Chevy 454 engine in 96 GMC suburban miss

[GMC454K2500]

I am new to the forum but have 25 years experience as a do it yourself mechanic maintaining my own vehicles.

My Suburban has 125K miles on it now and has recently developed a miss under load. I did the usual tune up things, plugs, wires, cap, rotor, fuel filter, but the problem persists. After getting to the end of my patience with replacing parts and not fixing the problem, I took it to a mechanic I trust and he has had it for a week trying to find the problem. I know he has tested the EGR system and it tested good, and he cleaned the injectors, the problem persists.

I was noticing the miss mostly under load, such as when driving at an RPM where the transmission is just about to downshift, or when driving up hills and very noticeably when towing. The miss manifests as a jerking type of power loss. Even though it feels kinda like a failure to downshift problem, the problem continues if you romp down on it and make it downshift.

Any ideas on how to fix the problem? I used high quality aftermarket replacement parts for tune up.

[GMC454K2500]

My Suburban has 125K miles on it now and has recently developed a miss under load. I did the usual tune up things, plugs, wires, cap, rotor, fuel filter, but the problem persists. After getting to the end of my patience with replacing parts and not fixing the problem, I took it to a mechanic I trust and he has had it for a week trying to find the problem. I know he has tested the EGR system and it tested good, and he cleaned the injectors, the problem persists.

I was noticing the miss mostly under load, such as when driving at an RPM where the transmission is just about to downshift, or when driving up hills and very noticeably when towing. The miss manifests as a jerking type of power loss. Even though it feels kinda like a failure to downshift problem, the problem continues if you romp down on it and make it downshift.

Any ideas on how to fix the problem? I used high quality aftermarket replacement parts for tune up.

[GMC454K2500]

Mechanic found nothing. Wiring to sensors inspected, ok. O2 sensor readings, ok. no vacuum leaks found. Fuel injectors cleaned.

I am really suspecting a fuel delivery problem but not sure where or why.

Fuel pump
In tank filter/strainer
Throttle position sensor
Obstruction in fuel rails

I'm about to go old school and jerk the injection manifold and distributor and put in a Holley and HEI.

Any ideas before I go that route?

[kronhenry]

you say its doing it during loads and down shift if its jerky it could be your torque converter getting ready to fail

[zombie1969]

Have your mec take a look at the fuel trim with a scanner.It should be 128 or very close to 128 within one or two.If its higher than 128 its running lean and you should verify fuel pressure is good.If fuel pressure is good suspect bad injectors.If the fuel trim is good suspect a bad dizzy.Of course check the basics first.You did the tune up so can rule that out but the condition of the engine should be checked with a compression test and check for vacuum leaks.

[GMC454K2500]

I am considering sticking in a HEI distributor temporarily to try to determine if I have a fuel problem or ignition problem. I have the truck back from the mechanic now so diagnostics will be without advanced equipment.