2008 Focus low fuel pressure?


#1

Hello everyone, My Focus has 102,000 miles. It has thrown a p0171 code and I have been attempting to fix it. So far this is what I have done:
Changed MAF, air filter, PCV Valve, plugs, Valve cover gaskets, Bank 1 O2 sensor and fuel filter. All new.
I have cleaned the throttle body, fuel rail intake manifold.
Cannot hear vacuum leaks and tested for leaks with carb cleaner.
Symptoms: engine cranks but has a hard time starting, usually on second try. When tank is about half full i get Rough acceleration, higher RPMs (doesn’t change gear) and engine stalls on hard left turns. Idle gets rough at times but usually stabilizes at around 700 rpms. At the beginning, #1 spark plug chamber had a lot of oil in it. It’s been a few months and no oil after Valve cover gasket and PCV change.
Tested coils and the plugs get a spark.
Before I start messing with the fuel pump, am I missing something else that I can do?


#2

The most common cause of the p0171 on this Focus is the PCV hose turning to mush. You’ve already replaced that though. How about the air intake boot? Is it okay with no tears or holes?


#3

Upon visual inspection, the air intake boot is free of defects. I had sprayed carb cleaner around it without any change in idle. I’ll go out tomorrow for a propane torch for the propane test. I hear that works better. I did blow smoke through one of the hoses attached to the intake boot, no smoke left the system (engine off). Then ran the engine and tried to see if any smoke would enter any part of the manifold, boot, around throttle body and PCV hose with no luck.
I do have an OBD II reader with bluetooth. I just don’t know normal values to compare them too. Internet searches resulted in not being able to pin point what they are for a Focus.


#4

My question really is, since all the repairs that were done didn’t solve the problem, is the most probable cause be the ignition coils, fuel injectors and/or fuel pump?
I’ve also just read in the manual that I should use 85 octane and not the supreme octane (i think its 94). I doesn’t seem that would cause the problem though.


#5

P0171 isn’t for low fuel pressure, it is a lean code. The computer thinks the mixture being injected into the cylinders has too much O2 or too little gasoline. Compared to what it expects.

One idea, there have been some Ford posters here who’ve had a problematic DPFE sensor, and that could cause this code I think. Those sensors seem to not be super-reliable, especially after 60-80 K miles or so. So if your Focus uses that sensor, that’s something to check at least.

Next in line is a vacuum leak. Vacuum leaks aren’t always audible, and sometimes won’t show up by spraying a fuel around to see if the engine speeds up either. Particularly if it is vacuum operated device that has sprung a leak. Each vacuum operated device needs to be tested with hand held vacuum pump to verify it holds vacuum. What does the warm engine intake manifold vacuum measure at idle btw?

The stuff you’ve done are all steps in the right direction. One further step is to measure fuel trim with your obdII reader. For a lean condition, the fuel trim will probably measure positive, 5-10% probably. Measure fuel trim at idle, then measure it at a higher rpm, 2000-2500 rpm. Does the fuel trim drop considerably? If so, that’s an indication of an as yet undiscovered vacuum leak.

No solution yet? Well, that pretty much leaves a problem with the fuel delivery. Incorrect fuel pressure, faulty injector pulsing, or clogged injectors. Start with measuring the fuel rail pressure.


#6

I.ve taken screen shots of the manifold pressure and Fuel Trim (as a %).
The fist pic is the Fuel Trim reading. First at idle then rev to 2500 RPMs.
The second pic is the manifol pressure. At idle it was about 4.5 (Torque Pro said it was psi) and the change is when rev to 2500 rpm.
I’ll do some research on the DPFE sensor. Thanks


#7

The fuel trim (on average) seems to go from 5 % to 0% as you speed up the rpm. That would indicate some kind of vacuum leak. A small leak at idle creates a bigger difference at idle than at higher rpms b/c at higher rpms the amount of air the engine needs just to run at all is a lot more, so it swamps out a small vacuum leak. Plus at higher rpms the intake manifold vacuum is less, so it tends leaks less. The only thing unusual from your FT plot on the left is that I’d expect the fuel trim to be positive with a lean code at idle --meaning the computer is having to inject more gas than it thinks it should. The scale on your plot doesn’t indicate which is positive or negative as far as I can tell, but you’d think toward the bottom would be negative. Maybe the instructions that come with the instrument clarify which is which.

The second plot showing intake manifold pressure seems to show the more negative pressures on the top of the chart, so maybe that’s just the way this gadget works is all.


#8

4.5 psi is around 10 inches of hg, which isn’t enough for a normal engine running at idle. That looks like an abnormal measurement. 18-22 is a normal idle measurement. For some reason mechanics measure negative pressure ( vacuum) in inches of HG, but positive pressure in psi. Go figure. An idle vacuum of 10 inches HG indicates a vacuum leak of some kind, an exhaust restriction, or possibly a compression/ring/valve problem.

