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Old 01-22-2019, 03:35 PM   #1
dvallis
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MS2 idle runaway?

Hello

I'm tuning a stroked turbo M20 setup with:

- Wasted spark
- Return fuel system w Nuke rail, Aeromotive FPR, 60lb injectors
- ICV delete
- MAF delete
- TPS
- Spring controlled waste gate
- MSPNP2 + Tunerstudio MS Ultra 3.0.28.

The car starts fine, idles briefly around 900 RPM then runs away to the rev limiter, without touching the throttle. We made incremental changes to the things below, but it did not help.

Throttle Position Sensor - calibrated
Cranking pulse
Afterstart enrichment
Warmup enrichment
Idle advance
VE table
Spark table
Basic fuel required
Rev limiter

Googled the heck out of this. I'm out of ideas. What causes RPM runaway in Megasquirt without touching the throttle?

My Tunerstudio project is here.

Click picture for video
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Old 01-22-2019, 05:34 PM   #2
haaken675
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Could there be a significant vacuum leak somewhere?
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Old 01-23-2019, 05:48 AM   #3
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If you have icv deleted, megasquirt has nothing to do about it. It can't magically get more air to engine to make it rev higher. You have vacuum leak after thorttlebody or major thorttlebody related problem.
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Old 01-23-2019, 08:11 AM   #4
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Thanks guys. Vacuum leak was Varg's diagnosis as well. Will start chasing it down.
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Old 01-23-2019, 10:53 PM   #5
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edit: Tried to make this concise and clear. Lots of thoughts on your issue.

1. Is your MAP sensor hooked up properly and calibrated? You have vacuum with the car off according to your boost gauge on your laptop. This issue won't cause run away like you're experiencing, but this MAP value is simply not correct. It would be helpful to be able to see a log with good data.

2. I am curious if your TPS is wired correctly because the signal looks noisy and you have said that it has been calibrated--yet it's having ~5% swings when you fire the car and turn it off. Please confirm the TPS wired to the 5v rail from megasquirt and grounded to a megasquirt sensor ground(not an engine ground!) using the proper pins.

3. The fact that you can start the car cold (75F coolant) without touching the throttle and no IACV makes me think the same as others: vac leak. Or your throttle stop is simply too wide. All of the cars I have seen with no idle control valve require some amount of throttle angle to start on a cold motor. Moreover, you wouldn't need a lot of angle on a large aftermarket throttle body to create enough area to rev the motor to 5k+ with no load. A way to eliminate your throttle body as an issue would be to back off the throttle stop entirely and only control the motor only with your foot when you start it. If the engine doesn't run on its own with the throttle body closed entirely, you don't need to look any further.


After inspecting your throttle body stop, TPS wiring, and fixing your MAP sensor issues, perform a datalog of your start and post it. You've already gone through the hardest part of megasquirt: getting the motor to run.
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Last edited by Jaxx_; 01-23-2019 at 11:34 PM.
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Old 01-24-2019, 05:02 AM   #6
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They aren't visible in the video so I thought I'd mention: the indicators at the bottom of the TunerStudio dashboard are very helpful in determining what factors are being applied at the moment, i.e. if warmup and afterstart enrichments are applying, if your TPS signal is jittering enough to activate acceleration enrichment, things like that.

Sounds like you have some good ideas for troubleshooting already, so report back when you can.
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Old 01-24-2019, 10:04 AM   #7
dvallis
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Quote:
Originally Posted by Jaxx_ View Post
edit: Tried to make this concise and clear. Lots of thoughts on your issue.

1. Is your MAP sensor hooked up properly and calibrated?

2. I am curious if your TPS is wired correctly

3. The fact that you can start the car cold (75F coolant) without touching the throttle and no IACV makes me think the same as others: vac leak
1. The MAP sensor is acting weird. We had it set as MPX4250 which is supposedly the default sensor in MS2PNP. However, that made the MAP reading sit at -4.8 PSI. We went to "custom" calibration and set value at 5V to 400%. This reset engine off state to 0 PSI, 100 KPa. Not happy with it though.

2. TPS is wired correctly, since I can get smooth 0-100% swing on it with the gas pedal. However, it is noisy. Not happy with that.

3. Yes. Looking at VAC leaks this weekend
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Old 01-24-2019, 03:49 PM   #8
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Thanks for the indicators tip. Will look at them closer next time we start it.
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Old 01-27-2019, 02:11 PM   #9
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Root cause was weak throttle body spring. See this post.
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Old 03-10-2019, 10:16 AM   #10
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Where did you get the intake manifold setup?
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Old 03-11-2019, 11:31 AM   #11
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It's a one-off. Bought it on eBay. Might be making a new one depending on tuning results. We'll see.
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Old 03-16-2019, 09:17 PM   #12
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in for your opinion on it when that time comes about
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Old 03-16-2019, 10:50 PM   #13
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Folks on here have the right idea. Without an ICV the MS canít add (or remove) air so it must be too much air with fuel. On my second iteration of MS I put back my ICV which was soooooo much better.


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