TBI and oil press. questions

[87swb]

how ya'll doing. been awhile since i posted but i need some info. first off my motor is 305 tbi. the oil pressure has been acting funny. start up is 60, warmed up driving 45 to 55 mph is around 30 and stopped and idled is a little above zero. doesn't knock or anything like that. could it be the sending unit? another thing is it idles kinda funny sometimes when ive been driving and warmed up, not at a cold startup though. when i'm stopped and sittin there it'll go up and down like it's loading up and sometimes it will go dead. or even when i back up and stop it will sometimes go dead. seems when im moving and start using my brakes to stop suddenly its worse bout going dead. and can someone explain the relationship between the oil pressure switch and the fuel system. i heard they were tied in together somehow and could this be a problem?

[VileZambonie]

Once the engine has started the oil pressure switch means nothing. On that note the oil pressure switch is not the same device used for the gauge. That is the oil pressure sending unit. As far as your stalling, and poor idle stability I would check the tbi base gasket quickly before you dig too deep.

[87swb]

now that you mention it you can hear sort of a whistle around the tbi gasket as if it leaks. so you think the oil pressure is ok? thanks for replying by the way.

[Jim Rockford]

try a new sender for the oil pressure , its cheap.

[Stewart G Griffin]

Viley, i thought that on a TBI system if the oil pressure drops below a certain level, the fuel pump will shut off?

[Jim Rockford]

No
 that is a myth, what it does is if for some reason the computer doesn't trigger the F/P relay, the oil pressure will engage the relay and let the truck start. Only way it would shut the engine off is if the computer doesn't activate the relay like its suppose to.

[Lt.Del]

i always thought the crank sensor allowed the fuel pump to operate, in case of accident, the crank sensor sees the crank stopped, and kills the fuel flow.

[VileZambonie]

As a back-up system to the fuel pump relay, the fuel pump can also be turned ON by the oil pressure switch. The oil pressure switch is a normally open switch which closes when oil pressure reaches about 28.0 kPa (4.0 psi). If the fuel pump relay fails, the oil pressure switch will power the fuel pump.

[87swb]

so my fp relay could be bad and i not know it...

[VileZambonie]

You'd have no key on engine off fuel pump and an extended crank.

[Jim Rockford]

no . the relay either works or it doesn't,  just change the sender. sometimes the sensor just gets weak.

[Stewart G Griffin]

So, in case of an accident and the ignition key is still on, what turns OFF the electric pump?

a) As far as the crank position sensor, i know that when i turn the key on my cavalier to the first position i can hear the pump hum, but engine has not cranked therefore the cps "sees" that the crank is not moving but the pump still works.   So, i'm not sure if that is the correct system to control the fuel pump.

[VileZambonie]

The ECM energizes the fuel pump with Key on engine off to prime the syatem. There is no crank sensor for our trucks, it uses the distributor reference pulse. The ecm will not turn off the fuel pump unless the engine stops. There is no inertia sensor.

[Stewart G Griffin]

i.c.,

It is because of reasons like this, i will be using carbs proudly, unequivically, and without apology for a long time to come;

Engine stops running---fuel pump stops pumping.  Simple.

[Captkaos]

Leaving the pump on isn't a big deal.  It only keeps the feed line presurized.  The regulator bleeds off the extra pressure and circulates it back to the tank.