On 2012-02-29, wrote:
On Tue, 28 Feb 2012 21:07:33 -0600, "Pete C."
wrote:


Ignoramus14054 wrote:


"Ignoramus10095" wrote in message
I have a Chevy Kodiak dump truck that I am trying to fix.
Pictures of the truck and carburetor are he
http://igor.chudov.com/tmp/Chevy-Kodiak/
Onboard computer bad ?
Check fuel line pressure ?

Looks like throttle body injection (TBI), lots of stuff to go wrong.


Tom, and others, I took some time off this problem, and also took some
time to work on it and diagnose it properly.

Yes, this is based on TBI (throttle body injection).

We replaced the fuel pump, at least some fuel filters, and injector
bodies.

It did not help. Even with new injector bodies, the fuel would, at
best, drip from the injectors. drip, drip, drip

Then I decided to NOT take shortcuts and diagnose things properly,
using proper troubleshooting techniques, organize my workplace etc.

As you know, before the injector bodies, there is a pressure
regulator. It has a spring inside. For testing purposes, I removed the
spring, essentially disabling pressure regulation.

We cranked the engine again, again to the same result. Dripping, not
enough fuel.

Then I measured the voltage supplied to injector bodies, it sort of
varied, but would hover around 2 volts. It seemed low.

Then, I took out the wires coming from the engine, plugged in my
little alligator clips, and supplied 4 volts to the injector body,
using my DC power supply.

Voila, the fuel came out in some force (but not as a shower), the
engine would start and run fine.

I put the regulator back in and, guess what, no fuel again,
even with 4 volts applied to injector bodies.

I ended my experiment right there and went home to think.

Clearly, without the pressure regulator (at full supplied fuel line
pressure), and with more volts supplied to injector bodies, the engine
runs. But what des it mean?

My current thinking is that I seemingly have two problems

1) Lack of fuel pressure
2) Lack of voltage supplied to the TBI.

I have a feeling that both items are a manifestation of the same
problem, which is lack of voltage in the fuel subsystem. I think that
both the fuel pump is not getting enough volts (so not enough fuel
pressure), and also, the injectors are not getting enough volts (so
they do not open properly).

Makes sense?

If so, what could it be? Bad ground?

i

Bad grounds could certainly do it. The pressure regulator may be ok, but
the fuel pump may have a low voltage condition as well and not be
keeping up the appropriate flow.


You say 2 volts and 4 volts.
This does not compute. WITH THE INJECTORS DISCONNECTED what voltage
do you have on one side of the injector with the key turned on?

I was measuring voltage between two terminals going to the injector
body, with an averaging voltmeter.

It SHOULD be roughly 12 volts from terminal to ground. When the
computer triggers the injector, the computer GROUNDS the injector for
a short time. Measuring across the injector will give you a low
voltage that varies with fuel requirement.

Right.

A bad ground on the computer will cause a low voltage drop across the
injector which translates to low injector current and reduced injector
opening. The same will happen with high resistance in the power supply
circuit.

Which is what a bad ground is, kind of .

The voltage on the "feed " side of the injector should remain at 12
volts even when cranking (well, it WILL drop some - but it should
remain "battery voltage"). If it drops below battery cranking vultage
you have a bad connection somewhere in the power feed circuit.

Sounds like a good thing to check, voltage on the feed side
vs. ground/body potential.

To test the fuel pressure regulator pinch the rubber return line and
see what happens to the pressure.

Another great idea, will try to find it.

Those regulators have a bad reputation, for a good reason. Note -
the RETURN line, not the feed or the vacuum signal line.

That vacuum line, I think, is disconnected, may be an issue.

The
injection pressure is regulated to provide the same pressure drop
across the injector under WOT (low vac, or high manifold absolute
pressure) as under low throttle (high vac, or low manifold absolute
pressure) so a given period of injector opening provides the same
amount of fuel regardless of engine fuel demand.

OK, why do the regulators have a bad reputation?

Thanks a lot

i