I have been working on a 1991 Chevrolet 2500 with a 5.7. I have replaced the fuel pump, Ignition control module, MAP sensor and fuel pressure regulator. The vehicle has a good spray pattern while cranking the engine over, but will not start. It is getting spark and fuel with the proper pressure. If I unplug the vacuum line from the back of the throttle body to the Map sensor and plug it at the TBI, then pour a little gas in the TBI it will start and stay running, but will not start unless gas is poured into the TBI. It is also blowing alot of white smoke if I try starting it with everything hooked up the way its supposed to be. Any help would be appreciated as I am completely out of idea's at this point and time.

Thanks for any help in this matter
Corey

 

you checked fuel pressure, you checked for spark, injectors are firing.
now the only thing you havnt checked is compression but you can get it running so that kind of takes care of that.
lets start with the basics, when it is running is it lacking any power?
hows it running?
have you tried holding the pedal down to the floor to start it? as weird as it sounds it puts the ecm into "clear flood mode"
when it is running do you notice black smoke out of the tail pipe?


Troubleshooting sensors and how to test

 

I have not pulled it out of the shop and took it for a drive while it is running, but when I hit the throttle it responds very well with out any hesitation. When idling its running smooth. When I hold the pedal down with the MAP sensor vacuum hooked up it back fires through the TBI. When I do it with it unhooked, if i crank on it long enough it will eventually start and run, but the minute I hook the vacuum back up it runs for 10 to 20 seconds and then dies. As far as smoke, it blows white with a strong odor of fuel either way, trying to start it with the MAP sensor hooked up or with it disconnected.

 

when you leave it unhooked are you keeping the plug plugged in? if so i would test that just to see where its sitting at (good or bad) normally if you unplug a sensor it will set the engine into a safe mode for that sensor as to limit the possible damage to the engine. does it trip a "check engine light"?
Obd1 code retrieval and decoding

 

Im leaving the electrical connector plugged in, just unhooking the vacuum from the back of the TBI. Its not setting any check engine lights or codes.

 

unhook the sensors pigtail

 

I unplugged the pigtail and no change, I also unhooked the exhaust to rule out a plugged muffler and no change. I also noticed that while the exhaust was unhooked at the manifold Im getting raw fuel running out and onto the floor of the shop.

 

check your coolant temp sensor with that first link i posted. it might be bad and dumping fuel, but i just had a similar issue with a truck i was just working on. long story short i tested every sensor even swapped a few with the ones on my 94 and added new fuel. battery died so i swapped it with the one on my 94, once i changed batteries she started right up and ran great. put the old battery back in after a charge and she still ran great. i never figured out what the issue was. summed it up to either bad fuel that worked its way out or a bad battery connection.

 

Havent got the temp sensor yet, but i just also wanted to put it out there that I started with the Map sensor unplugged to move it out of the shop and the minute I drop it in gear it dies out. So no power at all.