Mercruiser 5.0 backfire up intake

[ibrw1]

Here's another SWAG... Carbon build up on the piston gets hot and when fuel is added it fires off while intake valve is still open. Power tune or Seafoam to clean up carbon. Can't hurt.

Perfect, so, I'm going to get sea foam. Let's say I have 20 gallons in my tank. How many OZ s would you say would be the correct amount?

 

[Watermann]

I usually do a soak treatment first, put the seafoam in a spray bottle or pour it down the throat if you're brave, warm engine to op temp and squirt half a bottle in the carb until it dies or is trying too and turn it off. Let it sit for 15 or 20 min and fire it back up. Repeat with the rest of the seafoam. Not sure about the ratio for the fuel, haven't put it in the tank but some guys do.

 

[ibrw1]

And another piece of the problem, thought you were going flat out and you were just crusing


You are running lean IMO, because you get a back fire at higher rpm and slowing down allows the carb to fill back up with fuel. You take back off and the fuel pump can not keep up so the carb starts running low.

Start by checking your fuel pump pressure (4 to 7 psi). If you can leave the gauge on the line so as you drive you can see if the pump is keeping the pressure within the range. If the pump test low, you probably need a fuel pump or you have a restriction in the line

Originally yes, then I put in new plugs. Got AC-MR43LTS, problem was gone for 1 small 30 minute trip. The star of this post stars with I thought I had it fixed. Man, what a drag. I'm just offering current issue as not to confuse. A few weeks ago, I had 12 people in the boat, wide open and backfire started. Anyway I'm pulling plugs tonight to see what's what with those. Thanks!!!! Sorry for confusion.

 

[ibrw1]

To add, I just pulled my new spark plugs. Getting ready for compression test, I know the engine is supposed to be warmed up but I've had great success performing this test cold on boat engines in the past. Im looking for OMG, what's that? Anyway I'm thinking the plugs are too new to be of any help or direction. They look new. I was out two weeks ago for 4 days on the Mississippi. Ran 60 gallons through it. (Had to run either fast or slow) Since then I only take it out to test, totally afraid of what's blowing up in there! Won't backfire in driveway because RPMs must be up. Plugs look really new. Another clue. When tubing since day one, everyone said the fumes were not good. This is my second boat, but same tow line and tube. No complaints with old boat. Lots with this one.

 

[ibrw1]

At least its cheap
http://www.walmart.com/ip/14644669?... a&wl6=55136637649&veh=sem?reviews_limit=9&

Thanks, ordered the vacuum / Pressure pump.

 

[ibrw1]

have you checked the anti-siphon valve? could be debris in the tank and a partially plugged anti-siphon valve.

when you changed the fuel/water filter, did you empty the contents into a clear container to inspect? could be water in the fuel making it to the carb.



I thought of making a comment on the boat being in the Mississippi (Too thin to plow, too thick to drink) however you probably heard them before.

I got the brass anti-siphon valve out. It is difficult to draw a fluid through it. Not sure how to test. I will search online. The siphon tube and wire mesh was clean.

 

[ibrw1]

I got the brass anti-siphon valve out. It is difficult to draw a fluid through it. Not sure how to test. I will search online. The siphon tube and wire mesh was clean.

I ordered a new one. Brass, $12.00

 

[Rick Stephens]

Yours is one of the tougher tests of character I have read. Hang in there. You will get it running properly and be glad for all the knowledge you've gained on your new boat.

Rick

 

[NHGuy]

My 5.7 did that exact thing before I did the wires. I'm normally a fan of OE but I used these ones from Tungsten Marine. Stainless core and they look cool too. Bright colors instead of OE black. http://shop.tungstenmarine.com/Marin...EST-TM8804.htm

 

[NHGuy]

If you want to check the old ones start the motor somewhere shady and spray mist some water on the wires. I bet you get arcs to ground.

 

[ibrw1]

Yours is one of the tougher tests of character I have read. Hang in there. You will get it running properly and be glad for all the knowledge you've gained on your new boat.

Rick

Thanks Rick, I appreciate that, I actually thought I knew something about boats until this issue. Man, anyway, Lots of wonderful people have passed on an enormous amount of information and support. I will be very busy this weekend with all the tests, maintenance procedures and upgrades. THANKS EVERYONE. Not one piece of advice will I ignore. I'M CHECKING EVERYTHING

 

[ibrw1]

My 5.7 did that exact thing before I did the wires. I'm normally a fan of OE but I used these ones from Tungsten Marine. Stainless core and they look cool too. Bright colors instead of OE black. http://shop.tungstenmarine.com/Marin...EST-TM8804.htm

HEY, the wires are on sale, like $32 bucks! Thanks. I hope to have them by Saturday. Putting boat back in the water SUNDAY after all the upgrades, Maintenance and procedures are finished. THANKS FOR THE LINK TOO

 

[ibrw1]

Hey Watermann, I used to live in Waterman, Ill. anyway, I got some SeaFoam and going to follow your procedures this Saturday, and many others. Thanks Again

 

[ibrw1]

So,

Electrical; I JUST put on a new OEM cap and rotor, new wires, new plugs AC Delco as suggested by Mercruiser, set BASE timing to 10 degrees before.

