99 Tahoe - Trouble Accelerating / Hesitation


jrbueiz
11-28-2008, 01:38 AM
Over the last month my 99 4WD Tahoe has been giving me problems
accelerating when I give it a heavy foot. When I hit the gas down it loses acceleration almost like it's
being starved of air or gas. When I hit the gas the engines just bogs down, begins to hesitate, and will not accelerate.

It'll drive ok if I'm light on the pedal; and the engine feels smoother than ever.

This morning I took it out on the interstate (after is sat overnight - cooled off) and ran it hard and it was MOVING - GREAT ACCELERATION. I left the truck running for an hour or so, driving around town and sitting idly in the drive way to let things warm up and I took it back out on the interstate. This time....the hesitation was back. Could my fuel pump be overheating? I changed it about 4 years ago. The symptoms only got worse after an 11 hour road trip. It got progressively worse over the last 4 hours of my trip. In the end I had to be light on the gas pedal to get it to speed; at speed it ran great, but if I had to accelerate for any reason (passing, inclines, etc.) it wouldn't GO! Just had to cruise.

Any ideas?

MT-2500
11-28-2008, 08:39 AM
How many miles on tune up?
How is fuel filter and air filter?
Good Ac delco plugs?
What engine?
What is engie vacuum at idel and at 2K RPM?

Airtex or el cheapo aftermarket fuel pump?

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 and fuel pressure testing guide line.
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.

j cAT
11-28-2008, 10:02 AM
in addition to the fuel pressure test,,,, the cat may be restricted...or any exhaust restictions/defective muffler etc...

j cAT
11-28-2008, 06:33 PM
Now if it would only let me delete my double post....:banghead:

click on edit then delete,then confirm delete down a bit on page

Reflash
11-30-2008, 10:16 AM
Another thing you can check is what amps the fuel pump draws, especially since you said it got worse after running for a while. Should be no more that 8-9amps IIRC for that model/year.

My opinion based on your symptoms is a restricted cat conv. Pull one of the 02 sensors...pre cat...and put a low pressure gauge in the hole using a rubber plug. Run the engine up to about 2K rpm and note the reading. Anything over 1 1/2" is suspect.

j cAT
11-30-2008, 11:37 AM
Another thing you can check is what amps the fuel pump draws, especially since you said it got worse after running for a while. Should be no more that 8-9amps IIRC for that model/year.

My opinion based on your symptoms is a restricted cat conv. Pull one of the 02 sensors...pre cat...and put a low pressure gauge in the hole using a rubber plug. Run the engine up to about 2K rpm and note the reading. Anything over 1 1/2" is suspect.

on that pressure reading what in PSI would the correct pressure be?

some have just operated the vehicle with the upstream O2 removed from exhaust and then test drive..

777stickman
11-30-2008, 07:22 PM
For some reason the edit button will not work for me. The quote button works fine, but that does me no good. I am not new to internet forums, in fact I use to be a moderator on one until I got tired of being not much more than a babysitter at a daycare.

The edit button just doesn't work for me. :frown:

If you're using Firefox browser it has issues with this forum. I have to use IE to post and/or edit.

jrbueiz
11-30-2008, 07:40 PM
How many miles on tune up?
How is fuel filter and air filter?
Good Ac delco plugs?
What engine?
What is engie vacuum at idel and at 2K RPM?

Airtex or el cheapo aftermarket fuel pump?



Here's what I have so far. I just changed the fuel filter (got one from Autozone) and cleaned and oiled the K&N. I cleaned the MAF. I am running AC Delco $6+ Platinum plugs with MSD ignition. 5.7L engine, just changed the oil and tuned it up about 6K miles ago.

I haven't checked the engine vacuum - I presume there's a gauge for that I can pick up at Autozone? The fuel pump in the truck right now (changed once), I was told was a GM fuel pump - the shop who replaced could have told me wrong though....who knows.

Anyway, this is what I got. The fuel pressure at engine idle is about 55psi, and at high revs it's 60psi. I ran the truck with the gauge attached and it ran at 60psi - 55psi when coming to a stop. Whenever I encountered my current problem on the interstate the pressure dropped to 50psi. Fuel Pump?

I've read and heard elsewhere that it could be the Catalytic Converters? I am hearing a woofing sound coming from under the truck that I did not hear before the trip. Could it be the Cats?

777stickman
11-30-2008, 08:04 PM
Another indication of restrictive exhaust (cats/muffler) is higher than normal coolant temp. If you don't have that then suspect fuel delivery.

The "spider" injectors on the 5.7 are also a suspect. The fuel pressure readings are a bit low also.

