Gm tbi: im stumped

So, 1990 Silverado 5.7 tbi 700r4

Initial throttle tip in bog, down on power, "runs like crap" was the complaint. 

Checked ecu: swapped out with a known good. No chamge.

Map sensor: swapped out with a known good. No change. 

Ignition contol module: swapped out with a known good. No change. 

Timing in spec, and responds to throttle with change.

Tps: tested good, and swapped out with a known good for E36 M3s and giggles. No change.

The datastream (through winaldl) shows cts response is right, and all sensor readings in line with what they should be.

Tested for vaccum leaks. Fixes all, but no change.

When i say no change, i mean to the initial complaint. After swapping tbe map sensor around, its stopped idling. Stalls every time. 

The only thing that i cannot test are the hall effect sensor in the distributor, and fuel pressure. 

My gut says its out of time, or vacuum leak. But if thats the case, its hiding from me.

Sounds like fuel pump going away

Fuel pressure is usually the first thing to check

How about changing the fuel filter?

The only reason we didn't check the fuel pressure is because TBI uses a special funky adapter that we did to have. My buddies going to get the adapter tonight and check fuel pressure and get back to me. About the only thing that makes sense to me as well.

Yep, if the regulator is failing internally, fuel pressure won’t respond to changes in engine vacuum.

A dirty or partially clogged injector(s) could also be a culprit, but a fuel pressure test along with a new filter should help.

The TBI regulator does not use vacuum-referencing, it's a fixed pressure always.

Pressure should be around 9-13psi (though you'll want to be on the higher end of that).

They're a pretty stupid-simple system.  If all the sensors are doing the right thing, it just chugs along.

Try TunerPro for better datalogging than WinALDL can do.  www.gearhead-efi.com is a good source.  Lots of TBI stuff as well at www.thirdgen.org

It really does sound like a fuel pressure regulator problem.  As a test,  you can incrementally pinch the return line with a pair of vice grips to raise the pressure and see if throttle response gets better.  

Definitely swap out the fuel filter.   My 90 ran fine except for the same symptoms until one day it started stumbling while steady state.  When i disconnected the outlet side of the fuel filter I got essentially zero leakage,  but as soon as I cracked the fitting on the inlet the bottom of the truck was sprayed with gas; there was that much blockage. 

Unplug the EGR valve and try it.

Did that as well.

We tested the fuel pressure this morning. 4psi intermittent. Other times, dead.

So, pump and filter on the way.

Next time i see you remind me to beat it into your head that fuel pressure is number one test on any tbi issue.  

I had one actually run with 1psi.  

In reply to Patrick :

Yeah, wasted 5 hours by NOT checking the fuel pressure first.....

The fuel pump relay on the firewall is a known weak link too, the contacts get corroded and the pump doesn't get reliable power.  Probably worth swapping that and the filter out before you lift the bed. 

Note I don't suggest dropping the tank. Lifting the bed enough to swap the pump is a half day project all done with minimal chance of getting stuff in your eyes.  Dropping the tank is a miserable weekend of hating the truck. 

Be careful what pump you put in if it's in tank.  Cheap autozone ones the small hose from the pump to Hardline can't handle being submerged in ethanol.  Which is an excellent material choice for a piece of hose that will spend 100% of its life in a ~10% ethonal saturated environment.  It will swell and leak and you will be back to 4 psi in a year. 

Last time I did mine I bought a OEM pump for <$50.  That was 3 years ago and it's still fine.  The hose that came with was labeled as being good for immersion in ethanol blend based on the code I found printed on the hose. 

Alternatively you could source high quality hose for a cheaper pump.  The sock gasket and isolation oring swelled/disolved also but they really aren't necessary for the pump to work.

FWIW mine did exactly as you are describing.  Idled perfect but would stumble on any throttle.

This tomorrow when you have some spare time