Logo

Twin engines die at low RPM underway

bar none III

New member
I have twin 1986 454 model 350's with Rochester quadrajets. Both engines start on first crank and idle smoothly at about 800 & 1000 rpm. Not extremely confident in Tachs. I can run them in neutral at the slip for 45 to 60 minutes at various rpm's no problem. Whenever I attempt to go for a cruise, running at 1200 rpm for 15-20 minutes, trying to get out of the harbor, the engines die and will not restart immediately. There is no sputtering, the engines just cutout. After 15-20 minutes the engines will restart, but will only run 5-10 minutes under load and die again.
Sounds temperature related, maybe vapor lock. However, I have re-plumbed the entire fuel delivery system and installed the lines away from the block on the port engine and the issue still exists. There are no temperature issues with either engine.
Could the vapor lock occur within the mechanical fuel pump? This is obviously mounted on the engine block.
I have also installed electronic ignition and new coils as someone suggested heat could effect coils.
I thought I had read/heard that there are safety sensors installed for coolant temp, oil pressure and transmission temp, but they only provide an alarm. They will not shut down the engine. Is this the case?
Haven't been out yet this year. Anxious and aggravated.
Thank you in advance for any recommendations.
 
Welcome to ME.com...

you are correct on the alarm system...turns on horn/buzzer only - no shutdown of the engine.

is there one common fuel tank for both engines or do they each have an independent source of fuel?
 
Welcome to ME.com...

you are correct on the alarm system...turns on horn/buzzer only - no shutdown of the engine.

is there one common fuel tank for both engines or do they each have an independent source of fuel?


They have independent fuel sources. Vents have been checked. No blockages.
New fuel/water separator cartridges were installed in the spring.
Fuel is from last season. Treated with Stabil to winterize. If I could get to the fuel dock I would fill it with fresh gas.
 
ok...when "... the engines die and will not restart immediately..." do they die simultaneously or in rapid succession?

Also, once they have died, have you verified lack of spark on both?
 
The engines do not die simultaneously. The time is inconsistent and varies for each motor.
Unfortunately I am alone when I take the boat out for test runs, so I have been unable to verify available spark. I need to buy a remote start switch and wire it in for my next test run.
 
Same symptoms but only on one engine...it was that anti-siphon valve.....the ball bearing shut the fuel off due to "crude" from the fuel tank and after a 15 minute wait I could start the engine and then 10 minutes later do the same thing...but only under load. Earlier a "mechanic", who stated that my "old" 1991 boat did not have anti-siphon valves, ran the engines for 4 hours after changing a bunch of stuff and not a thing happened........ but under load the stbd engine would quit about 1 hour after leaving the marina .

I started at the fuel tank and worked toward the engine and found that $5.00 part that I changed...... when I got back to home port and in the mean time the mechanic ran up a bill that cost me over $1000.00 by changing parts that did not need to be changed.

At least I now know the workings of the entire engine fuel system now.......:D
 
Thank you Willow. I never would have considered that. It doesn't make a lot of sense that the siphon valve would work for some time and then quit. it seems like a device that either works or it doesn't. I will definitely try that inexpensive fix first. If it works I will owe you big time.
 
the other item to consider, if the anti-syphon valves are ok, if crud in the tank accumulating on the pickup tube's screen...
 
Thank you for your recommendations makomark. I am considering new pick up tubes, and will be inserting a tube to the bottom of the tank and removing some gas to see how much crud is lurking down there.
New sending units and gauges are most likely going to be replaced in the near future.
 
Hi makomark, I pulled the pick up tubes and there are no screens on them. 3/8" copper tube pick ups. There didn't appear to be significant debris at the bottom of the tank. The anti-siphon valves are clean and clear as well.
Looking at the fittings on the fuel system I have to wonder if the engines are fuel starved. It doesn't fit the symptoms, but the Crusader manual states that a 3/8" fuel line is required. The port of the shut off valve and interior diameter of the anti-siphon are barely 1/4". Then there is the shut off at the fuel/water separator and the inlet and outlet hose barbs. All with a 1/4" diameter. Racor filter is rated at 60 gph.
Haven't been able to get it on plane yet, but when I do I wonder if I'll have fuel issues.
 
based on these revelations, I'd thinking you recently acquired this hull so there are a lot of unknown?

The factory line size accounts for a reasonable number of fittings and most use 1/4" NPT fittings with the 3/8" line...

I'd suggest you verify lack of fuel/lack of spark when the engine(s) die...no reason to pump money where it isn't currently needed....
 
Back
Top