Hi Guys,
Sorry for the recent hiatus on this thread. I got really sick just before Thanksgiving and only recently recovered so the truck has been sitting with no work done. Yesterday I was able to change the battery (needed replacement) and I went ahead and checked fuel pressure at the fuel rail Schrader valve. My maintenance book says I need to check pressure at the coupling on the fuel supply line that goes into the fuel rail. Is it not valid to check pressure at the fuel rail Schrader valve? Does the dampener reduce pressure or something?


Anyway, the pressure was in the 33-35 psi range at the Schrader valve which I believe is low. Also, do you know if fuel pressure should hold at the fuel rail? when I turn the truck off the fuel pressure drops fairly quickly. I could quantify the rate but preliminarily it looks like 1-2 psi per min pressure decay. Does this seem normal.


My truck is now really not running very well and I have to disconnect the MAF if I want it to run without lugging out. Oddly enough when I disconnect the MAF sensor it runs absolutely fine. Seems to me like a fuel system issue would show symptoms regardless of whether MAF sensor is plugged in.


I'm going to proceed troubleshooting the fuel system, change filter and see where I'm at after that. I am really starting to get nervous that there may be something wrong with my PCM or something.


35 psi at the fuel rail is too low though right?

 

Glad you are feeling better.

1999 Ranger should be 60-65 psi at the fuel rail, and yes the schrader valve is there to test pressure or bleed it off if working on fuel system.

Ford switched Rangers to Returnless fuel system in the late 1990's, it will have a Damper at end of rail, this is just a diaphragm that absorbs pressure waves caused by injectors opening and closing, if they were to "reflect" from the end of the rail it could cause a build up of high and low pressure waves causing more or less fuel to flow when an injector opened, a nightmare for the computer trying to balance fuel mix, lol.
Damper does have a vacuum hose attached, this is a safety measure, if diaphragm should leak it would drip gas on hot engine parts, not a good idea, so vacuum hose would suck leaking Damper fuel into intake, causing rich running(and a code for Rich) and lower MPG.

Earlier Ford fuel systems did run at 35-40psi, they had a Fuel Pressure Regulator at the fuel rail and a Return fuel line attached to that.
If you don't have that, then yes your pressure is low.

And yes, fuel pressure should hold above 50psi for a few months, not minutes or hours.
When you first turn on the key the Computer will run fuel pump for 2 seconds, it won't run again until engine starts, or you turn key off and back on again.
This 2 seconds is good for about 10-15psi of pressure.
So Ford and the computer expects fuel pressure to hold at no less than 10-15psi below rated pressure.

In your system there are 3 system parts that hold pressure.
Fuel pump has built in Check Valve, fuel can flow out but not back in
Damper, sealed diaphragm just absorbs pressure waves.
Fuel injectors, should be sealed tight when engine is off.

Then there is the fuel line, it does have fuel filter in frame rail under drivers seat area with 2 or 3 connections, some had a return line on filter.
You would most likely smell a leak in the fuel line.

If injector or damper was leaking your MPG would be way down, and you would usually get a rich code, because computer is adding LESS fuel than calculated, because unreported fuel is leaking into engine.

If check valve was leaking then Lean code would be common, because computer is having to add MORE fuel than calculated, i.e. an injector open for 1 second with 60psi of pressure pushing fuel thru, vs 30psi of pressure, so less fuel flows in with lower pressure, computer expects 60psi so its calculations based on that is wrong.

Unplugging MAF sensor allows computer to use basic fuel mix ratios in memory which are slightly rich and then adjust that mix based on O2 sensors, so it should run a bit better but with slightly lower MPG.

You could pull main fuel line and run pressure test on it to determine if it is the check valve, but from your reports I would think that's what it is.
Yes, whole pump has to be changed, it isn't a separate piece.

