Fuel pressure drops!

[Angry7M]

So a little history on my car, for a few months it had been getting code 25 (running Lean). So a 2 months ago I replaced my fuel pump with a Walbro 255lph from horsepowerfreaks.com. The code didnt go away, then i found out it was my fuel pressure regulator that was faulty. So now I had a chance to replace the stocker FPR with an Aeromotive Fuel pressure regulator, I installed it and set the Fuel Pressure with the VAC hose off. It was set to 27 psig. The gauge seem to be reading fine, so I lefted at that.

After I drove it to Autozone to get some stuff i neede, i opened up the hood, and I notice it was reading 24 psi, then it got me thinking, is the Fuel Pressure suppose to stay the same at idle?

So I set the FP aggain at 26 psi, got to work, tuned off the car, and after work i checked what the fuel pressure read again, and it read 30 psi, so i went for a test drive, got it the engine temperature, and chekced it again and it went down to 24 psi at idle.

And now im also getting code 25 and 26 (running rich, and Lean)

Is this normal? Is fuel pressure suppose to change like that after driving it? What can cause this issue? Also the wiring to the fuel pump is stock.


Supra Turbo 1989 5 speed swap with 5 speed ECU:
Stock CT at 10 PSI
Intercoler and 2.5 Hard Pipping
Walbro 255 LPH (STOCK WIRING)
Bypassed J-tube
and some other stuff

 

[shaeff]

Rev the motor from the engine bay. Watch the fuel pressure gauge. What happens to the gauge when you rev the motor?

Also keep in mind that those liquid filled fuel pressure gauges become inaccurate to an extent the more they heat up.

 

[jdub]

FP should be 33-40 psi at idle with the vac line removed and blocked.

FP should be 23-30 psi at idle with the vac line attached.

If there is no change with the vac line on/off...check the vac line routing to the VSV and/or test the VSV.

You also want to make sure the system holds 21 psi for a minimum of 5 minutes after you shut the motor off....if it doesn't, you have a pressure bleed somewhere.

If you are running a stock AFM and injectors, I suspect the codes are the result of you hitting the ECU compensation limit due to low FP...it sounds like you are not getting vac to the AFPR and it is not raising FP as engine speed increases. Confirm you get the above FP values with the vac line on/off and raise the FP with vac off to 36 psi. When you attach the vac line, it should drop to ~27 psi. You will get *some* variation in idle FP depending on vac the motor is pulling at idle speed...i.e. cold fast idle vs warmed up normal idle.

 

[grimreaper]

there should be a port on the liquid filled gauges to vent them. make sure its facing up and pull the cap off. the reading should go back to normal.

i went to town adjusting fp psi up then down before i realized my mistake

 

[Angry7M]

Thanks for replying guys, after a little search I found that its normal for these type of Glycol fillled gauges to be inaccurate depenind if hot or cold. I guess its normal to see the change in pressure readings?

But to the other problem, what im more worried about is code 25 and 26 (rich and Lean). Before I use to only the code 25 (running lean), now I get both of them. Maybe i should adjust my fuel pressure?

I set the fuel pressure at around 35 psi with the vac line off, and with the vac line on it gave me like 26 or 28 psi, if I remember correctly.

I also did a throttle snap test with the vac line off, and its ok, drops no more then 1 psi.

I reallly just want to get rid of code 25 and 26.

Also after I shutt off the engine, it holds fuel pressure perfectly.

I have a 2 month old Walbro Fuel pump, Aeromotive fuel pressure regulator, Jtube bypassed, and also im running on the FUel pump stock wiring, and the rest of the fuel system is stock, also the fuel filter was replaced not to long ago.

Could it be the fuel pump wiring? maybe I need the 12 V mod?

Also I use to get this code sinche I bought the car like 2 years ago, but I use to get this code with the Automatic ECU, I have been running the Manual ECU for 5 months now, and I was only getting code 25, untill now.

 

[Angry7M]

Okay guys now that I remember I did adjust the fuel pressure in these 3 days that I took fot the codes to come up. I think I has adjusted the fuel pressure about 5 times, and at one time I did set it at 27 psi with the vac line on when hot, as soon as the engine was cold it went to 30 psi, and it was set like that for about a day, and thats when the code came up. Could me setting the Fuel Pressure several times cause code 25 and 26?

 

[teedoff00]

I had these same problems (fp fluctuations) when I first installed my AFPR. Get rid of your liquid filled gauge and get a mechanical one from Autometer, it will cost you under 30 bucks and it makes tuning a lot easier! Then you can really tell if you have a pressure bleed or not. It is hard to measure correct fuel pressure when your gauge changes depending on temp.
Good Luck!

 

[shaeff]

The fuel pressure shouldn't really drop, as vacuum decreases fuel pressure should increase. I only asked because when I was working on my car quite a while ago, I happened to get a nest of spiders living in my brand new fuel filter.

When I revved the motor, fuel pressure dropped because it was literally starving the motor of fuel. However, my drop was significant; 10+ psi at a blip of the throttle. I swapped out the fuel filter (damn spiders ruined a brand new one), and the problem vanished.

 

[Angry7M]

The fuel pressure shouldn't really drop, as vacuum decreases fuel pressure should increase. I only asked because when I was working on my car quite a while ago, I happened to get a nest of spiders living in my brand new fuel filter.

When I revved the motor, fuel pressure dropped because it was literally starving the motor of fuel. However, my drop was significant; 10+ psi at a blip of the throttle. I swapped out the fuel filter (damn spiders ruined a brand new one), and the problem vanished.

Yeah if I rev the engine the FP will go up and then down to normal.


LOL spiders, well im going to try to get a different gauge, and for now Ill set the FP cold, and see if I get those codes again. BTW I need a Widband! :biglaugh:

 

[wardog]

Hey REDMARO when you installed your walbro did you secure it with a clamp to the pump holder? I just changed mine cause i had low fuel pressure , it would dropp when i tap the throttle . Came out to be that when i secured my last walbro to the pump holder with the clamp it broke then nipple of the pump and that was my low fuel pressure problem . Just checking to see how you did yours...

 

[Angry7M]

Hey REDMARO when you installed your walbro did you secure it with a clamp to the pump holder? I just changed mine cause i had low fuel pressure , it would dropp when i tap the throttle . Came out to be that when i secured my last walbro to the pump holder with the clamp it broke then nipple of the pump and that was my low fuel pressure problem . Just checking to see how you did yours...

Yes it was secured, it looked like a tight fit.


Ok so lets get this straight, how would I set the fuel pressure then? If the pressure reads depending on temp, then how would I set it? Isnt there suppose to be a little more pressure when the vehicle is just started? And once the operating temps are reached it will go down a little?

 

[Angry7M]

there should be a port on the liquid filled gauges to vent them. make sure its facing up and pull the cap off. the reading should go back to normal.

i went to town adjusting fp psi up then down before i realized my mistake

I'll try that tomorrow, thanks!