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Series 5 NA with Street Port. Stock ECU.
I've read through multiple threads trying to figure out why my idle is bouncing around 1500rpm. I have replaced all the vacuum lines, changed to a new TPS sensor (calibrated it with the banzai tool), replaced the MAP sensor, and also made sure my BAC is within spec per the FSM. I am no longer running the coolant hoses from the water pump to the BAC and to the cold start mechanism on top. I recently converted to an EWP. But the bouncing idle was happening before the conversion. I also bought a new water temp sensor that my ECU reads from. I have tried adjusting the idle screw on top but that doesn't seem to do anything as well. I have tried spraying brake cleaner on anything vacuum and it doesn't affect the idle. I'm kind of at a loss for ideas at this point.
**I have an S4 but my insight might help. I believe S5 throttle body is pretty much the same, but 'upside down' or reversed. All pictures are taken from the S5 service manual.**
You said you disconnected the coolant hose to the cold start system on top. Are you referring to the coolant hose running to the thermowax? If so, your thermowax is never going to extend and actuate the 'fast idle cam' mechanism.
COLD ENGINE
If you disconnect your coolant hose running to the thermowax, it will more or less always remain in this position whether the car is "warmed up" or not. At this position, the fast idle cam will hold your throttle plate OPEN quite a bit. The thermowax needs hot coolant running through it to EXTEND.
HOT ENGINE
Hot coolant running to the thermowax is what allows it to function. Notice how the little roller (circled in red) has moved position. This happens when the thermowax rod EXTENDS. When this roller moves to this position, it will allow your throttle plate to CLOSE.
This is why everyone says to set your TPS with the car fully warmed up. If you set your TPS while it's cold, your throttle plates are essentially OPEN, but your incorrectly setting/telling the TPS "the throttle plates are CLOSED".
When the car is warmed up, the thermowax extends and the throttle plate CLOSE. When you set the TPS now, you are correctly setting/telling the TPS "the throttle plates are CLOSED".
So what I'm guessing is that you set your TPS, but your throttle plates are actually still slightly OPEN because the fast idle cam is holding it OPEN. You can do a little experiment and press down on the little bracket holding the screw, to simulating your thermowax extending and pushing the screw (red arrow below). When you press down on this, look at your TPS setting screw, you will notice it moving. And if that screw is moving, you can see how any setting of the TPS while cold engine will be an incorrect setting.
From what I've read/know, you can remove the BAC coolant hose because it just helps things from sticking/freezing. But the coolant hose to the thermowax/throttle body actually serves a pretty big function for the thermowax/fast idle/TPS setting
My car has a slight bouncing idle during warm up, but once everything warms up and settles in (thermowax extends, throttle plate closes, TPS now outputting 1.0V to ecu) my bouncing idle goes away.
That's the thing though, if you adjust your TPS without the thermowax extended, your TPS setting will be incorrect. Even if your engine is hot after going for a drive, if there's no HOT COOLANT (we're talking like 180F hot) running through the thermowax, it will never really extend and your throttle plate will always be open. And if you're throttle plate is open you can never really set your TPS. This is why when people do the throttle body mod when remove the thermowax, they ALSO remove the fast idle cam. The thermowax is not a sensor, it is a mechanical component that is critical to the fast idle/cold warm up system. It is essentially what makes the FC idle at 1500-2000RPM when cold, then when it warms up drops the idle down to 750RPM.
Try the thing I mentioned of pushing down on the bracket with a flat head screw driver. Maybe even take your intake off so that the throttle plates are visible. When you press down on the bracket, you will see the primary throttle plate close. When you release the bracket, you will see the throttle plate open up again. When you set the TPS, you're telling the ECU that the throttle plate is CLOSED, and this is a problem if the throttle plate is still open, because the engine is getting more air.
If a video may help, I have a spare throttle body that I can show you what I mean. It just might take me a day or two to me to get to. Let me know.
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Possible it can also be a result of using stock ECU with a street port?
