97 Suburban - loss of power on hard acceleration

Vehicle is 97 Suburban 1/2 ton 4x4 with 350 Vortec.

I have a hesitation under hard acceleration problem. It is bas enough that it is a noticeable jerking of the vehicle. Also, under hard acceleration, it really bogs down. If I let off and feather the accelerator, it wil gradually gain speed, but pretty slowy.

About a month or two back, I had to replace my fuel pump do to no start condition. I replaced it with the Delphi pump after reading threads here. Truck seemed to run fine, and then last week I started noticing a gas leak from the top of the gas tank. I dropped the tank again and found that the pass through electrical connection that is supposed to seat against an o-ring on top of the sending unit had come loose. It would not stay in place at all. Checked to see abotu replacing the wiring and plug but only the whole sending unit could be purchased. So I bought some gas tank repair epoxy and secured the plug in place. The hesitation actually occurred before this repair as well, but seems like it is more noticeable now.

I went ahead and replaced the wires and plugs (due anyway) and also another fuel filter just in case. Still has the hesitation. I also added some HEET and STP fuel cleaner thinking maybe bad gas, but I am doubtful that is it.

Air filter is pretty recent. Can't remember if PCV has ever been changed honestly.

I got home tonight and got my gage out and this is what I have (can't remember what is OK and can't find my book):

turn key on - snaps up to 60+psi and then within about 2-3 seconds, it drops down to 15 and holds.

start engine - snaps right up to 50 psi and maintains while engine is running.

I let the engine run for a few minutes and then shut it off and then pressure immediately bled down to about 15. I think this is telling me I have a leak somewhere, but where to start looking? Could this be a bad FPR? How bad is that to replace on a 97?

Thanks,
Shane

oh, vehicle has 200,000 miles on it.

Sounds like a bad pressure regulator, but could be a bad check valve in the fuel pump. Try pinching the return line off and run the fuel pump for just a second or two and see what the pressure is. It should be above 75psi and hold steady. If it doesn't get to 75 the fuel pump is week, if it doesn't hold, you have a leak somewhere between the pump and where you pinched the hose or you have a bad check valve in the fuel pump.

Well I was up parts of the night wondering about my problem(s). I am wondering if I have 2 issues here. I know that the immediate pressure drop (both after shutdown and after the initial spike when key is turned on) is not a good sign, although, my car has no problem starting up. Could this pressure decrease somehow be affecting my hesitation and loss of power at speed issues. When the car is running, the pressure is stable at 50. I am going to tape it to my windshield this morning and drive to work and see if the pressure fluctuates at all.

Still wondering if I might have a MAF or O2 sensor issue though.

Sorry, just thinking out loud...

Sounds like a bad pressure regulator, but could be a bad check valve in the fuel pump. Try pinching the return line off and run the fuel pump for just a second or two and see what the pressure is. It should be above 75psi and hold steady. If it doesn't get to 75 the fuel pump is week, if it doesn't hold, you have a leak somewhere between the pump and where you pinched the hose or you have a bad check valve in the fuel pump.

I will check this out when I after I get to work. Not to sound totally ignorant, but is there a better or safer place to pinch off this return line for that check?

And once again, does anyone think a bad FPR would cause what I am seeing while driving?

Thanks again.

That year and make has two injector systems.
CSI specs is 60/66 lbs pressure. should have 65-66 under full load.
SFI specs are 56/62 and should have 61-62 under full load.

For the regulator to maintain pressure check the pump full direct pressure with return line blocked or plugged off.
But only for a few seconds for a test.
Full pump pressure should be 95 -105 lbs.

Guide lines for checking pump pressure.
SFI pressure will be about 4-5 lbs lower.
The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold or hot and fuel pressure testing guide lines.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding 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 in system.
Post back fuel pressure readings.

Thanks MT-2500... now for my next dumb question...

how can I tell which FI system I have?

Could this pressure decrease somehow be affecting my hesitation and loss of power at speed issues. .



And once again, does anyone think a bad FPR would cause what I am seeing while driving?



Yup. Possibly...

Do the pedal down test, gauge taped to the windshield, like you were. FPR should respond to pedal down vacuum pressure, unless the diaphragm is ruptured in the FPR. If this is the case, pressure will NOT increase accordingly, and fuel will get by the diaphragm, and into the vacuum line.

At idle, rev the motor a few times. Kill it, then pull the vacuum line from the FPR. Get yer schnozz right on the vacuum line, and snort. Smell gas?

Yes? Ruptured FPR diaphragm.

No gas smell? Weak FPR spring, and no fuel pressure 'maintenance'.

Just some things to check...

OK, I have a couple of questions:

There are two lines running up towards the engine from the area of the fuel filter. One of them from the filter is obviously the supply line. The other one runs with the supply line through a rubber holder on the chassis, and then goes from stainless to rubber hose. This line is the one I pinched off to do the pump check (the one not coming from filter)...

That said, with the line pinched off, and assuming my clamp was tight enough to seal it off, I get the following:

turn key on - snaps up to about 60-65 and then back down to 15 and holds

start engine - snaps up to 53 and holds. Then turn off engine and immediately decreases to 15 and holds.

