4.3L MPI V6 Vapor Lock (again)!

Gunn

Well-Known Member
TECHNICAL Contributor
Oct 4, 2006
2,335
Potomac River - MD
Boat Info
2003 280DA and 1995 Sea Ray 175
Engines
Twin 4.3l and 3.0l, all w/ AlphaI GenII drives
I've about had it. This is now the 3rd time this year I've experienced the dreaded vapor lock issue with my 280DA with twin 4.3L MPI's. Experienced it once last year, our first full year with this boat.

This time, it was after dark about 20 nm from home with my family of four, and three family-guests on boat. Lightning in the distance. We had been trolling for fish for a few hours. Air temperature about 86F, high humidity. Water temperature about 80F. I had arrived to our fishing location using both engines, generator running, and then shut one engine down. Bilge blowers running the entire time. Fished for three hours or so. Then shut remaining engine down while we fished while drifting and ate dinner. Blowers still running along with generator.

After an hour or so, fired up both engines. The engine that we had been trolling with stayed running. The other that had been off died after about 15 seconds or running. Couldn't get it to restart. Classic vapor lock that I've experienced in the past. Shut down generator. Started home under single engine power with engine hatch now open. After about 30 minutes, trying every 5-10 minutes or so I finally got it started and kept it running. And just like that, it was fine. Thankfully we never got caught in a thunderstorm!

I've tried every recommendation that Mercruiser has mentioned in the vapor lock tech bulletin 99-07 concerning the issue. Fresh fuel, blowers always running, even leaving the hatch up when we leave the boat for a brief period. Thought I had it under control until this. The only other recommendation is to install fuel "booster" pumps in the line nearer to the tank to more of the fuel line under pressure so it can't vaporize. But this is an expensive option. Nonetheless, I'm about to trailer the boat for the rest of the season and use our remaining fuel account allotment for these boost pumps (~$400.00 each). But will it really work?

Has anyone fought this issue...and solved it? Has anyone had to add the boost pump(s) and had it actually solve the problem permanently? How about any other novel solutions to the problem? Obviously while talking around the coffee mess at work, non-boaters offer various solutions. Just brainstorming here:


  • Add electric water pump that I can circulate seawater to keep things cool, even with engine off? Possible?
  • Add primer bulb like on an outboard boat to squeeze out the vaporized fuel? Possible?
  • Higher replacement CFM blowers (Mine are 200cfm now which seem to be the highest I have been able to find)?
  • Adding blowers or other methods to cool the engine room and bring in more fresh ambient air?

This is super frustrating to say the least. And I don't want to be worrying if the boat will start, every time I shut it off (like I do now).

Tom

Mercruiser 99-07 Tech Bulletin
 
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Tom, I have had mine for over 4 years and luckily, have not experienced this. I have been running it on very hot days all summer long. When it is hot, I usually leave the blower on just to help air circulation. I am sure this is frustrating. I wonder if the cooling lines in the fuel pump could be blocked? Just a guess?
mike
 
Tom,

Although, I have twin 4.3 Volvo Pentas, I had a very similar problem this summer. My problem ended up being a clogged fuel cooler. Not sure if you have fuel coolers but my mechanic cleaned the cooler and now no engine stall and starts right away. Good luck.
 
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Gunn, contact susanandlance on this board. He had similar problems and I believe his fix was inexpensive and he hasn't had the problem since.
 
you could try clothes pins and tinfoil on the lines like we used to do in the olden days :grin:
worked well on cars that were prone to vapor lock
 
Gunn- I have the same boat and had the same problem last year-here's what I did -changed the blowers -stayed with the stock model and yes the new ones push more air -the old ones where tired . I turn on the blowers before I start the boat and never turn them off until I am done for the day . Next I changed my oil to royal purple marine full synthetic - (I know there will be stone throwers on this) any way I have put close to 40 hrs on the boat this year and have not had one problem

KK
 
Thanks for the info so far. I actually had a fuel pump failure early this year on the starboard engine. So I had to purchase the entire "cool fuel" assembly from Mercruiser which included the electric fuel pump, and the fuel cooler (which is a copper tube that raw water flows through, with narrow internal passages for the fuel). That engine started fine for me this time out...and was the one that was under power during our slow trolling. When I removed the old fuel pump and fuel cooler, it was not clogged or plugged at all.

I had a hand-held laser thermometer and when the engine failed to start, the hottest location I could find was 130F... I didn't climb down in the ER since the other engine was spinning away.

I think the real design flaw here is mounting the fuel pump underneath the engine, up close to the block. Mercruiser says when the engine is shut off, the hot water from the block can siphon back and to the bottom of the engine causing the fuel in the cooler and upstream to vaporize. There is obviously several feet of fuel line from the pump to the tank that can get hot and vaporize. There must be some regulation against putting the fuel pumps inside the fuel tank due to safety, as this would entirely eliminate the vapor lock issue. The boost pump that Mercruiser sells tries to pressurize more of the line by placing the pump as near the tank as possible.

Tom
 
KK, Do you have the 4.3's also? What weight Royal Purple did you go with?

I have the 4.3s MPI and I used the 10-40 marine -there was no change in the oil pressure even at idle - from where it was when I used the Mercrusier brand oil.
 
