Low Fuel pressure and HP

befarrer · Post by **befarrer** » Sun Feb 10, 2008 11:11 am

I found out the other day, that I have the wrong fuel pump in my Fiero. I have one for a 2.5L in it. I did replace the pump when I did my engine swap, I used a fuel pump for a 1990 Grand Am with the VIN A Quad 4 (HO), but the listing in our computer, and the manufacturers (Carter) website is wrong, it calls for a 24 GPH 12PSI pump for the Vin A Quad 4, and the 2.5 Grand Am, and a 16 GPH 50PSI pump for the Vin D Quad 4 (LO), the only difference between the VIN A and D Quad 4 is that the Vin D has lower compression pistons, smaller intake and exhaust manifolds, different cams, and lower flowing injectors. All other years and models of cars with the Vin A Quad 4 list the 50PSI pump, but 1990!! I ordered another pump, but I ordered one for a 92 Grand Am with the Vin A Quad 4 HO. It puts out 48-57GPH, and 78-92PSI (I dont know why there is a range??) I figured it would be better since the 92 and later Quad 4's have a different fuel pressure regulator, and I have that style regulator, and all Quad 4's are supposed to have 41-47PSI of fuel pressure at idle. This way I have more flow and pressure from the pump if needed for boost or nitrous.

Anyway, I have driven all 25,000KM since the swap with this 12PSI pump, where it was supposed to be 41-47PSI, I guess the low pressure must have affected my performance somewhat, but how much I wonder? Do you think I will feel a seat of the pants difference with the new pump?

The Dark Side of Will · Sun Feb 10, 2008 2:34 pm

If you had a pump that was only capable of 12 psi in your car, it would be VERY obvious.
As long as your car seems to drive ok and have the power it should, you have the right pump. I suspect an error in the catalog listing (have found them before).

p8ntman442 · Post by **p8ntman442** » Sun Feb 10, 2008 6:18 pm

Just check the pressure at the rail. No way would the car run with 12 psi. I had a fitting on a pump in my passat leak and at 25 psi at the rail instead of the rated 35, it would idle ok, but wouldnt move under its own power. I'd say you have the right pump, and the listing has the correct part number for the 45psi pump but the wrong text for the 12 psi pump. Also the range in pump volume/minute is probably from 12-14 volts, or varing on fuel system. A highly restrictive system would limit the flow more than a short low resistance system.

Bottom line is get a gauge on that schrader and confirm before dropping the tank. Then go aobut correcting the problem with the listing.

Pyrthian · Post by **Pyrthian** » Mon Feb 11, 2008 10:13 am

the pressure differences have to do with the pump & motor design. they work at the most effeicent at that pressure. does NOT mean they cannot attain other pressures. the pressure is handled by the regulator up at the fuel rail - NOT the pump. yes, I have doubt the impeller design of a 12 PSI pump will make it all the way to 40 - but I dont doubt it hits 30 pretty handily.

The Dark Side of Will · Mon Feb 11, 2008 10:59 am

Pressure and flow from a constant pump are inversely related.
Even if the 12 psi pump could hit 30 psi and allow the engine to idle (poorly), it wouldn't be able to maintain that pressure when fuel demand went up.

Like I said, if your car seems to run fine, then you have the right pump and it's a catalog error. If you want to check fuel pressure to feel safe, go right ahead.

befarrer · Post by **befarrer** » Mon Feb 11, 2008 2:21 pm

When I first did my swap, i had the fuel pressure checked, and it barely made his gauge move, it had maybe 15PSI at idle, so I replaced it, and had it checked again, since my symptons did not change at all, and it was higher, but it took awhile to build pressure, but still wasnt where it was supposed to be, that is when (and why) I replaced my regulator with the new style, which didnt have a schrader valve on it. The only way I can check the fuel pressure now is to tap into a line with an adapter or something, and they are like $60. But, I want to upgrade my fuel lines to AN fittings, and have a fitting for a permanent gauge there, so I will check the pressure there. I forgot that I had the pressure checked after I replaced the pump, but the garage did tell me with my old pump, it was very low the fuel pressure.

Blue Shift · Post by **Blue Shift** » Sun Mar 09, 2008 7:15 am

Drive everywhere at WOT. If it blows up, you had the wrong pump. :la:

At like 12 lbs, you'd have 1/4 the fuel delivery. Even if it made it up to ~30 lbs before it couldn't pump anymore, the [fuel pump] motor would probably be burning out if it's in a constant near-stall all the time. I'm pretty sure you'd feel something way off. You should just get a fuel pressure gauge with a T barbed fitting, and check it out to maks sure.