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Both Engines in Limp Mode (4.3L EFI)

Ominous Z06

New member
Other than fuel system, is there anything else which would cause RPM limitations / limp mode on both engines?

Engines: Twin Mercruiser 4.3L EFI TBI (Gen+) w/ 377 hours ea.
S/Ns: L686113 & L686115


Symptoms:
• RPMs when running on both engines is limited to 2,500 under load and at WOT, will not get on plane & max speed of 9-10mph.
• Each engine is limited to 2,000 RPM under load when running in isolation (other engine turned off).




Already Checked / Performed:
• Ran Diacom. No stored codes in either engine.
• Both engines rev to 4000+ in neutral
• Replaced distributor caps & rotors on both engines
• Replaced both fuel pick-up lines in tank
• Replaced both anti-siphon valves (port one was full of crud)
• Clean flame arrestors
• No lose wires or sensor wires
• Fresh fuel fill & negative on water in fuel test
• Ordered Schrader fitting adapters (P/N 91-806901 ) so I can hook up a fuel pressure gauge and monitor under load


Other than fuel supply, is there anything that would affect both engines? While I await the fittings to check fuel pressure, any other likely culprits or parts you recommend I check?


Thanks,
Eric B.
2000 Regal 2660 Commodore
Port of Cincinnati OH
 
Yes, they did. This is my third season with this boat. Prior 2 years ran fine. I've struggled with this condition all this season. Boat was winterized and stored indoors during the winter and most of spring.
 
Do you have 1 or 2 fuel tanks? If 2 tanks, does the problem happen with both? Have you checked your fuel tank vent(s) to make sure they aren't clogged and causing a vacuum as fuel is drawn out. This might be a remote possibility, but at least it's easy to check.
 
Good questions. My mechanic follows spec process winterizing engines (under my watchful governance) by draining all block petcocks & coolant lines, then refilled with antifreeze. Single fuel tank. Checked vent hose & ran compressed air through with zero restriction out hull.
 
Yes, fuel was stabilized with Red Stabil, but should have run through it already. I always buy non-ethanol Marathon. I have never fogged the engines since I've owned it.
 
4.3's are gutless, if one engine is not running up to power, neither engine will. With your isolation test, one running one off, you saw the rpm limited to 2000.

One of your engines has an issue, compression test is in line to determine which is the problem.
Compression on all 12 cylinders.
 
No solid proof but its seems that overwhelming number of posts to this forum are related to 4.3 problems, followed closely by the 3.0 4 bangers.
 
Chris - I ran both motors in isolation. Port maxed at 2000rpm under load with starboard off. Starboard maxed at 2000rpm under load with port switched off. Neither have a problem in neutral. That is what lead me to believe fuel related. Unfortunately, the EFI TBI generations don't have inline schrader valves to test psi underway. Adapter fitting enroute.

I gather from the searches on this site and other 4.3L problems are frequent. I just bought a '99 Sea Ray 330 with twin 7.4L. Need to get the Regal to point I can sell it, but a 29' boat that won't get on plane is limited on resale value.
 
Chris - I ran both motors in isolation. Port maxed at 2000rpm under load with starboard off. Starboard maxed at 2000rpm under load with port switched off. Neither have a problem in neutral. That is what lead me to believe fuel related. Unfortunately, the EFI TBI generations don't have inline schrader valves to test psi underway. Adapter fitting enroute.

I gather from the searches on this site and other 4.3L problems are frequent. I just bought a '99 Sea Ray 330 with twin 7.4L. Need to get the Regal to point I can sell it, but a 29' boat that won't get on plane is limited on resale value.

This is not a test. You 99 with the twin 7.4 (probably L29 Mercs) will spin up about the same with your test.

Start with a compression test.
 
In the test that you ran,i.e. with one engine at a time, the engines were in effect over propped. You are trying to get your boat to run with only one engine. Your engines are set up and propped on your hull to run with both engines. The only test that is valid, is running with both engines, and in your case your limitation (without any apparent changes???) to 2500. I saw that you did a lot of things, which of them did you do BEFORE you saw the problem? For example if you changed disti caps pre launch, and made the a mistake with wires,that could be your problem.
 
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