HI TOM what Mercruiser told you about hot water siphoning back when the engine is shut down is correct . I have a 1999 210 sd with a 5.0 T.B.I and had the same problem with vapor lock. I was told about all the fixes you talked about, the guys at Parker boats of Orlando Fl where I bought my Sea Ray suggested I install a check valve in the inlet water hose between the thermostat and fuel cooler. I do not remember the merc part # but your dealer will know what you are talking about or look online. Anyway I think it was under 75 dollars with two clamps. All it is, is a heavy plastic one way check valve that will not let the hot water from the engine siphon back down to the fuel cooler. After I installed the check valve myself that took about 1/2 hour I have not had any more vapor lock problems for two years now. I live in the Florida Keys with hot air and very warm water most of the year. I hope this info helps. My dealer said to try this first before a boost pump or other costly things Good Luck Rick.
 
HI TOM what Mercruiser told you about hot water siphoning back when the engine is shut down is correct . I have a 1999 210 sd with a 5.0 T.B.I and had the same problem with vapor lock. I was told about all the fixes you talked about, the guys at Parker boats of Orlando Fl where I bought my Sea Ray suggested I install a check valve in the inlet water hose between the thermostat and fuel cooler. I do not remember the merc part # but your dealer will know what you are talking about or look online. Anyway I think it was under 75 dollars with two clamps. All it is, is a heavy plastic one way check valve that will not let the hot water from the engine siphon back down to the fuel cooler. After I installed the check valve myself that took about 1/2 hour I have not had any more vapor lock problems for two years now. I live in the Florida Keys with hot air and very warm water most of the year. I hope this info helps. My dealer said to try this first before a boost pump or other costly things Good Luck Rick.

I do not think that will make a difference on the 280 -Here's why: This past weekend the seal broke on the water distribution housing and it was leaking water everywhere -when I took the hose off between the cool fuel and the water distribution hosing so I could remove the hosing (my boat is slipped in the water) there was a positive flow of water with the engine off -the boat sits so low in the water -that the water has a positive flow. I had to block that hose off to stop the water from coming in the boat. So if there is always a positive flow of water there I do not think it could siphon back.....well at least on mine anyway
KK
 
...where I bought my Sea Ray suggested I install a check valve in the inlet water hose between the thermostat and fuel cooler. I do not remember the merc part # but your dealer will know what you are talking about or look online.

Rick, thanks for the info. After my first vapor lock issue of this year, I found the Tech Bulletin and read bout this check valve. I went back and forth with Mercruiser tech support on this item. It looks like it is not available for my engine. I have the single-point drain system where all hoses run into a manifold along the bottom of the engine. A single valve is used to then drain the entire engine of seawater. So there is no single hose to place this checkvalve in between thermostat and fuel cooler. The fuel cooler is directly attached to this manifold... When I looked at parts lists, they show this check valve for those engines with multiple point drain systems, but not for mine.

Tom, you said you checked the temp of the coolant hoses with the infrared right? Did you also check the temps on the fuel lines?

Yes, I did. And they never read that hot where I'd be concerned. The engine hadn't run in several hours. The temp gauge at the helm showed around 130-140 (it had maxed out at about 200F or so about 30 minutes after shutting it down, then ever so slowly had been dropping).

I found this post on another forum with a user experiencing vapor lock symtoms: http://my.boatus.com/forum/forum_posts.asp?TID=62384&PN=66 . The very last entry he mentions that the anti-siphon valve on top of the fuel tank was the culprit. It was partially open allowing fuel to drain back into the tank creating a air gap in the fuel system. Does this sound possible? Or do the 280's not have these anti-siphon valves since we have the fuel shutoff solenoid contraptions?

Thanks all,

Tom
 
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You might be on the right track with the loss of fuel pressure when you shut the engine off. Perhaps you can get a pressure gauge and hook it up to the fuel rail to see if you have a significant drop in fuel pressure when you shut the engine down. I believe there is a port on the rail towards the stern. I had an ignition problem a few weeks back and the tech checked for fuel pressure by taping into the port. I believe I had somewhere around 40-50 PSI with the ignition on.
 
I'd say if it ran for 15 sec. after start up it's not vapor lock.. Rig up a line to attach to the shrader valve on the fuel rail, and see what's coming out when this is happening.
 
I purchased a fuel pressure guage and used it to diagnose my fuel pump failure early in the year. I even had it on the boat, but as it was dark and the other engine was spinning at 2000rpm to get us towards home, I decided against climbing down in there, hooking it up and introducing the risk of spilled fuel from the shraeder valve. And I wasn't about to shut the running engine down! :smt001 I would have suspected though it would tell me that I had very low fuel pressure that oscillated as the air passed through...

Running for a brief period after starting seems to be a classic sign of vapor lock. The fuel under pressure from the pump to the injectors will initially start the engine just fine and supply it with some fuel to run it for anywhere from 10-60 seconds or so. Then it will die as it loses pressure. Then it will not restart. Or it will restart at very low RPM and chug and hiccup and backfire no matter what throttle is given to it.

The way I finally coaxed mine back into running was to use throttle only and give it just a bit to initially start. Then basically swing the throttle from 3/4 to near idle at a very fast rate (2-3 times per cycle per second). It would run at around 200-300rpm and chug. It would often stall. But eventually it ran up to over 1000rpm and responded as it should. Back to idle and all is good.

Tom
 
Tom, did you try turning the safety valve to bypass mode? I was looking at the top of the tank while i was hooking up the floscan. I believe there is a check valve in that fitting.
Mike
 
I guess bypass mode would leave the valve open, thereby increasing the chance of fuel siphoning from the lines going to the engine, back into the tank? I guess it's possible that the valve is stuck open, but....both engines exhibit the vapor lock issue and each has their own electric valve.
 

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