Strange Spikes in Fuel Rail Pressure

[3VVV]

Hi everyone!

My log shows strange spikes in the fuel rail pressure and I can't make sense of it - I hope you guys can?

My 2008 GT Automatic had issues with loud pinging/detonation on hot days.
I flashed the stock software back on and ran some logs to see what was going on (engine is otherwise stock).
LTFTs were high under load and fuel pump duty cycle went up to .48 (96%), so obviously a fuel delivery problem.
I changed the fuel filter, cleaned the MAF and reset the KAM. Now LTFTs look fine (0-3%) but fuel pump booty cycle is still high at .44 (88%) WOT and .26 (52%) idle.
I ordered a new sock filter/restrainer for the pump - but there is something else going on that I don't understand:

My fuel rail pressure increases dramatically for up to 10 seconds whenever I let off from high RPM (marked red). WHY?

Another issue is that whenever load is applied, the fuel rail pressure drops for a second (marked yellow). I guess that could be fuel restriction and will hopefully disappear once the sock filter is changed?

Anyone having an explanation for the spikes?
Thanks and best regards!

 

[86GT351]

Stock fuel system?
Check the Fuel Pump Driver Module

 

[3VVV]

Yes, the fuel system is stock. What makes you think it's the Driver Module and are there ways to check it (apart from replacing it)?
Thank you!

 

[86GT351]

Yes, the fuel system is stock. What makes you think it's the Driver Module and are there ways to check it (apart from replacing it)?
Thank you!

Check voltage on the wire coming off the module to the pump itself as shown on schematic. I don't know if that is the specific concern. I know that it is in line to the pump.

 

[TripleFJeff]

None of that is unusual. The long spikes up happen when you go into decel fuel shutoff. The engine stops using fuel, but it's returnless so the pressure builds up since the pump is still going. Note how the FPDC keeps going down to try and compensate. The downward spikes are also normal. Remember that this is differential pressure of the fuel rail vs the manifold pressure. So off throttle you might be at 35psi fuel and -5psi manifold (made up numbers as an example) to be at a target 40psi. When you hit the throttle manifold vac drops, so you might instantly go to 35psi fuel and 0psi manifold which would show as a drop in differential fuel pressure. Also, the sudden change in fuel demand means the pump has to ramp up (notice the pump DC going up) and it takes some time to catch up. The opposite is the reason for the short spikes up when you get off throttle. Nothing to worry about.

 

[Pentalab]

On my 2010 GT automatic, with the small Roush M90 blower, one summer day it would gag, but only when trying to get into boost. The FPDC was pegged, and the fuel rail pressure dropped from it's normal 40 psi...down to 5 psi.

Turns out the problem was that sock filter, that goes over the pump. It was totally clogged. Once we cleaned that up, problems vanished.

Fuel rail pressure stayed steady at 40 psi at all times, even idle or wot. However, with wot, the FPDC was > 90%. My fix for that was the 10 ga wire upgrade kit.... which is a single 10 ga wire, directly from the battery terminal to the trunk. Ok, now the FPDC is 80% with blower on (5 psi boost), high rpm at wot...... and just 40% at idle.

I had previously changed the fuel filter to no avail.

 

[3VVV]

@TripleFJeff
"None of that is unusual." - Wow, that would be great! I hope at least one other person can confirm that.
What I still find puzzling, maybe you could elaborate: If the fuel pressure after let off has nowhere to go (only through the injectors which are closed) - how can the short spike in pressure be instantly released, but not the long one? How can the long pressure spike suddenly disappear while load remains zero with injectors still closed?

@Pentalab
Ha! I think I read your thread while googling, that's why I ordered the sock filter (still on the way)!
"Fuel rail pressure stayed steady at 40 psi at all times, even idle or wot" - And did it look like mine? Did you have similar spikes (up and down)?

Thanks!

 

[Pentalab]

@TripleFJeff
"None of that is unusual." - Wow, that would be great! I hope at least one other person can confirm that.
What I still find puzzling, maybe you could elaborate: If the fuel pressure after let off has nowhere to go (only through the injectors which are closed) - how can the short spike in pressure be instantly released, but not the long one? How can the long pressure spike suddenly disappear while load remains zero with injectors still closed?

@Pentalab
Ha! I think I read your thread while googling, that's why I ordered the sock filter (still on the way)!
"Fuel rail pressure stayed steady at 40 psi at all times, even idle or wot" - And did it look like mine? Did you have similar spikes (up and down)?

Thanks!

With the dirty sock filter, and foot into it, rail pressure dropped like a rock. Back off the gas, and rail pressure goes back up.

