Disclaimer: The following true account
does NOT fall into the category of
“proper diagnostic procedural order”.
The purpose
of its presentation will be obvious to some now and clear for all on the
follow-up (Wednesday)
A soiled 1996 S10 came in on the hook…
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This is kind of a strange
ticket… No diagnostics? Just replace the fuel tank? I grabbed my creeper and
crawled underneath to take a look. Quite a gash in the front of the tank;
probably lost a couple gallons of space. The center of the tank is only
slightly dented in and the pump and sender probably do not have any physical
damage. Oh yeah… this looks familiar! This is a fleet truck and it was in the
other day for brake work. It looks familiar because I was visiting my
neighbor (brake tech) as he was working on it. Strange, it was running fine
then… |
I asked the service writer what was up. He just assumed it needed a fuel tank and pump thinking that
the physical damage to the outside of the tank damaged the fuel pump. What do we get when we assume?
It makes an A$$ out of U and ME <g> So what the heck, lets check the fuel pump
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Hmm… amp draw is ok… the Hz reading on the right image (AC waveform) is off though due to
the dual trigger settings. Instead of adjusting it, lets just count humps and look at the time divisions.
Looks ok to me. Now keep in mind that these poppet nozzle injectors are touchy and fuel pressure
is very important. Good thing we didn’t replace that tank and pump yet. Now what?
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Lets go high tech and do some current ramping! I pulled the fuse that fed the injectors and primary ignition and inserted my Fluke 865 GMM. I can graph amp waves without using a probe. Love this thing…. |
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Ok the center of the display is always zero amps. With the amp division I have setup, it looks like the primary is firing with about 9 amps. I popped the coil wire from the coil tower and cranked the engine. Nice strong spark coming out of the tower.
Notice those little peaks on top? Betcha that is the fuel injectors. One way to find out… |
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I unplugged the ignition coil power feed to disable the majority of the primary circuit. Yep, lookee there… Notice my amplitude divisions are smaller so as to get a good look. The injector circuits are firing just fine. I wonder… we have had more than our share of plugged fuel injectors due to our wonderful California gas. More than a few times, EVERY fuel injector was stuck closed. Being a 96, I cannot do an injector balance test with the Tech 2. I will need the special J harness and I sure as heck don’t want to go look for it. |
There has GOT to be a better way! And then suddenly, I was struck with the answer. It was so simple.
I grabbed my fuel pressure gauge and hooked it up. Upon keyup, the fuel pump ran and the gauge went
to about 63 psi. Then it started sinking slowly…. And settled where you see it now.
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I need to say here that this is quite common. What happens is that the fuel pump check valve just doesn’t hold properly. I would not replace it. If I replaced every one of these (pumps) that bled down (pump side), I would be replacing fuel pumps night and day. I’m not going to worry about it. Now, what if I were to unplug the fuel pump and crank the engine? With no additional fuel being added and the injectors firing, wouldn’t I expect the fuel pressure to drop? Lets try it! |
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See that connector? That feeds the rear of the truck and more specifically, the fuel pump. Besides being a great place to check for pump amp draw, its perfect for what we want to do here. So I disconnected it and cranked the engine over for 20 seconds. |
The fuel pressure held steady,
it did not drop !!!
Jim! UDA MAN!!! Whoa, I just can’t believe what a genius I am some times… What? Why are you
shaking your head? I outsmarted this
hunk of iron…. Didn’t I ?
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