That's the thing though, if you adjust your TPS without the thermowax extended, your TPS setting will be incorrect. Even if your engine is hot after going for a drive, if there's no HOT COOLANT (we're talking like 180F hot) running through the thermowax, it will never really extend and your throttle plate will always be open. And if you're throttle plate is open you can never really set your TPS. This is why when people do the throttle body mod when remove the thermowax, they ALSO remove the fast idle cam. The thermowax is not a sensor, it is a mechanical component that is critical to the fast idle/cold warm up system. It is essentially what makes the FC idle at 1500-2000RPM when cold, then when it warms up drops the idle down to 750RPM.
Try the thing I mentioned of pushing down on the bracket with a flat head screw driver. Maybe even take your intake off so that the throttle plates are visible. When you press down on the bracket, you will see the primary throttle plate close. When you release the bracket, you will see the throttle plate open up again. When you set the TPS, you're telling the ECU that the throttle plate is CLOSED, and this is a problem if the throttle plate is still open, because the engine is getting more air.
If a video may help, I have a spare throttle body that I can show you what I mean. It just might take me a day or two to me to get to. Let me know.
~
Possible it can also be a result of using stock ECU with a street port?
Since I am no longer running the coolant hoses to the thermowax, are you suggesting I close fast idle screw? to mimic the car being at operating temp. Then actually warm up the car and adjust the TPS?
If you don't have the coolant flow to the thermowax anymore (although I recommend keeping it since it serves a useful function), remove the thermowax.
Wilff has already explained this next part but just to try and clarify:
The way it works is that the little piston in the thermowax extends as it heats up. This pushes on a screw connected to a cam on a roller. This linkage props open the primary throttle blade. As the car gets up to operating temp the cam falls off the roller and the throttle blades close down to zero. Then idle air is controlled entirely by the idle bypass screw and BAC valve (if you still have it). Your TPS is connected to the other side of this linkage, and it needs to be set when the throttle blades are 100% closed (idle).
Your problem is that the throttle blade will never be 100% closed since the thermowax needs coolant flow to do so. If you remove the thermowax you solve this problem, though you may end up needing to remove the double throttle blades for the secondary barrels at the same time (not 100% sure on that, but the thermowax seals the end of that throttle shaft on my S4 car).
All of the emissions components on the throttle body are related, so trying to remove only some and not others can be a pain. I highly recommend keeping the thermowax functional. Otherwise you need to maintain the idle manually with the pedal for a few minutes as your car warms up.
Disclaimer: I don't want to tell anyone to mess with any of throttle body screws because I know they can be finicky once adjusted from factory settings. This is kind of how I ended up with a spare throttle body and how I came to a better understanding of how it works on the FC lol.
With that being said, I think that would work. Adjusting the fast idle screw so the roller is off the cam to mimic it being at hot temperature. But if you do this, I feel like you will most likely have to hold the throttle down with your foot while warming up your car to prevent it from dying while cold, as WondrousBread says.
What I would test in your situation is...
1) Warm up car.
2) Push down on fast idle cam screw with screw driver (what I mentioned in my first post)
3) While holding down with the screw driver still, note if the linkage to the tps adjustment screw moves.
4) While still holding down with the screw driver.... set TPS to 1.0v..
5) Does bouncing go away? (with screwdriver still being held down)
6) Bouncing gone? Release screw driver. Is bouncing still gone?
7) Diagnose...
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On WondrousBread's comment, I believe if you remove just the thermowax, the springs will cause the cam to rotate counter clockwise. And your fast idle roller will end up behind the cam lobe. I think this will cause things to jam up. See below. Figure 1
I'm fairly certain that (at least on my throttle body) you can just remove the thermowax and nothing jams. But then I could be wrong, or the S4 NA throttle body might be different from other models.
Either way it's moot if you can just adjust the fast idle cam in the way you're describing and leave the thermowax in place. In which case I would do that since it makes it easier to restore the system later.
Disclaimer: I don't want to tell anyone to mess with any of throttle body screws because I know they can be finicky once adjusted from factory settings. This is kind of how I ended up with a spare throttle body and how I came to a better understanding of how it works on the FC lol.
