Time to get a tune up?

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MatSLO

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You haven't said the total miles your truck has done or why you are replacing the spider. I just changed the inlet manifold gaskets (they were shot) but skipped changing the spider as I run mainly on LPG. I did have to run on gasoline last week and it ran sweetly.

What I'm saying here is unless you absolutely need to change the spider don't. Instead, leave it until you have to change the inlet manifold gaskets and do it then (if at all). And change the inlet manifold gaskets at the first sign of them failing.

The truck now has 192 kkm (cca 120 thousand miles).
Perhaps I don't need to change the spider yet but I probably have a bad fuel pressure regulator. The truck won't start when hot unless I fully depress the gas pedal on startup. And I figured if I pay the mechanic to dig this far into the engine, I might as well get new injectors too...

I also sometimes feel like the engine has slightly poorer performance than before (in gasoline or LPG mode, regardless).


The reason why I changed ignition components and the the inlet manifold gasket in early 2017 is because my truck died in late December 2016. I don't know why I thought the gasket was to blame, or ignition, but I went ahead and purchased it all. After Christmas, my mechanic found out my fuel pump died... oh well, so much for my diagnostic skills.
Anyway, the fuel pump was replaced then, along with the ignition components (I bought the truck in late 2014 and had no idea when any of them was last changed). And the intake manifold gasket (already purchased it and the truck was in the shop anyway so I thought, why the hell not).

I definitely didn't regret changing the ignition components - while it seemed like my truck ran OK before (until it died), after that I noticed a huge improvement, much smoother running and better perfomance. And it wasn't just because of the new fuel pump, because it ran just as nice on LPG as on petrol.
 

Pinger

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I'm taking on board your comments on ignition parts and moving them up my to-do list. LPG is demanding on ignition so needs to be kept good.

Last year I had a situation where my truck wouldn't start on petrol - despite the fuel pump being good. It came alright - immediately after an inlet backfire while I was messing about with my LPG calibration. I think what happened is that with constantly running on LPG and with the fuel pump running, the FPR was wide open and stuck there - hence no pressure for the spider for starting. The inlet backfire shocked it free. Since then I mounted a switch on the dash to interrupt the signal from the PCM to the fuel pump relay and use it to switch the pump off when I'm running on LPG. Recommended!

The above (switched fuel pump) and some additive cleaner in the fuel and you'd stand a chance of freeing off your FPR - a lot easier and cheaper than replacing the spider if there's still life in it. Your FPR sound like its more stuck closed than open - so maybe just the additive cleaner in the fuel for now?

No need to regret changing the inlet gaskets - they would have gone bad on you at some point. I used the same Felpro ones - one less thing to worry about!
 

Schurkey

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I mounted a switch on the dash to interrupt the signal from the PCM to the fuel pump relay and use it to switch the pump off when I'm running on LPG. Recommended!
Are you sure?

The fuel pump will run even with the fuel pump relay damaged or REMOVED. All you need is oil pressure--the oil pressure switch bypasses the fuel pump relay.

You'd need to interrupt the wiring downstream of the relay AND the oil pressure switch to turn off the fuel pump.
 

454cid

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Are you sure?

The fuel pump will run even with the fuel pump relay damaged or REMOVED. All you need is oil pressure--the oil pressure switch bypasses the fuel pump relay.

You'd need to interrupt the wiring downstream of the relay AND the oil pressure switch to turn off the fuel pump.

I thought that was a TBI thing? I think I've had my relay out to shut the pump off, unless it was a fuse..... scared myself, thought my brand new pump was dead for a minute. This was awhile ago, so maybe I'm mis-remembering what I did.
 

Pinger

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Are you sure?

The fuel pump will run even with the fuel pump relay damaged or REMOVED. All you need is oil pressure--the oil pressure switch bypasses the fuel pump relay.

You'd need to interrupt the wiring downstream of the relay AND the oil pressure switch to turn off the fuel pump.

Yep - pretty sure though it's crossed my mind before. Kind of confirmed when it was running (with oil pressure up) and I pulled the fuel pump relay out and it quit. That was when I was looking for a quick way of killing the engine from under the hood when flushing the PAS fluid.

When next I'm out in it (if I remember) I'll run it on gasoline on the road and switch the pump off from the dash and check if the engine quits.
 

MatSLO

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TI apreciate to all the replies, especially from Pinger regarding LPG - thanks mate!

Perhaps I don't need a new spider assebly that urgently - I'll wait for a few months...
 

Schurkey

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I thought that was a TBI thing?
It is. It's also a Vortec thing, as confirmed by the '97 C/K shop manual--page 6E-558. It's also a W-body thing.

It's pretty-much "GM standard procedure". The oil pressure switch bypasses the fuel pump relay, either one can turn the pump on, or keep the pump running if the other fails.

it was running (with oil pressure up) and I pulled the fuel pump relay out and it quit.
If the fuel pump quit running when you pulled the relay out, you've got a failed oil pressure switch, or a failed wire harness.
 

Pinger

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If the fuel pump quit running when you pulled the relay out, you've got a failed oil pressure switch, or a failed wire harness.

Switch or sensor?
There's an oil pressure gauge on the dash - which implies sensor more than switch. Do I have one or the other or both?
In any case, the gauge makes sense - 60psi on the road cold, 40psi when hot, and 20psi at hot idle.
 

Schurkey

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My '88 TBI has a one-wire sensor or "sending unit" by the oil filter that drives the dash gauge. High-failure item. There's a two-wire normally-open switch by the distributor for the fuel pump. Never had to dick with it.

My '97 Vortec has a three-wire unit by the distributor that does both. But it's the "switch" part of that assembly that powers the fuel pump.

Either way, it only takes 4--5 psi to close the switch to power the fuel pump.
 

Pinger

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Checked it today running on gasoline, 60 psi on the gauge and used my dash switch that interrupts the signal to the relay and the motor quit.

My '97 Vortec has a three-wire unit by the distributor that does both. But it's the "switch" part of that assembly that powers the fuel pump.

Either way, it only takes 4--5 psi to close the switch to power the fuel pump.

So, conclusion is that the switch part of my sender (or associated wiring) is goosed. Which I can live with as there's nothing to be gained by having the pump running when I'm using LPG - and I think it promotes sticking in the FPR being held in the same position for hours and hours. I should really remove the possibility by disconnecting the oil pressure switch.
 
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