 

Hi RonD and Everyone,
Quick update on the truck. So I've got the bed off (atleast slid back about 18" and decided to pull out wiring diagrams and do some voltage checks before changing the fuel pump. Wanted to make sure that the problem isn't that my pump is getting low voltage since the pressure is running low rather than not there at all. I started from the PCM relay and worked my way back to the pump. All voltage readings are referenced to battery ground, I ran a long lead from the negative battery terminal as I moved back to the pump. Also, all voltage readings were taken with key in the "ON" position. Here are my results:

1. Pulled PCM Relay - 11.75 Vdc at pin 30 (input pin to switch) and 10.3 Vdc at pin 86 (input to magnetic coil).

2. Replaced PCM relay and pulled downstream fuel pump relay - 11.75 Vdc at pin 1 (input to magnetic coil) and 11.76 Vdc at pin 3 (input to switch).

3. Replaced fuel pump relay and pulled downstream connector to inertia shutoff switch - 11.65 Vdc at Gn/Ye cable initially when key turned on, drops to 155 mVdc after about 1 second (is this normal, does it just power the pump for 1 sec to prime the system, then drop the voltage down?).

4. Replaced the inertia shutoff connector and pulled connector at fuel tank - 12.26 Vdc at pin 7 (pump pwr) initially when key turned on then drops to 155 mVdc after about 1 second.

5. I decided to check the two ground wires on this connector also to see if there was a voltage potential difference between them and the negative battery terminal. When key is turned on there is initially 155 mVdc difference and drops to about 50 mVdc steady state.

Assuming the drop after 1 second is normal, it seems to me like the pump is getting good power. Any input from anybody? I'm going to go out and pull the pump so I'll let y'all know what I find. I'm guessing in addition to the bad check valve, the screeen will probabl be pretty gummed up. Any input is greatly appreciated. Thanks!!!!

 

Yes, the Ford Computer turns on the Fuel Pump Relay for 1-2 seconds each time key is turned on, then there is no/low power on that wire until engine starts(above 400RPM), on some models there can be 1 to 4volts on this wire, that is from the computer circuit that monitors if fuel pump power is active when computer closes the relay.

This is a safety feature, if a fuel line should break or a fender bender should happen, causing a fuel leak, the engine would stall and fuel pump power would be cut off by computer because RPMs would drop below 400, even if key was left on.
Inertia switch is a backup to this feature, inertia switch will also cut fuel pump power but only if there is a sudden stop or rollover.

Did the fuel pump run for those 1 second power periods?

 

Hi Ron,
Yes, it does sound like the pump runs when i turn the key on and it feeds about 32 psi of pressure when running but it doesn't hold pressure (check valve) upon shutdown. I just pulled the pump assembly. The strainer definitely is pretty dirty, you can see dirt on the papertowel where the strainer is leaking fuel out and I can feel the sediment in it when i pinch the strainer. I think I just need to go ahead and replace the entire assembly because even if a gummed up strainer is causing the pump outlet pressure to be low, the system won't hold pressure.

I took a look in the fuel tank and there is sediment in the bottom of the tank (not a ton but it's definitely there). Do you think I should siphon the fuel out and clean the tank before I swap pumps? I haven't ever looked in a fuel tank to know if a bit of sediment is normal. Thanks for all the help again Ron!

 

While sediment is not "normal" it is expected from years of filling up at gas stations.

Sediment will stay at the bottom of the tank for the most part and not bother anything, fuel filter will pick up any sediment that does get stirred up.

If you siphoned out the remaining gas how would you get the sediment out?
To clean the tank you really need to pull it out, rinse it out and let it dry, so unless you are prepared to do that I wouldn't bother siphoning the gas out, just replace pump and see if it will hold the pressure.

 

Update: Hi Guys,

Wanted to give you all an update. I have replaced the fuel pump and the check engine light has been cleared for about 200 miles now. Fuel pressure is about 60 psi at idle. I started running into issues starting the truck. I would have to play with the battery cables every time to get it to turn over. I removed the cables from the terminal recepticles and cleaned everything thoroughly....including the wires themselves. Starts up every time now. Seems that disconnecting the batteries reset my emissions monitors so when I took it in to smog check they showed me a couple monitors were incomplete. Along with that, the mechanic told me that there is a pre-cat exhaust leak (hooray!). I assume he could hear it, but I'm struggling to find it myself....let the next adventure begin!

