Surging at 3-4k RPM

[Flyingwombat]

So this started a while back on my 855i it started surging around the 3 to 4k RPM range. It was due a service so I did.

- New Battery, Oil and filters, Spark plugs
- Replaced all the worn gaskets in the airbox and around the IACV
- Cleaned Throttle bodies, IACV, Throttle springs etc.
- Balanced throttle bodies
- Replaced fuel filter
- Replaced ruptured oil drain tube in airbox
- Replaced fuel hoses
- Reconnected the airbox to fuel tank hose (which for some reason was unattached)

Was absolutely grand for about 500 miles then the surging returned.

- Balanced the throttle bodies again (balance was lost), added some loctite because screws felt loose.

After about 50 miles the surging returned.

- Was not far out of balance.
-Stripped down screws, cleaned springs, reassembled, balanced the throttle bodies, added more loctite and some silicone gasket to the screws for good measure.
- Replaced vacuum hoses.

Riding was absolutely fantastic for 80 miles, then surging worse than ever, almost bloody dropped it on a tight manoeuvre. If I hold the throttle at about 3k it's surging by 500 RPM either side, which means it's practically unrideable, and deteriorating.

Surging seems to stop at around 6k RPM.

Any ideas?

[Lee337]

Don't know if the fuel map on the ECU could be the issue. It seem sodd that the surgeing keeps returning, almost as if the ECU is resetting itself, especialy given the distance it appears to take to upset the balance.

Sadly, I'm no ECU guru so am clutching at straws here.

[Flyingwombat]

Also forgot to add why I replaced the vacuum hoses, the bike was cutting out when the clutch was pulled once warm. This isn't happening now, even though the surging is occuring.

I have plugged in TuneECU. It tells me that I have an un-registered bike and to contact them...

I have thought about the 12 minute tune but probably best not when it's 5C outside as it recommends a much higher air temp.

Now you mention fuel map. When I got the bike the fuel pressure regulator to airbox hose was not fitted. I thought that might be the mechanic, because they did a few other weird things too.

[ghulst]

If the fuel map is off, that might explain it, but then the 50-80 mile rides feel very random.
Have you tested for air leaks around the connection between carbs and engine?

[Flyingwombat]

I sprayed a bit of wd40 around the IACV to see if there was a leak there because the housing at the screw is cracked. Seems OK though. I also sprayed around the throttle body, where I could reach and the hoses once I fitted the new ones to see if they had leaks.

I'd be surprised if it's a mapping issue because the map has not been deliberately changed since it was bought, and I trust the previous owner on that one.

[Flyingwombat]

Possibly also worth saying no engine warning lights are on.

[wyaTT37]

Some standard things that I'm sure you've checked...make sure vacuum hoses are on the correct IACV nipple. One of those nipples is intentionally blocked. Also, the hose that goes from the air box back to the computer should be installed. Lastly, does your bike have an O2 sensor delete plug installed?

[London_Phil]

Fuel pump or fuel pressure?. can you check the actual pressure?, I noted you mentioned fuel pressure pipe was disconnected. maybe it was disconnected for a reason?

[Flyingwombat]

Fuel pump or fuel pressure?. can you check the actual pressure?, I noted you mentioned fuel pressure pipe was disconnected. maybe it was disconnected for a reason?

Yup I thought it might be, but according to other forums it does nothing because the pressure differential is basically zero. So I think it less likely. The surging seems odd if it's a pressure problem, would indicate pump ramping up and down...

[the slow heart]

It is always hard to diagnose issue when lots of components are disturbed. Did you clean the eventual tank slime sediments and clean pump strain mesh?

"- Reconnected the airbox to fuel tank hose (which for some reason was unattached)"


I have the same bike, but can't figure the hose you mentioned. May you explain more, please.
That must be the vacuum hose to the regulator?

[Flyingwombat]

It is always hard to diagnose issue when lots of components are disturbed. Did you clean the eventual tank slime sediments and clean pump strain mesh?

