Misfiring under acceleration

Misfiring under acceleration - It's Back...

This is a bit of a long story so please bear with me.
About 2 weeks my '99 VFR developed this 'misfiring' issue (I'm not sure if it is misfiring but thats what it seems like). It only happened when under heavy acceleration. Usually when I was throttling past 5-6000rpm the revs would just drop right down to 1000rpm then flick back up, jolting the bike violently. It only happened when under heavy load; while just puttering around normally the bike didnt miss a beat.
It took a few times until the FI light actually came on, so I checked it and it said the AIT sensor was the cause. I checked this in accordance with the manual and found nothing wrong, reset the memory and ran it, the FI didnt light up. After a test ride the same problem happened and the FI light diagnosed the AIT again, so I double checked and again found no problem.
After another test ride the bike seemed fine, the problem had disappeared.

I've only riden it a handful of times since and no problemo.
<Fast forward to today>
Rode the bike to work this morning, 30min trip on the fwy no probs. This arvo on my way home it starts misfiring again, not only under heavy load but just normal cruising and low speed. I got it home and checked for the FI light but it hasn't lit up.
So any idea what would cause this? My nuts are taking a flogging with the constant slamming into the fuel tank >_<

-Its a '99 800
-owned it for about 6 months now and never experienced this before except one time when the vac hose to the MAP sensor came lose.
-powercommander 2
-I also checked the throttle position sensor and that was fine too.
-Spark plugs recently replaced
-Leads still in good nick
-injector cleaner ran through a few weeks ago

 

Update: Problem seems to disappear some days and reappear other days. That said I have noticed some sort of pattern in that it usually happens when the bike ambient temp gets up around 88-90+ C.
Also I haven't had the FI light come on again.

Any help on this would be great.

 

fuel cap vent plugged causing vaccum in tank?????

Pop cap open and listen for vaccum "wooooossshh!"
try running with cap open to test.

 

Theres a fuel cap vent now? I'm guessing it would be found somewhere in the vicinity of the fuel cap?

When I open it to fill up I've never heard the vaccum pressure release.

So to 'test' it as you say with the fuel cap open, what should I be looking for?

 

Speaking from experience... most faulty electronics and sensors like to fail at temperature. Is there by any chance you can just do a sensor swap and try it. It's also not uncommon for sensors to give readings of "ok" when testing even if there faulty. Unless you can test that sensor "right" when your having the issue I would count on a 100% reliable test.

Not saying that the computer is right in telling you the AIT sensor is the issue... but I would troubleshoot it a bit more. Swap one if you can. Or if you can buy one and return it if it doesn't work even better.

 

Return an electrical part?? Not around here!

 

Test the sensor when the bike is hot. Heat can cause an electrical component to show as failed, but show as good when cold.

Sounds like a fuel issue to me. Check all fuel related issues. Check the simple stuff first (fuel filter etc...)

 

Are you talking about the IAT sensor? The checks in the manual (mines an 04 but I think they may be the same) look fairly straight forward, unplug it and check the FI light, check resistance of the sensor, voltage at the plug then at the harness. If all the that checked good I would look in to the resistance at different temps. Start with the IAT sensor cool and then heat it up, you should see the resistance change with the temp. If the resistance shows a big change when you hit the temp where you usually have problems you've found it. If you still can't find the problem, start looking at wiring from the sensor to the ECU harness. Good Luck!

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Hello again.
Well the problem escallated to the point that whenever the bike reached about 75+ deg C it would litterally cut out and would not start again til the temp dropped substantially. When it cut out the error code would appear for the AIT sensor. So I called up honda to get a new one, $82.50
I don't know how Honda justify their prices or sleep at night knowing they have an 8000% retail mark up. Anyhoo, stuck the new sensor in (the wrong way at first) while still muttering to myself about what I just paid. Chucjed the lid on, took the bike for a long spin and it ran perfectly :smile:

 

Bah! Same prob still persists. I find it weird that it was working perfectly fine for two days straight, then all of a sudden it starts jolting and missing again. Though today I discovered something bazar, I was riding on some roads that badly need to be resurfaced, potholes, rasied bitumen, drain holes etc. And every time I went over a major pothole that gave the bike a good jolt, it missfired. Any ideas what would cause that?

 

Still sounds electrical... A loose connection perhaps, maybe a broken wire. Since the fault code was leading to your sensor it could be in that circuit. It could simply be at the sensor connector. Maybe when installing the new one it temporarily fixed up the connection until the next pothole.

 

It might be worth getting some electrical cleaner spray (Radio Shack), unplugging every connection that you can find and/or get to, and cleaning it, particularly in the areas that are subject to heat, as that was an issue at one point.

It could also be the On/Off switch on the handlebar or the ignition switch. It sounds exactly like a dirty or corroded electrical contact on the ignition side of things.

Was the injector cleaner ran through with the new spark plugs, or were they changed afterwards? Spark plugs can be "semi-fouled" and do really funny things sometimes, especially heat and load related, although it tends to be misfiring versus a complete cutout. I know of one case where an air cleaner element was cleaned (with a spray cleaner of some type) and not allowed to dry long enough, and it fouled a new set of plugs. It started fine, idled fine, and ran up through the RPM range fine under LIGHT load. As soon as a heavy load was a applied, it started cutting out and missing constantly. New plugs were put in, and all the problems went away. Not that this is relevent here; maybe more of a longshot.

 

I've read this before. The last guy just started up his bike and shook all of the connections and bracket points throughout the electrical system until he reproduced the fault. It was a broken wire, if I'm not mistaken. Once he fixed that, ran fine.
Now that I think of it...I had the same problem with my Yamaha 600. Mine was due to a faulty fuse connection at the fuse box. I could reproduce it by wiggling the wires. Fixed connection, fixed problem.

 

Wow,some really good sugestions,I am always surprised by the depth of knowledge you guys have, Anyway,did you check the resistance of the plug wires,or for a broken plug wire. If not that; it could be a faulty spark plug,sometimes a spark plug can be a real gremlin.How about doing a resistance check of the coils. I have had experiences when an engine is under load, if it has a weak spot that is when it will show up. When you find the problem please let us know . eddie

 

Wiggle the key, sometimes the ignition connections get dirty.

 

Yo Mason, What the F is that supposed to mean?

 

On the bottom of the ignition switch are copper contacts. Sometimes they get dirty, corrode, get hot - all conditions that inhibit good electrical contact.

If wiggling the key reproduces the symptoms the owner described, then you've found your problem. If not, then it's one more thing you don't have to look at.

The pothole causing the misfire is definitely is a loose electrical connection. I was just trying to help him narrow down his search for it - among the many connections in the wire harness.

 

Hey RockVFR:

Believe it or not, after you clean all your electrical connections I would test your electrical system.

It could very well be a bad regulator/ rectifier working intermittantly!

BZ

 

Hey 45; Sorry for the misunderstanding,I do recall him saying that,but I still cant figure out why its worse under load. Usually the missing under load is coil,coil wire or spark plug related. Also I really didnt fully understand if its a miss or and engine shut off. If its a shut off, then I do agree with you,its most likely a wire making intermittant contact. eddie

 

Rock,
If you haven't already done it, clear all the old trouble codes out of the ECU memory. Then next time you have a problem recheck the codes. Since it seems to be an interment electrical problem the ECU may have stored some codes that would give you a good place to start. For example, if it gave you a code for the temp sensor, you could try putting an ohm meter on the wires that connect to the sensor and then move them around to see if one is bad. Good luck, intermittent problems can be a pain to track down.

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