 

I also have P0171 and P0174. Initially I did have a vacuum leak with rough idle and stalling which was solved but replacing the fuel rail gasket and upper intake gasket, truck runs like new now, nice solid idle. BUT I am still getting the codes. I tried unplugging the IAC which someone suggested in this post, idle instantly drops to 500 and nearly dies which i assume this confirms that there is no vacuum leak. My STFT looks normal where as my LTFT stays at +25 and will only drop to 24.2 with rpms held at 2500. I do not have a fuel pressure tester as of right now so i can not test that at the moment. The OP of this threads symptoms seem very similar to mine. So now Im guessing it could be my FP or my FPR. The only difference in our two cases i can see is that the OP's STFT was High and my LTFT is high, could this still point towards a FP or FPR on it way out?

 

Hi Houston,
Good news...my truck passed smog today with flying colors. So yay, issue completely fixed for me.

Regarding your issue. If your STFT is normal now, you may just need to reset the PCM so that the LTFT will drop, otherwise it might take a while for the codes to reset because the LTFT will remain high. When I did my gasket replacements I disconnected the PCM for about 10 seconds and it reset the LTFT. So I would disconnect battery ground, disconnect the PCM (should be located in the engine bay on the firewall, mine is on the passenger side) and wait 10+ seconds. Then reinstall (be careful not to bend any pins) and see if the LTFT is back down to 0.

I am by no means an expert so this may not solve your problem, but worth a try. If your fuel pressure was low you would see the STFT would still be high, I believe.

 

STFT, short term fuel trim is instant, well within a few seconds of real time, STFT averages set LTFT over a few weeks/months

LTFT, long term fuel trim is like it says long term, if it was at +25 because of vacuum leaks then it will stay at +25 until STFT averages slowly bring it down.
LTFT is really only used in Open Loop, before O2 sensors are available to use.
As an engine gets older fuel pressure goes down, injectors get dirty, spark gets weaker, vacuum leaks go up so the computer needs a way to set fuel levels to use on this specific engine when it doesn't have feed back from O2 sensors, thats what LTFT is for.
It should slowly come down with each drive cycle.
1 drive cycle = engine warms up to operating temp, then cools down to outside temp.
Could take 20 drive cycles to get LTFT down
Wouldn't be of any valve if it changed faster

Yes while the LTFT is high you will get the same codes, but they will go away as LTFT returns to the +5 range.

The Rear O2 sensor, O2 bank1 sensor 2 also adjusts LTFT but only slightly

A Ford dealer scan tool can reset LTFT if you are concerned.
Regular OBD2 scanner can't clear it, for good reason.

 

Thanks for the suggestion glreynol, I will go try this and see if it resets itself.


Thanks for the suggestion also RonD, I will try and see if what glreynol and suggested works and if not i will wait a month or so and see if anything changes.

On a side note, I just realized that I did not include one other symptom/other problem i have been having is cold starts, it takes about 5 seconds of cranking for the engine to fire. Maybe cause it has been colder recently, not sure.

 

I can't seem to get the PCM unplugged, is there a special way to do it?

EDIT: I figured it out, i was confused on whether or not i needed to unscrew the 10mm bolt.

 

Yep, you must remove the bolt...be careful not to overtighten it when you reinstall.

 

Well no luck, I disconnected the negative battery cable then disconnected the PCM for 10 minutes and hooked everything back up, let it idle for 15 mins then took it for a short easy drive then pulled the fuel trims, LTFT still idles at +25 but now drops +15 when I hold rpms at 2500. Guess I'll be waiting for that check engine light to come back on now.

So Im not sure if this actually reset the PCM or not. Maybe I will just have to wait and watch and see if it slowly corrects itself.

 

One more thing I've read that you can try to reset the PCM is to disconnect the battery ground, then connect a jumper wire from the positive battery terminal (still connected to battery) to the negative wire (disconnected from the negative battery terminal). Make sure you put something over the negative battery terminal to keep the ground wire from contacting it. Leave the jumper wire connected for 5-10 mins. Should drain the PCM capacitors which will reset the trims.

 

Thanks for the suggestion glreynol.

