30 Lb Injector Fuel Pressure Question

pearlnotchback

pearlnotchback

5 Year Member
Dec 18, 2011
319
44
59
Tennessee
#1
Im running a 306 with Edelbrock heads, B cam yeah i know, ported Cobra intake,255 in tank pump, 6 lbs boost and ive been running 24 lb injectors.A buddy of mine gave me a set of 30's and a calibrated mass air. I put it all together and wouldnt idle worth a crap. After hrs of resetting the idle and so on I started turning up the fuel pressure and to my surprise it started idling better. Now my pressure is set at 50psi and it idles fine and runs fine. I thought maybe my gauge was off or something so i checked it with a fuel pressure tester and nope its 50. Anyone have any thoughts on this. I could be wrong but it just seems a little high to me.
 
#7
Honestly I would like to see the car get tuned and ditch the FMU. I have no problems with the base fuel pressure being 50 so long as a/f ratios under boost are good. Where is the total timing under boost? Where is the fuel pressure at under boost? I've run base fuel pressures as high as 70 lbs trying to band aid too small of an injector.
 
#8
84Ttop said:
Honestly I would like to see the car get tuned and ditch the FMU. I have no problems with the base fuel pressure being 50 so long as a/f ratios under boost are good. Where is the total timing under boost? Where is the fuel pressure at under boost? I've run base fuel pressures as high as 70 lbs trying to band aid too small of an injector.
Click to expand...
I will have to drive it tonight after work to check the fp under boost. I just don't understand why I would have to have higher fuel pressure at idle with 30 lb injectors over the 24's. I thought it would be the opposite.
 
#9
It all depends on the calibration of the MAF, everyone has different tables and calibrations. The car will run differently from Maf to Maf while still being for the same 30lb injectors. All maf's are not created equal and that is why they come with tables that show where the air flow counts map out and at what voltages. This is valuable info for a tuner as they can look into the calibration that is setup in the maf before they start tuning.
 
#10
I would assume since the stock EEC is programmed for 19lbers and you went from 24 to 30's , the computer is still mapped for 19 and the signal and mapping is too much a difference- you got away with the 24's for whatever reaon probably by playing with the FPR.. I'll bet a good tune will clear it up and you can also ditch the FMU.
 
#11
pearlnotchback said:
...I put it all together and wouldnt idle worth a crap. After hrs of resetting the idle and so on I started turning up the fuel pressure and to my surprise it started idling better. Now my pressure is set at 50psi and it idles fine and runs fine. I thought maybe my gauge was off or something so i checked it with a fuel pressure tester and nope its 50. Anyone have any thoughts on this. I could be wrong but it just seems a little high to me.
Click to expand...
When I went from 19's to 30's, mine won't idle at all, without feathering throttle, until EEC goes closed loop (i.e., engine warm). It fires right up & dies if you don't feather the throttle.
Is your problem cold, warm or both?
The MAF is not even looked at by the EEC at idle, warm or cold.
http://oldfuelinjection.com/?p=3

FMU does nothing at idle, only comes into play under boost pressure, nothing at vacuum. You can demo with bicycle pump, pressure will go 100 psi, easily, when the return line is restricted.
I can't even reduce the pressure below 40 with the 255Hi pressure pump.
I don't know why it would idle better with higher pressure. With stock EEC, it would be running very rich, since injector pulse width would be the same as the 19's. Maybe it's ok when cold, but warm?
 
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#12
Blown88GT said:
When I went from 19's to 30's, mine won't idle at all, without feathering throttle, until EEC goes closed loop (i.e., engine warm). It fires right up & dies if you don't feather the throttle.
Is your problem cold, warm or both?
The MAF is not even looked at by the EEC at idle, warm or cold.
http://oldfuelinjection.com/?p=3

FMU does nothing at idle, only comes into play under boost pressure, nothing at vacuum. You can demo with bicycle pump, pressure will go 100 psi, easily, when the return line is restricted.
I can't even reduce the pressure below 40 with the 255Hi pressure pump.
I don't know why it would idle better with higher pressure. With stock EEC, it would be running very rich, since injector pulse width would be the same as the 19's. Maybe it's ok when cold, but warm?
Click to expand...
When I changed to the 30s it cranked right up and was running and started surging and hanging at 1200 then drop to 600 and so on .I reset the idle with unplugging the iac and unhooking the battery like we all do and I got it to finally idle steady at 600. It was struggling to idle though and my car has always liked to idle just a hair under 1000. So I looked at my fuel pressure gauge and it was at about 44 psi so I started to lower it to 40 and the idle started dropping so I said hmmm. I started raising the fuel pressure and it started idling back up. So with it idling at 900 now which is just a hair low from where it likes to be the fuel pressure with the vacuum line attached is right at 50.
 
