Bypassing low fuel pump relay

[chrism3784]

Is it harmful to a 90 mtx to bypass the low fuel pump relay and leave it as a 12v power wire to the fuel pump? I would do this by adding a jumper between the resistor located on the back firewall. I think this low/high fuel pump relay system is whats screwing up my car. Fuel pressure isn't enough or the high relay isn't not working and not bumping up the voltage to fuel pump when needed.

If bypassed where 12v is always sent to fuel pump when cars running, and pumps on full speed at all times, with this have any damaging effect to the fuel system? I put a new 155lpm fuel pump and brand new fuel pressure selenoid.

 

[SuperchargedSHOguy]

Make sure you verify voltages/FP before you do this.
I don't think you want to bypass it to be honest- sounds like you would burn up your pump...this is why they sell voltage regulators for aftermarket pumps- so they don't see full voltage all the time.
I've seen lots of aftermarket pumps needed to be sent back to manufacturer for rebuild (Aeromotive/magnafuel) because they were seeing 12-16volts all the time and it burnt up the motors in the fuel pump...

 

[K-Dawg]

It won't hurt anything, but I think you'll be wasting your time because that's not causing your problem.

 

[chrism3784]

ill bypass it later today and give it a drive to see if thats the problem, then come up with some solution. looking at the wiring diagrams, the regular taurus's with J module send full voltage to fuel pump at all times cars running. only taurus's with m module have the two fuel pump relays

 

[Bob Gervais]

Have you checked your fuel pressure at the rails?

It's important to understand flow vs. pressure here. Because your pump is getting full voltage and spinning faster, does not necessarily mean you'll get more pressure.

At any rate, I'm with Kelvin. I don't think this is your issue.

 

[K-Dawg]

ill bypass it later today and give it a drive to see if thats the problem, then come up with some solution. looking at the wiring diagrams, the regular taurus's with J module send full voltage to fuel pump at all times cars running. only taurus's with m module have the two fuel pump relays

What CCRM do you have?

The M CCRM is for 89-93 MTX cars. The J CCRM is for V6 ATX cars.

Only 89 and 90 SHOs had the two speed pump.

IIRC, using a bigger pump without the low speed capability and a stock regulator will actually cause lower fuel pressure.

 

[gmorrell]

IIRC, using a bigger pump without the low speed capability and a stock regulator will actually cause higher fuel pressure.

Fixed that for you.

Honestly, with a 155lph pump, defeating the two-speed drive might cause higher low-load fuel pressure due to over--running the stock FPR, but probably not, usually this becomes a problem with the 255lph pumps.

 

[chrism3784]

So I should be good with the 155lpg. Doesn't any taurus thats not an 89 90 sho send full voltage to pump at all times. I'm just not getting this system of low and high fuel flow. This may not be the problem, but its a good start diagnosing my problem.

Its not spark, so has to be fuel. I changed pump, press reg, and a year ago injectors. It really does feel that it lags cause of a drop in fuel pressure. I connected a test light to the fuel pump pnk/blk connector and I can see the difference from high voltage to low volts, the light will be brighter. I see it goes from full voltage to low voltage very flucuating, and when it stalls out its blinking back and forth very fastly. Maybe its the M relay box. I have 2, one from junkyard and one of them, stalls the car out just tapping it. The other may have a bad high fuel pump relay. I don't have the money for a new one, so my bypass method will at least let me know thats it.

Edit: Bypassed it, saw with my test light full voltage to fuel pump at all times. Didn't fix anything, still cuts out, a lot. It only seems to do it when cars warmed up. Then it gets really bad. Now I know at least its not that.

 

[K-Dawg]

Fixed that for you.

I meant lean. :angelnot: