Hey guys, so like the title says I have a CCC L69 305 engine that's going to become a 350 with a Holley Double Pumper.

Question is, what do I do with the in tank fuel pump? I'm ditching the ECM, but I'm not sure if the ECM controls the fuel pump, or if I can leave it. The new motor will also have a mechanical pump on the block.

 

Yes
but it can be wired to run off the key


Need to remove the intank pump as the mech one on the engine won't pull through it

 

He can run the electric, since the car was carbed the Holley can handle the pressure. I believe the pump runs off a relay not the ecu, but I could be wrong.

 

I have heard differing opinions. Consensus seems to say that the pump runs off of ecm for first couple seconds to prime the system, and then the oil pump sensor (or something like that) takes over and runs it. And if that's true, it's no big deal because the carb'd system doesn't need to be primed (so i've heard)

 

The ECM runs the pump relay and once the cranking oil pressure comes up then the pump runs off the oil pressure SWITCH ALSO;
when the engine is running the pump is getting power from 2 places so if the primary source ( the relay ) fails you can still get the engine to start by extended cranking to operate th epump from the OP switch.

 

In that case why would you bother with a mech pump if you already have elec one installed ?

But what happens when you let you car sit for a week and all the fuel evaporates out of the system and you have to crank for 5 mins
until your mech pump pulls the fuel up to the carb , all the while hoping the dodgy battery will keep cranking over?
With a elec pump , you hit the switch , instant fuel to the carb.

A smart person would use the elec in-tank pump , wire it to run only off the stock OP switch fulltime and then have a push button
to bypass the OP switch so the carb can be filled when empty like on very hot days or the car has been sitting.
Works like this.
Crank engine , car starts on fuel in the carb bowls, oil pressure comes up while cranking over , pump runs.Turn engine off , OP drops , pump stops .
If the car has been sitting a while , hit the manual push button or switch for 15 sec 1st to fill the carb bowls, pump pedal ; instant start.

Wired that way it also works as a pump kill switch because if the engine stops , no OP so no fuel pump running; even with the key on.

No need to remember to turn off a switch , can have the key on if needing to check something without having the pump running fulltime
( most popular way on here is to hotwire the pump to the Ign switch )

 

Like you just said, when the oil pressure comes up while cranking the OP sensor then trips the in tank pump. So by that logic, the mech pump wouldn't be doing all the work. The in tank pump would just come on a couple seconds later and prime the system.

Unless I'm missing something, in which case please correct me. I'd like to make sure this works right the first time.

 

Vetteoz, my other question with that statement is that carb'd cars got along without a priming pump for a few decades. Most '84 third gens did as well. Why does it all of a sudden take 5 minutes to get fuel to the carb? If you let it sit for a year or more I can see that, but I've had race car engines sit for a couple of months and fire right off. Fuel doesn't evaporate from a closed system as quickly as you'd think.

Again, however, please correct me if I'm wrong.

I don't mean any of this to come off sounding condescending, I'm just trying to make sure this works for me, so please don't take any offense.

 

The qjet isn't a closed system. The carb vents to the evap canister.

Older qjets WILL leak through the casting plugs on the bottom into the intake. Some well placed epoxy can usually fix this.

The carbed f bodies had a tendency to boil fuel in the lines after a hot soak. Observe the proximity between the fuel lines and the right exhaust manifold. The in-tank pump was added to prevent this issue. It is a low pressure pump and probably not strong enough to feed the carb on its own.

Maintaining the stock system with in-tank, mechanical pump and return lines will pay off in the end.

 

That's the intent. I'm basically arguing for not removing the pump. But I'm also arguing for not having to wire up any kind of switch, because I don't believe the priming of the system is needed. Or at least, I think it can wait 2 seconds for the OP to come up when turning the motor over.

 

I've not had any issues with my 85 TA that has no in-tank pump. It's sat two weeks or more and still fires up.

If you've got the pump, though, keep it.

 

Like I said, that's the plan. It'll run once the oil pressure switch trips it. I'm just not going to wire in a bypass to prime the system, unless someone can give me a reason that my car won't run correctly without it.

Side note, vetteoz you may be able to answer this, where is the OP switch? I've heard it's on the oil pump, but I'm not familiar with it. Is there any specific wiring that has to be done, or is it involved with the OP gauge in the car?

 

OP switch should be connected to the fitting that goes into the china wall behind the dist. On carbed cars it tees off the fitting with the OPSU. The switch will be the one with the two wire connector.

 

Not how you originally asked the question, otherwise I would have given a different answer.

 

I was hoping I could get away with leaving the pump. I was worried without the ECM that it wouldn't work correctly, but from everyone here it sounds like I can get away with it.

So vetteoz, you would say that I should be alright as is? Or do you think it's a necessity to wire in a switch of some sort, or can I just use the OP switch?

 

However , you have wiring that goes to the now redundant fuel pump relay you can easily rework to install a prime switch with little work.
Even having the switch under hood is handy IF you need to prime / purge the system