TBI question

[TommyD11730]

Guys, I have. 91 5.7 TBI powered K5 that runs and drives fine.
Recently I had an auto start added. It works fine with 1 exception. On the first start of the day it will crank and not start. If you cycle the auto start again it fires off. What I suspect is the fuel system is loosing prime (I mean how long should it hold prime to begin with). In an effort to confirm the theory I cycled the key on and off cold. However unlike other FI trucks I did not hear the fuel pump come on. (If it did it was Damn quiet!). Needless to say cycling they key did not help it and you still had to auto start it twice to get it to fire.
Then it dawned on me. Does the oil pressure sender have to read OP before the fuel pump relay closes? Just wondering if that is the case then extending the crank time (on auto start) would solve my problem.
Thanks!

 

[bp71k5]

The TBI oil pressure switch is a backup for the fuel pump relay. It keeps the pump running if the relay dies. If your relay is bad, that might be why it takes a bit to start and why you don't hear the fuel pump when you turn the key on.

My TBI system doesn't stay primed more than a few minutes. Assuming your relay is ok I'd try turning the ignition on and letting it stay for a few seconds before actually trying to start it.

 

[mckustomfab]

If the relay is bad you should be able to jump the single red wire hanging from the fuel pump relay back to 12v and you could check to see if the pump kicks on....

 

[Russell]

Sounds like your fuel pump relay is bad and that your oil pressure switch is doing all the work!

 

[TommyD11730]

Ok, I got to see this for myself. Does anyone have a link to a diagram of the fuel pump circuit? Oh and just so I know what I'm looking for is it a standard 5-pin relay under the hood?
Thanks!

 

[mckustomfab]

Here is a link to almost all the info you could want, download the wiring manual. I will upload a pic in a little while, I cant access photobucket from my work laptop.

Link

 

[ktmoutfront]

One other issue might be, the oil pressure switch does not come on first thing when you start cranking. Takes a few seconds for the pressure to build. If the auto start does not crank long enough, the fuel pump is not staying on. And on my 87, the fuel pump only runs for 2 seconds when the key is turned on and not cranked.

For some reson it runs for 15 seconds when the truck is turned off, but that is another issue.

 

[TommyD11730]

Thanks man. FWIW I can feel my relay click key on. I'll have to get someone behind the truck with young ears to listen!