4.3L 4bbl hard cold start issue

[bruceb58]

Re: 4.3L 4bbl hard cold start issue

No so on Mercruiser 4.3LH and LX carbureted engines. They have one +12V wire supplied from the ignition switch that goes to one side of the oil pressure cut off switch and there is one wire from the other side of the oil pressure switch that goes to the fuel pump. Refer to items 7 & 8 in the ‘B’ block in the wiring diagram on page 4E-4 of Mercruiser Service Manual #18 or page 4E-8 of Mercruiser Service Manual #25.

Are you ignoring the wire that goes to the fuel pump from the starter? That is what I am referring to.

Really no need to add a timed relay if you have that connection in place. Will take less than a couple seconds of cranking to fill the carb bowl. Very possible that a previous owner replaced a starter without the correct number of terminals on the starter solenoid. I would be checking that before adding a relay.

 

[fen-isl]

Re: 4.3L 4bbl hard cold start issue

Dont get me wrong I love your idea, didnt know others had a similar problem. Thinking i might have to revist and try the timed relay idea.

 

[bruceb58]

Re: 4.3L 4bbl hard cold start issue

Even with a timed relay in there, why wouldn't you just wire the output of it to the fuel pump directly and leave the original wiring from the ignition switch to the oil pressure switch intact? Seems that would be way more reliable. If your relay failed, your fuel pump would still operate as it does now.

 

[flynnhart1]

Re: 4.3L 4bbl hard cold start issue

bruce58,

With the Weber/Edelbrock 1409/etc 4bbl carburetor on the 4.3L engine it can take several seconds of cranking over the engine before the carburetor fills with gas. I don’t want to have to depend on my starter cranking over for so long to fill the carburetor. It sounds to me like hard cold starting is somewhat common with the Weber/Edelbrock 4bbl versions of the 4.3L. The Weber/Edelbrock 1409 are basically the old Carter AFB design that was used on many cars coming out of Detroit in the 60’s. It was a common complaint with the Carter AFB’s being hard to cold start. It seems like there is something inherent in that design that makes the float bowls go dry after sitting for long periods. I’m guessing it’s mostly from evaporation, especially with modern gasoline formulas. I took the carb off, filled it with gas and laid it on a piece of paper on my workbench overnight. Overnight AZ temps were in the 90’s. The very next day the carb was almost empty and there weren’t any signs of gas on the paper. Not a conclusive ‘experiment’ by any means but it did point to evaporation.

 

[flynnhart1]

Re: 4.3L 4bbl hard cold start issue

Yes, I could have done that. I left the stock wiring in place so in the event of a failure it takes a no more than a few seconds to unplug the relay and plug the stock wiring back in. I put in a high quality relay that is spec’d for something like 100,000 cycles. Transistors and SCR’s are pretty dependable too so hopefully this will be a reliable circuit. The relay would have to fail in a completely open position for there to be no voltage getting to the fuel pump. If it were to fail in the N.O. or N.C. contact position, +12V would still get to the fuel pump. My wiring better not fail mad, I soldered and taped all of the ring connectors on to the wires.

 

[flynnhart1]

Re: 4.3L 4bbl hard cold start issue

I stand corrected. bruce58 is correct in that there is another wire from the starter that will supply +12V to the fuel pump when the engine is cranking over. This still means the fuel pump/carburetor is dependent on the starter/battery, etc. to fill the carburetor. With some of the 4bbls being used on the 4.3L engines it can mean a lot of starter cranking before the carb fills which is not something I want to have to do.