Rear cylinder switches off when hot.

Energy One
Roaming Wolf

Roaming Wolf

Member
Hello,
2018 K-9 VFI. Rear cylinder switches off when hot and the bike is idling. It happens after about 30 min riding. No check engine light, just the bike looses power. Checked for the spark - nothing. If the front cylinder wire is disconnected the bike dies. The coil is relatively new -2 years old I believe. The wires are the same age, Screaming Eagle. If I switch the bike off, then start again and ride on 1 cylinder, after about a minute the second one switches on and the bike runs fine. I understand the rear cylinder gets hotter due to its location, especially now as the heat is coming back over here. The ECU that I have which was installed by Big Dog is locked and I can't change any settings there. I contacted S&S, they confirmed this and offered me a new one that can be adjusted. It is quite expensive though. So, before I buy it I want to check if anyone has had a similar issue. I understand most of the bikes here a crab ones. I have been thinking of downgrading it to a carb. This will require quite a lot of mods though, according to S&S. In such case buying a new ECU might make more sense. Can there be an issue with anything else? So far I have: coil, wires, ECU. O2 sensors a very unlikely. I changed one to original Bosh and they usually through a check engine light. Also, the spark should remain on the second cylinder.
 
Jwooky

Jwooky

Well-Known Member
Roaming Wolf said:
Hello,
2018 K-9 VFI. Rear cylinder switches off when hot and the bike is idling. It happens after about 30 min riding. No check engine light, just the bike looses power. Checked for the spark - nothing. If the front cylinder wire is disconnected the bike dies. The coil is relatively new -2 years old I believe. The wires are the same age, Screaming Eagle. If I switch the bike off, then start again and ride on 1 cylinder, after about a minute the second one switches on and the bike runs fine. I understand the rear cylinder gets hotter due to its location, especially now as the heat is coming back over here. The ECU that I have which was installed by Big Dog is locked and I can't change any settings there. I contacted S&S, they confirmed this and offered me a new one that can be adjusted. It is quite expensive though. So, before I buy it I want to check if anyone has had a similar issue. I understand most of the bikes here a crab ones. I have been thinking of downgrading it to a carb. This will require quite a lot of mods though, according to S&S. In such case buying a new ECU might make more sense. Can there be an issue with anything else? So far I have: coil, wires, ECU. O2 sensors a very unlikely. I changed one to original Bosh and they usually through a check engine light. Also, the spark should remain on the second cylinder.
Click to expand...
Do you have a laptop with Protune on it? If not, this would be the absolute first step.

You can see any faults but also read all the parameters in real time, such as cylinder temperature. You can also log data and review it after the ride and capture an event such as cylinder deactivation.

I can share, over 10 years of riding even idling in traffic, I can’t recall once the rear deactivated.

There is a temp sensor in the rear head, I did have to replace mine years ago, so that could be a suspect. You can read it with Protune.

Would not even consider downgrading to a carb, would be an extremely costly downgrade.
 
Roaming Wolf

Roaming Wolf

Member
Thank you.
I will try to replace the coil first. I have a slight doubt though that the coil is the real problem. If it heats up an a crack expands, I believe several things should happen:
1. there will be a spark leakage that should be visible at night.
2. the spark should not appear in the rear cylinder plug after a minute of riding with the front cylinder and then keep working fine.
3. a wet bike will have a spark issue at any time as water will get into the crack. Rode the bike in a heavy rain a month ago, no spark issue whatsoever.
 
Mikeinjersey

Mikeinjersey

Well-Known Member
Roaming Wolf said:
Thank you.
I will try to replace the coil first. I have a slight doubt though that the coil is the real problem. If it heats up an a crack expands, I believe several things should happen:
1. there will be a spark leakage that should be visible at night.
2. the spark should not appear in the rear cylinder plug after a minute of riding with the front cylinder and then keep working fine.
3. a wet bike will have a spark issue at any time as water will get into the crack. Rode the bike in a heavy rain a month ago, no spark issue whatsoever.
Click to expand...
A simple check can be to swap your spark plug wires to see if the problem moves. I agree that the coil is susceptible to breaking down from temperature and a strong candidate for the problem. Check the connections to the controller especially pin # 11 that it hasn't backed out of the connector. Intermittent is never fun.
 
Roaming Wolf

Roaming Wolf

Member
Removed the coil cover today and found these 2 sensors: the small one and the bigger one. The small was installed in the place of intake manifold bolt, holding it, in the front cylinder; the bigger one in a different place in the rear cylinder. Both of them seem to be some temperature sensors. Can they cause the issue. My main concern is about the bigger one (rear cylinder).
 