This reply was done trying the Safari browser.

j cAT
12-01-2008, 11:21 AM
the fuel is low..50 psi is not good..make sure the gauge is accurate if so the fuel pump/wiring/relay may have issues...so before replacing pump make sure the ground and feed to the pump is secure and correct...poor ground at fuel tank frame rail is known for this..

try operating the vehicle with the upsteam O2 sensors removed and see if this improves the hesitation...

if it does then you must replace the cats as well as the O2 sensors..the O2 sensors are rated for 100,000mi max life...

operating new expensive cats with old O2 sensors is a waste of your money/time..

jrbueiz
12-02-2008, 03:45 PM
I had to get back home yesterday after already missing a day of work.

I went and had the Cats checked out. The guy said they seemed ok other than a MASSIVE leak coming from their weld (replacement cats); he fixed that weld along with replacing the O2 Sensors (all 4). Thru in some complete fuel system cleaner. Also changed the fuel pump with a Bosch one from Advance Autoparts with lifetime warranty and the truck is not hesitating any more.

Could I have patched the problem temporarily, or did I fix it? The O2 sensors that were in there were the originals so I am sure they weren't helping the problem. The cats are about 2 or 3 years old. The entire exhaust system is cr@p, so I'll have to save up for a nice catback exhaust with headers.

I drove it home 11 hours, stopping only to refuel, and no hesitations. I'd call it good.

j cAT
12-02-2008, 04:33 PM
I had to get back home yesterday after already missing a day of work.

I went and had the Cats checked out. The guy said they seemed ok other than a MASSIVE leak coming from their weld (replacement cats); he fixed that weld along with replacing the O2 Sensors (all 4). Thru in some complete fuel system cleaner. Also changed the fuel pump with a Bosch one from Advance Autoparts with lifetime warranty and the truck is not hesitating any more.

Could I have patched the problem temporarily, or did I fix it? The O2 sensors that were in there were the originals so I am sure they weren't helping the problem. The cats are about 2 or 3 years old. The entire exhaust system is cr@p, so I'll have to save up for a nice catback exhaust with headers.

I drove it home 11 hours, stopping only to refuel, and no hesitations. I'd call it good.

this all sounds great..the only thing that is an issue is the BOSCH fuel pump is /has been a problem.. delphi pumps are recommended..

roon65
12-22-2008, 01:09 AM
I have the exact same problem on my 99 Tahoe. It starts out running great then within a mile or so I have to start feathering the accelerator to make it run and the farther I go the worse it gets. Here is another thing I can turn the ignition off for 10 seconds or so and it does ok for a couple of miles before it starts again. I have been dealing with it for 6 months.

I have replaced so far

fuel filter

map sensor

tps

coil

spark plugs

both oxygen sensors

ignition module

fuel pump (that went out on me).

radiator and thermostat (Guy at the shop said it was leaking and the thermostat was stuck open not allowing the engine get up to temp)

Oh yeah another thing when it's cold outside it does better. The hotter the worse it is. It is 19 degrees where I am right now and it drove great on the way home from work tonight.


Help me I'm tired of spending money!!!

MT-2500
12-22-2008, 10:34 AM
I have the exact same problem on my 99 Tahoe. It starts out running great then within a mile or so I have to start feathering the accelerator to make it run and the farther I go the worse it gets. Here is another thing I can turn the ignition off for 10 seconds or so and it does ok for a couple of miles before it starts again. I have been dealing with it for 6 months.

I have replaced so far

fuel filter

map sensor

tps

coil

spark plugs

both oxygen sensors

ignition module

fuel pump (that went out on me).

radiator and thermostat (Guy at the shop said it was leaking and the thermostat was stuck open not allowing the engine get up to temp)

Oh yeah another thing when it's cold outside it does better. The hotter the worse it is. It is 19 degrees where I am right now and it drove great on the way home from work tonight.


Help me I'm tired of spending money!!!

Welcome to AF
Proper testing before throwing parts at it.

On your problem start a New Thread/post
\Give all of you car info.
And someone will help get you started on some proper testing.

roon65
12-22-2008, 12:47 PM
5.7 liter

1999 chevy Tahoe

MT-2500
12-22-2008, 12:56 PM
5.7 liter

1999 chevy Tahoe


The New tread button is upper left.http://www.automotiveforums.com/vbulletin/newthread.php?do=newthread&f=1452

kacey4life
03-08-2011, 05:43 PM
I had a similar problem with my 99 tahoe, I replaced:
the fuel filter twice,
new distributor, with cap and rotor
and finally a new fuel pump. It was the fuel pump that was actually causing the problem. I hope this saves someone some time and money in parts.

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