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6th gen VFR800 surging gone with IAT unplugged?


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I bought my first VFR in February and I love it, but...it has that surging issue that I've seen a lot of people dealing with. I finally broke down an installed a PCV and installed the stock Dyno tune. That did help, but what actually made the surging disappear was the fact that I forgot to plug the IAT sensor back in while digging around under the airbox.
So what am I getting at? Is there a way to  simulate whatever it happening when the IAT is unplugged using the PCV? I'm honestly fine with leaving it unplugged if that's what works, but I would prefer to have the PCV do its job and the IAT do its job as well. Thanks in advance, you've all been such a huge help already!

 

 

 

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IAT is Intake Air Temperature sensor. It is used to adjust the fuelling based on air temp. So a bit richer in cold air & a bit leaner in warm air.  The 6th gen runs fairly lean. So without checking the manual I'd suggest it defaults to a richer AFR when IAT is AWOL. Thus lean surging is reduced. 

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Just a thought - try plugging it back in and using the key method to defeat closed-loop control.  Key on, kill switch on, let ECU boot and injectors fully prime (whine noise stops).  Turn key off...turn key on, but hit starter button before the whine noise stops and the dash lights finish their start up sequence.  No closed loop control after that, no highway drop off and surging for me.  Stock 2002 with nothing (until I install a RB and headers in the next few weeks).

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FWIW, when I forget to do the key dance, I am reminded how annoying closed loop control is as soon as I get on the highway!

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So far I haven't gotten the key trick to work for me. Installing the power commander did help a little, so now I guess I just need to figure out how to build a tune around however it is my bike is running without that IAT sensor installed.

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Us old guys just unplug the O2 sensor and stick a 30 cent resistor in the connector (leaving the actual sensor in place).  No more closed loop.  No more surging.  A bit better off idle throttle.  I still get 45 mpg on roads that aren't twisty enough to stay above vtec threshold.  The only downside is spending all that money on the mod.  😝

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You can just unplug O2-sensor and forget about resistor.

All that does is connects heater-circuit and prevents ECU from throwing code.

Functionally same open-loop operation either way because data-circuit is disconnected.

 

Measure and test IAT according to service-manual.

Bad out-of-spec IAT will cause ECU to adjust fuel incorrectly because it'll think you're riding at -20C or 100C all the time.

I suspect your test of unplugging IAT shows it's bad. And/or its wiring to ECU is bad.

 

You really need to disable O2-sensor if you want to use PC for tuning.

Factory ECU will use O2-sensor to correct AFR back to factory levels.

It "learns" that fueling has been modified and will implement an opposite adjustment to fuel-maps of what was adjusted with PC.

Basically it'll negate all PC adjustments after while.

 

You may also have vacuum leak.

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1 hour ago, DannoXYZ said:

You can just unplug O2-sensor and forget about resistor.

All that does is connects heater-circuit and prevents ECU from throwing code.

 

 

Exactly.  I don't like seeing idiot lights on the dash.  I think 30 cents is well worth the investment.  😉

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I have both O2 sensors unplugged and have 30 ohm resistors in place. I guess I'll start by replacing the sensor since it's fairly inexpensive at $35.

 

I would love for the issue to be a bad sensor, but wouldn't it throw a code when plugged in?

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1 hour ago, AnfernyRocks said:

I have both O2 sensors unplugged and have 30 ohm resistors in place. I guess I'll start by replacing the sensor since it's fairly inexpensive at $35.

 

I would love for the issue to be a bad sensor, but wouldn't it throw a code when plugged in?

 

I'm not 100% certain, but I'm fairly confident that the resistors need to be 330 ohms, not 30. They should be 1/2 watt or better.  Did you slip a keystroke or are they actually 30?  If so, that may be the problem . . .

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Yeah, that's a typo haha. 330 ohm resistors per the instructions I found elsewhere on this board. Right now my action plan is just to replace the IAT sensor and test. Will report back ASAP.

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A vacuum leak is a distinct possibility. I ordered some vacuum line and will be pulling the throttle body in order to replace as much of it as I can. The bike is nearly 20 years old at this point and just needs new hoses, so I'll be doing that and the coolant lines.

 

I guess what's weird is that the bike only starts surging and leaning out once it gets to temp. Or is that weird?

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4 hours ago, AnfernyRocks said:

A vacuum leak is a distinct possibility. I ordered some vacuum line and will be pulling the throttle body in order to replace as much of it as I can. The bike is nearly 20 years old at this point and just needs new hoses, so I'll be doing that and the coolant lines.

 

I guess what's weird is that the bike only starts surging and leaning out once it gets to temp. Or is that weird?

 

That typically points to O2-sensor as being issue because ECU will stay in open-loop with rich AFR. Only when everything's warmed up will it consult O2-sensor feedback and go into closed-loop operation. Then if you've got bad data coming from O2-sensor, it'll make incorrect AFR adjustments and cause stumbling. Then it tries to correct in opposite direction, and back & forth.

 

 

Not just hoses, but anything rubber in intake-tract can crack or shrink over time. Rubber insulators holding intake-manifold are common source of leaks. 

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7 hours ago, DannoXYZ said:

 

That typically points to O2-sensor as being issue because ECU will stay in open-loop with rich AFR. Only when everything's warmed up will it consult O2-sensor feedback and go into closed-loop operation. Then if you've got bad data coming from O2-sensor, it'll make incorrect AFR adjustments and cause stumbling. Then it tries to correct in opposite direction, and back & forth.

 

 

Not just hoses, but anything rubber in intake-tract can crack or shrink over time. Rubber insulators holding intake-manifold are common source of leaks. 

The 02 sensors aren't plugged in, so I'm not sure they're the issue. I'll be sure the check the rubber manifold insulators when I'm under there. Thanks!

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With the IAT unplugged aren't you seeing a flashing 9 code? Something definitely wrong if you're not!

 

With the IAT unplugged the ECM will assume a default temperature of 25degC 77degF. Actually the IAT is used for intake air density calculations.

 

There are some simple checks in the Service Manual you can carry out for the IAT.

 

Without the IAT the further your intake air temp is away from the default of 25degC then the further your A/F ratio could be skewed.

 

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15 hours ago, Grum said:

With the IAT unplugged aren't you seeing a flashing 9 code? Something definitely wrong if you're not!

 

With the IAT unplugged the ECM will assume a default temperature of 25degC 77degF. Actually the IAT is used for intake air density calculations.

 

There are some simple checks in the Service Manual you can carry out for the IAT.

 

Without the IAT the further your intake air temp is away from the default of 25degC then the further your A/F ratio could be skewed.

 

I am getting 9 flashes, yes. I've got a new sensor on the way as I don't have a ton of time for diagnostiscs ATM. I'll post something in this thread when I get a result.

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  • 2 weeks later...

UPDATE, APRIL 29, 2021

 

I don't have a ton of time to tinker, so I just rode my bike with the IAT sensor unplugged while I waited for a new one. I finally got the new one installed and.....solved, I think! I've only gotten out once so far but it doesn't surge anymore with a new IAT sensor! It may be worth a look if you're having issues....

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