Engine fuel pressure regulator or fuel pump?

[white91]

Background:

I have a 1991 GT with 25000 miles on it. I have owned it since it was new.

It has only been on the road for about 3.5 years. It has been stored on and off for the past 25 years.

11 years ago I installed GT40y303 heads, 1.72 rockers, and a GT40 tubular intake with 30lb injectors, with a ProM75 mas calibrated for the injectors.

It also has a Vortech A-trim supercharger.

I also installed a BBK FPR and a Walbro 307 pump.

It started and idled fine, but I never had it on the road.

Problem:

So, I did all of this prior to having a family with kids. I now have the time to get the car out and use it.

I finally got it out last week, on the road, and once it hits 2000rpm, it bogs down.

I disconnected the Vortech in order to troubleshoot the problem and eliminate one of the variables.

I put a fuel pressure gauge on it, and it shows that the fuel pressure drops off to less than 10psi after 2000rpm.

I have already refreshed the gasoline and replaced the fuel filter, I am now thinking that it may be the fuel pump or the fuel pressure regulator.

The fuel pump is really noisy. You can hear it above the engine, exhaust, and blower whine at idle, and in the car when moving.

The fuel pressure at idle is set at 41lbs with the vacuum off, and 35lbs with it on.

I also let it idle without the vacuum on, and when I revved the motor above 2000rpm, the fuel pressure dropped down to 10lbs.

Regulator or pump ? or something else..?

Any help is greatly appreciated.

If you need any more information in order to properly diagnose, let me know.

Thanks,
Matt.

 

[General karthief]

I vote fuel pump.

 

[Wayne Waldrep]

Does it have a fuel management unit? Can't remember if the A trim kit used one or not.

 

[Potomus Pete]

Fuel pump = fast

 

[white91]

Yes it does have a FMU.

Since the FMU is on the return line, I would not think that it would cause a drop in pressure on the supply line.

Also, since this fuel problem is occurring at such a low rpm, and with the blower disconnected, the FMU should not be operating at all.

I am going to drop the tank today and have a look at the pump and the inside of the tank.

When I changed the fuel filter, a bunch of redish junk flowed out of the filter inlet, so I think my tank might have rusted, which may have clogged the pump and sock.

I will report back what I find.

 

[JD1964]

I had a kinked return line before. It caused a bog down right around the rpm you describe. 2k or thereabouts

The front end alignment dude had pushed it out of position and linked it.

So if the fmu is in the return line, it could be part of the problem.

But I’d be suspicious of the noisey pump too. And maybe the little rubber hose that connects the pump to the sender inside of the tank. They get brittle and split

 

[jrichker]

Background:

I have a 1991 GT with 25000 miles on it. I have owned it since it was new.

It has only been on the road for about 3.5 years. It has been stored on and off for the past 25 years.

11 years ago I installed GT40y303 heads, 1.72 rockers, and a GT40 tubular intake with 30lb injectors, with a ProM75 mas calibrated for the injectors.

It also has a Vortech A-trim supercharger.

I also installed a BBK FPR and a Walbro 307 pump.

It started and idled fine, but I never had it on the road.

Problem:

So, I did all of this prior to having a family with kids. I now have the time to get the car out and use it.

I finally got it out last week, on the road, and once it hits 2000rpm, it bogs down.

I disconnected the Vortech in order to troubleshoot the problem and eliminate one of the variables.

I put a fuel pressure gauge on it, and it shows that the fuel pressure drops off to less than 10psi after 2000rpm.

I have already refreshed the gasoline and replaced the fuel filter, I am now thinking that it may be the fuel pump or the fuel pressure regulator.

The fuel pump is really noisy. You can hear it above the engine, exhaust, and blower whine at idle, and in the car when moving.

The fuel pressure at idle is set at 41lbs with the vacuum off, and 35lbs with it on.

I also let it idle without the vacuum on, and when I revved the motor above 2000rpm, the fuel pressure dropped down to 10lbs.

Regulator or pump ? or something else..?

Any help is greatly appreciated.

If you need any more information in order to properly diagnose, let me know.

Thanks,
Matt.

Check fuel pressure:
The local auto parts store may rent or loan a fuel pressure test gauge if you don't have one.
Disconnect the vacuum line from the fuel pressure regulator. Check it for evidence of fuel present in the line by removing it and blowing air through it. If you find fuel, the fuel pressure regulator has failed. Reinstall the line; leave the fuel pressure regulator end of the vacuum line disconnected. Then cap or plug the open end of the vacuum line and stow it out of the way.
Connect the fuel pressure test gauge to the Schrader port located just behind the alternator.
Turn the ignition switch on & start the engine. Observe the pressure: you should see 38-41 PSI at idle.
Turn the ignition off; reconnect the vacuum line to the fuel pressure regulator. Then disconnect the fuel pressure test gauge. Watch out for squirting gas when you do this.