Attachments

Mickmorris

Mickmorris

Well Known Member
Supporting Member
Roaming Wolf said:
Removed the coil cover today and found these 2 sensors: the small one and the bigger one. The small was installed in the place of intake manifold bolt, holding it, in the front cylinder; the bigger one in a different place in the rear cylinder. Both of them seem to be some temperature sensors. Can they cause the issue. My main concern is about the bigger one (rear cylinder).
Click to expand...
Go back to post#3 jwooky. I believe he mentioned a temp sensor in the head
 
pknowles

pknowles

RETIRED
[UOTE="Snuffy, post: 1034128, member: 21440"]
I would think if a sensor on one head or the other read hot it would shut it all down. Not just one of the cylinders. I would still think coil. But I could be wrong and I usually am. Best of luck with it.
[/QUOTE]
Limp mode kills the rear cylinder
 
Snuffy

Snuffy

Well-Known Member
Supporting Member
Yep. I was wrong. That just makes no sense. It’s running hot or something is wrong so let’s keep one of the cylinders running so you can be sure to fuck something up. Lmao. I wasn’t aware the was sensors anyway. You learn something new everyday.
 
Roaming Wolf

Roaming Wolf

Member
Mickmorris said:
Go back to post#3 jwooky. I believe he mentioned a temp sensor in the head
Click to expand...
Yes. In my case the front sensor is the CHT and the rear one is the knock sensor. I contacted S&S and got the most elaborated answer)) Online search gave no result. There is a similar CHT from Big Dog, but the connector is different. There is a place in the front cylinder with a thread where I can possibly connect a different type of CHT. In the picture my sensor is connected in the blue spot and there is another place in the red spot with a wider hole.Screenshot 2026-05-05 at 8.50.33 AM.pngtempImageqvgFhE.png
 
Mickmorris

Mickmorris

Well Known Member
Supporting Member
Roaming Wolf said:
Yes. In my case the front sensor is the CHT and the rear one is the knock sensor. I contacted S&S and got the most elaborated answer)) Online search gave no result. There is a similar CHT from Big Dog, but the connector is different. There is a place in the front cylinder with a thread where I can possibly connect a different type of CHT. In the picture my sensor is connected in the blue spot and there is another place in the red spot with a wider hole.View attachment 139408View attachment 139407
Click to expand...
Interesting. So your sensor is actually the intake manifold flange bolt. The other opening is for the temperature sensor I am pretty sure. Hope you get it figured out. Definitely test your coil with a multimeter.
 
Jwooky

Jwooky

Well-Known Member
Roaming Wolf said:
Removed the coil cover today and found these 2 sensors: the small one and the bigger one. The small was installed in the place of intake manifold bolt, holding it, in the front cylinder; the bigger one in a different place in the rear cylinder. Both of them seem to be some temperature sensors. Can they cause the issue. My main concern is about the bigger one (rear cylinder).
Click to expand...
The bottom pic is the Engine Temp Sensor (ETS) I referenced. It goes in the rear cylinder securing the intake.

I have never seen or heard of the other sensor, nor a knock sensor. Mine for sure doesn’t have it, nor any of the other EFI bikes I’ve seen.

Was this possibly a California EPA bike? Maybe aftermarket?

Also, no references in the service manual nor wiring schematic


IMG_1751.jpeg

IMG_1752.jpeg
 
Roaming Wolf

Roaming Wolf

Member
Jwooky said:
The bottom pic is the Engine Temp Sensor (ETS) I referenced. It goes in the rear cylinder securing the intake.

I have never seen or heard of the other sensor, nor a knock sensor. Mine for sure doesn’t have it, nor any of the other EFI bikes I’ve seen.

Was this possibly a California EPA bike? Maybe aftermarket?

Also, no references in the service manual nor wiring schematic


View attachment 139418

View attachment 139416
Click to expand...
Yes, the throttle body looks similar. I changed all the sensors except for the injectors). My bike is VFI, not sure if it has any connection to California. The bike was originally made to be shipped to Monaco and then it came to Dubai, where I bought it. The previous owner only rode 4,000 miles and then realized he was more into Ducati. He wouldn't bother with any mods or repairs, the guy is too high profile, so he just gave it to a local Custom Bikes shop to sell where I bought it. No matter how much I love the bike, and I am even selling my loyal FXDWG now and keeping the chopper, the quality of the electrical parts and the way they were mounted by Big Dog is SHIT. There are not many Big Dogs here, and I know their owners, and they all have issues now or then. It might be connected to the scorching weather in summer, I am not sure, but Harleys have no issue riding in such heat, though most of them are GCC (local) specification.
 
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