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Why can't I bypass the inertia and oil pressure switches on my fuel pump?

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HalLucy Avatar
HalLucy Silver Member Hal Levison
Longmont, CO, USA   USA
Hi Everyone:

I have a '80 TR8 with a Holley 600cfm carburetor. I am having problems starting it after it sits for a while. It appears that it is not getting enough fuel. I am thinking that it might help if I can prime the carbs. But, unlike my older cars, the fuel pump will not run unless I cranking the engine.

I am thinking of adding a new circuit that will bypass the inertia and oil pressure switches. I will add a switch on the dash that turns it on and off. See attached. I plan to run it for 30 sec or so before I try to start the car and turn it off after.

My question is why can't I do this? It seems like an obvious solution, and yet I can't find anywhere where someone discussed doing it. Am I missing something?

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TR8_fuel_pump_wiring.jpg

Bergie Bob Berg
Tarpon Springs, FL, USA   USA
I would replace the oil pressure swtich and rule that out...you want to know if you have an oil pump issue and low pressure is cutting out the fuel pump with a hot engine...are you sure its not flooding and fuel percolation?

TR8todd Avatar
TR8todd Todd Kishbach
Mass, USA   USA
1977 Triumph TR7 "Rally Fraud"
1978 Triumph TR8
1979 Triumph TR7
1980 Triumph TR8    & more
You could do that. You could also add, or replace all together with an electric in line pump. Then again, you could crank for a few seconds, let off the key, and let the fuel pump continue to run for a few seconds. Then pump the pedal and crank again. The OP switch will continue to send power for several seconds after the engine stops cranking. Sounds like the fuel is leaking out or evaporating from your carb while the car sits idle. Might also be a case of no check valve on the fuel pump. The originals had a check valve, but many if not all of the new pumps with superceeeded part numbers don't. If thats the case, all of the fuel in the line will drain slowly back to the tank. Requires much more cranking to get fuel to the carb before it can start topping off. In line check valves are cheap enough and easy to install.

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HalLucy Avatar
HalLucy Silver Member Hal Levison
Longmont, CO, USA   USA
Thanks for the quick replies.

In reply to # 1585715 by Bergie I would replace the oil pressure swtich and rule that out...you want to know if you have an oil pump issue and low pressure is cutting out the fuel pump with a hot engine...are you sure its not flooding and fuel percolation?

Well, after sitting, the car will usually starts after I inject starter fluid, so I don't think that I am flooding it. The car runs great after it starts and has no problem when it is hot. Also, it starts fine after sitting in a parking lot for a few minutes to a couple of hours. So, I don't think that the gas is boiling away. I will try replacing the oil pressure switch if I can find one. Does anyone know a source?

In reply to # 1585738 by TR8todd You could do that. You could also add, or replace all together with an electric in line pump. Then again, you could crank for a few seconds, let off the key, and let the fuel pump continue to run for a few seconds. Then pump the pedal and crank again. The OP switch will continue to send power for several seconds after the engine stops cranking.

I did not know that. I have tried to let the car sit for a few second after cranking it, and it does not seem to help. Makes me think that I am thinking about this all wrong.

Are you suggesting that I leave in the original pump and put an additional pump in the fuel line? If so, how do I wire it? Do I put it in the same circuit as the old pump or bypass the inertia and oil pressure switches? Can you suggest a model?

In reply to # 1585738 by TR8todd Sounds like the fuel is leaking out or evaporating from your carb while the car sits idle. Might also be a case of no check valve on the fuel pump. The originals had a check valve, but many if not all of the new pumps with superceeeded part numbers don't. If thats the case, all of the fuel in the line will drain slowly back to the tank. Requires much more cranking to get fuel to the carb before it can start topping off. In line check valves are cheap enough and easy to install.

This sounds like a great idea. I will put one in.


Thanks again to both of you....

tr8 Avatar
tr8 Paul m
ottawa, ON, Canada   CAN
The TR8 coupes originally have a one wire oil pressure sensor, for the idiot light. The fuel pump works when you turn on the ignition. Makes for easier starting, I have modified the TR8 convertibles that I have had to do the same thing.

HalLucy Avatar
HalLucy Silver Member Hal Levison
Longmont, CO, USA   USA
In reply to # 1585780 by tr8 The TR8 coupes originally have a one wire oil pressure sensor, for the idiot light. The fuel pump works when you turn on the ignition. Makes for easier starting, I have modified the TR8 convertibles that I have had to do the same thing.

Do you have a wiring diagram for the early coups that you could share?

tr8 Avatar
tr8 Paul m
ottawa, ON, Canada   CAN
here you go


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tr8 Avatar
tr8 Paul m
ottawa, ON, Canada   CAN
closer


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HalLucy Avatar
HalLucy Silver Member Hal Levison
Longmont, CO, USA   USA
Thanks!

bowen6951 Avatar
bowen6951 Gold Member rob bowen
Fontana, CA, USA   USA
Hi Hal,
you need to figure out where the fuel in that carb is going. The Holley has fuel bowls on it and they should not be empty after a few days unless they are leaking. Do you have a mechanical or electric fuel pump? Thanks, Rob

Bergie Bob Berg
Tarpon Springs, FL, USA   USA
You can put a fuel pressure gauge on the line near carb and confirm your pressure is constant >5 PSI....do you have original pump in tank or external pump? Could be low fuel pressure issue....when I added an external electric fuel pump I also added a one way check valve to prevent fuel leak back from carb line to tank...

tirebiter Jeff Garber
Dighton, MA, USA   USA
Gasoline can evaporate right out of the fuel bowl. Does the aftermarket carb -zero evaporation control equipment I am assuming - at least have any rubber fuel bowl vent tubing connected to it ?

Bergie Bob Berg
Tarpon Springs, FL, USA   USA
Also retighten the holley fuel bowl screws....

tirebiter Jeff Garber
Dighton, MA, USA   USA
I'm pretty sure if the oil light goes out while cranking the engine, then the fuel pump is getting power.

Bergie Bob Berg
Tarpon Springs, FL, USA   USA
Jeff is correct...cranking the engine should build up oil pressure and if your oil pressure switch is working properly the oil light will go off when pressure is above 7-9lbs and then the switch closes energizing power to the fuel pump...if the light doesn't go out after a minute of cranking then replace the oil pressure switch...its cheap and there are some threads in here on which one to purchase..also its easier to get off with an oil pressure switch socket...

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