Ford Mustang Forum banner

1994 GT Fuel pressure issues-pump? injectors?

8K views 6 replies 4 participants last post by  wbrockstar 
#1 ·
I recently acquired a 1994 GT with ~120,00 miles, auto with 2.73 (I assume) gears and completely stock. I realize this car isn't going to feel that fast, but it feels like its down on power. Fuel filter and pressure regulator are both new (Motorcraft).

A few of the spark plugs looked a little dark and slightly wet when I changed them, but the electrodes were so worn that I attributed the issues to the gaps being huge.

I put a fuel pressure gauge on it yesterday and observed the following:

-KOEO: Sometimes pressure jumps to ~38 psi, but immediately drops to about 8 psi, and then continues to drop very slowly. Other times the pressure only jumps to between 6 and 12 psi, and then immediately drops. I never could get the KOEO pressure to hold at the "normal" ~35-45psi. The inconsistency bothers me as much as anything.

-KOER: The car starts right up and holds steady at 32psi at idle. It drops to about 31psi at 2000rpm in park. I don't have plates on the car so I can't road test it right now. Holds 32psi for at least 20 minutes after shutting down.

I've done a lot of reading on Mustang and Ford truck forums and can't find a definitive answer about what fuel pressure is actually right. I know my KOEO symptoms aren't normal-do they indicate a dying fuel pump?

Is 32psi at idle a problem? I've read that 30-40psi while running is within spec, but a lot of anecdotal "real world" comments suggest 35 is a better minimum.

Any thoughts?
 
See less See more
#2 ·
32psi isn't bad for idle. Maybe a little low depending who you ask,but not problematic. As far as KOEO, when you cycle the key on the fuel pump runs for a few seconds then cuts off, so it's normal to see pressure jump up then fall. As far as it varying the psi it jumps to, kind of odd it jumps around but if it starts right up every time I wouldn't worry about it.
 
#3 ·
Pressure should be higher with the KOEO than it is with the KOER at idle because vacuum from the engine reduces the spring pressure inside the regulator making it easier for the valve to be opened,sending fuel down the return line to the tank.
KOEO= 35-45psi
KOER= 30-40psi
The pump may/may not be bad.

Usually the best way to check for a defective fuel pressure regulator,bad check valve inside the fuel pump or a leaking injector is by doing a check of bleeddown pressure.However you stated the pressure holds at 32psi for 20min after the key is turned off.That should indicate those components arent leaking.The test is done as follows:
1) Let the engine idle til it reaches operating temp,then shutdown.
2) Turn the koeo,record pressure,turn key off.
3) Wait 60 seconds,record pressure again.
4) Did pressure drop more than 5psi in that 60 seconds?
5) If yes,grab a rag and vice grips and pinch a soft section of the return hose closed
6) Redo steps 2-4
7) Did pressure drop more than 5psi?
8) If no,the fuel pressure regulator is leaking
If yes,the check valve in the fuel pump is bad or injector(s) is bad.

I would either replace the plugs or remove them,dry with an air compressor,re-gap,then start the engine and make sure each plug wire & plug is firing.This is especially important to do on the plugs you found that were wet.You wanna make sure the plug is firing before doing the next step so you wont think an injector is leaking when in reality its a non firing plug/plug wire.
Once you verify the cylinders are firing,start the engine and let it run til it reaches operating temp,shut it down,wait 5 minutes,then pull the plugs you noticed were wet during your original check.If theyre wet again,the injectors for those cylinders are leaking down.
Another thing you can do is listen to the injectors to see if theyre pulsing,rather than being dead or stuck open.The suspect injectors (if present) would most likely be stuck open though,since youre getting wet plugs vs dry.With the engine idling,you can hold a mechanics stethoscope or a wooden dowel/handle against each injector body and you should hear a repetitive clicking noise,if the injector is firing.If this noise isnt present,but the injector harness is pulsing,that indicates the injector is bad.The bad news: reaching every injector is almost impossible with the upper intake in place.
 
#7 ·
You can check the balancer with a timing light & double check the timing while youre at it.
1) Pull the #1 plug and stuff a piece of tissue paper into the hole slightly.
2) Locate the 0° & 10° btdc timing marks and draw a line across both of those with Witeout.

10 ¹ ¹ ¹ ¹ 0* ¹ ¹ ¹ ¹ 10*

3) Use a torque wrench/breaker bar & socket and rotate the crank CW til the paper blows out(hissing noise will be heard) then slowly continue rotating til the 0° mark aligns with the timing pointer.
4) Remove the distributer cap.The rotor should be pointing at the #1 cylinder on the cap.It should point just like shown in the picture below.
5) If its pointed correctly,reinstall the plug and wire,start the engine,pull the spout plug,aim the timing light and the 10°(if its set to the factory position) balancer mark should be aligned with the timing pointer.
6) Its possible the previous owner could have set it above 10°,so if its sitting anywhere between 10-14°,thats fine as long as youre not getting detonation (pinging/rattling/knocking)
Now that youve started the car & checked timing,did the timing mark stay aligned with the pointer with the engine running and the spout plug removed??
If yes,the balancer should be fine.If the outer ring is spinning on the hub,the timing marks will be bouncing around.

Another thing I would do is clean the maf sensor with CRC MAF Cleaner.Remove the sensor from the housing.Let it air dry for 5-10 minutes before reinstalling.It can cause the symptoms youre having.
Check the tps voltage at the gray/white wire.Use the black/white wire for the ground.It should increase smoothly from the closed to wot to closed position.
Closed throttle= .80-1.00 volts
WOT= <4.84 volts
If voltage spikes or dead spots show up,the tps is bad.

Low battery voltage to the fuel pump can also cause lower pressure.Voltage at the pump should be very close to battery voltage.
Just as a double check,pull the vacuum hose from the fuel pressure regulator while the engine is idling and make sure the hose doesnt have fuel in it and the regulator isnt seeping fuel.Either condition= bad fpr.New/Remanufactured parts can be faulty too.
 

Attachments

This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top