Is your gauge accurate? Are you trying to read a 9--13 psi spec, on a gauge calibrated to 100 psi?
Greetings Anchor-Enthusiast,
The overall success of a sharp troubleshooter is due as much to the
quality/accuracy of their
test equipment in addition to having a working knowledge of the subsystem they are working on.
Schurkey is making a very valid point about trying to get actionable info out of a 0-100 psi
fuel pressure gauge on a 'low-pressure design' TBI system. (I define 'actionable' as data that's
reliable enough that I'm willing to change parts because of what I'm being shown.)
EDIT: I see that you replied that you are using a HF 0-10 psi test gauge. Those gauges were
designed to test fuel pumps feeding carburetors, for they typically worked best at ~5-6 psi.
If it's a vintage test gauge from the last century, it could be lying to you.
For what it's worth, to me this would be the (affordable) gold standard in the 0-15 PSI fuel pressure
tester:
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Here's the current Summit listing for this ~$37 + shipping gauge: (
OTC vacuum/pressure test gauge)
NOTE: I would be willing to use a cheaper gauge, but ONLY after I could test it side-by-side with one of these
and verify that it agreed. (Same thing for a bang-around daily use cheap multimeter -- only after it's proven
to agree with my 'reserved for reference use' Fluke. :0)
I forgot to elaborate on why it made sense to me, the guy at Napa said the pressure in the large fuel line is always lower than what's actually going into your engine, as the line chokes down pressure increases so in that big wide line 5-6psi would equal 10-12psi in a line half the size. It made perfect sense to me when he said it but if you guys think that's wrong I will check again with a better tester.
Charitably speaking, that's a well-meaning but misleading explanation. I wouldn't recommend using that line of reasoning
while troubleshooting your fuel delivery subsystem. I'll try to substitute a concise overview of what you want to look for.
There are 2 variables that we need to be thinking about in our fuel system. The first is how high the pressure climbs to,
and the second is how much
volume that the fuel pump is capable of delivering to the TBI. For example, even a good pump
can be constrained by a dirty inlet strainer, similar to a human sucking on a smashed soda straw.
Or, assuming that the inlet sock is good, the fuel filter on the pressure side between the pump outlet and the TBI nozzles
is partially plugged. Thinking about it, with the fuel pump running while the engine is stopped, you might measure 12 psi
before the
filter as well as the same 12 psi
after the filter. Because there is no flow (volume) ...you are simply measuring the 'deadhead' (no flow)
pressure that Schurkey has referenced.
But let's say that you start the truck, and idling there is now a 2 psi drop across the partially-plugged filter. A bit oversimplified,
but having a larger vs smaller diameter hose between the filter & TBI nozzles isn't going to make a difference at what the
injectors see/feel.
Taking this thought experiment a little further, I'm going to state that at 10 psi (nozzle) the truck will idle OK. And it will
drive during light throttle/light loads OK. But with more demand (longer injector pulsing) we could watch the pressure
drop in response, and quickly we'll reach a point where insufficient fuel will be delivered to the air rushing past, and the
bucking/stalling will begin. (NOTE: If we were to check the fuel pressure *before* the fuel filter, no doubt it would remain
steady. But the TBI shower nozzles don't care about what's going on between the fuel pump & plugged filter. They only
care about the fuel pressure at the input to the injectors. (!)
In this case, simply changing the inline fuel filter will remove the pressure drop/fuel flow bottleneck
across the new filter,
and (assuming a good pump) the pressure will always be more than needed for the FPR to be able to hold the pressure
steady at the nozzles, no matter how high or low the engine demand is.
****
The reason why I dragged us through the above is that a static fuel pressure check alone is just a partial go/no-go answer.
When I see fuel pressure, this tells me that I at least have some electricity being sent down to the pump. But in order to
figure out IF we have enough fuel pressure x volume to cover all possible demands, the best answer is to (temporarily) install
a
Tee that allows me to monitor the fuel pressure while the vehicle is being operated, preferably under the same loads as
when you are doing the daily driver thing.
NOTE: Since outboard fuel volume test equipment is spendy, we instead use a fuel pressure gauge AND also monitor the fuel
trims reported by the computer.
*** CAUTION CAUTION CAUTION ***
It goes without saying that safely testing the delivery pressure of a flammable liquid in the vicinity of a hot exhaust
system is serious business. This is not a dribbly duct-tape connection & hit the road scenario. Elsewhere in this forum
members have documented some slick/safe ways to safely tee into their fuel delivery system. Deadhead testing a fuel pump
in a stone cold engine bay is both easy & gives you a quick go/no-go answer. But in order to observe the system under load
is quite a bit more involved.
Be aware. Leak-free test connections and an onboard fire extinguisher is a prudent bare minimum.
****
If I were to guess, I'll bet that Schurkey's mindset is that if your system can only deliver 6 psi
before the engine starts
to draw fuel, then it's not going to take too much gas pedal to force a fuel delivery error at the nozzles.
Apologies for the length, but given what the helpful NAPA dude was telling you, I wanted to try to help explain why we
need to worry about pressure
and end-to-end volume delivery capability, from inlet sock > fuel pump > inline fuel filter > TBI nozzles.
And a good quality 15 psi fuel gauge tee'd in (plus fuel trim data from the computer) will tell you everything that you need
to know. EDIT: If a trustworthy gauge still tells you 6 psi, then a new pump (and/or fresh wiring) is in order since you've
already tried a new filter. Fingers crossed that the new/accurate gauge shows you 12-15+ PSI at the input to the TBI, and
that the MAP sensor was 100% of your original driveability issue.
Best of luck keeping that GMT400 on the road and earning it's keep.