I will give this a try tomorrow with some extra speaker wire I have laying around and see if that does the trick.

I will post back with results.

 

The 1994 and older PCMs(EEC-IV) could sometimes have the LTFT cleared if battery power was removed.
1995 and newer, EEC-V, can not be cleared that way, Ford diagnostic tool can do it, but a very expensive tool.

Cold start, turn key on count to 3 then turn key off
Repeat 3 times then try to start.

The key on and off gets fuel pump to run 6 seconds(2 seconds each time key is turn from off to on), this would build up any lost pressure in the system.
If engine seems to start better then you are loosing pressure when pump is off, if MPG is still good then most likely issue is check valve inside fuel pump, not a big deal but.............

 

Thought I'd piggy back here since there seems to be some similar issues..

I've been getting P0171 and P0174 on and off for quite sometime, they come and go.

Problem now is that it won't high idle on start, especially not on a cold start.
I did the unplug the IAC when warm test and nothing changes...
I've also cleaned the MAF a few times, once fairly recently. The IAC and TPS aren't that old. I've replaced them.

It also seems to be stumbling a bit, especially at lower RPMs/Speed but smooths out at speed and higher RPM.

Fuel pressure seems to be something I should check but maybe there's something else I should check first

 

Welcome to the forum

If you unplugged IAC Valve's power then it should close and RPMs should drop, so either IAC Valve is not working or you have a vacuum leak.

Cold engine, remove IAC Valve
Plug it in
Turn on the key
IAC Valve should open all the way, it gets full 12v from computer for Startup with key on
Unplug it's wires, it should Close all the way, no power
Plug wires back in and it will open all the way again
Repeat as you see fit to make sure it is working

"New" just means "Never Tested", it no longer means "it works"
So could be new IAC is bad


ECT(engine coolant temp) sensor tells the computer when engine is cold so it can set higher idle and richer fuel mix(choke)
Yours may be telling computer engine is always warmed up, so engine idles low and stumbles until it does warm up, it would also set Lean codes, because cold engine needs more fuel

There are TWO temp units on fuel injected engines
ETC sensor, used only by computer
ECT SENDER, used only by dash board temp gauge

They look similar, sensors always have at least 2 wires
Senders usually 1 wire, but later models had 2 wires on each

Without knowing what year and what engine that the best I can do

 

Going to check the IAC now.

4.0L mazda b4000... 1999

Rich

 

IAC doesn't move... so this would be a good place to start.

Going to get one today

 

Yes, and if you have a Volt meter check IAC's 2 wires, one should be 12v and the other a Ground with key on

 

Yah I didn't do that and I have a volt meter, kicking myself... didn't end up buying a new IAC because I didn't check... but I drove to get one, not smart.

 

I have this 1998 Ranger 4.0 manual XLT step side 105 original miles everything on this truck is original including the paint.

I have been trying to locate cause of high fuel trims in the Plus 15 to 25 range. That will pop off p0171 p0174 codes. I just finished replacing all the intake gaskets fuel rail gasket, cleaned all injectors and ring seals, upper intake gaskets.Replaced all external rubber vacuum hoses, did smoke test and sprayed intake cleaner none caused a rev in engine but I went for whole intake change because its 24 years old and figured why not. Before that I replaced mass flow and all o2 sensors for the same reason might as well they are old but did not correct issue . I changed fuel line filter and got constant 65 psi fuel pressure even at 2500 rpm.

whats weird is that my long term fuel levels on scanner while I drive are lower then my short term at revs and they are both moving up and down while reving which makes no sense to me either.

It runs ok once it warms up. After doing intake I did discover one fuel injector wire the insulation was all deteriorated off. wire was intact but it was laying on surface of metal intake. I retaped that back up snug and 1 port on coil was blue green and the wire connector I clean all that out and the truck for sure runs better before doing all the intake work., I even spayed carb cleaner through all injectors adding 12 volts to injector to pull fluid through.
I have no leaks in manifolds or any alone the whole exhaust . There is a new stainless exhaust from cat back..

Anything I miss? I love this truck we tow it behind our motorhome when we are home its stays in garage.. Can it be the computer only thing I think I left from doing.