With that being said, I think that would work. Adjusting the fast idle screw so the roller is off the cam to mimic it being at hot temperature. But if you do this, I feel like you will most likely have to hold the throttle down with your foot while warming up your car to prevent it from dying while cold, as WondrousBread says.
What I would test in your situation is...
1) Warm up car.
2) Push down on fast idle cam screw with screw driver (what I mentioned in my first post)
3) While holding down with the screw driver still, note if the linkage to the tps adjustment screw moves.
4) While still holding down with the screw driver.... set TPS to 1.0v..
5) Does bouncing go away? (with screwdriver still being held down)
6) Bouncing gone? Release screw driver. Is bouncing still gone?
7) Diagnose...
~~~~~~~~~~
On WondrousBread's comment, I believe if you remove just the thermowax, the springs will cause the cam to rotate counter clockwise. And your fast idle roller will end up behind the cam lobe. I think this will cause things to jam up. See below. Figure 1
For measuring the TPS 1.0v are you disconnecting it from the harness? or are you measuring it from the three prong connector near the air box? I've been using the Banzai TPS light.
Personally I've found the light method to work best, although the voltage method is fine too. You can only measure for the voltage method at the TPS connector itself. The three prong connector by the airbox is for the test lamp (it switches depending on what the ECU is reading from the TPS). Since the test lamp uses 12V you can't actually use this connector for the voltage method; it's designed to tell you when the ECU is seeing "idle" from the TPS sensor, not to actually tell you the sensor voltage. This is why I really like the lamp method. It takes all the ambiguity out of the resistance and voltage methods where the wiring harness and connectors may be adding resistance into the system.
Adjust TPS so that one light is lit at idle, not both. Then push the linkage a few times checking that it always returns to idle with only one light lit. Adjust as necessary until it maintains one light at idle. The linkage is a bit sloppy so I find it takes a few tries to get it to stay.
For measuring 1.0v I use a multimeter at the TPS connector, probing the back of the connector. I don't disconnect anything.
I have used the light method too. And I think it's an okay method as well because as WondrousBread said, it's based off what the ECU is seeing. With the light method when there is only one bulb lit, I believe the ECU is pretty much reading 1.0v. The ECU is seeing 'okay we're idling. throttles plates are closed'.
Some people don't like the light bulb method because when you turn the screw, and have it at one bulb. You can still turn the screw maybe half a turn, and it will stay with only one bulb on. So there is a question of 'when do I stop turning the screw? right when it's at one bulb? or do I add a quarter of a turn, or turn until two bulbs then back off?' I just stopped using it because one of my bulbs blew out and I didn't realize it as I was adjusting. And I ALWAYS had one bulb or no bulbs.
With bulb method maybe try this... 1) Warm up car. 2) Push down on fast idle cam screw with screw driver (what I mentioned in my first post) 3) While holding down with the screw driver still, note if the linkage to the tps adjustment screw moves. 4) While still holding down with the screw driver.... set TPS to have one bulb on. 5) Does bouncing go away? (with screwdriver still being held down) 6) Bouncing gone? Release screw driver. Is bouncing still gone? 7) Diagnose...
Let us know if you have any findings.
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@WondrousBread I check in on your build thread every now and then. Good stuff!
So here's an update. I actually had a spare S5 throttle body, and I removed the fast idle cam and thermowax per the FC3Spro article. I installed it on my car, warmed up the car. Set the TPS with the Banzai Light Tester. Still nothing. The idle is jumping up and down between 1200-1500rpm. At times for maybe like 1-2s it sounds like the idle is getting steady then it returns to a bouncing idle.
Okay so update. I ended up putting my original throttle body on. I adjusted the cam to be in the hot setting (25°) to mimic the car being warmed up. I then set the TPS using the Banzai Racing test light. Started the car up and bam it started idling below 1k rpm. I then fiddled with the idle screw on top of the throttle body to get it close to 750-800rpm.
05-18-21, 11:47 AM
