CamarosRUS

1991 TPI converted carb. Been all over the 135 threads with HOT and START as well as quite a few others.

I'm having a hot start issue when the motor exceeds approx 200 degrees. Anything under that and she will fire and re-fire at will.

She will crank (a few times until the volts drop too much and the starter just clicks) but will not fire. After a few seconds and a few pumps of the pedal she will sometimes start.
I wanted to believe it was the ignition causing my problems. Still might have other issues there, HOWEVER...


Had a helper try and start the car while I was in the bay and I verified (at least this time) that I am getting spark but the fuel pressure gauge on the carb read zero!

You could not hear the fuel pump running when she was cranking the motor. Now the fuel in the bowls should get it started and it acts like it wants to when you give it gas but there has to be something else going on.

This is the factory fuel pump setup and relay when the oil press sender senses oil pressure it turns the fuel pump on.


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I can think of a few things, let me know what you think:

1. Hot oil while just cranking the motor doesn't give enough oil pressure for the factory oil pressure switch to excite the relay for the fuel pump. Probably not the cause and I can eliminate this quickly because I have mech oil pressure gauge in the engine bay I can double check AND have a manual override switch to turn the fuel pump on whenever I desire.


2. The heat is causing some part of the fuel pump circuit/electrical system in the car to go haywire (no ideas what here...oil press sender??).


3. I see some sort of fog/mist/vapor expelling from the carb when I turn the car off. This could be vapor lock, however, my carb bowls and stainless braided lines near the headers aren't very hot to the touch
Even if I was experiencing vapor lock, the fuel pump should still prime helping to combat the issue.

Vader

The fuel retained in the carb bowl shoudl be adequate to start the engine. Actually, you don't even need a fuel pump until the engine is running. Most cars operated that way for about 90 years with no fuel starvation problems.

If the fuel is percolating from the carb due to engine heat soak (which sounds like uyour description of the vapor rising from it), you have a different problem. This is just one of the reasons many of us old guys have turned our backs on carbs in favor of injection - We fought all these kinds of issues for too many decades and just don't have patience for inadequacy any more.

If you discover it's a heat soak/percolation problem, you may be able to shield the carb from excessive heat if you use a non-metallic spacer. I've even used aluminum plates under fibe spacers to help dissipate heat from the intake, but it can only do so much.

CamarosRUS

I originally thought the carb and fuel might be getting too hot but now know the fuel bowls stay cool. There is no evidence of fuel boiling through the sight plugs, either.

camaronewbie

There will be no priming of the pump since your ECM is gone - thus the fuel pump relay is actually not doing anything right now if you still have it - the pump is operating solely off the oil pressure sending unit - so you are at the mercy of it. Might try replacing it.

To NOT be at the mercy of the OPSU, I cut the fuel pump wires off the OPSU connector, and I ran them to the original fuel pump relay so that now, my pump runs with the key on, rather than only when oil is detected by the OPSU.

Here's what I did to rig the relay to run the fuel pump instead of the OPSU:

https://www.thirdgen.org/forums/carb...ml#post4301540

CamarosRUS

Thanks, I seem to have found my no hot start issue but cannot find out why. Fuel is seeping into the carb and on the hot intake floor causing the fog. It so bad it puddles on the throttle blades.

I was stumped because I originally thought after cranking the car, the oil pressure would excite the relay. I don't think this is the case and am pretty sure the relay is only excited when the motor fires up.
That's a good idea, however, and when I get this thing right I may follow your lead to bypass the commonly faulty OPSU.

camaronewbie

The relay is not "excited" at all if you've removed the ECM - the ECM "excites" the relay, and since you've swapped to carb and are not using the ECM, the relay isn't doing anything at this point.

CamarosRUS

Yes, the relay is what gives the fuel pump power. No relay "excitement", no fuel pump power. Giving (I think it's 4 psi) to the oil pressure sending unit excites the relay and gives the fuel pump power.

