Bogging down when hot

[freakazoid]

My '96 R1100RT has developed a strange problem. A couple months after I first bought it from my father in law, it started hesitating when I'd accelerate from a stop after coasting to a stop with the clutch engaged. Revving the engine once or twice while stopped would usually clear that.

 

The problem gradually got worse over time, and eventually the bike started bogging down once it got hot. It appears to suddenly start running very lean. My morning commute is about 50% city streets followed by freeway when I'm going to work, so the bogging down starts about the time I get on the freeway, but last night I was riding a shorter distance to come home so the problem didn't start until I was almost home. Once the problem starts the bike will start to slow down and I will have to downshift to stay at freeway speed, and I will have no power to pass. The engine also idles rough and pops a lot more when I let off the throttle.

 

The reason I'm pretty sure the problem with accelerating after coasting to a stop and the bogging down when hot are related is that the problem has now evolved three times: the first time, then twice after San Jose BMW thought they had it fixed.

 

The first time I brought the bike to SJBMW, they adjusted the TPS, replaced the air cleaner, synced the throttle bodies, changed the plugs, and adjusted the valves because the bike was about due for all of that anyway. The problem stayed away for 4-5 days.

 

The second time, they did some diagnostics on the O2 sensor at my request and then replaced the coil thinking something might be wrong with the ignition. The coil had a crack in it so it was plausible that expansion when the bike was hot was causing problems.

 

The problem definitely seems temperature related: it only seems to happen between five and six bars on the oil temp gauge. Frequently after I get off the freeway and start riding around town at a more leisurely pace the problem seems to go away.

 

Does anyone have any idea what might be going on? Nobody at SJBMW seems to be able to figure it out, though fortunately they have not been charging me for overlapping diagnostic work; they seem pretty good about that sort of thing. I just dread bringing the bike back to them again and being without it for a week and then getting it back and having the problem start again after a few days.

[bmwmick]

How many miles on it? When was your last fuel filter change? Is the fuel pressure good? 43psi is what it should be.

 

Mick

[ninermatt]

The oil temp sensor is starting to go bad I had a similar experience on my GS. The dealership couldn't figure it out I'm pretty familiar with the the way motronic works(automotive applications) and replaced it myself even though the dealership told me it wasn't the problem and basically I didn't know what I was talking about. that was about 3 years ago and haven't had a problem with it since. the parts about $30 and maybe a half hour of work including Tupperware.

[Chip]

I had similar problems with my '96 RT. Problem went away with new plug wires....never in a million years would I have thought they were so important. I replaced them as a last straw. It worked.

[Phill]

I had same with my 96 rt in my case found it to be bad electrical contacts both on injectors also on plug leads at coil. Used electrical cleaner on any electrical connection i could find and problem disapeared. Hope you have same luck.

[freakazoid]

Wow! Thanks for all the great replies!

 

A friend of mine suggested fuel tank venting as well and said that the oil temp sensor is easy to disconnect, so I will try those two. I *think* they put in a new fuel filter in November, but I'll have to check my records (I'm at work right now). SJBMW doesn't seem to have checked anything fuel related like pressure so I'll have them check that and replace the fuel filter if it has not been replaced.

 

My friend also mentioned the plug wires and suggested wrapping electrical tape around them, I guess the insulation has a tendency to degrade. I don't know off the top of my head if SJBMW changed the wires when they did the plugs so I'll check that when I get home.

 

Again, thanks a lot. It's great to know that I'm not the only one who's experienced this problem; I hadn't really been able to find anything but surging problems in my searches so far.

[Ken H.]

Tank venting occurs to me too. Ride it once with the filler lid popped open and see what happens.

[freakazoid]

Well, with the cap open the problem did not occur, but I have not yet reached the point where the problem is happening reliably and when I closed the cap the problem did not start again. Tomorrow morning I'm going to try riding to work with the spare key in the cap to see if there is a sucking sound if I pop it while the problem is occurring or if that makes the problem stop.

[freakazoid]

Popping the gas cap while the problem was occurring did not seem to make it go away, but at this point it feels more like standard surging when I'm on the freeway, and the major issue is that when I do any amount of coasting with the throttle closed and the clutch out the engine acts like it wants to stall when I rev it again.

 

The problem primarily seems to happen between 3000-4000 RPM too, though the "wanting to stall" thing happens from idle sometimes.

[Rider1200RT]

Is the charcoal canister still on the bike if it has one??

[4wheeldog]

Popping the gas cap while the problem was occurring did not seem to make it go away, but at this point it feels more like standard surging when I'm on the freeway, and the major issue is that when I do any amount of coasting with the throttle closed and the clutch out the engine acts like it wants to stall when I rev it again.

 

The problem primarily seems to happen between 3000-4000 RPM too, though the "wanting to stall" thing happens from idle sometimes.

I had a similar issue when I had water in the tank. Just a thought.