The fuel trim chart is consistent w/that, going from 5% to 0 % as you bump the rpm, . I’d have expected at idle the fuel trim to be positive, rather than negative, but that might just be how your reader works. Do the instructions clarify what up and down mean in that chart? Usually for a “lean” code, the ecm will be injecting more fuel than it thinks it should have to, and that would be a positive fuel trim. But since it goes from 5% to 0 % as you bump the rpm, that also indicates some kind of vacuum leak.


#9

I take it as 4.5 psi absolute pressure, which is about 20"HG vacuum, right?


#10

The percent indicates above the long term trim. The long term trim is at 25%, so when a short term reads 3%, its at 28%. Now, what those percents mean, I don’t know.
Insightful, another obd program puts it at 20.4.
I disconnected some tube, I believe it is to the DPFE. A line comes from the manifold into the device. I disconnected the bottom hose and found suction. When I disconnected the top hose (to the manifold) the car loses vacuum and stalls. I’m about to look it up in the manual.
Right now I blocked off the port to the manifold and engine is running fine.


#11

At normal idle the manifold pressure is at 20 inhg, but when the p0171 code was thrown, its at 8inhg. So I’m still inclined to believe I have a leak. Could the leak be linked to the EGR system?
I do appreciate all the help, and I am in no hurry. Thanks.


#12

Since you can watch fuel trims why not use spray while watching short term looking for changes. When you hit a leak, the numbers will go negative. If you have 3% ST at an idle with a warm engine, then your leak is when it’s cold. By the way the short term and long term aren’t added to give a sum total. Long term is just that. Long term. short term is what it’s doing immediately. As in now. Long term is more of an average over time. Fuel trims in the single digits are considered normal. The long term at 25% is the problem.


#13

@insightful … if the reading on the OBD II reader of 10 mmInchHg represents an absolute positive pressure above a complete vacuum, then I think you are right, that would be 20 mmInchHg of vacuum compared to atmospheric pressure. Normal atmospheric pressure is around 30 mmInchHg. At least that’s what my weatherman on tv says when he reports the barometric pressures.

And fuel trim values are offsets of the long term fuel trim? And the long term fuel trim is a whopping 25%? Seems that OBD II reader needs to come with good instructions, otherwise the numbers it posts are difficult to interpret.


#14

I’m not sure how that DPFE gadget works OP. I think it is some kind of EGR modulator Ford uses, to optimize the EGR response and minimize nitrogen pollutants while maintain acceptable acceleration response. You might search on that term in the search bar above, see what other posters here have to say about it. The way I test the EGR on my early 90’s Corolla, I apply vacuum to it, and if it stalls the engine at idle, I know it is working.


#15

@Brewjar

If your long term fuel trim is +25% and it’s not starting normally, that could be a fuel pressure problem

It would help if you measured fuel pressure and reported back, but I’m pretty sure your fuel rail has no test port, so you’ll have to tee into a line with adapters

Did you say your short term fuel trim at idle is +3% . . . ? That is great. That means you don’t have any vacuum leaks. Even if you did have vacuum leaks, they primarily affect the fuel trim numbers at idle

If you post your fuel pressure numbers, I’ll try to tell you if you have a problem or not, as far as that goes

The nice thing about hooking up a gauge, is that you can visually see if the pressure is holding or dropping, when the engine is shut off

“but when the p0171 code was thrown, its at 8inhg.”

Sounds like you’re looking at freeze frame data. What was the rpm when the code was thrown? Might be the engine wasn’t idling, hence only 8" of vacuum

What engine do you have?


#16

I’ll look around for a gauge with adapters (you are correct, there are no ports). Long is 25%, it takes a coue attempts to start and when tank is half full, gets sluggish acceleration at stop.
Does that leave me with injection, and/or fuel pump as my issue?
The 8 inhg was while driving, seems to hold steady at 20.4 when idling. And yes, freeze frame.
The engine is a 2.0 inline 4 beast (OK, not a beast).
Thanks for your help.


#17

Hard to start, lack of power and lean code under load sure sounds like insufficient fuel supply.


#18
if the reading on the OBD II reader of 10 mmHg represents an absolute positive pressure above a complete vacuum, then I think you are right, that would be 20 mmHg of vacuum compared to atmospheric pressure. Normal atmospheric pressure is around 30 mmHg.

Change “mm” to “inches” and you’re good to go…


#19

Thanks for catching my error on the units @insightful . Of course you are right, they should be Inches of Hg instead of mmHg … I just corrected it above.

An aside, on some of my prior work assignments I directed scientific software computer programmers, and for some reason it often doesn’t occur to them that it is important to the users of the software to produce plots with units on all the axis, or they’ll put the wrong units, or the units will be off by a factor of ten or something … Pretty much every error conceivable to make on plot units, I’ve seen. You probably recall failing to take into account the dimensional units of the measurements is what caused a NASA spacecraft to crash into Mars, instead of a soft landing.


#20

@Brewjar

I’m leaning towards low fuel pressure, due to a weak fuel pump, based on everything you’ve told us so far

But it would be best to verify this with a gauge, before condemning the pump just yet

According to a source . . . which may or may not be correct . . . your fuel pressure should be 35-55psi key on engine off

20" of vacuum at idle is great. No worries there

If the pump is indeed the problem, are you planning on replacing it yourself?