89 TBI problem

I have a TBI related question. My truck now will not start on its own, if you pour a little fuel down the throat it fires right up like it's not getting the initial prime shot. At first I figured fuel filter, or pump. changed the filter and had no change. the fuel pressure is 12lbs running and does not drop even when the vehicle starves of fuel then stalls. If you open the TPS up when it starts to stall, you see the injectors pattern get larger and the vehicle doen't stall right away,but you must keep increasing the amount the TPS is open and it wont stall untill the tps is eventually bottomed out (wot position) and the fuel just dribbles out. This led me to believe that it is a volume issue, but yet the pressure did not drop with the snap on tester inline. the box is currently off, and the fuel pump is easy to get to, so I changed it as well, with no change. all this led me to believe that there was an obstruction of some sort in the tbi unit, so I disassembled the tbi unit removed injectors and what not and there is no visible obstructions and all orifices were blown out with compressed air including feed line from tank. Both injectors seem to work fine, you increase the pulse rate and they open right up, but then the pressure drops at the injector nozzle and it just runs out of fuel at the injector, the line still has full pressure! I could see no wrong inside the tbi assembly but for some reason fuel is not keeping volume between the inlet fitting of the tbi and the injector. when I break the feed line loose after it runs the tbi out of fuel, it still has line pressure in it but yet fuel ran out at the injector. I am lost, it doesnt make any sense. please help as this was not a gradual thing, one day it ran, next day it didn't

If you look at this diagram http://www.slantsix.org/articles/dibiase_efi/gm-ign-module-schm.jpg you will see how the basic GM TBI Fuel Injection works (note that the ECM pin number do not match your truck).

The signal from the pickup coil is converted to digital and that signal is sent back to the ECM which in turn uses it to control the fuel injectors.

A bad pickup coil or igniton module could be causing your issue. The pickup coil can become cracked and then corrodes. The magnet on the pole could be also be bad causing a weak signal.

Also see if the dist has too much play in the upper shaft bushing since this will cause the pickup coil signal to be weak.

//2000CAYukon

Ohm test the injectors.

Fuel pressure is too low. FPR might be at fault. Pinch return line to get static pressure readings for pump.

Also, pull vacuum line from FPR, and smell for gas in the vacuum. Smell means ruptured diaphragm in FPR.

No gas in vacuum line? FPR spring might be weak.

Ohm test the injectors.

Fuel pressure is too low. FPR might be at fault. Pinch return line to get static pressure readings for pump.

Also, pull vacuum line from FPR, and smell for gas in the vacuum. Smell means ruptured diaphragm in FPR.

No gas in vacuum line? FPR spring might be weak.

Pressure is fine, it doesn't have a vacuum line to the regulator like a multi port has, but the whole tbi unit has been dissasembled and checked all components including the diaphragm and all are fine. what I've come up with this afternoon is that voltage is dropping at the injector gradually. when cranking, pulse looks good, but as soon as it fires up(by adding fuel by hand) the pulse almost seems steady and if I open the tps up the voltage drops even more quickly the more open the tps is. the injectors ohm test fine as well. so I guess tommorow I'll start trying a few other things now that it's getting narrowed down.

Ignition timing? Distributor movement?

If it happened all-of-a-sudden, it's gotta' be electrical. Mechanical stuff doesn't happen like that - usually mechanicals become a problem gradually.

today I swapped the distributor and ecm out of my other 89 that runs into the dead truck and it changed nothing. the components from the non running truck work fine on the other as well. I guess I could have a broken wire somewhere maybe? that carb is looking better and better.

I hope Grumpy posts his solution. I think I have the same problem. 90 k1500 starts but will die. The pump runs fine will start and run with carb cleaner.

It might be better to start a new thread. You need a special tool to check the fuel pressure and you need to see if the pressure is good (9 - 13 PSI for TBI).

//2000CAYukon

All this was caused by a faulty MAP sensor, never thought it could do all these but it did so there ya go.