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Discussion Starter #1 (Edited)
TL: DR; I've been having a rough time getting my k4750 to idle on it's own when cold and I've had to hold the throttle open for a few minutes to get it warmed up before it'll hold an idle without stalling.

Long description:
Over the last few months I've been having progressively worse issues getting my bike to idle when cold (and now sometimes even when warm). I adjusted the cold and warm idles to no avail.

Problem: When I start the bike the revs jump up to 3-4k then after 10-15 seconds drop down to the normal idle 1200-1400rpms but then after another 4-5 seconds the idles lopes and goes down below 1k then the bike stalls. Restarting it takes giving a bunch of throttle then the same thing will happen again, high idle, drops to below 1k stalls. To get the bike to run I have to hold the throttle open and keep the idle at 1.2 1.5k for a few minutes. When the issue started the bike would idle normally and run normally after the temps reached about 85-90 degrees but over the last few weeks i've had to wait till the bike is well over 100 degree's before it would hold an idle on it's own and even then it'll still stall on me occasionally when i hit the first 1 or two stop signs or red lights, then it'll run normally for the rest of the trip.

I've also noticed that my fuel mileage has dropped drastically. Before this issue i was getting 35mpg pretty consistently now I get 25mpg at best, so i believe the bike is running really rich (though I don't really smell gas from the exhaust).

I've tried new spark plugs, adjusting the idles both warm and cold and adding some seafoam (which helped make the bike run smoother when cold but hasn't fixed the idle problem).

Any idea's of what I should check next? I've read similar problems on other bikes can be caused by the throttle position sensor but not sure if that is relevant on our bikes. Any help would be much appreciated. The bikes old and i'm ready to start replacing pieces as needed just want to know what order I should start checking things. Thanks in advance for any help.

Similar problem on Yama's: Throttle Position Sensor Issues (Are you Experiencing Poor Idle/Mileage, or Stalls?) - Sportbikes.net

I'll add a video this evening.
 

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Try disconnecting the IAP sensor electrically (i.e. unplug its electrical connector, not the hose attached to it) and see if it helps. If it doesn't idle at all after disconnecting it, try holding the throttle manually to keep it at 1400 rpm or so. If that works try adjusting the idle to make it idle there, all the while keeping the IAPS disconnected. If you can get it to idle stably at 1400 or so rpm, after possible adjustment, with the sensor disconnected, then you probably have leaks in the vacuum hoses attached to it, or a failed sensor.

Consult the service manual on where the IAPS is and what it looks like.
 

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Captain Obvious ... because obviously it’s obvious
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If it's not a faulty IAP, it could also be that one of your injectors might be stuck partially open, hence the absurdly rich conditions. Leakdown test is also a good thing to try.
 

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Less than 2 but whose counting? But it's what I saw when I searched the problem. I was hoping the OP might see this 🤞and say whether or not he found the problem.
 

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If you're not going to make your own post, post what you've done.
 

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As of now all I did was played with the idle just before the end of last riding season. Set it to 1600rpm once it was warmed up and that didn't help. Everything else in the original post is what my bike has been doing. Which is why I didn't see a reason to start a new thread. The STVA is functioning . It seems like the exhaust smell has gotten a little worse over the past 3 seasons or so, my gas mileage went down which I just attributed to changing sprocket gearing. the idle problem started probably 2 seasons ago and gotten worse. By which I mean the temp at which it will idle on it's own has gone up to about 115-120F.
 

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My 750 K5 has always done this if a certain time has passed since the last service. To get a steady even idle that withstands winter starts without stalling the bike needs to be in a perfect state.

From experience I can say that the 5 things that improve this situation (in this order of importance) are:

New / clean air filter
Synchronise throttle bodies
TPS set correctly
Valve clearances in spec
Clean / new spark plugs

After servicing mine, the problem goes away until a few months or a few thousand Kms later, then it starts getting worse again. I just see it as the bike's way of telling me it needs a service.

On the other hand I do think that Suzuki were optimistic with the duration of the high idle function on cold starts, it backs itself off to early and too abruptly for anything other than the perfect bike in an optimal state. We have to consider that this is the first GSXR generation with an ECU controlled cold start / fast idle function, up to the K3, it was manual.

Playing with the throttle stop screws to raise the idle will only cause the warm idle to be too high, and also mean that you have to back them off again after servicing it.
 

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My 750 K5 has always done this if a certain time has passed since the last service. To get a steady even idle that withstands winter starts without stalling the bike needs to be in a perfect state.

From experience I can say that the 5 things that improve this situation (in this order of importance) are:

New / clean air filter
Synchronise throttle bodies
TPS set correctly
Valve clearances in spec
Clean / new spark plugs

After servicing mine, the problem goes away until a few months or a few thousand Kms later, then it starts getting worse again. I just see it as the bike's way of telling me it needs a service.