Fuel; since issue started I bought another carburetor, and adjusted mixture at 650 rpm, measured fuel pressure at 6 psi after NEW anti-siphon valve was installed and new water separator and pulled tube out of the tank and cleaned mesh strainer and replaced back into the tank not to mention cleared clog in gas tank vent.

Mechanical; I measured compression of each cylinder at 164 to 177. I pulled all plugs out and with a wrench spun the CRANK back and forth while staring at the rotor, I couldn't see any SLOP, by that I mean I spin crank CW then back and the rotor did not hesitate to follow. The timing chain looks good then.

I filled tank with new gas, I dumped in 2 cans of sea foam and sprayed 1/4 can down the carb while running.

All that, and the problem still exists. Backfires up intake. Only difference, the range is 3200-3800
And only after warm up. It flys like bat out of hell, idles like a dream.

 

[ibrw1]

I have to pull the valve covers off and measure the throw of each rocker, I will do that tomorrow. I think that all the time and money was well spent. STRANGEST THING, I put the boat in the water this morning, cranked it up, 15 minutes before it started acting up, AND, AND, AND the range that causes the backfire, MOVED higher in RPMs, was 2900-3500, now 3200-3800 any ideas?

 

[ibrw1]

Hammer down from idle to 4800, not one pop, if I gradually move into 3800-4800, POW, one after another, and not equal distant. Random pops, any more ideas?

 

[biggjimm]

This is just a thought. We had a engine in one of our drag cars that had a similar problem. Spent countless hours & $$ trying to figure it out. Ended up pulling plug wires off one at a time & found the cylinder where the problem was occurring & after tear down found a tiny burnt spot on one of the valves. Yours might not be directly related to one cylinder but then again it might & removing the wires may reveal at least the offending cyl as it did in our case. Good luck. Jim.
By the way, this particular problem did not show itself in the many compression checks we performed before finally figuring it out.

 

[ibrw1]

Biggjimm, if I narrow down to which cylinder I would still have to pull head anyway? If that's true true, then pull em both right. But if the issue seems to be related to all cylinders then keep going forward. Pulling heads would not be beneficial. That's good stuff. My questions continue. What if engine has two issues? One being its running too hot, and two, who freakin knows. If I run at 2000 rpm it's not hot enough to cause issue (bumping speed up into danger zone) funny that issue has a range? (3200-3800) Did yours? Was yours tied to temperature? I will look up recommended temperature range of 1999 5.0? So, Is there truly such a thing as hot spots? Muck and scuz sucked up into engine, settles in, makes hot spot. I got a guy who says he sucks engine clean as part of winterization process. Is this a load of monkey dung?

 

[biggjimm]

If you could narrow it down to a particular cyl then pulling the head would probably be in order. But you would know it wasn't fuel or ignition related but rather something in that one cyl.
I'm not sure about hot spots in the coolant passages or sucking out engines, I've never heard of that but maybe it is a common practice??? You can get hot spots in the combustion chamber as someone mentioned earlier, carbon buildup in there that stays hot & tries to ignite the incoming fuel while the intake valve is still open.
It's been so long ago that we had that issue on our race car that I don't remember any of the particulars as far as a certain temp range or rpm range.
I wish I could help you more but pretty much everything has been mentioned & you've tried already. Well, except one thing, & if we knew what that was you wouldn't be having this problem anymore, huh?
I really thought alldodge had it nailed early on when he suggested it was running lean but it seems you've ruled that out. I'm at a loss. I've got to take some parts to my machinist tomorrow & I'll pick his brain some, he's been racing alcohol injected dragsters for years & he knows more about the inner workings of these internal combustion engines than any 5 guys I know. I'll see if he has any ideas. Good luck

I just thought of something, you might want to try adjusting the valve lash??? Definitely if you narrow it down to one cyl I'd try that before tearing it apart. Your symptoms could perhaps be a rocker arm that is a tad bit out of adjustment. I think there is a thread in the stickies at the top of the main page that details how to do it if you're not familiar with the procedure. It's not hard but you have to pay attention to which cyl's you're adjusting & when.

 

[ibrw1]

BiggJimm,

Wish I would have thought of the plug pulling yesterday, although the river was full of boaters, it was difficult to test everything with all the traffic. I'm going out to the Mississippi in two weeks for 4 days loaded with test equipment and tools. I will replace the thermostat with the 160 degree as required by Mercruiser and replace the impeller "lower unit" before I go. I have a alpha-1 Gen-2 that is 2 years old. Replaced entire unit the day I figured out what a wing dam was, and where it was. My boat has electronic ignition and the module is attached to the Port side of the engine. Does anyone have any experience with this device?