Vapor lock?

[kmelford]

Engine: 1996 175 EFI, Serial number 0G361770

I've been chasing an issue for about two years now. I have an inconsistent but persistent issue that I can't resolve. My motor starts fine every time, but it will sometimes just run at what seems like a rough idle even after I throttle forward. It will suddenly take off and run perfectly, but then suddenly cut out and run in the rough idle state, then take off again. I've checked and replaced almost all recommended items related to fuel and spark.

If this is a vapor lock issue, I was thinking of moving my primer bulb so it's outside of the bilge area just in front of the engine cowling for quick and easy access every time I start the engine. Being a competitive angler, time is valuable and we make several stops throughout the day. Is this an issue? Currently the primer bulb is about 10" from the gas tank and lies horizontal.

Thank you.

Fyi - here's most of what I've done to date (all OEM):

• Replaced fuel lines and primer bulb as recommended
• Replace fuel filter
• Replace voltage regulators
• Rebuilt mechanical pump (three times)
• Replace spark plugs
• Replaced trigger (wasn't getting spark on two cylinders) - solved
• Checked fuel pressure at idle
• New throttle cables (one broke) - works great
• Sent ECM to Brucato for testing - passed every test more than once. I really thought this was the issue based on the way it cut out and back up as if the fuel modules may have been dislodged

Engine: 1996 175 EFI, Serial number 0G361770

[leonreno]

I would also suggest possibly removing idle stabalizer, they can go bad and cause such issues. Will have to reset timing, which if you replaced the trigger you should know how to do.

[kmelford]

Thanks Leon! Will do. So you don't think there's an issue moving the primer bulb away from the gas tank?

[leonreno]

Well, it’s not a short term problem, the only real problem is it being exposed to sun, will degrade it faster. But if it offers a competitive edge, maybe worth it, some others with more expert knowledge may have other suggestions.

[JOE54]

the less exposed fuel line you have to direct sun light the better if you actually do have a vapor loc problem (i think you dont but) it will increase the chances of it happening more frequently , wouldnt do it dont think its your problem vapor loc doesnt just take off and run fine , if you can duplicate this problem with a fuel psi gauge installed on engine you can see a vapor loc problem very easily

[fly by night]

Joe, do you remember back in the day when we built a little ice basket on top of the fuel pump on race cars to prevent vapor locking, kept the gas cooler? Some used clothespins on the fuel lines too, forgot what that did.

the less exposed fuel line you have to direct sun light the better if you actually do have a vapor loc problem (i think you dont but) it will increase the chances of it happening more frequently , wouldnt do it dont think its your problem vapor loc doesnt just take off and run fine , if you can duplicate this problem with a fuel psi gauge installed on engine you can see a vapor loc problem very easily

[JOE54]

sho do made lots of cool cans outa paint cans and copper tubing , 68 big block 5;13 4 speed close ratio camaro eleven twentys daily driver no slicks with muffins on 1979 was a bad street car one of many ive owned 7

[kmelford]

I’m not moving it yet. Tomorrow I will try pumping the primer bulb before every start to see if it makes a difference. After that, I will remove the idle stabilizer.

[leonreno]

Have you read on here how to remove the idle stabalizer? It’s not just simply remove and go, I believe their is a bias wire that needs attention and you then have to set the timing.

[kmelford]

Yes, I read it before but need to find it again. I recall one question. It says I need to jumper one pair of wires to each switch box. When I look at the owners manual it looks like that pair is already wired to and from each switch box? I'm not sure I understand the reference to 'jumper' the wires?

On the vapor lock note, I ran my boat yesterday and pumped the primer bulb before every start and I only had the issue once on my first run early morning. After that it ran great without out issue all day. Interesting. I did fill up the gas tank in the morning but not sure that has anything to do with it.

[leonreno]

Has the pulse fuel been rebuilt recently with new gasket and check valves? The pulse pump is basically what primes the fuel to the motor and the electric fuel pump.

[silverbullet02]

Check your fuel pressure when it acts up. If fuel pressure remains constant it’s not vapor lock.

[Hez]

I have experienced issues with my motor very similar to what you described.

It ended up being a couple of compounded issues.

1. My mechanical fuel pump (that I rebuilt like 4 times) needed to be replaced
2. very, very slight air leak from my rear tank that allowed the fuel system to lose pressure


This is one of the most annoying problems to have, and even worst when it comes to troubleshooting.

It haunted me for a couple of years.

Even after posting the below thread - I still continued to have issues until I hired a mobile mechanic to come check it out:
https://www.bbcboards.net/showthread...884&highlight=

[JOE54]

Check your fuel pressure when it acts up. If fuel pressure remains constant it’s not vapor lock.

exactly ^^^

[kmelford]

Hez, thank you! I will take a look at your thread to see what you did to address the air leak. That sounds encouraging so far.

I've rebuilt the pulse pump three times so far and as recent as a month ago.

I will keep you posted.

[kmelford]

Hez - I read your thread and it doesn't mention replacing the pulse pump or what you did to address the air leak. Maybe I'm missing it? Did the mechanic figure this out? Thanks again.

[EuropeanAM]

kmelford: Be careful relating someone else's engine/symptoms to your engine. Every once in a while it's helpful, but more often than not it simply complicates things.

This is one of the reasons that we have a STRICT "One engine per thread" policy here.

[kmelford]

Thanks Don. I understand. There are several of us that have been chasing a very similar issue for more than a year and have spent time and money trying to solve it. Any indicator or new idea that may lead us to the answer give us hope.

[kmelford]

I found the instructions from Don on how to remove the idle stabilizer (see below). Question, the owners manual (page 2D-5) shows the switch boxes are already connected with a white/black wire. If this is the case, do I need a "JUMPER" as mentioned below in the instruction?

Thank you!

3. Complete and total removal of the module, and all related wiring. Note: It's critical that there be a JUMPER between the two switch boxes (white with black stripe) which ties the BIAS circuit (allowing the two switch boxes to interact for timing).

*** A COMPLETE Sync-n-Link Proceedure must be completed AFTER removal of the module, with SPECIAL ATTENTION paid to the WOT TIMING, and idle speed (idle speed would be set IN GEAR, IN THE WATER).

[JOE54]

you will not need to add anything just make sure they are jumped with white/blk wire just remove any other wires connected to that terminal , all swithbox mtrs have a wire that connects both boxes at that terminal for BIAS circuit