I removed my sock filter, cleaned it out, and re-installed it. After that, pressure did not budge.

 

[GlassTop09]

@TripleFJeff
"None of that is unusual." - Wow, that would be great! I hope at least one other person can confirm that.
What I still find puzzling, maybe you could elaborate: If the fuel pressure after let off has nowhere to go (only through the injectors which are closed) - how can the short spike in pressure be instantly released, but not the long one? How can the long pressure spike suddenly disappear while load remains zero with injectors still closed?

Hi 3VVV,

I can confirm that this is more or less normal w\ this Ford S197 OEM returnless FPDM controlled fuel delivery system as TripleFJeff has described. Usually just after you've fully let off the APP which will tell the ECU to go into full DFSO (the 1 item you don't have selected in your graphs is the APP which is the initiator of all the other graphed items......the item that you--the operator of the engine--is telling the ECU what your wants are). You're dropping from a high load to a no load situation in a very short period of time.......like during a gear shift, the ECU command to the FPDM to shut the FP down then the FPDM's reaction time to carry out this command just isn't fast enough to prevent this from happening a majority of the time.

As for the long spiked fuel pressure off DFSO dropping off while still in DFSO, how many folks still have the pprv (positive press relief valve) installed in the fuel pump hi press side-to fuel pump low press side & if they do (std in OEM Ford fuel returnless type systems), do they still not leak by (I know mine does--tested & verified leaking.......the FP assembly is 15 yrs old--the original FP assembly the car had when it rolled off the assembly line--doesn't hurt anything by it leaking by as this part's sole job is to prevent excessive high line press deadhead between the FP & the fuel rails.....like say 100 psi depending on the press the pprv was set to relieve).

If the fuel pressure after let off has nowhere to go (only through the injectors which are closed) - how can the short spike in pressure be instantly released, but not the long one?

When the APP (accelerator pedal position) is still at .000 A\D counts the fuel system will stay in DFSO during decel until the engine RPM's drop below 1,000 RPMs.......or until you press on the APP & it starts to increase A\D counts......this tells the ECU that you--the operator--wants to start applying TQ again thus the ECU will suspend DFSO & reengage the fuel injectors thus momentarily dropping the system fuel press until the FPDM can react to ECU command & spin up the FP again.

The key is to monitor the APP A\D counts output in relation to all the rest of the outputs......then it will all come together.

Hope this helps.

 

[GlassTop09]

I changed the fuel filter, cleaned the MAF and reset the KAM. Now LTFTs look fine (0-3%) but fuel pump booty cycle is still high at .44 (88%) WOT and .26 (52%) idle.
I ordered a new sock filter/restrainer for the pump - but there is something else going on that I don't understand:

This is 1 of the issues that can occur w\ the stock fuel delivery system simply due to not having enough sufficient amperage (note I didn't say voltage......voltage is electrical press needed to physically move electrical amperage down a wire--actual electrical electrons the FP motor needs to actually spin)--available at the FPDM due to the approx 24' of 16 awg wire causing some voltage droop (resistance) across it's length.......causing the FPDM to call for more amperage thus will raise the FP duty cycle higher (also causing the FP to run hotter) to try to meet the ECU command for more fuel delivery.

This is where the FPDM wire mod will help to alleviate this issue (running a larger gauge fused wire from batt 12v+ to a 40A relay using the existing fuel pump power 12v+ 15A 16 awg wire to trigger the relay to send the 30A fused 12v+ power from the larger gauge wire to the FPDM that has far more amperage available at the FPDM under the same 12v+ voltage due to essentially no electrical resistance for the amperage to freely flow down this larger wire (min of 10 awg but I'd recommend to run 8 awg for any future growth plans so this is done 1 time) as opposed to the OEM 24' of 16 awg small stranded wire.

If you need to get more info, please watch JeremyH's YouTube videos (JeremyH owns\runs S & H Performance where he specializes in fuel delivery systems for hi powered\racing Stangs & is a long tenured forum member.......also is an electrical engineer by trade) & you'll see\learn everything you'll ever want to know about fuel delivery........I bought my 8 awg FP wire mod kit from JeremyH after seeing his video demoing just what I've typed earlier using a Walbro 400 series FP output simulating the voltage\amperage & heat resistance buildup flowing thru 24' of 16 awg stranded automotive grade wire vs flowing thru 15' of 8 awg stranded automotive grade wire w\ FP held at the std 40 psi 13.5v+ power mimicking the FPDM control ......the results weren't even close & he didn't have to say anything else to me as he states that even stock OEM S197 returnless fuel systems can benefit from a FP wire mod......lowers the operational FP duty cycle load\operating heat load on FP motor while maintaining the needed FP fuel delivery output.