#13
I also found the paper work that came with my aeromotive fuel reg. and they suggest to start with the base pressure at 48 with the vaccum line off. I don't know if this is relevant or not just thought I would throw it out there.
 
#14
So many people get this wrong (not saying you), here it is directly from the Ford 1988 Car Shop Manual

Ford specifies the following in the Shop Manual for 5.0L SEFI HO (p. 24-35-7)
Engine Running: 30-45 psi (i.e. vacuum line attached)
Engine Off (Ignition On, Pump On): 35-45 psi (i.e. no vacuum line attached)
Ford also says: Balancing one side of the fuel pressure regulator diaphragm with manifold pressure maintains a constant fuel pressure drop across the injectors.
------------------------------------------------------------------------------------------------
Convert manifold vacuum to psi
20 inHG (inches of Mercury) vacuum = 9.8 psia (about 10 psia, pounds per square inch absolute pressure)
10 inHG (inches of Mercury) vacuum = 4.9 psia (about 5 psia, pounds per square inch absolute pressure)
Ford spec allows for a 5 psia drop, since manifold vacuum can vary.

According to you above: 50 psi with vacuum line attached (line on)
You are only 5 psi above the Ford Range.

Aeromotive says: 48 (line off) - 8 = 40 (line on)

40 psi is the generally accepted value with line on. Pulling the line generally goes to 35 psi. Most all FPR's won't go below 35 psi, line on or off.

pearlnotchback said:
When I changed to the 30s it cranked right up and was running and started surging and hanging at 1200 then drop to 600 and so on .I reset the idle with unplugging the iac and unhooking the battery like we all do and I got it to finally idle steady at 600. It was struggling to idle though and my car has always liked to idle just a hair under 1000..
Click to expand...
Once it warms up everything changes, O2's are monitoring, tells EEC to adjust injector pulse width for proper idle based on AFR programmed into EEC.

You have to catch it while it's still open loop (cold to warm) or you're just chasing ghosts. It's pretty hard to do with only 1 person, unless it's a cold winter day. Haven't had one of those here in almost a year.
 
Last edited:
#15
Blown88GT said:
So many people get this wrong (not saying you), here it is directly from the Ford 1988 Car Shop Manual

Ford specifies the following in the Shop Manual for 5.0L SEFI HO (p. 24-35-7)
Engine Running: 30-45 psi (i.e. vacuum line attached)
Engine Off (Ignition On, Pump On): 35-45 psi (i.e. no vacuum line attached)
Ford also says: Balancing one side of the fuel pressure regulator diaphragm with manifold pressure maintains a constant fuel pressure drop across the injectors.
------------------------------------------------------------------------------------------------
Convert manifold vacuum to psi
20 inHG (inches of Mercury) vacuum = 9.8 psia (about 10 psia, pounds per square inch absolute pressure)
10 inHG (inches of Mercury) vacuum = 4.9 psia (about 5 psia, pounds per square inch absolute pressure)
Ford spec allows for a 5 psia drop, since manifold vacuum can vary.

According to you above: 50 psi with vacuum line attached (line on)
You are only 5 psi above the Ford Range.

Aeromotive says: 48 (line off) - 8 = 40 (line on)

40 psi is the generally accepted value with line on. Pulling the line generally goes to 35 psi. Most all FPR's won't go below 35 psi, line on or off.


Once it warms up everything changes, O2's are monitoring, tells EEC to adjust injector pulse width for proper idle based on AFR programmed into EEC.

You have to catch it while it's still open loop (cold to warm) or you're just chasing ghosts. It's pretty hard to do with only 1 person, unless it's a cold winter day. Haven't had one of those here in almost a year.
Click to expand...
That is some real good info. So basically 50 psi is nothing I should worry about?
 
