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Old 03-04-2014, 12:21 PM   #1
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Default What causes "Herky Jerky" at parking lot speeds?

I tried searching this site and google but maybe I am not using the right terms. Basically I am having a problem where sometimes at low speeds of about 3-15mph I encounter problems where my engine is idling so low that the car bucks which causes my foot to press the accelerator ever so slightly which in turn pulls my foot off the gas which slows the car down which allows my foot to press on the gas again...

My "fix" is to push in the clutch and let the car settle and start over. Obviously it will never crawl like an automatic, but is there something I am doing wrong with my tune that is causing this low speed jerkyness? It happened with the stock ecu but it's worse with my megasquirt. I replaced the motor mounts when I installed the turbo with firmer mounts but I didn't do the diff mounts if that means anything.

I thought originally that firmer engine mounts would make this better because the engine would not "spool up" as easily in a mounting sense.

Any thoughts?
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Old 03-04-2014, 12:23 PM   #2
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There are a few things, accel enrichments and eae being what caused it the most on my car.
Too much vvt advance too low in the rev range was the next.
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Old 03-04-2014, 12:39 PM   #3
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fueling.

also, sounds like your idle control needs tuning. You should be able to keep the car in first and climb up a parking garage ramp without any maniuplation.
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Old 03-04-2014, 12:57 PM   #4
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Yeah I spent 3 hours in an autozone parking lot in Colorado messing with the shitty 1.6l mechanical idle control afr unit. Had the exact same problem. Fixed it after advancing the unit a couple clicks leaner.
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Old 03-04-2014, 01:11 PM   #5
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never lift
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Old 03-04-2014, 01:17 PM   #6
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Idle and Accel Enrichment sound like likely places to start. I will have to read more into this or find a local expert...
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Old 03-04-2014, 02:06 PM   #7
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You have a Reverant MS3 too, don't you Chiburbian?

While I love the ECU, I must say that it is not good for a beginner. I haven't learned ANYTHING about the idle-control stuff because I haven't had to touch it. Out of the box it works perfectly. And since it works, I'm too lazy to learn it by re-doing it.

Don't get me wrong, I'm glad it all worked out of the box. It's just not ideal to learn when it all works, it's much easier to learn when it doesn't and you have to fix it.
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Old 03-04-2014, 02:46 PM   #8
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Light load fueling and injectors coming out of decel cutoff.

Light load fueling depends heavily on the injector offsets.

Injectors coming out of decel cutoff depends on accurate setting of TPS min voltage, the MAP decel cutoff setting, and RPM.
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Old 03-04-2014, 02:53 PM   #9
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Quote:
Originally Posted by thenuge26 View Post
You have a Reverant MS3 too, don't you Chiburbian?

While I love the ECU, I must say that it is not good for a beginner. I haven't learned ANYTHING about the idle-control stuff because I haven't had to touch it. Out of the box it works perfectly. And since it works, I'm too lazy to learn it by re-doing it.
Thanks, I'll be quoting this when people ask me.
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Old 03-04-2014, 02:59 PM   #10
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He speaks the truth. He did the final install at my place, and when we were done with the wideband, IAT, and getting the big bastard up under the dash, it just fired up and he drove him. From what i understand, driveability was pretty much like stock, but with more power.


I was honestly a little disappointed, i was hoping i'd get to learn something about Megasquirt that day. I didn't.
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Old 03-04-2014, 03:02 PM   #11
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Quote:
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I was honestly a little disappointed, i was hoping i'd get to learn something about Megasquirt that day. I didn't.
****, I want my money back!
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Old 03-04-2014, 03:05 PM   #12
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HAHAHAH that's like the best "criticism" ever
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Old 03-04-2014, 03:07 PM   #13
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I don't have a Rev built MS3. I self-built mine and as such I have thus refrained from asking for "cheat mode" msq's from people who have actually paid money to Rev. I know it's not exactly trade secrets but he spent time and effort putting them together...

Unfortunately the roads are once again covered in snow and salt so true road testing is going to have to wait a couple weeks. That's how it works though. I get bored and want to tinker when I can't be driving, and once I can drive I don't want to tinker.

I'll post my msq later for input.
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Old 03-04-2014, 03:09 PM   #14
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You have mine, and that was after I solved the jerking/bucking, so it should give you at least some help
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Old 03-04-2014, 03:11 PM   #15
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"I am really disappointed in this Glock... It was so well put together and reliable that I never had to figure out how much powder to load, never had to worry about how to prime the pan (frisin?), and and while everyone else got to learn how to solve malfunctions by having actual malfunctions, I had to 'pretend,' I feel jipped."
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Old 03-04-2014, 03:33 PM   #16
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This is why I love rotaries. Damn piston engines are too reliable and you never get to learn how to rebuild them!
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Old 03-04-2014, 03:42 PM   #17
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Quote:
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This is why I love rotaries. Damn piston engines are too reliable and you never get to learn how to rebuild them!
That's what ebay turbo kits are for!

--Ian
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Old 03-04-2014, 05:00 PM   #18
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Also, look at overrun cutoff. That's what does it on mine.

All fueling, just like Brain said.
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Old 03-08-2014, 12:31 AM   #19
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Driveline "slop" will aggravate the situation as well- worn engine an diff mounts.
Mine does it every once in awhile, but not enough to bother me.
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Old 03-08-2014, 10:13 PM   #20
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Well, motor mounts are new as of last July but the diff mounts are still stock. There is a bit of play in the driveshaft but I don't know what is considered acceptable driveline play. Plus, driveshafts seem expensive. This is the first (RWD) car I have ever had that didn't have user servicable parts in the driveshaft.

It should be warming up enough to drive it next week and I will give the suggestions here a try.
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