ted-hurst

I have a 1988 Pontiac Formula 5.0 TPI/700R4. On start up the fuel pump comes on for a short bit (like 5-10 seconds) then shuts off, then once the engine starts it stays on continuous. My question is, is there a fuel pump relay that allows the fuel pump to come on and stay on or lets the initial pump run last a little longer to build a little more fuel pressure.




Thanks
Ted

RBob

iTrader: (1)

Look for a 'hot fuel module' from a truck. It is an additional piece in parallel with the relay and oil switch. At key-on, engine-off it will run the pump for 20 seconds.

Note that a larger then stock fuel pump can cause the drive transistor to burn out.

RBob.

Abubaca

iTrader: (3)

Sounds like your pump is working as it should. I believe the few second "prime" is determined by the ECM. There IS a relay, but it doesn't turn anything on or off. It just behaves like....well....a relay. Basically just a glorified switch. On my 89 it's mounted in the driver side corner on the firewall by the brake booster.

OrangeBird

iTrader: (1)

I'm sorry my Friend , but this is an inaccurate statement .

The relay does indeed turn something on or off , it turns on the fuel pump for the two second prime at startup . What the question this thread poses is this ; Why do you want to alter the pump's running parameters ? Is there some deficiency in the way it starts/runs that is causing the interest in making the pump run longer at key on ?

ted-hurst

EVERYTHING in the fuel system is new and stock but on a cold start it takes a bit to start. If I turn on the key, turn it off and turn on again it starts fine. I'm just looking for something that'll allow the fuel pump to run a bit longer and wasn't sure if it was the ECM or Relay that controls fuel pump. When it's running I have around 42-43 psi and when it's running it runs fine.


Ted

OrangeBird

iTrader: (1)

Hi Ted , The way it works is this , the ECM "knows" that you just turned the key to the on position and it sends a command (12 volts DC but at a tiny current , on the order of milliamps) to the relay's coil to close it's switching contacts and send 12 volts at at least a few amps or so to the fuel pump . So in effect yes the ecm and the relay are both controlling the pump at this point (key on but engine not yet started) , the ECM "tells" the relay to engage and turn on the pump for a couple seconds .

Now , if your having cold start issues the very first thing you've gotta do is to measure the fuel pressure when it's just sitting there cold , not having run for several hours or more .Yes indeed if everythings truly OK with the fuel system there is supposed to be pressure in the fuel rail , and if there isn't it's a sign that either an injector is slightly leaking , the fuel pressure regulator is slightly leaking , or the one way check valve in the pump is slightly leaking . Now if you do have pressure when sitting cold , note the pressure and then turn the key to on , does the pressure rise or stay relatively the same . Say you find 40 ish PSI when it's sitting cold that only goes to 44 ish when you put the key on , the rail would be considered to be holding pressure . You said it runs with 44 ish PSI but what it sits at when cold & not running is what is important here to your diagnosis .

Bottom line = if it has no or very low pressure while sitting parked & cold , and that's why it needs a longer pump run to start quickly , the problem is in the injectors , or regulator , or pump (barring , of course , an externally leaking fuel line , but I'm sure you'd have already noticed that if it were the cause) . If it sits with 40 ish PSI cold then your next check should be the temperature sensor that the ECM uses to determine fuel ratio based on engine temp. The ECM does have it's own dedicated temperature sensor and if the engine is cold the ECM "sees" that and richens the mixture for starting , just as a carb car would have a choke (in our case we have no carb , there is no choke , and the ECM just pulses the injectors a few milliseconds longer when the engine is cold) . If that sensor goes bad it can make the ECM "think" the engine is warmer than it actually is , causing no fuel richening at cold startup .

It is not normal to have to "double key" it to get a good cold start , so there is a problem even if each and every fuel system component has been replaced . Brand new is never , ever , to be taken as "Oh 100% it couldn't be that , it's brand new" as I have seen MANY failures of brand new components and the only "proof" of a properly functioning component is to actually see it in action , flawlessly performing it's designed for application .

The fuel pressure of the rail when several hours cold will be the most telling factor into what your next move should be ...

Abubaca

iTrader: (3)

I can see how my statement was taken wrong. (assuming we're on the same page.) What I meant by not turning anything on or off was that the relay was simply the on/off switch, and the ECM is what CONTROLLED it. Meaning the time, 2 seconds, 5 seconds, 20 seconds etc. was controlled by the ECM, and not the relay. The OP asked if the relay controlled the time.

Now....perhaps THAT statement is wrong....and if THAT is true....well then I'll change my post and I will have learned something.