Fuel Pressure Drop

IHPowerstroke7.3 · Sep 30, 2015

Not too familiar with aftermarket setups and could use some help! Work at a Ford Dealer, customer brought in a 7.3 with concern of fuel pressure dropping under load. Not sure of exact details, has stage 1 injectors (no idea on size), driven diesel regulated return that retains stock filter housing,t-500 hpop, hydra with Bills tunes i believe, stock fuel pump. Had 5/8 pickup and line to stock pump. Removed that and went back to stock pickup lines and installed a new bosch pump. Swapped in a known good regulator, tried known good sending unit for his gauge. Fuel pressure drops from 60psi down to 35-40 psi at wot after a couple seconds, comes back up as it shifts into 3rd then when it hits O/D it will jump back to 60 psi. Only does this at wot, any other time it holds 60 psi rock solid.

Customer requested to go back to stock lines, thinking 5/8 line was causing issues with stock pump. 5/8 line was siphoning out when i removed it from the pump, so I can't see that being the issue. From what I have read, seems a stock (and new at that) pump should hold stage 1 injectors.

Anywhere else I should be looking?

fordman67 · Oct 1, 2015

I have seen several people have issue with the stock pumps not big enough.
It needs a gsl392. The lines were not the problem.

IHPowerstroke7.3 · Oct 1, 2015

To me it seems the pump can't keep up, but customer says he has been running this same setup for a few years.

Dmstrucks02 · Oct 1, 2015

I would bet it's plumbing size after the pump and the tiny lines going to the heads. And the fittings at the heads with the screens in them. Another member local to me has a full strictly fuel system with the stock pump still that holds pressure fine

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Powerstroke Man 6.4 · Oct 1, 2015

I had 160/100's, stock pump couldn't keep up it was dropping pretty bad. (Original 305,000 mile pump)

Changed to a walbro and held 60 psi all the time. Stock fuel bowl, lines.

I also had a sump and AD-100 (for filtering purposes).

97f350stroker · Oct 1, 2015

Dmstrucks02 said:
I would bet it's plumbing size after the pump and the tiny lines going to the heads. And the fittings at the heads with the screens in them. Another member local to me has a full strictly fuel system with the stock pump still that holds pressure fine

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^agreed. I'm running a single sd pump with 230/200's and it never drops below 60psi. Running 1/2" to a y block in the valley then 3/8" to the back of the heads though.

IHPowerstroke7.3 · Oct 1, 2015

I agree with what's being said, the factory 3/8 lines going to the heads would be a huge restriction in volume. Just doesn't make sense why the truck suddenly started dropping fuel pressure now when this same setup had zero issue before (according to the owner).

Owner talked to someone at strictly, believes hpop may be getting weak, not holding pressure so it is increasing pulse width to make up for it. I can't see it, but someone else mentioned this happens. Need time to test it further.

Magnum PD · Oct 1, 2015

Pulse width and fuel pressure are two different things. They don't interact with each other.

IHPowerstroke7.3 · Oct 1, 2015

I understand that, pulse width is duration of injection. Fuel pressure is just supply pressure to the fuel side of the injector. It was explained that PCM is increasing pulse width in order to meet mass fuel desired because Icp is low. I call bs on that because I'm certain the PCM isn't smart enough to do that and also it's not setting a code for low Icp like it should (if it's really that low). I can hold my foot to the floor and when it shifts into overdrive at wot, fuel pressure returns to 60 psi.

Just trying to wrap my head around what could have changed to start dropping fuel pressure now if the setup hasn't changed. Not being my truck, I can only go by customers word.

Dmstrucks02 · Oct 1, 2015

How old is the filter

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psduser1 · Oct 1, 2015

IHPowerstroke7.3 said:
I agree with what's being said, the factory 3/8 lines going to the heads would be a huge restriction in volume. Just doesn't make sense why the truck suddenly started dropping fuel pressure now when this same setup had zero issue before (according to the owner).