Fuel pump pressure test
Disconnect the larger of the two fuel lines up by the Schrader valve. It is the return line and does not have the Schrader valve on it. Find a piece of rubber fuel hose and clamp it on the return line coming from the regulator. Stick a bolt in the other end of the hose and make sure that all your connections are tight and leak proof as possible. When this powers up, you don't want fuel squirting everywhere. Hook up the fuel pressure test gauge. Turn the ignition switch on and watch for leaks. You may want to use a helper inside the car to cut the switch off quickly if you have a leak. To trick the fuel pump into running, find the EEC test connector and jump the connector in the Upper RH corner to ground.

Caution!!! You have blocked the return line for the fuel pump! Pressure will rise very quickly past safe levels with a good pump
If the pressure goes up past 55 PSI, the pump is good and the fuel pressure regulator is bad. If the fuel pressure does not hit 55 PSI or more in a few seconds, the pump is bad or you have electrical problems.

 

[Shuttleman83]

Prime it 3-4 times before starting.

 

[white91]

Well, I dropped the tank today, and found what I think is the problem.

It would appear the inside of the tank is extremely rusty, and the scale has clogged the fuel pump strainer.

Hard to believe the inside is so bad, since the outside looks so good.

It looks I will be ordering a bunch of new parts tomorrow.

Thanks for everyone's help.

Does anyone know what size of fuel pump hanger I need ? The parts websites list one with 5/16 lines and the other with 3/8.

 

[Potomus Pete]

Omg is that how my tank looks.My tank looks good outside also , but my meter doesnt work How common is that.

 

[General karthief]

More so with cars that sit for extended time. (Taking about the rust and the fuel meter not working.) Ethanol in todays fuel absorb moisture like a sponge
Measure your fuel outlet for the size but I believe it will be the larger one.

 

[Potomus Pete]

More so with cars that sit for extended time. (Taking about the rust and the fuel meter not working.) Ethanol in todays fuel absorb moisture like a sponge
Measure your fuel outlet for the size but I believe it will be the larger one.

I use ethanol fixer , Enzyme stuff from Star chemical. Im sure my tank does not look good. What would one do instead of new, flush and dump.

 

[General karthief]

New tank is what I would do, you can try a flush.

 

[white91]

Ya, I was surprised too!

I guess I should have kept my tank topped up, but the car was literally parked for over 10 years, and I had it stored on a lift, so it was not convenient to start, and I got lazy and forgot about it, even though I walk past it every day.

Funny thing is that when I started it for the first time in the spring, in over 5 years, it fired right up and idled smooth.

I guess I now have to play catch-up on the past 30 years of maintenance that I overlooked since it wasn't on the road.(excluding oil changes and coolant changes)

It goes to show you that sometimes it's the age and sometimes it's the mileage that wears out the parts.

Also, the Intake of the pump had a piece of pump strainer in it, combined with the dirty strainer, I would assume that this is why the motor wasn't getting fuel above 2000 rpm under load.

I also changed my fuel regulator back to the stock one, since the BBK one was leaking.

General do you know what the outside diameter of the pump hanger line should be ? Looking at LMR they only sell one pump hanger and its a 5/16. It's just that autozone lists both a 5/16 and a 3/8.

Thanks,
Matt.

 

[Wayne Waldrep]

Like I clearly said in my post above, it's probably a clogged up fuel pump, filthy sock, and crud inside the tank!

 

[white91]

Yep, new tank, sender, pump, pump hanger, grommet, fuel filler neck, and filter. This is my last time dropping a tank on this car, so it will be all new parts going back in.

 

[General karthief]

See which size wrench fits, use the open end of a 3/8 and a 5/16
i'll run out to the shop out back and look at a fuel pump I got and see.

 

[General karthief]

Like I clearly said in my post above, it's probably a clogged up fuel pump, filthy sock, and crud inside the tank!

lucky guess!

 

[General karthief]

Just measured mine 5/16
That's measured at the fuel pump outlet.

 

[Onefine88]

I noticed that you mentioned above that you are using a Walbro 255lph #307 pump. Normally that would be fine but under supercharged conditions you really need a Walbro 255lph#344 pump. It's basically the same pump as the one you have but is rated for higher pressure for boosted applications. While you have your tank down I recommend changing to this pump.
Good Luck