With the alleged return line not clamped off, the action is about the same, but maybe 2-5 psi lower on the higher readings.

The pump in the tank is a 2 month old Delphi pump. Rather than throw out my postulations, I would simply ask what you think I should check next.

Thanks

Yup. Possibly...

Do the pedal down test, gauge taped to the windshield, like you were. FPR should respond to pedal down vacuum pressure, unless the diaphragm is ruptured in the FPR. If this is the case, pressure will NOT increase accordingly, and fuel will get by the diaphragm, and into the vacuum line.

At idle, rev the motor a few times. Kill it, then pull the vacuum line from the FPR. Get yer schnozz right on the vacuum line, and snort. Smell gas?

Yes? Ruptured FPR diaphragm.

No gas smell? Weak FPR spring, and no fuel pressure 'maintenance'.

Just some things to check...

Thanks, I will check the vacuum line

Thanks MT-2500... now for my next dumb question...

how can I tell which FI system I have?

SFI is the one with a injector for each cylinder.

OK, I have a couple of questions:

There are two lines running up towards the engine from the area of the fuel filter. One of them from the filter is obviously the supply line. The other one runs with the supply line through a rubber holder on the chassis, and then goes from stainless to rubber hose. This line is the one I pinched off to do the pump check (the one not coming from filter)...

That said, with the line pinched off, and assuming my clamp was tight enough to seal it off, I get the following:

turn key on - snaps up to about 60-65 and then back down to 15 and holds

start engine - snaps up to 53 and holds. Then turn off engine and immediately decreases to 15 and holds.

With the alleged return line not clamped off, the action is about the same, but maybe 2-5 psi lower on the higher readings.

The pump in the tank is a 2 month old Delphi pump. Rather than throw out my postulations, I would simply ask what you think I should check next.

Thanks

Two lines to engine.
Large from filter to engine and.
Small is return.
Better to check direct fuel pressure at the in line to filter.
Remove it from filter and hook gauge direct to it.

Thanks, I believe I have the CSI then (which is funny as my 911 has CIS). I have what looks like a TB unit on top of the intake. I am going to go out and check the vacuum line on top of the FPR nad also check the direct pressure fromt he suplly from fuel pump. I will post findings in about 30 minutes.

Looks like I am off to the auto parts store. The fitting on that fuel filter is a flared fitting nad my test gage set does not have a fitting the right size. Hoping the APS does. I guess I will pick up a manual as well. I cannot figure out where the FPR is exactly to check for the gas smell in the vac lines. I feel like an idiot,but it looks like it could be one of two choices.

Guess I will report back in about an hour.

Looks like I am off to the auto parts store. The fitting on that fuel filter is a flared fitting nad my test gage set does not have a fitting the right size. Hoping the APS does. I guess I will pick up a manual as well. I cannot figure out where the FPR is exactly to check for the gas smell in the vac lines. I feel like an idiot,but it looks like it could be one of two choices.

Guess I will report back in about an hour.

The fuel pressure regulator is inside the upper intake cover.

Does your have two wires to the inside injector unit or 12 wires to top of cover.

The fuel pressure regulator has no rubber vacuum line on it.

Oh boy, I found a fitting to work on the filter. Got everything hooked up (used my fluke with pressure transducer this time). With cranking the car, I am getting 57 psi tops. I cannot believe this is the problem again. The pump that is in there is only 2 months old and is the Delphi unit. I am really irritated about having to drop this stupid tank again, but I am getting pretty effiecient at it now!

What amazes me though is why I have not been having any problems with it starting if pressure is this low off the pump? Does this still make sense, or could I be overlooking something stupid?

Verify fuel pressure with a gauge.

Also check voltage and power and ground to fuel pump.
The Delphi-AC pump should be under warranty if it is bad.
Good luck and let us know how it goes.
Good Luck

Went to town and exchanged my 2 month old Delphi pump for a new one. Brought it back, installed it and checked pressure again at the filter. 80+ psi! I am still shaking my head about a pump like this failing in two months, btu the truck seems to be running like a champ again.

Thanks for all the help and information. It really helped me stay on track as I would have still tried to discount the pump if I had not been guided to check the actual pump discharge pressure and not just by crimping off the return line.

One note though for any who see this and do this in the future. You need to add a fitting to your collection in your fuel pressure test gage set (assuming you have one to start). The filter fitting is a flared fitting and as such will not thread to anything other than a flare fitting. You should be able to get these brass flare fittings from your favorite hardware store. I got a 90 deg fitting going from flare to NPT. I think it is 3/8" but I will try to double check and report back. Right now, I need to get home.

So then, any ideas on my other 2 posts. My transfer case problem is really bothering me as my wife is about to head west to SD. Would like to have the 4WD option again! ;-)

I also am looking for a recommendation on shocks, but have a separate thread for that.

Cheers all!

Shane

You are welcome and.
Thanks for posting back how it went.
Glad you got it running.

I extra thing do not run your tank low or below a 1/4 ttank of fuel.
Low fuel is hard on fuel pumps.
Good luck