Fuel PSI drop after 5500 RPM's

[UpAllNight]

Its a 96 GT with a PI head/cam/intake, swap all of the bolt on BS and crap as well as a dyno tune.

I started to notice just lately that at full throttle it can start to ping just a little past 5000 RPM's and mainly only in higher gears like 3rd or 4th.

I checked and replaced the fuel pressure regulator on the fuel rail and that helped slightly. I hooked a pressure gauge up and took the car for a ride. At idle its at 29 which is good. Full throttle it goes to 39 which is good but I noticed the PSI can start to drop some after like 5300 RPM's and by 6000 it goes to down around 35 PSI.

Is this normal? do I need a new or better fuel pump? Would a aftermarket regulator help?

 

[wmburns]

What size (flow) fuel injectors are being used?

35 PSI under load does seem low.

IMO, a better regulator will not help if the fuel pump can't keep up.

However, I think it is important to understand how much power you are making and what your plans for the future are.

Just to make sure not missing something easy, is the battery, battery terminals, and alternator in tip/top shape?

Consider having the alternator tested for excessive AC ripple.

Consider monitoring battery voltage to see if dipping along with fuel pressure.

Also note, the if spun too fast, alternator "brush bounce" can cause the voltage to drop. The voltage drop can cause a whole range of other issues.

 

[trinity_gt]

When was the fuel filter last changed?

 

[UpAllNight]

Thanks for both replys.

The build is as far as Im gonna go as it is used as strickly a nice daily driver. The injectors are stock 19 pounds. I will check to make sure the voltage is staying up the par during higher RPM. Thats a good catch and tip. It does have under drive pulleys but I dont think that would matter.

As for the filter. I took a quick look and it looks pretty much old and stock. So it will get changed out in the morning. If the voltage is good and the filter makes no differance than I'm thinking maybe the pump? If so would a larger LPH pump effect my tune or would everything be alright?

 

[bhuff30]

Replacing the fuel filter would be the next easy step. Especially if it is factory! I think the fuel pump should be capable of handling your fuel requirements if it is working correctly, but installing a bigger fuel pump wouldn't change your tune.

Thanks for both replys.

The build is as far as Im gonna go as it is used as strickly a nice daily driver. The injectors are stock 19 pounds. I will check to make sure the voltage is staying up the par during higher RPM. Thats a good catch and tip. It does have under drive pulleys but I dont think that would matter.

As for the filter. I took a quick look and it looks pretty much old and stock. So it will get changed out in the morning. If the voltage is good and the filter makes no differance than I'm thinking maybe the pump? If so would a larger LPH pump effect my tune or would everything be alright?

 

[2002BLGT]

put a new 255lph pump in and call it a day , and new fuel filter when you do the pump

 

[trinity_gt]

put a new 255lph pump in and call it a day , and new fuel filter when you do the pump

That'd be a waste of time and money if the current pump is okay and the problem lies elsewhere.

A new filter is no-brainer no matter what, especially if the one on there now is so old as be possibly OE. If the problem goes away, there's no need for the larger pump. Only if the problem remained and all other avenues of diagnoses were exhausted would I recommend dropping the tank and installing a 255lph unit.

 

[sneaky98gt]

I have this problem, too, to a more minor extent. With the vacuum line off the FPR, it reads 39-40 psi at idle, and reads 30-31 with the vacuum line on. All through first gear, the fuel pressure is right at about 39-40; it stays like that through 2nd gear, too, until about 5500, when it sometimes will drop back to like 37ish. It only does this on long, full runs. Hitting it from 60 and running to the top of second gear doesn't do it. Only on a full 1-2 gear pull will it do it. And even then, unless I've run the crap out of several times back to back to back (i.e., got the blower heat soaked), it doesn't ping any. So maybe it's nothing to be concerned about.

It started doing this after my fuel tank fiasco a few months ago. Before, I had a SVT Focus pump and with the Fastlane tune it was perfectly fine; I put a new tank and a new SVT Focus pump in after a rock in the road ruined the first one, and have had this 'problem' since then. Fuel filter makes sense, as it is possible some debris from the pump/pump assembly got into the lines (they were completely shattered). I'll try and think to get a new one sometime and get it installed and see if it solves the problem.

 

[UpAllNight]

Update-
The filter replacement actually did the trick. My pressure now stays at 39 all the way up and in all the gears (well 1st, 2nd, and, 3rd is all I actually tried up to 6000 RPM's) So thats good.