"- Reconnected the airbox to fuel tank hose (which for some reason was unattached)"


I have the same bike, but can't figure the hose you mentioned. May you explain more, please.
That must be the vacuum hose to the regulator?

Yes, vacuum hose to fuel pressure regulator. I have read it does very little, so doubt that's the cause.

Cleaned sediments and mesh.

To answer above from another poster, all vacuum hoses on the IACV are attached to correct nipples.

[the slow heart]

I would try to isolate the problem by checking/replacing just one item at a time and then the next etc.

New battery? Would attach volt meter and observe what happens round 3k rpm, to verify alternator is ok.

New fuel filter? Would test with bypassed pressure guage to confirm 3+/-0.25 bars, to verify also pump is ok.

Vacuum regulator? Would deattach the hose and test.

iacv lines, are they soft silicone that might shrink on suction increase or be pinched by oher items?

ecu mapping, why would not reinstall the standard map for the model and be able to see all the tests and diagnostic data? Prev setup does not matter since all is disturbed in fuel system. I had similar issue with outdated map.

Note carefully the throttle grip position for this 3k rpm in neutral. Then remove the tank and airbox, and observe in detail the throttle items movement when you adjust throttle arround this position. Any slack anywhere? Stripped threads are no good.

All sensors have their nominal testing parameters in the manual. You could check them also. I would start with the tps.

[Flyingwombat]

I would try to isolate the problem by checking/replacing just one item at a time and then the next etc.

New battery? Would attach volt meter and observe what happens round 3k rpm, to verify alternator is ok.

New fuel filter? Would test with bypassed pressure guage to confirm 3+/-0.25 bars, to verify also pump is ok.

Vacuum regulator? Would deattach the hose and test.

iacv lines, are they soft silicone that might shrink on suction increase or be pinched by oher items?

ecu mapping, why would not reinstall the standard map for the model and be able to see all the tests and diagnostic data? Prev setup does not matter since all is disturbed in fuel system. I had similar issue with outdated map.

Note carefully the throttle grip position for this 3k rpm in neutral. Then remove the tank and airbox, and observe in detail the throttle items movement when you adjust throttle arround this position. Any slack anywhere? Stripped threads are no good.

All sensors have their nominal testing parameters in the manual. You could check them also. I would start with the tps.

General update - tested while stationary

- TPS appears to be at 0.6% when closed, so need to adjust this.
- When at 3500 rpm TPS is constant but RPM is fluctuating by +/-200, IGN values also constant.
- When I turned the ignition key for the first time since my original post it made an odd noise, either the IACV or the fuel pump? Started fine though.
- Map is 9880 but I have not read it to break it down.
- Detached vacuum regulator hose, no appreciable difference.

Will take tank off later today to inspect vacuum lines and re-check balancing.

[the slow heart]

Tough fight you have    

Is there any chance for debris in fuel system?
I have not cleaned injectors for instance, but two years ago sipped a bottle of Bardhal injector cleaner in the tank, just for the peace of mind.

[Flyingwombat]

For more info, I noted the throttle bodies were very slightly unbalanced, I have balanced them to dead on and used instant gasket and threadlocker to try to prevent them slipping, maybe this time it will work! I have re-tubed one of the vacuum hoses that was slightly kinked, but wasn't pinched shut. Airbox went back on then I reset the TPS.

Idle is now at 1200 RPM and fluctuating by +/- 80 RMP rather than +/- 50 RPM.
Sounds quite rough idling even though the engine is warm.
Revving to 3000 RPM, there's about a +/-100 RPM fluctuation, better than +/-500 RPM...
Adjustments : IACV set to 120, Idle fuel trim @ 16.4, Long term fuel trim @ 0
Still with 1601 Indicator Lamp error code.

Debris in the fuel system... Possible but I worked clean when I changed the filter. Will try running some injector cleaner through it.