. The leak could be temperature sensitive. A leak results in a Lean rough idle and even stalling.
This very often does not affect cold starting and still allows hot starting, then rough idle shortly thereafter.
I would start the motor as a full cold start and observe the running condition, then let it run to full hot to see how it might change.
There could be clues to this interval , in how it ends up idling.
.
To review cold and hot start sequences:
On cold starts the PCM looks at the values of the IAT and CHT to determine the fuel injection level, opens the IAC and advances the ignition timing. All result in a higher idle RPM, then gradually drops to normal idle.
Only difference with hot restart is the PCM still looks at the same sensors which are now in a warm state (different signal values) so less fuel, less ignition advance are set and IAC opens to a lesser degree.
Anything that interferes with the sequences will be noticeable.
.
Using this info as a base, trouble shooting should begin to recognize the issues.
Sometimes you have to get deeper into the diagnostics to solve the issues.
It does no good to dwell on what if's, whys or other positions trying to talk the issues away with out full testing to find them.
I'm not trying to be smart with the last sentence but people fall into this line of thinking that never helps when the issues are still ongoing.
Hope this might be help.
Good luck.

 

First off, Bluegrass, thanks so much for taking the time to compose such a detailed and informative answer. This should prove helpful.

Did that, just to verify that the fuel pressure does change according to the amount of vacuum applied. Not sure if doing that test actually proved that the FPR is not sticking, but it does respond as expected.

I'm seeing about an 11 psi. delta.

Prior to chasing this problem, I've really had no good reason to own a scanner. Unfortunately, the one I recently purchased does not give me the ability to monitor LTFT, so I will have to take it to a shop to have that done. To get a meaningful result it will have to be done on another extra-warm day. Still, the fact that this only seems to occur during unseasonably-warm winter days, but not in the hot summer weather, is pretty strange.

You are correct, of course, and by "noodling" this out with people smarter than I am, I've been able to eliminate a few potential causes & narrow down the possibilities. My hope has been that mine is not a unique problem, altho this specific behavior does seem to be. Actually, altho the problem hasn't been fixed yet, thru this forum and others, I've learned quite a lot about how this truck operates; it would be easy to take it to a shop and let them worry about it, but diagnosing & fixing it myself would be much more satisfying!

Thanks again.