Misfire caused by a full tank of gas.

[Cali MD]

First time poster here and thanks in advance for all your help. Also, sorry if there is already a post about this, I found similar topics but they weren't related to fuel level.

Bike: I own a 2008 SV650S, it was laid down a couple times at low speeds and was pretty much all cosmetic. I'm the original owner and have been filling the gas tank the same way since 2008 and it currently has about 19k miles on it. This is a California motorcycle which means it has an evap canister on it.

Symptoms: After filling the tank above 65% full, it will misfire under load. It will idle fine, but i have to rev to above 4k so it doesn't stall. Also, when holding the throttle steady at around 5-6k it will intermittently misfire and jerk. Also, it blows white smoke after start up if it has a full(ish) tank, smoke goes away after warmup. again, these symptoms are strictly fuel level dependent

Story: I took it in to my local dealer who said i had gas in the oil, so they did a service. They also said my plugs were bad despite the fact they were brand NEW!!! Literally only had 0.5 mile on them before being towed to dealer, (bike start misfiring and blowing white smoke so I though I blew a headgasket or something). I had them replace the plugs anyway, the plugs they gave back had carbon deposits beyond what a half mile of misfire could do but whatever. They couldn't duplicate the problem and they didn't try filling up the gas tank after I told them it needed to be, (they blamed me for overfilling the tank). So now i'm looking for help here. Also, i'm a Mercedes mechanic so I can work on my own bike.

What I Have Tried: I R&R the evap canister to see if there was gas in it, which there was a tiny bit, i'd say maybe a cap full from a small water bottle. I let it air out over night, I also cleaned and lubed the check valve right before the evap canister, I attempted to shake it to see if the valve was stuck, but it wasn't.

I have no idea what to try next since everything I found online pertains to different problems or discuss components within the fuel tank, which is obviously inaccessible.

Thank you very much for any help.

[NTECUK]

Does opening and the gas cap change the symptoms.
Obviously being caution becaus the danger of gas coming out of the tank is very real.
As your aware it's easy to trap a hose or blockages to occur

[andy1472]

So they reckon you had fuel in your oil right or did I read that incorrectly? This is a separate issue then?


Did they even investigate this problem? I suspect not because they have fed you a line and also given you someone else's plugs back. Nothing worse than a bent garage!


Fuel in your engine oil (sump) means mechanical problems from valves worn, sticking, stem seals gone, piston rings gone ect ect. I suspect your bike has no mechanical problems like this. Fuel in your oil would also thin it causing major bearing damage. Get a 2nd opinion/compression test ect just be be sure.

[Cali MD]

Well, I ended up taking it to a different dealer, he said it is most likely caused by an intermittent misfire of one of the ignition coils. He will do a resistance test to see which one it is. Either way, I'll have hi, replace the bad one and put the old one in the back so if that goes out it will be easier to get to. Also, he said the fuel level is coincidental. I honestly don't believe that because the bike runs perfect %100 of the time when it's below a half a tank. That being said, I can't really think of anything else that can cause this, so if the resistance test really come back as being bad and it fixes the problem then that works for me.

Also, being a mechanic I always ask for the old parts back, so I'll be asking for this coil back also. If he is honest and does a good job I'll have him do the front fork spring and oil replacement also. (Since the bike has almost 20k miles on it).

Thank you very much for you help everyone.

[rictus01]

just something I've come across in the past along the "symptoms only show with a full tank" line, had a bike with a crushed HT lead, the weight of the tank when full was enough to make it run intermittently but symptoms disappeared when less than full, the coils were both checked and found to be fine, frankly left me baffled, eventually found it when the coils from another machine were used; careful examination showed a very slight distortion to the HT lead casing.

Just a thought ?

[Cali MD]

Ok, so the mechanic checked the coils, which ended up being fine, which baffled him also. He finally noticed that the front cylinder was flooding when the bike sat there, causing the white smoke and would then suck in air and run lean after warming up, which caused the misfire and stuttering. So, he found that the regulator on the fuel pump was bad, along with the seal, or so he says. Unfortunately for me, that regulator is roughly $350 to replace, not including labor. He said I can do the whole pump, but a whole pump is around $700 or so, geez. Anyway, I just hope after all this hassle it will run normal again.

Not I have to decide whether I still want to dump more money into doing the front fork spring and oil upgrade. I was hoping to keep my bike for a really long time, it has 20k miles on it now, 98% of which are from me (first owner), but I wonder if I should just get something different or stay with this.

[Bibio]

if you put your hand over the end of the exhaust and it sucks your palm in then you have an open exhaust valve.

white smoke is water vapour which could indicate water in the gas tank or a blown head gasket.

the other things worth checking especially if the bike has been crashed are the side stand and clutch switch.

one more thing is the TPS could have gone out of spec or is cracked inside.

have you tried putting the bike in dealer mode and seeing if there are any fault codes?

[atassiedevil]

Just a thought, there is a pad under the tank, it could be sagging enough to block the inlet under the extra weight of the fuel.
Have a look, might be something that simple, and its not at all obvious.
C.

[stuR]

Any luck? curious about this one!