Ok, so, I keep hearing differing #'s for what the correct fuel pressure should be. I have an 88TII with a walbro 255. Where is the correct place to test for the correct pressure and what should that pressure be at idle and at higher rpms? Will the pressure be different under load vs. revving in neutral...or is it just rpm based?

 

You tee into the fuel line between the fuel filter and fuel rail. With the fuel pump check connector jumpered and the ignition on (engine not running) you should see ~36psi (called static fuel pressure). When the engine is running fuel pressure depends on manifold pressure. Under vac it drops below the static pressure and when in boost it goes above it. The FPR keeps the fuel pressure a constant ~36psi higher than manifold pressure. So at idle (assuming 16inHg or 8psi vacuum) you'll see ~28psi fuel pressure and at 8psi boost you'll see ~42psi.

These figures are with the stock fuel pump. With a Walbro they'll be a bit higher unless you have an aftermarket adjustable FPR.

 

thanks...gonna try to get that checked today. Sorry for the ignorance, but how/where do you jumper the fuel pump check connector? Thanks.

 

bueller? bueller?

 

Near the AFM there is a plug that has a rubber cover and only two wires. Jump them with a paper clip, turn key to on and you should here the pump running.

Vince

 

ok...here's what got tested this afternoon. At the connector that hooks into the fuel pump by the rear driver's shock tower, with the car running and headlights, radio, efan, etc running I was getting right about 10v...the most I could get was about 10.7 with everything off, and revving the car up to 3500 rpm's or so. If I'm reading correctly, this is low correct? I think I read that there should be at least 12V. Is this correct? If so, I would think a rewire would be in order. Anybody with some tips on doing this? I read over the write-up on 1300cc and it isn't the clearest thing to understand...half the time it's 10ga, then it's 12ga...it's not clear on the relay wire up at all...Is there a writeup that's easier to read?

 

nevermind on the write-up...the diagram didnt' show up the first time i looked at the link...now it does and it makes sense. Anybody know if that 10-10.7V is low? I have a walbro 255 and the fuel pressure right off the pump is only about 72 psi. It used to be 90. Could this low voltage be causing that much of a pressure drop?

 

You need to read the FSM that is available at no cost, online. The Fuel chapter.

A S5 non turbo or TURBO car, OR a series four 87/88 Turbo car, has a fuel pump resistor relay. Normally at idle the voltage to the fuel pump will be only 9-10psi. If you jumper the fuel pump checK connector, you put a ground on the fuel pump resistor relay and bypass the resistor in the fuel pump resistor relay package. That should make the voltage at the pump connector, go up to whatever the alternator/battery voltage is About 12-14vdc.

The fuel pump deadend pressure is supposed to be from 71.1 to 92.4 psi WITH the fuel pump check connector jumpered. With the check connector jumpered the fuel pump should get more voltage and the fuel pump deadend pressure should rise.

If you have a low battery, the voltage at the pump connector is going to be, what???? Yes, low like the battery. Read the voltage at the fuel pump connector and then go to the battery and read across the terminals with the key still in the same place you had it when checking the fuel pump connector.

Your 72psi is in spec.

The FSM has the info I wrote but it's more easily read in the FSM. It's free, online at little cost.

EDIT: I changed some things. When I first wrote I thought you had a non turbo S5. My mistake. The corrections should make sense.

 

shouldnt' the fuel pressure be higher since it is a walbro? I'll see if I can get the pump checked tomorrow doing as you suggest. Can you give me the link to the online fsm? Thanks for the info.

 

Tell you what. Since it's a series four turbo, what you might want to do is put a meter in the backside of the large blue wire at the fuel pump plug to monitor voltage.

Then before you start the engine, put a 3.5K resistor jumper in the air temp sensor plug on the throttle body inlet duct. That simulates a HJT start condition. Then with the intercooler back on, start the engine and monitor the voltage at the fuel pump connectors Blue wire. It should be over 12vdc for approx 50 seconds, whereupon it will then drop to approx 9-10vdc.

On a series five car you don't have to jumper the air intake temp sensor with a resistor because, as you see in the online S5 FSM, you simulating the function of pin 3K of the ECU by putting a jumper in the fuel pump check connector (won't work on a S4 car, wired different).

Personally I just depin pin 3D on a S4 car to make the ECU bypass the resistor in the fuel pump resistor relay package. Take note that S4 and S5 fuel pump resistor relay package are a bit different as far as the output wire from the ECU to trigger the high/low voltage output of the fuel pump.

This post is to correct what I said in the first response about how putting a jumper in the fuel pump check connector will make the fuel pump resistor relay package bypass the resistor and putout full voltage. That only happens on S5 cars since they are wired a bit different.

You wouldn't have had to read this garbage if you had read the S4 FSM.

On a series four car, the best way in my opinion, to checkout the fuel pump voltage, is to drive the car while observing the digital meter . It will read only about 9-10vdc UNTIL you go into boost. Then pin 3D on the ECU will remove the ground from the fuel pump resistor relay and cause the voltage to go up to whatever your alternator is putting out.

Take note: do the above while driving with all accessories (sp) off. Then turn the heater/air conditioner, headlights on and read the voltage under boost conditions. There will be a voltage drop I guarantee you. If the drop is down below 12vdc, then check the alternator out. It might have one of the three diodes burnt out. Hell, I don't know.

 

The voltage in the above would not have been as low IF you had either depinned pin 3D on the ECU OR driven the car in BOOST. Going into boost is what triggers the higher voltage. Just reving in the driveway won't do it.

For the FSM, go to the first two or three threads on this forum and find the one with FAQ in the title. In that thread there are several places to download the S4 FSM. Only do the FUEL and Emissions section. That's all you need right now. Less time to download just a single section.

Sorry about the confusion. I'm multi tasking and didn't somehow read the part about it being a 88 turbo. I clicked on your profile and it said S5 non turbo (I think) so I started giving out S5 turbo info. It's a bit different than S4. Not much, but enough different. to cause problems.

https://www.rx7club.com/forum/showthread.php?t=494667

 

just an update...the rewire fixed all the problems. Had 110psi pressure after the rewire (it was roughly ~70psi beforehand) I was shocked at how much of a difference it made.