Should my TBI's fuel pressure act like this?

[RaysnCayne]

Hi all. I've got a mostly-original TBI system atop the mostly-original 305 in my '87 SWB.

It seems to be working fairly decent other than I feel it should be starting a bit quicker than it does (takes about 3ish seconds of cranking to light off when cold 2ish when warm) and should definitely be getting better mpg than it does (~13mpg). It's definitely running rich according to the tail pipe smell.

I put a fuel pressure test gauge on it this evening and found the (what I believe to be original) fuel pump quickly makes 12psi when I turn the key to the start position. But when I hit the key to start it, the FP just as quickly drops to 0-1psi and then takes about 3 seconds to build to 12psi at which point the engine starts. And then it holds quite steady at 12psi. When I shut it off, the psi drops to zero within a couple seconds.

Is all the above fairly normal? Or should the psi never drop after the key is turned on? And should it hold and then lose pressure more slowly after it's shut off?

(Full disclosure: I swapped a L31 roller cam into it. And installed my own dual exhaust. But it's totally stock beyond that.)

 

[gmbellew]

turning the key to ON will prime the pump for a couple seconds. after the prime, the pressure will bleed off. not sure why the pressure drops to 0 for the first couple seconds of cranking. I've never done that test with mine, but intuitively it seems like the pressure should be there during cranking....

 

[RaysnCayne]

I would think the same thing, thus my query to the TBI brain trust here.
A friend into LS's told me their fuel system does the same thing: 58psi key on, then drops to 0 and pumps quickly back up to 58 upon hitting the key.

 

[RaysnCayne]

Anyone??? Please!
Is fuel pressure supposed to dip to zero right after key on?
And it's falling quickly to zero after shut off normal? (Or a sign of a weak FP?)

 

[gmbellew]

Anyone??? Please!
Is fuel pressure supposed to dip to zero right after key on?
And it's falling quickly to zero after shut off normal? (Or a sign of a weak FP?)

right after key on, yes. it primes for 2-3 sec when key on and then the pump turns off. I don't know if it should drop to 0 during cranking, but that doesn't seem correct. have you looked in the emissions and drivability manual?

 

[SquareRoot]

Is fuel pressure supposed to dip to zero right after key on?

No. It should prime and hold some pressure until key is turned off. It will slowly bleed off afterwards.

 

[RaysnCayne]

Thanks guys.

 

[skysurfer]

Figured it was easier to film it than explain in text. Engine was cold but had been started a minute earlier while I tried to get the correct lighting. Gauge is attached to an Aeromotive 13301 regulator and supplied by a Walbro pump.

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[YakkoWarner]

Having played a lot with this part of the system recently, I'm wondering if what you're seeing is the fuel pump relay switching off while cranking, but then it starts after a couple seconds because the oil pressure switch is forcing the fuel pump back on once pressure builds.

Try disconnecting the 3 pin connection from the oil pressure switch temporarily, and see if it still starts after a couple seconds of cranking. If it does, then you're still getting power through the fuel pump relay - if it doesn't then that would indicate that fuel pump is running only from the oil pressure switch. Note that you won't have a valid oil pressure gauge while doing that test, but it will give a data point on where the fuel pump is receiving power from.

 

[Rusty Nail]

Sounds to me like fuel filters are really cheap!

You'll need an open-ended 5/8 AND a 1" wrench. Get ya a Sharpie and write the mileage on it.

 

[RaysnCayne]

Sounds to me like fuel filters are really cheap!

You'll need an open-ended 5/8 AND a 1" wrench. Get ya a Sharpie and write the mileage on it.

Put a brand new one on it just before I put it on the road about 6 months back.

 

[RaysnCayne]

Figured it was easier to film it than explain in text. Gauge is attached to an Aeromotive 13301 regulator and supplied by a Walbro pump.

Thanks for the video, John.
That's exactly how I'd imagine most any healthy FP (stock or Walbro) should act.
I'd pretty much already made my mind up that I should get a new FP (I have no idea how old my current one is). But your video pretty much seals the deal.

 

[RaysnCayne]

Try disconnecting the 3 pin connection from the oil pressure switch temporarily, and see if it still starts after a couple seconds of cranking. If it does, then you're still getting power through the fuel pump relay - if it doesn't then that would indicate that fuel pump is running only from the oil pressure switch. Note that you won't have a valid oil pressure gauge while doing that test, but it will give a data point on where the fuel pump is receiving power from.

This would be an interesting test. But I returned the fuel pressure test kit to Auto Zone yesterday - having already made up my mind that I should get a new fuel pump.
If I'm still having issues after the new FP is installed, I'll circle back to this post and try your test.

 

[Rusty Nail]

Ok got it.

The fuel pressure spring is broken. Eh?
Now is the time to improve fuel pressure and hook up that adjustable regulator thingy they do - with the improved sprang.

Well...we're waiting

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[RaysnCayne]

The fuel pressure spring is broken. Eh?
Now is the time to improve fuel pressure and hook up that adjustable regulator thingy they do - with the improved sprang.
Well...we're waiting

Assuming you're tapping your fingers waiting on me
The truck runs okay-ish as-is, and it's my daily, and I've got other things going on in life that are more urgent.
So I'm not in a big hurry to mess with having to drop the tank to replace the FP right away. It may be a week or 3 months before I get to it. (I'll try to remember to come back and update this post when I do.)

I did a gasket rebuild on the TBI a couple months ago and the spring was fine.

I've considered installing an adjustable FP regulator in the TBI. But I don't understand how you get an accurate reading on that? Every installation I've seen, the pressure gauge is before the regulator.