'97 305 runs like out of gas

[stenseltizm]

I replaced the intake gaskets on 2/19 along with the dist gear on my 97 305 vortec. It ran like a top with no troubles until a few days ago I got a call from my dad saying it acted like it 'ran out of gas' and now won't start.

I thought it was the fuel pump as I verified power to the pump, but didn't have the same loud whine it usually had, and didn't seem to have much pressure when pushing on the schrader valve on the intake(didn't have a gauge, so didn't actually measure).

I replaced the pump and made sure it had a good ground, it ran good for a couple days, now it will run like it's starving for fuel intermittently. It will idle fine, but when you press on the throttle, it will bog and pop and not have any power. I will check fuel pressure today(need to buy a gauge) But what else could be causing my problem?

Any ideas would be great, I'll report back when I learn the fuel pressure.

[stenseltizm]

Fuel pressure jumps to 62 when the pump primes, then drops to 10 after.
when idling the pressure is at 50-52, sometimes coming close to 60 while driving.

Is the pressure supposed to bleed off like that or could that be another symptom? I drove it around for a while, and the engine falls on its face when you mash the gas. It will ramp up to speed if you push the throttle very slowly, but it's still gutless and not running right. No 'popping' like I thought I heard the other day.

[Jeremy-WI]

fuel pressure should hold after the pump quits. You could try doing a pressure test with the return line crimped, there is a section of rubber fuel line behind the engine near the bell housing, if the pressure holds while crimped, you probably have a bad regulator

[stenseltizm]

I was wondering if it should hold or not. I'll check the regulator (where is it located if it's bad, somewhere on the return line?) And if it doesn't hold when crimped I assume I'll have a leak before the regulator. Either a leaky/stuck open injector, or where the fuel pump meets the fuel line. That's the only two places I can think of.

Thanks for the input.

[Jeremy-WI]

regulator is in the upper intake manifold(have to remove the black blastic piece the throttle bolts to), it's a good hour of fun to do. Pressure regulator is on the passenger side of the injector pod assembly. If the regulator is leaking fuel into the intake, there is usually a clean spot under its vacuum port. Everything usually has a nice layer of black soot from the egr valve working

[stenseltizm]

Ok, I want to revisit this problem. I had some questions about how this system works. As stated earlier in the thread the pressure should not drop after the pump cycles. Mine does pretty much instantly after the pump stops. I pinched off the return line completely, and the pressure still bled off. So then that leads me to believe I still have a leak somewhere.

Question, what keeps the fuel from just draining back into the tank, is there a check valve somewhere? Is it in the pump, or perhaps the filter?

If a leaking regulator makes a clean spot on the intake under it, then the pressure won't hold with the return line crimped. Because the fuel would have to be leaking into the intake mani, not past the regulator and back through the return line. So the eliminate the return test seems useless.

What should the fuel pressure be at idle, and wot?

[mr_mushroom]

sounds like the regulator is probably the best diagnosis. that's what i'd guess. assuming of course that you've checked the filter recently. i put bad gas in once and it acted like it was dying and it was just the shitty gas.

[stenseltizm]

Originally Posted by mr_mushroom

sounds like the regulator is probably the best diagnosis. that's what i'd guess. assuming of course that you've checked the filter recently. i put bad gas in once and it acted like it was dying and it was just the shitty gas.

Problem with that, is it's just another guess. I'm asking a few questions about how the system works so I can stop spending money throwing parts at it, and fix the leak.

It could be a miriade of parts that I could guess were leaking, I did just replace the intake mani gaskets, pump, and filter. Any one of those rusted connections underneith could be leaking, or I could have pinched an o-ring, etc.

I need to know what keeps the fuel from backflowing to the tank, a check-valve I assume, but where.

What is the minimum working pressure. I've heard varrying numbers around that people say the injectors won't be reliable below.

These are a few things I wanted to know so I can best test sections of the system at a time. I was uncertain about the connection between the pump and the line, and I'm waiting on a new 'dampener' to come in so I can put the pump back in. But I could blow backwards through the pump, so that got me wondering what keeps the pressure in the line once the pump is not energized.

Thanks for the help!

[MT-2500]

There is a check ball int the fuel pump thats keeps pressure or fuel from bleeding back into tank.
Best way to check is to let the pressure build up and when you get full fuel pressure pinch the fuel line from tank off as soon as pressure is up.
If pressure holds with feed line pinched off unpinch the line and see if it holds or bleeds off.
If it drops after unpinch it is leaking by fuel pump check valve.
But the fuel pump check valve does not affect the pressure the pump or engine is running.
Just lets pressure bleed off after engine is shut down.

