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Head Gasket Leaking?

It lives. I got the last parts. With the new tensioner on, the chain didn't slip. Got it started last night after some frustration with stuff not plugged in correctly. Apparently the crank sensor connector is the same as the acc outlet connector, and they are in the same general area. Ran nice and smooth. Just warmed it up before I quit. Didn't want to stay in the shop with the fumes anyway.

I still need to put the front suspension back together, and all of the covers & hood. But it might be about done.

Hopefully now it will stop loosing coolant and idle after it warms up.

I've started it a couple more times, but we got warm weather for the last week, so I haven't riden it yet. However when I bought it in December, I was informed of one small problem. He said it had a small oil leak he never found. When I got it home, it looked like the oil cooler o-ring was leaking, which I replaced when I changed the filter. Kept it in the shop half the winter and had no evidence of a leak. Assumed that fixed it.

Now I see a small oil slick under the engine. Only a couple ounces, but still a leak. After all I had apart, couldn't have been much more difficult to replace the pan gasket. But now that it's back together, stupid oil leak.

Good news is cold weather coming back Tuesday. I hate to say it, but after last winter I'm glad the ground will get to freeze a little bit before the snow starts. Maybe hold off until after Thanksgiving travel, then let it rip.

Last fall we got 2 inches of rain that transitioned right into 6 inches of snow. Ground under undisturbed snow never froze. First time in my life I'd ever seen that. But getting my loader stuck in Mud, in Northern MN, in January, while trying to pull my tracked skid steer out of the not frozen (hidden by snow) creek wasn't right. Tearing up my lawn in March (before warm weather hit) by clearing snow away from the house wasn't cool either.


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Finally have a few inches of snow and tested it out. Put a couple miles on and it seemed to run great. It did push a little coolant out, but settled about 6oz low after it cooled, and I had somehow forgotten to put the cap on all the way. I'll run it again soon, hopefully.

Oil leak didn't even cover the bottom of the pan I put under it (for the last two months), but it was almost a quart low. Filled the oil before I took it out, and parked it inside over a clean spot to see if it is still leaking. Don't know where the rest went, because the pan surely didn't have even a pint in it.
As most know we still haven't received much snow. I've been out once with the new sled and the trails weren't good. It's been so warm this week the snow is melting. So I figured I'd take the yamaha for a spin around the fields and park it back in the shop until the temp drops again. It hasn't moved since the last test I posted about.

It fired right up, then my daughter said she was ready for me to put a bale of hay in for the horses. 15 minutes later I'm back and it isn't running any more. Engine was warm, so it must have run a bit. It started back up, but reluctantly, and did not want to idle, just like before. I thought maybe it just needed to get moving for a few minutes, so I went for a spin around the field. It still had good power, and seemed to run well off idle, but would die almost as soon as I stopped. Rough idle at best, then die and be reluctant to restart. I had to just nudge the throttle to get it to start again.

Oil wasn't low at all, which was good. There was a small (3" diameter) oil spot on the packed snow under it. No coolant spillage like before when it would warm up. When I pulled it into the shop, it suddenly started idling perfectly again, but only for about 5 seconds, then ran rough and died. Before I assumed it was normal to idle a little rough because it seemed like it always did, and that is usually a side effect of squeezing a lot of hp out of a small engine. But the sudden change (and stopping) showed me a clear difference. It really isn't idling correctly when warm.

After a couple hours I started it up again, fired right up and idled perfectly again. I haven't been out yet today to check coolant or oil levels. It really doesn't feel like a mechanical problem to me. It feels like a sensor malfunction or injector failure, but only at temp. I wish I could see what the computer is seeing when it gets up to temp. Or could measure how rich it was cold vs warm.

Has anyone had a similar problem, or know what I could check? The 5 seconds of good idle really changes how I'm looking at the problem.

I like the power band this engine produces, the sled has great acceleration (for me). Rear suspension is good. There is a lot that I like about the sled, but not running properly is a no go in my book. Clearly I've lost too much brain power over the last 10 years to figure the problem out myself. I certainly can't have my daughter ride the yamaha on a trail ride. So it is currently useless to me. I need to find (and fix) the problem or give up and sell it.
Have you tried to adjust the idle speed? Maybe it's just set too low. On cold startup it will run at fast RPM and then slow down as it warms up. If you base idle is too slow, it could stall.

Then there the throttle position sensor that could need adjustment. There are a few threads on that subject. Just have to go into diagnostic mode and see what it's reading.
Yes. I adjusted the idle a couple times during the year last year, in an attempt to find my idle problem then. Then again after the head gasket, while it was running well and at temp. I have the full manual and made all of the related adjustments accordingly.

I went out today to see what it did exactly. I'll fill in details later when I have time, but long story short, it has a definite problem. Completely Intermittent, idles great one second, then very poorly (like on one cylinder), then back to great. I took a couple videos too.

The only good news is that the coolant level was still perfect. Oil level was good too, but I think that is a long term slow leak, that doesn't get worse while running.

The problem is very Intermittent, but definitely related to temp. Doesn't happen at all cold, to 95% of the time when warm.
I wonder if the temp sensor is off its readings. In diagnostic mode, start it and watch the temps as it warms up and if it changes when the motor starts running rough or at what temp.

Is it possible that an ignition coil is going bad where it's fine when cold but cuts out once warm? Seen that happen before. When it starts running rough, disconnect one and restart it. If it won't even start, that could be it. Reconnect it and disconnect the other. If it now starts but runs rough, then that coil is bad. If it does start but running the same, then that disconnected coil is bad. Another possibility is a bad injector in the same way.
Watch for an error code showing on the dash when it's doing this.
No codes present (not that I've seen). Temp sensor is showing a reasonable value when not running, and I haven't figured out how to start it while in diagnostic mode, but that would help I think. It would help to see the intake pressures while running, along with the temp, but it won't start in while looking at any of the sensor readings.

I woke up too early this morning and spent a while thinking about it. Most likely thing I could come up with is a coil. But I don't know how I'd test one while under the fuel tank and coil-on-plug. All I could do is hope to unplug one at a time if I can get a pick in from the side.

It is very unlikely to be mechanical (other than possibly an injector, but it would be the electrical side failing). So I'm looking for a failing sensor, failing ecm, faulty wiring (my second best guess), failing coil (best guess), or failing injector.

I am going to search the manual a bit, since I finally have a chance to sit down. Then maybe figure out how to post a short video of it running. It starts and idles around 24-2500 rpm cold, and slows down as it warms up. Once it hits a certain point it is idling just fine around 2100rpm. Then it remembers that it hates me and drops to about 1100 rpm, most likely running on once cylinder. Again pointing to a possible coil.
After looking through the manual and the pictures of the sensor readings. I see that it does have a code stored. Code 12, for crankshaft position sensor. Although I do remember when I went to start it the first time after I got it together, that I had the crank sensor wiring mixed up. So that is understandable. Intake temp was 11 degrees, and coolant temp was 80 degrees.
I just pulled the left panels. I might be able to unplug the LH coil, but in order to unplug the right, I'd need to remove the seat and gas tank.
Yeah, it is a PITA but after having done it 2 dozen times, it takes me like 10 minutes (15 with rear seat removal).

While there, do check the plugs. They might be fouled. You could put the tank back on loose to run it while checking the coils.

Now that code 12 could be pointing to the problem but it's just in history, not currently active. Not entirely familiar with that sensor but I believe it's in with the stator. It's another PITA to get to.
I'll clear the codes and warm it up tomorrow, then take the tank off and take a good look at the coils and plugs.