still a dog...

[RawbDidIt]

Alright, I'm at a loss as to what else could be causing my problem. I've replaced quite a few parts and it's been getting better, but still just not 100% yet. The problem is hesitation, and bogging under acceleration from a stop, particularly while cold. It gets better after warming up, but still doesn't feel right until after 1500-2000 RPM. FYI: It's not throwing any codes.

What I've done so far:
replaced EGR valve
replaced fuel filter
replaced fuel pump
replaced fuel pressure regulator
tested fuel pressure, everything in spec. prior to replacing FPR, it was low, around 45-50# with engine off, key on, 50-55# while running. now it's 60-65# engine off, key on, no pressure drop when pump shuts off. around 60# while running.
Ran some seafoam through the last full tank of gasoline, no noticeable improvement.
Cleaned the throttle body while replacing the FPR, nice and clean, turns smoothly from stop to stop, nothing gumming up the edges of the plate
tested the throttle position sensor, both still and while tapping it, checked out as it should.
Replaced PCV and PCV tube boot/elbow
Tested for vacuum leak: I don't have a vacuum gauge, did the spray test with starting fluid at all likely culprits. none found.
New fluid in both differentials, new transmission shift solenoids. (I know this isn't likely to fix the problem, just covering my bases at this point)

Now, to be fair, the only thing that didn't deliver a noticeable improvement was the fuel pump, and I knew the fuel system was pretty well screwed when I bought the truck. My fuel filter was pouring garbage when I replaced it, I cleaned the tank when I replaced the pump, and put a new filter in again, just to cover my ass, and because they're cheap. The only thing in the fuel system that it could possibly be is the injectors (please correct me if I'm wrong here), and I almost replaced them too while I had the upper intake off of it, but they looked like they had already been replaced. Maybe some of you more tenured guys can help me out here, but I was under the impression that the Vortec had a fuel injection system that was all controlled electronically at the spider, with no electronics at each injector. When I pulled the upper intake, I was surprised to find the same spider I had expected, but each injector also had a pair of wires going to it. At the time this led me to believe that they were upgraded at some point in the past, and therefore were not likely to be the problem, is it possible these injectors were actually meant for a different engine, and therefore don't pass through the appropriate amount of fuel at lower RPMs? Is there a way to test this without replacing my fuel injectors? I'm at a bit of a loss here, I'm curious whether the heads may be causing this, but I do not want to pull my heads apart if I don't have to, and I don't want to pull my injectors if I don't have to as well.

Note: I will be replacing the lower end in the next year with a 383, so the entire top end will be transplanted to a new block. At that time, I'm planning on rebuilding the heads, but I want to do all of this at the same time if possible, which is why I'm hesitant to just do it now. This also begs the question, would a 383 benefit from higher output fuel injectors, or will I not notice a big difference from the stock injectors?

 

[letitsnow]

Maybe the tps isn't working right? Or a temp sensor is causing problems, being as how it is worse when cold.

If there are wires to each injector then yes, you already have the upgraded mpfi spider.

I'd pull the spark plugs and look at them one at a time, looking for weird color or gunked up.

 

[RichLo]

Have you done a compression check yet? If you haven't, try doing it both with a cold engine and a hot engine and compare the results. There could be a sticking lifter or out of spec valve adjustment. This usually isn't a problem with hydraulic lifters unless you hear an audible tick from a stuck lifter but since it's been worked on by a PO (spider injector upgrade) somebody could have tried adjusting the valves and done it wrong or while the engine was hot causing them to fall out of spec cold.

I doubt its the problem but you've gone through the rest of the engine thoroughly already and its a relatively quick check.

EDIT: wait have you done anything to the ignition system yet?

 

[tahoe.mkIII]

Wires going to each injector sounds like the upgraded mpfi spider...not sure if you have checked it, but a bad coolant temp sensor can cause some of the hesitation and shifting issues as well

 

[SAATR]

MAF and MAP sensors would both be suspect. Clean the MAF and retest. Also could pickup a good, used MAP sensor at any scrapyard as the design didn't change until at least 2010. MAF can make all sorts of weirdness occur, but without a scanner capable of watching live data, there's no verifying operation. Temporarily swapping with a known good unit is a quick troubleshooting method. Timing moving around due to a worn distributor gear could account for the sudden jump above 2000rpm. Knock sensor could also be pulling timing due to a malfunction or something causing false knock, such as a loose heat shield, bolt, etc.

 

[Phillip Buessing]

Second what SAATR says about the MAF, clean it, especially if you run an oiled air filter like a K&N. You can actually just unplug it and take it for a spin and see if it runs better. Check your cmp retard with a scan tool, make sure it’s around 0*

 

[Schurkey]

Seems very silly to me to replace a heap of parts, without ever connecting a scan tool to verify operation.

The answer--or at least clue(s)--are almost always in the data stream somewhere. Maybe you even find some stored codes.

 

[97K1500Silverado]

Have you replaced o2 sensors?

Catalytic converters could be clogged, but that usually causes no power at 2k and above.

I still have original mechanical spider and truck runs excellent with 209k on it. There’s a few reviews on Amazon about the mpfi upgrade causing problems after being installed a few years.

I second the map and maf sensors, swap to a junkyard spare.

 

[RawbDidIt]

So sorry I haven't updated, never got notifications that anybody responded. To answer all questions:

Replaced all plugs and wires when I bought it, forgot to put that in the list of maintenance. Plugs showed signs of running lean, not a surprise considering the low fuel pressure.

Replaced "whole heap" of parts based on diagnostics on the fuel system, I don't have a scanning tool other than a cheap obd scanner, and it's not throwing codes, but the fuel pressure was low, so I worked from cheap and easy to expensive and hard getting better and better results until it was within spec. Turns it it needed all of those parts to make the fuel system run properly.

Just replaced air filter, MAF sensor and air inlet temperature sensor yesterday (could've cleaned the MAF, not sure if it would have helped or not, I just replaced it instead, I'm replacing the lower end of the engine soon, and don't want old problems in the way of diagnosing potential new problems). With the new intake system sensors, it has the power I expect from it. It'll chirp tires from launch, and with a limited slip differential, and 33"x10" tires, that's about as good as I can expect for now. I imagine a bit of power has been lost in the past 22 years, and 220k miles.

Thanks for confirming the upgraded fuel injectors, that's a load off my mind.

Thanks for all of the responses.

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