On the other hand I do think that Suzuki were optimistic with the duration of the high idle function on cold starts, it backs itself off to early and too abruptly for anything other than the perfect bike in an optimal state. We have to consider that this is the first GSXR generation with an ECU controlled cold start / fast idle function, up to the K3, it was manual.

Playing with the throttle stop screws to raise the idle will only cause the warm idle to be too high, and also mean that you have to back them off again after servicing it.
Thanks Clipper. I cleaned the filter around September. I ordered a valve cover gasket yesterday and I'll check the valves when I get it. I was going to pull the throttle body off and clean that up today but it's colder than I expected. Checked the tps in dealer mode and that's good. And I was just reading the TB sync procedure in the service manual last night since I've never done it and I've had the bike since '07. I still have to check the IAP and vac lines too. They look ok but I'll get a better look with the throttle body off.

Thanks for the tips Clipper
 

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I wouldn't be taking the TBs off. Too much hassle.

I reckon a full service will give you surprising results if you've not done one in 13 years!
 

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I wouldn't be taking the TBs off. Too much hassle.

I reckon a full service will give you surprising results if you've not done one in 13 years!
I planned on cleaning the TBs because they looked pretty gummed up last time I had the air box off.

I did have the valves adjusted twice but the last time was at least 4 years ago. The only thing I've never done that you mentioned is the sync. I had heard of it but always thought it was more of a set it and forget it kind of thing. Not something that had to be regularly checked.

Anyway I appreciate the help. I'll start knocking those things off the list and see how it goes.
 

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I wouldn't be taking the TBs off. Too much hassle.

I reckon a full service will give you surprising results if you've not done one in 13 years!
Just a little update. I adjusted the valves today and I test fired it with the airbox off. It started and idled great. I put the airbox back on and it was back to stinky exhaust and low idle.

So I pulled the IAP sensor plug and it stumbled for a second and the continued idling around 900 rpm. I then pulled the IAT plug with no change. Then I pulled the vacuum hose off the IAP and it came back up to a good idle. So obviously it's running way rich since a big vacuum leak makes it run better. Idk what this means but if any ideas come to mind I'm all ears. I'm still going to check the voltage on the IAP and sync the TBs but didn't have time today.
 

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I haven't really followed this (because, since it is a hijacked thread, I can't be sure about which symptoms belong to this or the original case) but you should note that a vacuum leak, "looks" to the ECU like open throttle, so it should lead to enrichment of the mixture. That said, I'm not sure the simple deduction that the mixture is lean applies here.

If the engine idles at 1600rpm and drops to 900rpm upon disconnection the IAPS, I wold be inclined to suspect that there is some issue with your vacuum metering, which you have compensated for by adjusting the idle speed. As I've said, I don't know which of the symptoms of the original case apply in yours as well, but if when you "start the bike the revs jump up to 3-4k then after 10-15 seconds drop down to the normal idle 1200-1400rpms" that would be an even stronger indication.
 

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I haven't really followed this (because, since it is a hijacked thread, I can't be sure about which symptoms belong to this or the original case) but you should note that a vacuum leak, "looks" to the ECU like open throttle, so it should lead to enrichment of the mixture. That said, I'm not sure the simple deduction that the mixture is lean applies here.

If the engine idles at 1600rpm and drops to 900rpm upon disconnection the IAPS, I wold be inclined to suspect that there is some issue with your vacuum metering, which you have compensated for by adjusting the idle speed. As I've said, I don't know which of the symptoms of the original case apply in yours as well, but if when you "start the bike the revs jump up to 3-4k then after 10-15 seconds drop down to the normal idle 1200-1400rpms" that would be an even stronger indication.
Yes I did hijack the thread but all my symptoms are the same as the discussion starter and there was no clear resolution to the problem in the thread. Like the DS, on a cold start-up it fast idles for a few seconds (only 2.5k but close enough), then as the STVA closes (or opens... I forget) it drops to 1.2k-1.4k idle for 2-3 seconds, then it continues falling until it dies. That's all only while the engine temp is cold. But the exhaust smells pretty bad at idle all the time which is why I assume it's too rich.

In my post from yesterday I wasn't very clear. When I fired the bike with the airbox off (IAP, IAT and vacuum line disconnected) it ran fine and came up to temp without stalling. The exhaust also didn't smell nearly as bad.

At that point I put the airbox back on and reconnected everything. When I started it back up the idle was already at 900 rpm and the exhaust smelled. I pulled the electrical connector off the IAP and it stumbled for a second but came back to 900 rpm. THEN I pulled the vacuum line and the idle cleaned right up. So with the IAP disconnected the ECU wouldn't know there was a leak and it would be getting air it didn't know about.

As I think about it I wonder if the IAP has a leak. That would explain a lot I guess seeing as the problem seems to be at it's worst at idle when the vacuum is highest. There is a diaphragm inside of it I assume? I do plan on checking the voltage on that.

Anyway, I wasn't sure if any of that information was relevant to finding a solution but I figured I'd put it out there with fingers crossed.

So I would like to ask forgiveness for my thread thievery and thank you all for any input you can give.
 

