albinoplatypus

Is it possible for a fuel pump to run slow rather than just stop?

here's the details. its an '89 pickup 3VZE. the other day it was idling rough in the morning, then eventually just shut off and wouldn't start back up. spark was fine and fuel pump is running. it would start up for about a second then die. I replaced the fuel filter (which looked like straight mud when i poured it out) and it ran great for a minute or 2, then started idling rough again and died. I pulled the new filter just to be sure it didnt clog again but it was still nice and clean. I switched out the air flow meter today with a known good one from my '95 4runner and it made no difference. after i put the old AFM back on i did figure out something.

If i stick my hand in the AFM and open the flap a little while someone starts it then it will start up and run fine. As soon as i let go of the flap it will die again.

I really cant figure out what that means, I'm just not that good with all the sensors and stuff involved in EFI engines.

Any help is greatly appreciated.

7m4x4dude

Yes it can run slow if the pump bushings are starting to.seize to the pump shaft. Ie tolerence is to tight.
Or if the pump is getting the wrong voltage. Most pumps run 12v's. I know the supra ran a two stage pump. Not sure if toyota did it with any other vehicles. They put a resistor inline with the main power source so it ran at 6v's. When the ecu wanted more fuel flow /pressure it would turn a relay on which allowed 12v's to bypass the resistor hence allowing the pump to go full speed. .


Tho the way the flapper door afm works is it provides power to the fuel pump(iirc) while the door to the afm is open. While the motor is running. But the circuit opening relay provides power while key is in ign/ motor off for about thirty seconds. Just long enough to prime pressure and allow you to crank the engine which forces air through the afm opening the door powering the fuel pump..

Check the rubber tube from the afm to the throttle.body for cracks and air leaks. If it is not sealed it creates a secondary air intake that by passes the afm. If enough air goes around the door and not through it. The door closes shutting the fuel pump back off..

annoyingrob

yeah if the wiring is corroded they can run slower, and not build up proper pressure. I had a problem like that on my Supra. Was a pain to diagnose. I ended up installing a fuel pressure gauge in my dash, and re-wired the pump.

It's possible that the filter sock in the tank is totally dirty too, and the pump isn't sucking properly. You say your filter was a mess, imagine all that gunk in the tank too.

albinoplatypus

thanks for the info. my intake pipe is actually in pretty rough shape. ive duct taped it up multiple times so ill check and see if i have any new cracks. i should have mentioned though it does the same thing whether the fuel pump is jumped or not so i assume the fuel pump switch in the afm if working properly

albinoplatypus

The junk in the tank is a good lead honestly. I've actually changed my fuel filter twice in the last year or so, and i've had to pull and clean the in-side of my injectors 2 or 3 times too. i guess i need to pull the tank and check it out.

Thanks

TNRabbit

You need to replace that intake tube, OR do an ISR mod.

I replaced my fuel pump after @ 260,000 miles & it made a HUGE difference in startup. They do just get "tired" after so many years. Pretty amazing that one can run that many cycles & still even WORK~

albinoplatypus

well i went ahead and pulled the tank. it was pretty rough in there so im gonna clean that out, put a new pump in, and clean my injectors.

By the way what's an ISR mod?

annoyingrob

Just for future reference, it MIGHT be easier for you to just remove the bed rather than drop the tank. The bed is just 6 bolts, three screws on the fuel filler, and a few wires, then it comes right off.

subyracer05

yeah it can run slow

albinoplatypus

Yeah in hindsight i really should have taken the bed off. im thinking about taking it off to put the tank back in.

scope103

If holding the VAF flap in the "running" position lets the truck run fine, what would make you think the pump was running "slow"? The pump is fine; as long as it gets power through the COR triggered by the VAF.

Now, jumpering FP to B+ IS the test for this, but you reported that it also stopped after a few seconds when jumpered. Sorry, the only way to reconcile these two observations is that you jumpered the pump incorrectly. (I suppose you could have some sort of intermittent piece of dirt moving around inside the tank just to mess with your head, but in the world of "most likely causes" this isn't one of them.)

If you actually jumpered it "correctly," you may have a wiring problem to the diagnostic connector (use your multimeter to check for 12v at B+ and continuity to ground at FP). But as long as your finger on the VAF vane "cures" the problem, well, you're pointing right at what needs fixing.

albinoplatypus

sorry let me clarify. with the fuel pump jumped, the engine will stop running after a few seconds, but the fuel pump is still running. I found out that the fuel pump was pretty much gone today. i had to bang on the tank to get the pump started up today which is the main reason i went ahead and pulled the fuel tank.

The AFM thing was just an observation but i dont really understand the cause. as long as the flap is held slightly open the truck will run. once closed it will sputted and die. this happens regardless of whether the fuel pump is running or not. I'm leaning towards an air leak being the cause but i haven't check it out yet. Im pretty sure i will be doing the ISR Mod regardless before putting everything back together.