Engine/Transmission (1994 - 1998) occasional engine miss after injector swap

[KMott]

Hi, i am getting ready to sell my '94 and pulled the upgrade 215 injectors out and put the stock injectors back in. The truck ran fine for about a week, and then it started to have a miss at idle- usually under load (in gear at idle with brake on or at stoplight etc). It only does it occasionally, seems like more if the truck was shut off for a short period of time and then restarted. It sounds just like if you cracked one injector (smokes white too when it misses) I assumed that the problem was an injector, so i put the 215s back in and it still did the same thing. I then put the stock injectors in again with new copper washers and still same thing. After talking to Piers, they told me it could be a line not on straight or bent. I then put a spare set of fuel lines off other truck and also changed fuel filter, and still same thing.

Truck starts great and has lots of power, runs just like normal other than the miss.

The truck has3 k gsk, #5 camplate, advanced timing, and entire fuel system had been gone through about 2 years ago. I also verified valve timing, fuel solenoid operation, inspected fuel system front to back and no obvious problems. Im running out of ideas, but i think it happened too soon after changing the injectors to start suspecting the injection pump?? It definately seems to be an injection problem since the truck runs so good otherwise. Thanks in advance for any help.

Kevin

 

[KMott]

Update- I replaced a portion of the supply and return lines and bypassed the fuel pre-heater, and truck seemed to be fixed, but then started acting up again after a few hours of running around. I was almost ready to pull the injection pump, but wanted to check over the supply first just to rule anything out. I attached a clear line into the filter head and ran the line into a can. When i fired the truck up it seemed to be pumping all clear and at a steady rate. I also check pressure off this fitting while driving and seemed to be between 20-35 psi depending on at idle or under load (but while truck wasn't acting up) The final test i did was disconnect the return line, and attach a clear hose onto the return from pump (run it into a can). It seemed to run very poorly when i started the truck, and as soon as i pinched the line off by hand the truck smoothed right out. Seems that the lift pump is occasionally getting too weak to supply enough fuel to the injection pump at idle. The overflow valve is almost brand new, and with shop air is popping around 25psi. It almost seems like its easier for the fuel to bypass the pump, than to enter it at idle?? So i stretched out the spring in the overflow temporarily and can update tomorrow on if it works or not. Seems like the real problem is the lift pump. Glad i did those last few tests, would have felt real stupid after bolting the spare injection pump on.

kevin