Fuel pressure testing 88 Civic CRX DX

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tich - 14 Jul 2005 10:19 GMT

Just wanted to add to the growing content in this group for
do-it-yourself mechanics.

I became frustrated trying to find a way of testing the fuel pressure
of my 1.5L honda DPFI (throttle body).

The bolt size of the "service bolt" on the fuel filter is 6mm. I
couldn't find a way of adapting to this port with the fuel pressure
tester that I had short of ordering a very expensive Honda part that
would be two weeks out. Hence, I went to the wrecking yard and got a
made-up fuel line that goes from the the filter to the throttle body. I
cut this roughly in half and installed a 1/4" barbed "T" with a
threaded hub (came in the Pressure test kit).
I replaced the original fuel line temporarily with the test line and
installed the pressure tester. I placed a two-by-four across the engine
compartment and under the hood, snaked the pressure test guage under
the hood onto the windshield and drove the car around the block a few
times and observed the pressures on the guage...when I accelerated the
guage pressure would decrease.

Would someone please explain the relationship between pump pressure,
regulator pressure and volume of gas used associated with acceleration?

Thanks,
Tich

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Michael Pardee - 16 Jul 2005 04:34 GMT

> Just wanted to add to the growing content in this group for
> do-it-yourself mechanics.
[quoted text clipped - 21 lines]
> Thanks,
> Tich

The regulator is a bypass regulator, so the pump is always (supposed to be)
putting out way more gasoline than is needed. The regulator shunts most of
it back to the tank and lets the injector system take what it wants.

(Background) In multiport injection, the regulator has to maintain a
constant pressure across the injectors. The injector tips are at intake
manifold pressure, so the regulator has a connection back to the intake
manifold. The rail pressure is lowest when the manifold vacuum is highest,
and the pressure goes up as you open the throttle because the manifold
vacuum drops. Pressures in the 40 psi range are typical.

I've only messed with one throttle body injector, and that was in a Taurus
(shudder!), but....
The injector operates at essentially constant pressure. IIRC, it was less
than 10 psi in the Taurus. The injector tip is at essentially outside
pressure so there wasn't any vacuum compensation. Yours may be different,
though. If there is a vacuum line from the throttle body to the fuel
pressure regulator, it may be normal to see the pressure drop at large
throttle openings. If there is no vacuum compensation line and your pressure
is dropping I would suspect a dirty fuel filter.

Mike

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