If a CSI fuel injection specs are 60/66 lbs of pressure.
If a SFI engine specs are 56/62 lb of pressure.
Here is some guide lines on fuel pressure testing.
Check cold start fuel pressure.
Check engine running fuel pressure.
Tape a gauge to outside of windshield and drive or run it for 20-30 minutes then.
Check engine running on the road fuel pressure.
And make sure it is not losing pressure after fuel pump is hot.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak.
Post back fuel pressure readings.
MT

[stenseltizm]

Thanks, MT
The pressure would get to 60-62 when the key was cycled on, then drop to 20 when the pump stopped.

Cold start the pressure sat around 50-55

Had a gauge clipped to the wiper arm all last week while I was waiting for time to work on it, lol. Only drove it a few times though.

Under light throttle roll the pressure would get up to 60, under load (hill with more than light throttle) the pressure drops back to 50-52

Shut-down the pressure held 55-62 while the pump was still on, then dropped instantly when the pump shut off.

I dropped the tank back off yesterday, I can blow on the output of the pump, and gas came out the backside, so I know that check-ball isn't working. Should be why the pressure drops instantly.

I believe The pump was not seating well into the sending unit either, so I think that's where my low pressure is coming from.

Thanks guys, I just wanted to know how the system worked so I could test it all out while the tank is down. I'll take the 'new' pump back, and I replaced the rusted jurry rigged sending unit, so I'm pretty confident these will solve my problems.

BTW what does CSI and SFI stand for?

I know it's for the type of injection, but I don't know which I have. I have the 8 injector cluster inside the intake mani, with the 8 tubes going from the cluster to the intake runners.

[MT-2500]

CSI fuel injection is the older tpye continuous fuel injector unit that injects fuel from the one or two injectors all of the time.
The fuel line goes into the back of intake on that one.
SFI fuel injection is Sequetial [Port] fuel injection system that has and fires a injector for each cylinder when the cylinder is at TDC compression.
The fuel lines and injector wires go into the top of the intake.
One injector for each cylinder.

On the fuel pump I always like to go a complete fuel pump modjual.
Less mess and fuss but more money.
Does yours have a fuel prssure damper in there between fuel pump and outlet line?
Good luck and let us know how it goes.
MT

[stenseltizm]

Originally Posted by MT-2500

CSI fuel injection is the older tpye continuous fuel injector unit that injects fuel from the one or two injectors all of the time.
The fuel line goes into the back of intake on that one.
SFI fuel injection is Sequetial [Port] fuel injection system that has and fires a injector for each cylinder when the cylinder is at TDC compression.
The fuel lines and injector wires go into the top of the intake.
One injector for each cylinder.

On the fuel pump I always like to go a complete fuel pump modjual.
Less mess and fuss but more money.
Does yours have a fuel prssure damper in there between fuel pump and outlet line?
Good luck and let us know how it goes.
MT

I'd say mine is SFI then, the fuel lines run into the top of the intake, right beside the large injector plug for the bundle of injectors.

Yes there is a pressure damper, That is where I suspect was a less than perfect seal on the old sending unit. That is also the part I am waiting on to reinstall this mess, as it didn't come with the new sending unit. Should be in by now so I can get back on this after work.

I really wish I would have gotten the complete module (assuming you mean fuel pump, dampener, and sending unit all in one package). Instead I listened to the older retired gentlemen whom was loaning me his personal lift to do the work, and only put a pump in first time round. (He pushes to reuse/restore everything, if it needs torn back apart then get the new part. but his current job/pastime involves his maintenance/restoration of his fleet of old cars). I was very hesitant at the hack job we performed on the old sending unit, but he was very pleased to save me $100 some odd dollars. lol

[MT-2500]

I see.
Some time you can save money which is good.
But sometimes saving money can cost you money.
New damper and a good new pump should fix it.
Good luck and let us know how it goes.
MT

[stenseltizm]

Well the 5 min test has resulted in:

62-65 psi when keyed on pump on,
drops to 58 when the pump stops

Running is 55 at idle, 63 when I give it a rev in park.

Shut off the pressure seems to be holding just fine, I'll give it some time sitting and see if it still holds.

I'll clean up, put some fuel in it and give it a longer test, but so far it works.

Thanks a lot for all of the information and suggestions

Tim