Owner talked to someone at strictly, believes hpop may be getting weak, not holding pressure so it is increasing pulse width to make up for it. I can't see it, but someone else mentioned this happens. Need time to test it further.

Factory 1/4" lines, pretty sure what you meant.
With a screen in the fitting at the rail connection, I assume?

Or an old filter.

bigfords · Oct 4, 2015

IHPowerstroke7.3 said:
I understand that, pulse width is duration of injection. Fuel pressure is just supply pressure to the fuel side of the injector. It was explained that PCM is increasing pulse width in order to meet mass fuel desired because Icp is low. I call bs on that because I'm certain the PCM isn't smart enough to do that and also it's not setting a code for low Icp like it should (if it's really that low). I can hold my foot to the floor and when it shifts into overdrive at wot, fuel pressure returns to 60 psi.

Just trying to wrap my head around what could have changed to start dropping fuel pressure now if the setup hasn't changed. Not being my truck, I can only go by customers word.

I believe the pressure comes back up in OD due to the drop in RPM. Mine does the same thing. It must be more difficult to keep psi at higher RPM, even though you are keeping it WOT in both gears.

bruce · Oct 4, 2015

Has it had the in-tank mods done? Also, low fuel pressure can cause low icp (pretty much what you described, longer pw to make up for low FP) but i've never heard of low icp causing a few psi issue. If so we prolly all would have had a fuel psi issue at one point.

IHPowerstroke7.3 · Oct 4, 2015

The change in pulse width from 3rd to 4th yes, but its not like the pump is driven by the engine, so RPM has no direct effect on fuel pressure. On a obs using the in valley pump yes that would affect it.

Only in tank mod it had done was a 5/8" pick up, with a 5/8" line that was reduced to 3/8" (or whatever stock is) to factory pump. He was told to put it back to stock and replace the stock pump with another. So that is what I did, blew out the factory line and re connected the line to the pump (it was capped off) Problem was there with the 5/8" pickup and when the stock line hooked up its the same.

As far as the low ICP causing low fuel pressure, I have a basic understanding of everything that happens inside a heui injector. Way I understand it is like this. Lets say ICP is supposed to be 3000 psi, fuel pressure is 60 psi and pulse width is 3.0 ms (using random numbers so don't read into it.) If the hpop drops pressure, lets say to 2500 psi, the injector is not going to inject the same amount of fuel into the cylinder as it would if icp was 3000 psi at 3.0ms. If the IPR is already maxed out, only other thing the PCM could do is increase pulse width to say 4.0 ms, holding it open longer to inject the desired amount of fuel into the cylinder to meet the mass fuel desired. Obviously the fuel pressure is going to drop a lot more than normal if all 8 injectors are draining more fuel than they normally would (if that makes sense) I'm using random numbers, tomorrow I plan to go out and log some data and see what is actually happening. Without knowing the tune, I can't say what pulse width the PCM is calling for under normal circumstances. But if I see ICP dropping off and I see a stupid number for Pulse width after the drop in ICP, then it may be the issue.

Sorry for my stubbornness when it comes to the line size, restrictions on all of that. But if there wasn't a problem with the truck before, I can't see something that can't physically change (line size) being an issue now. Restrictions such as a fuel pickup, yes, but changing the pickup did not have any effect on it. Changed the regulator, says the filter is new, i'll confirm that first thing tomorrow.

Justin@DP-Tuner · Oct 5, 2015

I would first confirm the size of injector. If it truly is a stage 1 (160/stock), then a stock SD fuel pump should have no problem keeping up.

Where are you pulling pressure from?

IHPowerstroke7.3 · Oct 5, 2015

Spent some time with the truck today, talked with Dennis from Strictly and discussed my findings with him. Think we figured it out.

