Fuel Pressure Dropping

[Dog House]

Having more trouble with my 89. Finally got it running good enough to drive it to work 1 time. Check engine light came on, checked codes when I got home and code 32 was present again and a lean exhaust as well. I cleared the codes and went in the house.
Two days latter went to go to work and the car would barely start much less run. Checked the fuel pressure and it went up to 48 and dropped to 15 within 30 seconds with a new FPR.
Did the procedure in the FSM pinching of the feed and return lines and had leak down pinching the feed line and when I pinched off the return line the pressure shot to around 65 and held at 60 according to the FSM it is a bad FPR.
I want to confirm that I have a fuel issue and not something else so my question is can you remove the MAF and using a spray bottle with gas in shoot it into the TB to see it will run properly?

 

[boomdriver]

Two days latter went to go to work and the car would barely start much less run. Checked the fuel pressure and it went up to 48 and dropped to 15 within 30 seconds with a new FPR.
Did the procedure in the FSM pinching of the feed and return lines and had leak down pinching the feed line and when I pinched off the return line the pressure shot to around 65 and held at 60 according to the FSM it is a bad FPR.
I want to confirm that I have a fuel issue and not something else so my question is can you remove the MAF and using a spray bottle with gas in shoot it into the TB to see it will run properly?

Nope...it might burp or gurgle but what will Probably happen is it'll spit a flame out thru the T-body and scare the crap outta ya..TPI's have an ongoing backfire with EGR gasses flowing thru the plenum so there is an ignition source there...

FPR's are easy to damage...that thin delicate diaphram can tear just by mounting it in place. Very fragile. If its adjustable the plunger can sometimes tear one if it was slightly twisted when installed. The plunger itself can stick also. The regulator could possibly stick open and allow the pressure to fall to nothing, not hold any after shutdown, and let everything go back to the return following the path of least resistance, right there on the rail, thru the return line.

The book spells out the process pretty good, with no return line (pinched off) the fuel has no other route to escape except:
BAck the way it came, so press would still drop...(bad pump)

or

leak thru the injectors and press drops as eng floods....(bad inj)

or

press holds because the inj are tight, the pump ck valve works. FPR has to be the source of the leak-down or return to tank because there is no other way and the return is at the FPR.

I know its some work, but you might try to disassemble your FPR and see if the plunger is stuck or a piece of the diaphram is in the plunger keeping it from seating. I would have a new one on stand-by. At this point there ain't much to loose,. its not running anyway.

 

[Dog House]

Thanks Boomdriver, does it mean anything that while turning the engine over the pressure holds at 45 but the car still doesn’t start?

 

[Dog House]

I think I have confirmed that it’s the FPR. I added some clear hose to the return line at the tank and as soon as the pump turns on fuel comes gushing out the return line.
Sound like a logical conclusion?

 

[vetteoz]

it'll spit a flame out thru the T-body and scare the crap outta ya..TPI's have an ongoing backfire with EGR gasses flowing thru the plenum so there is an ignition source there...

EGR only operates under certain specified conditions; certainly not at startup.
Have used a squeeze bottle to squirt fuel in through TB many times when trouble shooting without problems

 

[vetteoz]

does it mean anything that while turning the engine over the pressure holds at 45 but the car still doesn’t start?

If you have 45psi at the rail when cranking and no fire then;
you either have no injector pulse to fire the injectors or you have no spark
Have you ohm tested injectors? One bad injector will shut down the whole bank
If you spray some fuel in the TB and it kicks while cranking you know you have a fuel problem.
If it doesn't then I would be looking at the spark

FWIW.
My Aeromotive reg holds no rail pressure whatsoever and my engine will fire up fine on pump pressure alone .As soon as engine spins the oil pressure switch closes and runs pump putting full pressure back at the rail .This is how the factory designed it as a get home backup should the pump relay fail and not be able to prime the fuel rail ; key on

 

[boomdriver]

I think I have confirmed that it’s the FPR. I added some clear hose to the return line at the tank and as soon as the pump turns on fuel comes gushing out the return line.
Sound like a logical conclusion?

well, there is always going to be a good deal of return flow. The engine simply cannot use as much fuel as the pump can move when cranking. Volume has to be more than needed, while pressure has to be more exact. The pump will supply a vol and the eng uses what it needs, a metered amount, and the leftovers get sent straight back to tank. As long as there is "pre-tension" on the injectors, thats what the motor cares about.
NASCAR engines or a drag engine that was burning fuel by the lb at extended high RPM would know exactly how much their pump needed to move. I read that a top-fuel rail burned around 10-14 gallons per run. Our cars can run for about 25 minutes on a gal at around 2000 rpm.

If you are holding pressure while cranking, as Oz stated, its from the pump getting power thru the fuel relay, passed by the oil pressure switch and finally a signal from the ECM when it see's a distributer pulse. Otherwise, when key ON happens, the ECM only allows about 1-2 seconds of power to the fuel relay/pump to "prime" the injectors for start up.

Holding pressure to me sounds like distributer signal is there energizing the relay/pump, but the fuel injectors may not be opening. If they were, a tiny fluxuation would be seen in the pressure guage and you;d get some sort of burp as long as the spark was hot enough.

