Could someone please explain to me the purpose of

[arkie dem]

in the fuel tank fuel pumps on newer FI autos?

Had one quit on the wifes car, and I about fainted when pricing a new pump.

[HamstersFromHell]

1. Older cars with engine mounted pumps...

were also carbureted rather than fuel injected. Carbs. only require somehwere around 2-4 psi of fuel pressure, so the system was designed more so that the (mechanical) fuel pump could draw gas from the tank up to the engine, then push it the short distance up to the carb. under low pressure. The float in the carb. then kept the fuel level in the carb. at the correct level against a low pressure source.

Today's FI systems put it in the tank because it's easier to build a pump which doesn't have to draw fuel uphill a long distance and then pressurize it as well to the 35-50 psi needed to operate an injector. These systems also have a fuel pressure regulator as well as a return line back to the tank (this is where the excess fuel goes). Since FI systems can't have a "bowl" to store fuel locally close to the engine.

Mechanical pumps supply one "squirt" of fuel per 2 crankshaft revolutions (they usually run off a lobe on the camshaft, running at 1/2 crankshaft speed). This pulsing would cause no end of problems in maintaining a constant pressure in the fuel lines and accurate metering and control of fuel delivered to the cylinders with a FI system. In contrast, a electric pump running all the time supplying a constant regulated pressure insures very exact metering of fuel...something good performance and good emissions require.

Note also that Honda used electric low pressure pumps mounted at or in the tank for their carbureted cars for years...with a similar price to today's FI pumps.

Hope this helps!

[arkie dem]

2. Thanks

I understand the reasoning behind pushing the fuel instead of the pulling effect of the old mechanical pumps.

Why can't the manufactures mount the electric pumps between the tank and the fuel rail? This in the tank business seems unneccesary and an extra labor expense.

[HamstersFromHell]

3. Don't quote me on this but...

I think it's done for two reasons.

Cheaper to make. (Don't have to pull gas any distance at all, so no need to have a check valve to keep the pump from losing "prime".)

No need to "armor" the pump to make it resistant to road objects...this would also reduce cost a bit more. It's relatively "safe" inside the tank.

Just my thoughts on it anyway, for what they're worth.

[CO Liberal]

4. I Agree They Can Be Expensive

I had one go out on my '96 Isuzu Hombre pickup - the garage said Isuzu wanted $600 for the part!!! I asked them if a Chevy part would fit, since the Isuzu Hombre was based on the Chevy S10. They checked and found out the pumps were identical, except the Chevt pump was only $150!!

Needless to say, my Isuzu went back on the road with a Chevy fuel pump in its gas tank.

[arkie dem]

5. Thats Insane

I know it happens seems like the part suppliers like to gouge on the foriegn auto parts.

I called several parts stores concerning the fuel pump for my wifes car. The AC Delco pumps were $100 higher than the Delphi pumps. Considering Delphi was once owned by GM I figured all the pumps were made by Delphi, but who knows...???

The consumer takes in the shorts again...!!!

[CO Liberal]

6. You Have to Shop Around

The garage I took My Isuzu was willing to work with me. The Chevy dealer wanted $200-plus for the pump, but when I called around I found the exace same pump at Auto-Zone for $150. I went and got it and the garage installed it for me.

[arkie dem]

7. That's great to find a garage willing

to do that. Most make big bucks of parts mark-up. My wife found a shop that would replace the pump for $120 plus the list price of the pump, which is what I would have paid for it. I'd be willing to bet the shop buys the same pump for around half of what list price is. Had her car towed there yesterday, no way I would change it for $120. The difference between working in your driveway with set of jackstands and having a well equipped shop with a nice lift...!!!