Injectors are a Full Force stage 1, 160cc stock nozzle as per customer. Stock pump should not have a problem supplying them. Dennis had me verify some things. Checked power to the pump, its within 300 millivolts of battery voltage. Also asked me to check how much fuel the pump would push out the fuel drain on a couple key cycles. Compared to my stock 7.3, customers truck would flow almost a whole quart more than my truck would (my original 120k mile pump) on 2 key cycles.

Logged data, In the tune he brought it to me with (say its a hot tune) WOT in 3rd, 3200 rpm, mass fuel desired was 100mg, pulse width was 4.6-4.8 ms, ICP would hold between 2800-3300, IPR duty cycle was 44% from what I remember. Fuel pressure would drop from 60 to 40 psi.

Went to his tow tune, WOT in 3rd 3200 rpm, mass fuel desired was only 76-80mg, pulse width max I saw was 3.6 ms, ICP and IPR was the same. Fuel Pressure drop was 5-6 Psi max.

So just as I figured from the get go, too much fuel in that tune for the pump to handle. Asked the customer if he switched tunes, says it was the same in all tunes. Should have checked for myself, but didn't have time to go play around last week.

Going to go to dual sd pumps and re evaluate.

Thank you to everyone for the help.

Forgot to add, I'm assuming you mean where am I measuring fuel pressure? At the fuelab regulator port. Customer had auto meter gauge. I hooked up a mechanical gauge today to confirm the readings were accurate. Both gauges read within 1 psi.

Justin@DP-Tuner · Oct 5, 2015

IHPowerstroke7.3 said:
Spent some time with the truck today, talked with Dennis from Strictly and discussed my findings with him. Think we figured it out.

Injectors are a Full Force stage 1, 160cc stock nozzle as per customer. Stock pump should not have a problem supplying them. Dennis had me verify some things. Checked power to the pump, its within 300 millivolts of battery voltage. Also asked me to check how much fuel the pump would push out the fuel drain on a couple key cycles. Compared to my stock 7.3, customers truck would flow almost a whole quart more than my truck would (my original 120k mile pump) on 2 key cycles.

Logged data, In the tune he brought it to me with (say its a hot tune) WOT in 3rd, 3200 rpm, mass fuel desired was 100mg, pulse width was 4.6-4.8 ms, ICP would hold between 2800-3300, IPR duty cycle was 44% from what I remember. Fuel pressure would drop from 60 to 40 psi.

Went to his tow tune, WOT in 3rd 3200 rpm, mass fuel desired was only 76-80mg, pulse width max I saw was 3.6 ms, ICP and IPR was the same. Fuel Pressure drop was 5-6 Psi max.

So just as I figured from the get go, too much fuel in that tune for the pump to handle. Asked the customer if he switched tunes, says it was the same in all tunes. Should have checked for myself, but didn't have time to go play around last week.

Going to go to dual sd pumps and re evaluate.

Thank you to everyone for the help.

Forgot to add, I'm assuming you mean where am I measuring fuel pressure? At the fuelab regulator port. Customer had auto meter gauge. I hooked up a mechanical gauge today to confirm the readings were accurate. Both gauges read within 1 psi.

Sounds like you figured it out then. Dennis is a very knowledgeable resource, especially with fuel systems.

You may want to contact the tuner to see if they can make adjustments before going to dual stock pumps. A healthy stock pump should have no problems holding pressure on 160/stock injectors.

IHPowerstroke7.3 · Oct 5, 2015

Was a pleasure to talk to for sure, sounds very knowledgeable indeed. His help was very much appreciated from a Canuck in a different time zone! If i ever decide to upgrade my stock dinosaur, would definitely give him a call.

I mentioned a tuning adjustment to the customer as well. I believe he is using PHP tunes, not sure if it is live tuned or not. Customer was the one who directed me to Dennis, I asked him to call Dennis to discuss options, got the call back he had ordered the kit already. Just as with every customers truck, give them the options and let them decide what the wish to do.