Thanks for the reply though.

rgarcia63

If the you've removed the ECM take the wire from the Fuel Pump Relay Terminal "C" should be a Dark GRN/WHT wire and connect it directly to the pink wire that powers the Dizzy and ignition coil. This bypasses the ECM control of the Fuel Pump Relay. This is a one wire mod that keeps the OPSU as a backup to the Fuel Pump relay as it was originally designed to work.
FYI:
The OPSU actually supplies power to the fuel pump (not the fuel Pump relay) once the pressure closes the switch.
On FI engines If the fuel pump relay fails while the engine is running you won't know it until the next start cycle which will crank, but not start until the pressure closes the OP switch and powers the fuel pump.

camaronewbie

Nope - not true. The OPSU circuit to the fuel pump has nothing to do with the relay - the relay circuit to the fuel pump is a completely different and independant circuit. If there is no ECM connected, there is no use of the relay, period.

The OPSU circuit to the fuel pump is a straight connection from the battery, to the fuse behind the battery, to the OPSU, to the fuel pump - there is nothing in this circuit that connects with the relay.

The relay circuit had constant 12v power from the battery, a switched 12v source that is activated with the key, the power wires to the fuel pump, and a ground that activated by the ECM.

The two circuits are totally independant. This is why a bad OPSU can and will make your fuel pump run constantly, regardless of the position of the key. You could leave the key in Timbuktu with the car off, and as long as the battery is connected and the OPSU "thinks" it sees pressure, the fuel pump will run until the battery is dead.

You are right that the OPSU will direct power to the fuel pump when pressure is present - if you have no ECM, then that is how you are getting fuel pump power. Want to test it? - start the car, and remove the relay - it'll run all day long off that OPSU circuit.

Not necessary as NO ECM connection means no use of the relay anyway!


If you want to make use of the relay, you can rewire the fuel pump leads from the OPSU through the relay, as I describe in the thread linked above - but with no ECM connected, that relay is not doing anything at anytime.

Zwrench

Is there any chance that the stock TPI fuel pump pressure is too high for your carburetor? Maybe causing flooding? I assume that the pump does run at some point or you wouldn't have any fuel at all.

If you are indeed using the TPI fuel pump to feed the carburetor, do you have a pressure regulator?

CamarosRUS

Thanks, that's a good idea. Seems like it would make sense because, currently, I have a switch wired up to the fuel pump relay so I can turn my pump on whenever I desire (to prime the bowls for instance).



I eat my words. That makes perfect sense and I was under a completely different impression but haven't personally studies a schematic. That's good info! Hopefully someone searching this carb swap/fuel pump issue will stumble across this thread.

Here's my question then...
You said the relay does nothing at this point without ECM input, however, I have mine wired up to a switch where I can turn it on (via the relay) whenever I need. I *think* this was accomplished by simply giving the green/white (ECM feed?) wire 12v. That was so long ago I cannot recall for sure, however.


Running a Mallory return style regulator. Have the fuel pressure set at 6psi. There is an issue here, however. When testing the initial install of the regulator, I could not get a low enough fuel pressure recommended by Mallory when removing the adjustment screw completely (I think it's 3 or 4 psi). This is probably due to too small of a return line since the fuel pressure change from F.I. to carb is such a big drop.

Because of this my fuel pressure likes to jump from 5-7 psi back a forth back and forth under throttle. At idle it will rest at 6 psi. (This is all my best guess)

rgarcia63

Your however statement is correct:
That's basically what I stated.
Here's the Post I replied to and attached the Diagram it's the 5th reply up from the last reply. You may what to copy if for future reference. Independent circuits means that if you remove the power to one the other will still function. As you can see from the diagram this is not the case with the OPSU & FP relay circuits. The Hot at all times fuse powers both the FP relay and the OPSU.

True without the ECM the FP relay will not function, but that doesn't mean it can't be powered by a toggle switch, or any other type of switch for that matter with or without the ECM.