If you ever see a "NOID" light, buy it....its a simple little tool that will light up with the inj pulse so you can SEE it pulsing. It will also tell you if an inj is stuck ON, or not getting a signal at all. It's a $10 tool that will tell you alot.

 

[Dog House]

I have a Noid Light set and all inj are getting a signal?

 

[vetteoz]

all inj are getting a signal?

Pull a plug out and see if they are wet with fuel ( injectors obviously working )
If so start ,checking for spark problems

 

[vetteoz]

Holding pressure to me sounds like distributer signal is there energizing the relay/pump,

Possibly not.
If he has a spark problem ( say the module in dist is faulty ) and consequently no reference pulse to ECM , he would still get pump run and fuel pressure while cranking because OP switch is wired direct to pump ; not to the pump relay

 

[John Robinson]

New Ideas

First thing I think of is maye you are being mislead into thinking EGR or Fuel Preassure problems.

My first thought on this is that you have a ground issue or an electrical mechanical issue.

Check all the wires and connectors for all the sensors for continuity and be sure to wiggle them as you check them. I have experienced the wires breaking inside the plugs and also just being broken. Some of these wires are only 4 or 5 very thin strands and over time and vibration they will break.

Next I would check the TPS it will cause you to think you have fuel problems because of the way it works.

I know I have published this before but I can not stress enough the role this sensor pays in how the car runs.

If you get the car running wiggle the wires going into the TPS plug and see if the engine changes how it idles.

Throttle Position Sensor (TPS)
Copied from 1993 Service Manual
The Throttle Position sensor (TPS) is a potentiometer connected to the throttle shaft on the throttle body. It is a potentiometer with one end connected to 5 volts
from the ECM and the other to ECM ground. A third wire is connected to the ECM to measure the voltage from the TP sensor. As the throttle valve angle is changed (accelerator pedal moved), the voltage output of the TP sensor also changes. At a closed throttle position, the voltage output of the TP sensor is low (approximately .5 volt). As the throttle valve opens, the output voltage should be approximately 5 volts.

By monitoring the output voltage from the TP sensor, the ECM can determine fuel delivery (This is why I say you can be mislead into thinking you have a fuel pump or FPR problem) based on throttle valve angle (driver demand). A broken or loose TP sensor can cause intermittent burst of fuel from the injectors and cause an unstable idle, because the ECM detects the throttle is moving.

If the TP sensor circuit is open, the ECM will set a DTC 22. If the TP sensor circuit is shorted a DTC 21 will be set. A problem in any of the TP sensor circuits will set either a DTC 21 or 22. Once a DTC is set, the ECM will use a default value for TP sensor, and some vehicle performance will return.

A personal note, when my TPS failed it DID NOT set a code other than to tell me the ECM was bad.

​

 

[Dog House]

I pulled the FPR and it looked fine and the plunger seemed to work okay, Hard to tell it’s a pretty strong spring. I have another FPR but did not replace it.

I also pulled the fuel rails high enough to see if the injectors are leaking and they are not.

I had my wife turn the key so I could see if the injectors had a spray and the only one that sprayed anything at all was number one cylinder. So what would cause this? :W

I also checked to see if the injectors were getting a signal with a Noid light and they are.

 

[Dog House]

Okay I’ve put new injectors and two different FPR’s in and the pressure is still dropping. It is extremely hard to get started and will barely run.

I can pinch off the return line and it will run fine? Why is the pressure still dropping?

Obviously not getting enough fuel to the injectors it’s all going down the return line.

Still get 47#’s at the rail when you turn the key to on but drops right away.

How does the FPR work? What makes the valve open to allow fuel to go down the return line?

 

[John Robinson]

Still get 47#’s at the rail when you turn the key to on but drops right away.

When you say drops right away do you mean that it goes from 47# to zero in a matter of just a few seconds? When you turn the key on and build preassure it should hold preassure for several min. On mine it holds 45# for at least 45 min. after I turn off the key. You do not say how old the fuel pump is. Just a guess but I belive that the check valve in the fuel pump is not keeping the fuel from leaking back through the feed line.

Also if the dampner is not correctly placed on the fuel line it will releave the fuel preassure because it is in effect also a leak in the system.

I have highlighted the steps I think you have tried and the answears you have given.

 

[Dog House]

Hi John, for some unknown reason the car started and is running fine now. Not sure what happen I really didn’t do anything else to it? The pressure would go from 47 to 20 in about 30 seconds. I replace the pump just last week when I started having a problem just in case that was it. I think I’m going to purchase the strainer assembly and replace it at least that way I’ll know it’s new. Pretty soon there won’t be anymore parts to change. LOL :L

 

[John Robinson]

Absolutely replace the fuel pump pick up screen. While your in there again make sure the dampner is tight on the tubes. A little trick I used to help insure that it did not move was to take and put extra gromments on each side of it to keep it in place. As for the new fuel pump don't get boxed in thinking that it can't be bad. I fought my problem for almost 10 months before I pulled the fuel pump I had put in 10 months earlier.
Take a look at what I found.
<a href="Small Hole in Fuel Pump picture by John-68 - Photobucket" target="_blank"><img src="http://i285.photobucket.com/albums/ll72/John-68/Picture008.jpg" border="0" alt="Small Hole in Fuel Pump"></a>