Mercruiser 140 - low fuel pressure when hot?

[boozer1966]

1984 Mercruiser 140. Problems starting when engine is hot. Recently replaced cylinder head with complete tune up. Fuel system is in good shape. Replaced most everything. I installed a fuel pressure gauge. On start up, it runs 4-6 psi. All good. As the engine warms up, pressure star gas to fall off to almost nothing.

What would cause this?
Is this the cause of hard starts when hot?
How to I fix it? Fuel pump and filters are new.
Should I go electric fuel pump?

The boat is running great otherwise. So great in fact that my wife has almost agreed to go out on it again.

 

[nola mike]

Mechanical fuel pump shouldn't be affected by engine temp. I'd check a supply problem to the pump, start with the anti siphon valve. Presumably you're having issues when the pressure drops?

 

[boozer1966]

The only issue is it doesn’t like to start after it’s been run for a while then shut off for a few minutes and then I try to restart it. Runs great otherwise. Over the past couple of years I replaced the anti-syphon valve, all the fuel lines, pump, filter, and rebuilt carb.

 

[Scott Danforth]

open your filler cap. if you hear hissing, your vent is plugged

 

[boozer1966]

I even replaced the filler and vent hoses. Forgot to mention that. Also made sure tank was clean and pickup not dirty. Also replaced the fuel pressure gauge thinking it was bad.

Would converting to electric fuel pump help?

What about a carb spacer?

 

[Scott Danforth]

I even replaced the filler and vent hoses. Forgot to mention that. Also made sure tank was clean and pickup not dirty. Also replaced the fuel pressure gauge thinking it was bad.

New doesnt mean good. you could have a defective vent, you could have gotten defective hose, you could have installed with a kink in the P-trap loop

Would converting to electric fuel pump help?

No for many reasons. first, a mechanical fuel pump is better from a longevity point. it will out-last an electric fuel pump 5-10X over. Second, from a pure cost standpoint, a mechanical pump is $50, where a marine electric pump is $85, a relay and socket is $10, the oil pressure switch and wiring is another $20. short of a $500 electric fuel pump, the mechanical fuel pump will also out-flow the electric. Then there is the re-plumbing, add another $40 to re-plumb the motor for the electric fuel pump.

What about a carb spacer?

A carb spacer will not help a fuel delivery problem. it will help an intake plenum issue, however on the 4-banger, you wont notice anything.

for fuel pressure to drop, your fuel pump diaphragm could be stretched or damaged, your engine burn demand exceeds your fuel flow capacity (not likely), you have a restriction in the system that exceeds the flow capacity of your engine's fuel usage, or you have a blockage in the vent causing low-pressure in the tank after operation and the fuel pump can not overcome this restriction.

I would pull the cap and check

 

[boozer1966]

Great info. I will keep searching.

 

[Scott Danforth]

BTW, are you sure its fuel related?

Being a 1984, you have points. have you verified dwell and timing? also a bad condenser would potentially be intermittent with heat leading to hard starting when warm. having your timing off will also lead to hard starting when warm.

Another thing. you mention being warm. normal operating temperatures or elevated operating temperatures?

 

[boozer1966]

Ok, now this sounds totally crazy and probably is, but what if I installed a primer bulb, like for an external tank, in between the fuel tank and the fuel pump?

 

[Scott Danforth]

no, find the problem and fix it

if your fuel pressure is truly dropping, check everything I pointed you toward

if you want to test your motor running off a portable tank, that is acceptable for a test to isolate is it a pump problem or a tank/plumbing/vent problem. however you would not want to add a primer bulb.

 

[boozer1966]

I really do appreciate your time in responding. And knowledge. It helps temper my frustration and wanting to do stupid stuff like add a primer bulb.

 

[nola mike]

The only issue is it doesn’t like to start after it’s been run for a while then shut off for a few minutes and then I try to restart it. Runs great otherwise.

I don't think you have a pressure issue. Is this new behavior? What's the start procedure? What's the choke doing? When you restart, are you getting squirts down the carb when you pump the throttle? If so, not lack of fuel. (And likely too much fuel).

 

[boozer1966]

I feel like I’ve been chasing the hot start problem for years. It does have fuel at the carb. But I don’t understand the pressure drop as the engine warms up. Best start procedure when hot is either no throttle or very little throttle. Choke is wide open when warm.

 

[Scott Danforth]

I feel like I’ve been chasing the hot start problem for years. It does have fuel at the carb. But I don’t understand the pressure drop as the engine warms up. Best start procedure when hot is either no throttle or very little throttle. Choke is wide open when warm.

I will re-ask. what is your dwell and what is your timing currently at?

 

[boozer1966]

I switched to Pertronix a while back. Timing is at 6 degrees BTDC

 

[Scott Danforth]

put a dwell meter on to make sure the pertronix is still working properly. out of the two I used (including my 1984 OMC 3.0), they both failed, starting with hot-start issues. you may need to re-adjust the gap between the magnetic ring and the pertronix pickup.

what does your fuel pressure drop off to?

 

[nola mike]

put a dwell meter on to make sure the pertronix is still working properly. out of the two I used (including my 1984 OMC 3.0), they both failed, starting with hot-start issues. you may need to re-adjust the gap between the magnetic ring and the pertronix pickup.

what does your fuel pressure drop off to?

Any reason to not just check for spark?

 

[Scott Danforth]

Verifying dwell ensures you are getting adequate coil saturation

 

[nola mike]

Verifying dwell ensures you are getting adequate coil saturation

So does spark?

 

[boozer1966]

I get my cookies off the low shelf when it comes to most of this stuff. I thought dwell was set internally by Pertronix? And dwell was used to set the point gap?

Here is the great news, spent a great day on the water. No hot start problems at all. I stuck with no throttle start procedure and it fired right up every time. I’m going with the If it ain’t broken don’t break it philosophy.