Last edited:
#18
More specific here: http://oldfuelinjection.com/index.php?p=64
Cold Start & Warm Up
The engine has just started and we have to get adjusted. EEC's first reaction is to keep the throttle air bypass open 100% or else we'll be back to engine crank mode. RPM will drop however, usually to around 1000RPM after a few seconds, then drop down to base idle as she gets warmer (about 160degF). EEC looks almost completely at ECT and TPS; both indicate what will occur next. The colder the more fuel gets dumped in and the more timing added. When she gets warmer (about 170degF) she will lean out the mixture. This logic will speed up the time it takes to heat up the engine and converter. If you drive away with ECT under 185degF she jumps to cold drive-away strategy. If you stay asleep and leave everything alone ECT will cross 185degF in about 4 minutes and we are now in a warm idle.

Cold Drive-Away
So the TPS and PIP are going up, the MAF/MAP is showing greater load, and the ECT is under 185degF. The TPS and MAF/MAP tell EEC that we are now driving away. The goal at this point is to move the vehicle without stalling and warm up the engine as fast as possible. The IAB opens 100% for insurance from stalling if the TB slams shut suddenly. We have a rich fuel mixture that leans out as temp rises. Once ECT is greater than 170F the mixture is actually less than 15:1, this rushes us into normal operating temperatures quicker. Timing is advanced and retards slowly as temps rise. Once the ECT temp crosses 185degF the mixture should return to normal and relate to HEGO signal as long as you aren't accelerating enough to be in another strategy. After 4 minutes if the engine is warm enough to begin EGR function it's about time to pass command to the "Big-3". Warm cruise, Part-Throttle, and Full-throttle acceleration can take us down the road from here.

Warm Idle
This one tends to be more about emissions. Warm idle occurs if the engine starts and ECT rises above 185degF, we come up to a stoplight, or place it back into neutral. The computers is programmed with a target RPM for idle, typically it's around 672RPM. Typically the computer is struggling to achieve its target RPM. 255degF is the magic number to start complaining about overheating. Let's break it down by system. Fuel is now closed loop with HEGO switching. We are now aiming for 14.7:1 A/F ratio for perfect emissions, enrichment will occur only if the ECT is above 225degF. There will be no EGR operation at these RPM's. Timing is mildly advanced and will begin to retard after a minute. We need hot exhaust gasses to help complete combustion and converter operation. Air is pumped into the converter, but will dump to the atmosphere after several minutes to prevent overheating the converter. The Throttle Air Bypass will be controlling idle, we need a nice smooth idle. Not to high with A/T transmissions to prevent creep at stoplights. Idle will increases 100-200RPM if the ECT or ACT is over 225degF. This RPM increase will help burn the rich mixture cooling the combustion chamber. The faster RPM speeds up the accessories, specifically the water pump and fan. Belt fans will speed up, and electric fans will need more juice from the alternator.
 
#19
84Ttop said:
Likewise, MAF has plenty more functions than you realize. Idle is certainly driven by the Maf voltage signal and correlates directly to the injector count. Anyone who has done any tuning with the stock eec iv can attest to this.
Click to expand...

Exactly! And having tuned my own cars, just having someone say the MAF is not even looked at idle is beyond absurd. Then posting it on a forum as fact is makes it even worse. Lots of bad info out there for sure.
 
#20
Blown88GT said:
When I went from 19's to 30's, mine won't idle at all, without feathering throttle, until EEC goes closed loop (i.e., engine warm). It fires right up & dies if you don't feather the throttle.
Is your problem cold, warm or both?
The MAF is not even looked at by the EEC at idle, warm or cold.
http://oldfuelinjection.com/?p=3

FMU does nothing at idle, only comes into play under boost pressure, nothing at vacuum. You can demo with bicycle pump, pressure will go 100 psi, easily, when the return line is restricted.
I can't even reduce the pressure below 40 with the 255Hi pressure pump.
I don't know why it would idle better with higher pressure. With stock EEC, it would be running very rich, since injector pulse width would be the same as the 19's. Maybe it's ok when cold, but warm?
Click to expand...
You may want to reconsider your sources, to speak of some of this stuff as fact is very misleading and down right untruthful! Please don't believe everything you read on the internet ;)
 
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