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Try disconnecting the IAP sensor electrically (i.e. unplug its electrical connector, not the hose attached to it) and see if it helps. If it doesn't idle at all after disconnecting it, try holding the throttle manually to keep it at 1400 rpm or so. If that works try adjusting the idle to make it idle there, all the while keeping the IAPS disconnected. If you can get it to idle stably at 1400 or so rpm, after possible adjustment, with the sensor disconnected, then you probably have leaks in the vacuum hoses attached to it, or a failed sensor.

Consult the service manual on where the IAPS is and what it looks like.
Just realized you already said this. I'm a big dummy.
 

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Yes I did hijack the thread but all my symptoms are the same as the discussion starter and there was no clear resolution to the problem in the thread
I didn't mean to scold you about not following protocol or anything; it's just that hijacking presents certain problems that make troubleshooting more difficult than it already is. For instance, unless you're very careful in specifying that each and every symptom of the original case applies, and that you've already followed all the advice given in the previous discussion and got the exact same results, we either need to ask all the details all over again (which we could have done in a fresh thread, without the potential for misunderstanding introduced by the hijack), or read a lengthy and potentially irrelevant discussion and confirm which of the points there apply.

You can imagine that the likelihood of terminating a very mysterious, grasping-at-straws troubleshooting session with something like: "Ah, no, I missed the point about the TPS in the previous thread and it turns out it was that all along", is not negligible.

At that point I put the airbox back on and reconnected everything. When I started it back up the idle was already at 900 rpm and the exhaust smelled. I pulled the electrical connector off the IAP and it stumbled for a second but came back to 900 rpm. THEN I pulled the vacuum line and the idle cleaned right up. So with the IAP disconnected the ECU wouldn't know there was a leak and it would be getting air it didn't know about.
Hmmm. Assuming you're sure that you are actually disconnecting the vacuum hose from the IAPS after having already disconnected the IAPS' electrical connector, and it makes a difference, then I can only assume that you are indeed running too rich and the leak introduced in the intake by disconnecting the vacuum hose, draws fresh air and leans out the mixture. I wonder whether perhaps your TPS is set too high, leading to a rich condition which is compensated, or rather covered up, by a lowered idle. You might try this as a quick test. Leave the vacuum hose and IAPS connected and disconnect the TPS. See what effect that has.
 

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I didn't mean to scold you about not following protocol or anything; it's just that hijacking presents certain problems that make troubleshooting more difficult than it already is. For instance, unless you're very careful in specifying that each and every symptom of the original case applies, and that you've already followed all the advice given in the previous discussion and got the exact same results, we either need to ask all the details all over again (which we could have done in a fresh thread, without the potential for misunderstanding introduced by the hijack), or read a lengthy and potentially irrelevant discussion and confirm which of the points there apply.

You can imagine that the likelihood of terminating a very mysterious, grasping-at-straws troubleshooting session with something like: "Ah, no, I missed the point about the TPS in the previous thread and it turns out it was that all along", is not negligible.



Hmmm. Assuming you're sure that you are actually disconnecting the vacuum hose from the IAPS after having already disconnected the IAPS' electrical connector, and it makes a difference, then I can only assume that you are indeed running too rich and the leak introduced in the intake by disconnecting the vacuum hose, draws fresh air and leans out the mixture. I wonder whether perhaps your TPS is set too high, leading to a rich condition which is compensated, or rather covered up, by a lowered idle. You might try this as a quick test. Leave the vacuum hose and IAPS connected and disconnect the TPS. See what effect that has.
Ok that's understandable. I've just seen so many threads here and on other forums where the first response is "Did you search the forum?" so I thought I was actually doing the right thing seeing as it seems like all the same symptoms on nearly the same bike (k5 600 btw).

Anyway, the IAP was definitely disconnected before the vac line. And I did check the TPS in dealer mode but I can still try disconnecting it and see what happens.

I am leaning towards the IAP sensor as I wrap my head around exactly what is going on. I finally figured out it's exactly the same thing as a MAP sensor and it would make sense that a crack inside the sensor would cause an incorrect voltage reading without introducing any new air into the TBs like a vacuum hose leak would. When I read your earlier post about a faulty IAPS I just assumed that "faulty" meant not working at all and I expected I would have an FI light.

Thank you for the help.
 

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A couple things. These aren't really a MAP. The bike has a second atmospheric pressure sensor and uses a differential.

I'm a bit confused about the results. With the hose hooked up to the IAP, but the electrical disconnected, it should be using a static value in failsafe mode. Then you create a vacuum leak and it get's better?

At this point, if it were in my garage, I'd leave the IAP hose connected, but disconnect the electrical to it, the AP sensor, and the IAT sensor and see what happens. All three of those augment the fuel pulse, so force the ECM to use static values for all three. If it doesn't run somewhat clean here I'm inclined to think the ECM is faulty.

You said it started with a dead batter. Did you try to jump start it from a car or something? That's been known to fry an ECM.
 
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