Once installed & I dropped in a 30A slo-burn mega fuse for safety purposes, my FP operational DC went from 80% down to 67% at full on WOT OL PE loads handling E10 91 oct fuel (due to the 10% of ethanol this increased the total fuel delivery by roughly 4% over the same non-ethanol based 91 oct fuel to make the same HP\TQ...........)

None of us are leaving these engines\tune calibrations fully stock thus IMHO this should also be considered.

Hope this helps as well.

 

[3VVV]

Thanks a lot! I'm relieved that these spikes are normal. Regarding the high FPDC: I will change the sock filter/strainer on the pump and see if that has a positive effect. If FPDC remains high after that, I'll consider the wire mod, thanks for sharing!

One more question while we're at it: There is "Fuel Rail Pressure" and there is "Pressure Drop Across Injectors". The latter is a delta pressure taking the varying manifold vacuum into account with the system always trying to maintain 39.15 PSI at the injectors, right? So, is the assumption correct that there is enough fuel as long as "Pressure Drop Across Injectors" stays at 39.15 at all time?

Thanks and best regards!

 

[GlassTop09]

One more question while we're at it: There is "Fuel Rail Pressure" and there is "Pressure Drop Across Injectors". The latter is a delta pressure taking the varying manifold vacuum into account with the system always trying to maintain 39.15 PSI at the injectors, right? So, is the assumption correct that there is enough fuel as long as "Pressure Drop Across Injectors" stays at 39.15 at all time?

In general terms, yes this is how a FPDM controlled returnless fuel system is designed to operate.......but the assumption is not always correct. This is why a WB O2 sensor is generally used to ensure that the fuel delivery is actually there. But if you don't have 1, there is another way to get this info using your SCT LiveLink datalogging software.

I'm sure you already have the MAF airmass channel graphed, so now set up the FP flow rate channel in your graphs & graph this as well. Now if you can set up a math channel to do the math, set it up to divide the MAF airmass output by the FP flow rate (make sure that both outputs are set to use the same unit of measure...say lbs\min.......this makes the equation work properly) as the result will be the actual airmass to fuel ratio--or AFR--the engine is actually using (example: 21.67 lbs\min flowing airmass \ 1.89 lbs\min fuel flow rate = 11.47 AFR). Then compare this result to the ECU's in-tune EQ Ratio Lambda equation's fuel stoich AFR x commanded Lambda (example: 14.13 x .82 Lambda = 11.59 AFR).....now you will definately KNOW if the FP is actually properly delivering the necessary fuel delivery along w\ the proper FRP ref as in this example, the FP is actually delivering slightly MORE fuel than is actually commanded.....but the difference is so small between the 2 (.12 AFR difference) that it ain't worth messing with thus all is good.......you don't need a WB O2 sensor to find this out.

This is another method to ensure that you're actually getting delivered what the ECU is commanding...........

Thought I'd mention this to help you out.

Hope this helps.

 

[Pentalab]

Thanks a lot! I'm relieved that these spikes are normal. Regarding the high FPDC: I will change the sock filter/strainer on the pump and see if that has a positive effect. If FPDC remains high after that, I'll consider the wire mod, thanks for sharing!

One more question while we're at it: There is "Fuel Rail Pressure" and there is "Pressure Drop Across Injectors". The latter is a delta pressure taking the varying manifold vacuum into account with the system always trying to maintain 39.15 PSI at the injectors, right? So, is the assumption correct that there is enough fuel as long as "Pressure Drop Across Injectors" stays at 39.15 at all time?

Thanks and best regards!

The 10/8 ga wire mod is a relatively straightforward simple easy mod. It just starts at the battery terminal.... and runs through the firewall on pass side.... then neatly tucked under the edge of the carpet.... right to the trunk. But yes, put on a clean sock 1st.

 

[TripleFJeff]

What I still find puzzling, maybe you could elaborate: If the fuel pressure after let off has nowhere to go (only through the injectors which are closed) - how can the short spike in pressure be instantly released, but not the long one? How can the long pressure spike suddenly disappear while load remains zero with injectors still closed?

The decel fuel shutoff is on a timer. So the first, short spike is from the transition to off throttle but fuel is still on so it returns to normal quickly. The long spike is the decel fuel shutoff which happens after a certain time at closed throttle as long as the rest of the criteria are met. I don't know about automatics, but on a manual car of that era (RYA1) it's 4 seconds.