However, I'm still getting the pinging. I dont get it. Nothing has really changed on that car in like 6 months. So today I tried sea foam, put in a different coolent senser and air charge senser, and cleaned the mas... No help. I can pull the octane pin and the pinging is cured but I dont want to go that route.

 

[trinity_gt]

Update-
The filter replacement actually did the trick. My pressure now stays at 39 all the way up and in all the gears (well 1st, 2nd, and, 3rd is all I actually tried up to 6000 RPM's) So thats good.

However, I'm still getting the pinging. I dont get it. Nothing has really changed on that car in like 6 months.

Glad the filter helped. Re: Detonation: Is it possible you've got a tank of gas that doesn't have the octane you think it did? Scares me to think of this happening (coz I've got a 9# KB on mine and run Sunoco 94 relgiously) but given how tanks in the ground are filled it's not entirely impossible that midgrade or regular could end up in the premium tank...

I'd run this tank near-empty and re-fill it with premium before digging too deeply. I'd also try to avoid that detonation if you can since you can hurt brittle pistons with what amount to hammer blows...

I can pull the octane pin and the pinging is cured but I dont want to go that route.

"Octane pin"?

 

[UpAllNight]

Octane pin... The 96 and some other years have a octane pin. It looks like a spout pin but on these it just pulls 3 degress of timing across the whole curve.

 

[UpAllNight]

For search reasons, as I personally hate searching threads to find a bunch of loose ends....


I took the car to a friends house tonight who works at a shop and does diagnostics. He has all the stuff to hook up to my PCM and watch whats going on at the very moment (live date). We checked everything out sitting still then we went driving. We drove around for about 30 minutes while he was watching everything on his data logger or whatever its called.

He couldnt find anything wrong at all. Everything was good from the mas, to the fuel trims, to the o2's, at idle, part throttle, and full throttle.

He then explained that there are two fuel blends sold in the US, one is the summer blend and the other is the winter blend. He told me the winter blend is more volatile to help cold starts in the morning in very cold conditions. Well I live in florida. Right now it still feels very warm, but we have currently switched over to the winter blend as the rest of the US has. He claims this could make my car ping, at least for now untill we do finally get colder weather and then it should be fine. On a stock car anyone would be fine, but my car has high compression and a dyno tune at 93 octane so its basicly already at its fine line in perfect conditions.

I then pulled the octane pin which is claimed to pull 3 degress timing across the board. I wanted to verify this with his date logger. At idle it pulled 3 degress out like they say. We also check full throttle and it pulled 3 degress exactly as well. We then compaired the fuel trims, they were unchanged from octane pin in to octane pin out. So basicly the octane pin does only pill 3 degress timing and it does it from idle to full throttle. Thats all it does, nothing else.

He suggested I use my car with the octane pin pulled (retarding it 3 degress) for the time being, and as the weather get colder I should be able to put the pin back in with everything acting normal again.

 

[2002BLGT]

Octane pin... The 96 and some other years have a octane pin. It looks like a spout pin but on these it just pulls 3 degress of timing across the whole curve.

never heard of this , can you post a picture up , fuel pressure is not equal to a measure of volume , if the car still has the stock fuel pump then I would upgrade the pump IMO thats probably why its pinging , that or the dyno tune has too much timing added

 

[trinity_gt]

never heard of this , can you post a picture up

I'd never heard of it either. Did a google and saw:

Octain Pin question - Crownvic.net

fuel pressure is not equal to a measure of volume , if the car still has the stock fuel pump then I would upgrade the pump IMO thats probably why its pinging , that or the dyno tune has too much timing added

If fuel pressure at the rails is not dropping the pump's capacity is not the problem. I agree: He's got a dyno tune with too much timing. Pulling the "octane pin" -- reducing the timing a few degrees -- got rid of the ping:

... I can pull the octane pin and the pinging is cured but I dont want to go that route.

It sounds like the correct route to go is to have the timing addressed in the tune.

 

[UpAllNight]

Yes the octane pin is just how the link Trinty GT just proved, however on a mustang its on the passenger side near the MASS.


I did this dyno tune in the spring and it didnt ping thought the spring or summer, but now that we are starting to enter winter its started. I just cant find anything that would cause it except maybe the seasonal fuel change that was suggested to me.

It is pretty cool having a octane pin when you can use one though. Before I spend more money on aother tune I will see how things play out when winter fully sets in. For now the octane pin is pulled and she's running great.