TBI woes

[66gmcguy]

Hello folks. I normally post over in the 73-87 section, but since I have a TBI setup in my 1982 GMC, I figured I'd get more help over here, since 1987 was the first year for TBI trucks.

A few months ago I bought a 1987 Chevy R20 with a TBI 350 and manual transmission to use as a parts truck for my 1982 GMC. It seemed to run okay but the throttle was extremely sticky due to the flappers in the throttle body sticking. I tore down the TBI, bushed the throttle shafts, replaced the injectors and IAC valve (both with Standard Motor Products), and did a thorough cleaning, making sure to clean all vacuum ports and the IAC passage. I replaced every gasket, seal, and the FPR diaphragm. After doing so, I've never really been able to get this thing to run right since then. Here are a few problems I can't get ironed out. I have WinALDL and no codes are present.

At idle, the RPM fluctuates up and down. If I depress the throttle a little bit, the RPMs still fluctuate, just in a higher RPM range than in idle. I know this is either due to low fuel pressure, a vacuum leak, an IAC problem, or TPS problem. Thing is, I have a new fuel pump + hose, all new fuel lines, fuel filter, all new intake gaskets, and new vacuum lines. I've checked fuel pressure (11.5 PSI), and checked for vacuum leaks by spraying starter fluid around every possible source for a leak and found nothing. I've tried plugging every vacuum port with no luck. I have the timing set at 0 degrees (with the bypass unplugged). I must have done the IAC reset procedure 50 times, I've put the old injectors and IAC back on, replaced the TPS and set it to .5 volts at idle, all with no change. The thing is, if I turn the idle screw up so it revs around 900 at idle, it seems to stabilize (only when in closed loop), but the IAC counts are zero. Another thing I can do that seems to help is unplug the CTS which richens the A/F mixture. I can't tell if it's the richer mixture or the slight raise in RPM that's fixing the problem though. When the engine is cold it idles fine until the coolant temp reaches maybe 100ish degrees, then I get the fluctuating idle (until it goes into closed loop mode.)

Another thing is when accelerating, I have a very distinct, but momentary misfire at a certain RPM. It only happens under light to moderate acceleration though, right at around 2200-2300 RPM, and it's only for a split second. The best way to describe it is it feels like the engine shuts off for a split second then turns back on almost instantly. It doesn't feel like a typical lean-sputter, or backfire. I thought maybe this was a TPS problem, but it does it on all three TPS sensors that I've tried, all at the same RPM under same conditions.

The last thing. When the engine is cold, it drives like crap and has no power it feels like, and every now and then I get a CEL but with no codes. If I shut the engine off and shut it back on right away, the CEL turns off, and it seems to be fine after that. None of this happens after it's warmed up beyond maybe 120-150 degrees. As I mentioned above, it idles fine when cold.

Since having these problems, I've replaced the TPS, MAP, CTS, knock sensor, EGR valve, PCV valve, O2 sensor (3 wire unit), wire harness (Painful Performance Swap harness), distributor (and all internal components), spark plugs, wires, every single intake/exhaust gasket, etc. Pretty much everything short of the EGR solenoid, the ESC, and the PROM. All components used apart from the IAC and injectors are all ACdelco. I've even switched out the computer from another truck that I have with no luck.

I know this is long winded, but I am genuinely stumped, and would love some input. As mentioned, I have WinALDL and can datalog and post it on here if needed. I'm also open to the possibility that I did something wrong by either not doing a test/reset procedure properly or didn't install something correctly, etc, so if you think I should recheck something that I've already messed with, just let me know.