89 5.7 TBI sputters under load once warm

[spooderman]

Hello, I'm working on a 89 chevy with a 350 TBI and I'm trying to figure out this issue. The truck starts up fine and idles normal, I can rev it no problem upon first start up but as soon as she starts to get warm and also once up to temp when I rev or accelerate she starts to sputter and has no power. She'll go only if I barely give it gas. The other thing that has me stumped is that once the truck is warm if I turn her off and back on right away and rev she has no problems revving but right after the first or second rev she goes back to sputtering and having no power. I'm thinking it's leaning out because it hasn't backfired or popped at all.
I have replaced spark plugs, spark plug wires, fuel pump, and ignition control module. Tested the throttle position sensor and egr both are good. I also don't have any codes

If you guys have any ideas it would be greatly appreciated

 

[Road Trip]

Greetings spooderman,

Welcome to the GMT400 forum! We have a lot of members who are still driving their '88-'95 TBI
vehicles all over the world, so between everyone here I'm sure that this can get figured out.

Taking a stab at what you described, the engine is responsive when cold, but this goes away as
it warms up. Poor running cold, but good hot leads us to one area. But in your case, it sounds
like you may have a lazy/contaminated/worn out Oxygen Sensor that, once everything is up to
temp and the computer starts using it's (out of calibration) feedback, everything goes sideways
with the engine performance.

Q: How old is your O2 sensor? If recently replaced, what brand did you install? Did you buy the
new O2 sensor online, or from a Brick-&-Mortar store? While we are discussing the engine bay, what
is the state of the normal tune up parts? (plugs, wires, cap, rotor, etc)

All of the above play key roles in the quality of the feedback loop that your computer uses to
minimize emissions and also optimizing driveability. And as others will tell you, the O2 sensor
is like a spark plug, in that it's a wear item that requires periodic replacement. (Although if you
recently purchased the truck, the previous owner probably took the default approach: No SES light,
no problem! :0)

Let us know what's up with your O2 sensor & general tune up, and we'll go from there.

Cheers --

 

[JustinJ94FSB]

what procedure did you perform to test the EGR valve?

 

[Scooterwrench]

Have you checked/replaced the fuel filter? Trash in the filter will fall away from the element when you shut he truck off then after a few minutes of running the trash will get pushed back against the element and restrict flow.

 

[DeCaff2007]

I have this EXACT same problem. I'll be watching this thread to see if there's ever a solution.

 

[Schurkey]

89 chevy with a 350 TBI

The truck starts up fine and idles normal, I can rev it no problem upon first start up but as soon as she starts to get warm and also once up to temp when I rev or accelerate she starts to sputter and has no power. She'll go only if I barely give it gas. The other thing that has me stumped is that once the truck is warm if I turn her off and back on right away and rev she has no problems revving but right after the first or second rev she goes back to sputtering and having no power. I'm thinking it's leaning out because it hasn't backfired or popped at all.
I have replaced spark plugs, spark plug wires, fuel pump, and ignition control module. Tested the throttle position sensor and egr both are good. I also don't have any codes

I have this EXACT same problem. I'll be watching this thread to see if there's ever a solution.

The Usual Three:

1. Verify fuel pressure at prime, at idle, and under load. Most fuel pressure gauge assemblies have a push-button pressure release connected to crappy vinyl tubing. Route the tubing so it empties into a drain pan, then push the button while the engine is running. This simulates higher fuel demand if you can't check fuel pressure on the highway. Fuel pressure should remain steady even with fuel flowing down the pressure-relief tubing. How old is the fuel filter? Have you ever dumped a bottle of Chevron Techron Complete Fuel System Cleaner into the gas tank? (Recommended at every oil change.)
   
2. How old are the usual “tune-up” parts and procedures? Inspect/replace distributor cap, rotor, plug wires, spark plugs. Make sure the ignition coil will reliably fire a spark-tester calibrated for HEI when the coil is fully warm, and misted with water from a squirt-bottle. Cranking compression test of all cylinders while the spark plugs are removed. Verify EGR, PCV, EVAP, and Heated Air Intake (if used) systems for proper operation. Verify proper initial timing (TBI) and electronic spark advance (TBI and Vortec). Replace old O2 sensors unless you can PROVE that they're working properly—old O2 sensors get lazy, they don't provide accurate data, but they do provide “data” that fools people into thinking they're “working”.
   
3. Connect a scan tool (NOT a crappy “code reader”) and look for “codes”. More important, look at the data stream to verify EVERY sensor and computer output. Verify fuel trims during the time that the vehicle is not running properly. Look for misfire counts for each cylinder (OBD2 only.) “Codes” have official diagnostic procedures that will be found in the service manual set for your vehicle. The service manual set can be downloaded from the links in the Sticky thread section of the Engine forum on this web site.

 

[JustinJ94FSB]

There is a hose that comes out of your heater core pipes on the firewall and goes up to your intake manifold on the passenger side of the engine close to the firewall. If that fitting has coolant leaking from it, pull your distributor cap and look inside for corrosion. My truck was doing this exact same thing until one day it decided it wouldnt run at all anymore. Was a ton of crud built up on all the contacts in my distributor cap, and corrosion all over the top of the distributor. Replaced the cap/rotor and that fixed most of it. but i still noticed the issue where it runs great when i first start it and then it gets worse as it warms up to the point where it sputters. my father-in-law gave me a golf tee and told me to disconnect the hose going to my egr valve and stick the golf tee in the end of it. After doing that, it drove great. turns out even though my EGR appeared fine, and i could move the plunger and all of that, it was actually bad.