so my car is getting old, and ive been noticing that somthing has been off for a while with fuel or spark, but not enough to trace really. But i have most of this new stuff sitting aside for my supercharger so i was holding off because im pretty much replacing whole fuel and spark system anyway. But now its acting up so cant really wait much more. Heres issue.

Ive been losing power up top for a while. Today though, mid rpm range low gears wot it has a hesitation/miss(not always every time at the moment either). so i figure great spark issue. But it has 120k on it and factory fuel pump, and i have a gauge so id figure id start there. Warm at idle reads around 30psi, spikes 45 when i throttle it. But then i plug in my tuner and moniter my fuel rail pressure and its coming in at a steady 39-40 at idle with the sct tuner i can only assume that means psi. And then i took it out and romped on it and it peaked maybe 42 but 2nd gear wot higher up the rpm range drops to 36.

So i guess im just wondering is that the pump? a regulator issue? A sensor issue? just spark? it just seems a bit conflicting for information and now im confused and irritated. any help would be great

 

any ideas?

 

no one?

 

just throw the blower on and hope everything fixes itself haha, your putting new plugs in and fuel pump so there are two possibilities right there

 

The actual fuel pressure on the returnless systems will vary from 28 to 43 psi give or take a couple psi as the PCM and fuel pump control module vary the power to the fuel pump to maintain a 40 psi (39.15 psi in reality) pressure drop across the fuel injectors. This total pressure across the injectors is a combination of the rail pressure plus the manifold vacuum (which is a negative pressure).

At idle the manifold vacuum is 18-20 inHg, which is -9 to -10 psi, so the rail pressure will be (39.15 - 9 = 30.15 psi) give or take--also keep in mind this is not a precision system, not are mechanical or even electrical sensor fuel pressure gauges. An overall accuracy of +/- 2% would be greater that I would expect.\

Reading the PID with a scan tool will tell you the pressure drop across the injector (which is what the PCM needs to know), not the commanded rail pressure so it will always be near 40 psi (it can be higher if the fuel temperature is elevated, the PCM will command as much as 53 psi to prevent the fuel from vapourising in the lines).

That yours is dropping to 36 in 2nd at WOT may or may not be a concern--what is the AFR (wide band) when that happens? As the PCM knows what the pressure is, via the rail sensor, it can manipulate the injector pulse width to keep the mix as it should be.

I.e. the actual fuel rail pressure is far less important with the returnless system, as long as the pump can supply sufficient flow rate so that the PCM's calculated injector pulse can deliver the necessary amount of fuel.

 

and thats why i wanted your answer...that explains a lot about why the readings are different, although im not sure i understand why the rail pressure would dip to 36 at high rpm wot and not be a problem? because what im reading and understanding is that the pcm does what it can to maintain that 40 psi which it does for the most part until the higher rpm. Sadly my sct xcal 3 doesnt have a wideband. I think i can moniter the o2 voltage but im not sure what its suposed to look like i believe correctly working is somewhere between .4 and .6 on a heated o2, but its been a long time.

Also i thought what my sesor was reading was pressure drop on the injector? at least thats what it says, so am i getting injector reading or command rail? fuess im still a bit confused. Any tests that i can do id be happy to do and get back to you. Also the missing/ hesitation seems to be more part throttle now above 2500. leading me to lean more towards a few failing cops. thank you for the help.

 

I was under the impression from your earlier post where you said "i have a gauge" that you were reading the actual fuel pressure at the fuel rail, and also via the SCT's data logging capability, is that the case?

With regard to the narrowband O2 sensors, their output is nearly meaningless for tuning. They can only produce a usefully measurable output for AFRs between 14.5:1 and 14.9:1 (read more here) at very best (that's the "narrowband" part). That is why in closed-loop mode the PCM constantly varies the injector pulse e width to keep the O2 sensor's average voltage output around 0.45V where AFR = 14.68:1 = the stoichiometric¹ (ideal) mix for gasoline.

Is the missing/hesitation very slight, just enough to be annoying? If so I have seen this caused by two things; a missing MAF screen, or a sticking/stuck EGR valve. The missing screen being the more subtle of the two. When the EGR valve sticks the engine will almost buck at light loads and in the RPM range you mention.

There is no definitive test for the EGR valve, mine began acting up over a year ago now and I just disabled it in the tune--someday I'll get ambitious and do a full delete...

How does it pull at full throttle from say 3500 RPM upwards? If strongly with no misfire or hesitation there's nothing wrong with the COPs.

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¹ - The stoichiometric mixture of fuel and air, where complete combustion occurs, is also known as a lambda (λ) value of 1.0.

FWIW, a wide band O2 sensor is a narrow band sensor combined with a microprocessor controlled electro-chemical oxygen pump that can either add or remove O2 from the test chamber depending on the polarity and amount of control voltage/current applied.

The narrowband sensor first tests a sample of gas and tells the processor if the sample is rich or lean, the processor then commands the O2 pump to add or remove oxygen from the sample chamber until the NB sensor reports the mix is now at λ 1.0. Because the processor knows how much voltage/current at what polarity was needed to add/remove enough O2 to make the λ 1.0 it can calculate the λ (AFR) of the original exhaust gas sample.

 

yes that was the case, i may have just read your reply wrong. i have a gauge and was reading sct data. Now that i think about it it makes me want to say the miss may be 3k and higher at part throttle(part meaning half or more im sure.) Im pretty sure its not egr engine was apart not to long ago and i cleaned the whole thing it all seemed pretty kosher. Plus it would seem the miss would likly be up where egr wouldnt effect it. Also my maf never had a screen since ive owned it, always had an after market intake no screen that i saw. i should probebly test the cops, but im not sure at this small misfire id find one out of range even if it wasnt working? thank you cliff

 

So under what conditions IS it stumbling and hesitating--load, throttle position, hot/cold, what grade fuel are you running, do you know what the tune has done to change fueling and/or ignition timing? You have not mentioned any misfire codes, if the misfire monitor is not picking them up then they are very subtle

What condition are the COP boots in? I don't like to blame COPs without checking everything else as they are very robust units that generally fail only from abuse--such as being run for a long period with very bad plugs or boots that are "blowing through" and causing the COPs to dump its full charge and overheat. Ford issued a service advice some time back stating that overhalf of the COPs swapped out under warranty and sent back had nothing wrong with them.

If the boots are hardened, smell burned, or show signs of arcing then they can cause a subtle misfire (actually more of a "malfire") under heavier loads when the voltage required to ionise the plug gap is highest. The potential across the gap bleeds off through the bad boot and finally ionises the gap just as the COPs runs out of "steam"--of builds slowly resulting in a late and weak spark.

 

i guess ill do some more investigative reporting, and try to get you the load and throttle input information. I run super which here is 91 the tune is from sct and i have no idea what they changed im locked out of being able to advance my timing or lean out my fuel mixture, ive always thought it smelled a bit rich. gas is always from chevron or another big name station.

I havnt pulled them out but the cop boots are old, i could check to see if they hard and what not. The miss or whatever it may actually be i guess would be considered subtle because it wont set off a code. its not subtle to me because i know it shouldnt be there. i just figured with 120k hard miles and only one new cop that they were probebly having some sort of issue even if its just the boots?

Also i know you tried the weapon x stuff and eventually took them off because of the heat causing them to fail right? if thats the case how hot was that, because i was looking at doing these with my sc set up and around here 90s is the hottest it really gets in the summer very few days if any go above that.