Random Reving at Idle - FIXED

[dragbug]

Ok so I have an odd issue that just started recently. The car will randomly rev up as though I just pressed the throttle. It comes up a few hundred RPMs and then settles back down. Its not a lean surge because ...well its not running lean and its not surging. It only seems to be happening at idle and it just started doing it over the weekend. I don't have a log of it unfortunately. When I tried to log it, it wouldn't do it. Its happened when it was already warmed up and its happened when it was still trying to warm up. I will post up my tune this afternoon when I get home but looking for some ideas as to what might be doing it. Thanks!

[Matt Cramer]

Without a tune and data log, all I can do is offer some generic advice. Which I have put together in an article here:

http://www.diyautotune.com/tech_article ... uning.html

[dragbug]

Thanks Matt. The one time I didn't have my tablet to run a log (took the car in to get new front and rear window gaskets installed) is the time it started acting up. I will work on getting a log as I am pretty sure it will do it again.

[dragbug]

OK Matt, attached is my most recent log and tune. The log is long, sorry. The issue didn't happen the rest of the day so I wasnt able to catch it in the log but maybe something in the log will show where the potential for it to occur is. My guess is it has something to do with AE but I have not changed the threshold from 50 since I first started using it a week or so ago and never had this issue until this weekend. At the same time though, I started getting more sync loss suddenly this morning. Usually its rare and I get one at most when it happens. Today I got 12 in about 15 min. I moved the sensor closer to the wheel and as you can see in the attached log, no sync loss. So I don't know if that could have been related or not. I changed the threshold to 60 just this afternoon.

[Matt Cramer]

Unfortunately, I'll need to see this captured in a log. Nothing popped up in the files you posted.

[dragbug]

Ya I was afraid of that. Well...guess I will keep trying.

[dragbug]

I think I might have found the issue. One of the injectors are leaking and the top of the injector is acting like a small bowl. I think the fuel is occasionally dripping. However, in my experience with carbs, specifically a Holley, I have never heard a car rev up from fuel leaking. Usually it just runs pig rich. Although, this leak doesn't seem to be dripping with any consistency. As a matter of fact, I watched it for several minutes even revving up the engine and never saw enough fuel collecting on top to overflow and to form a drip and it never revved on its own during this time.

So, does this sound like it could be the cause of the revving or have I just coincidently found another problem? I intend to fix it either way but looking for any thoughts.

[Sam280Z]

In my experience, the only things that usually cause major fluctuations in idle are additional air and/or timing. I'd look there first.

Sudden additions of additional fuel would likely cause a stumble assuming it is idling reasonably well to begin with.

[dragbug]

Ya but I can't find any other cause yet. I have not been able to catch it in a log yet. Still trying though.

[dragbug]

I was finally able to catch this in action in a log. It happened at least 3 times. I think the stepper motor is either the problem or something is causing the stepper motor to move. Mat or anyone, if you could look over the log and tune and maybe let me know if its a bad stepper motor or something else I would appreciate it. Thanks!

[elaw]

The Megasquirt is losing sync... look at "lost sync count" in MLV. Lost sync basically means the MS is not seeing what it expects from your trigger wheel, and is often caused by the VR pots (assuming you're using a VR crank sensor) being set incorrectly.

I thought it had been fixed in newer firmwares, but it's a quirk of the code that when sync is lost then regained, the code basically acts as if the engine was just started. So the idle valve goes to cranking position then tapers back to the right settings, thus causing an RPM "flare".

[dragbug]

ok, so the idle valve is a result of the sync loss, not the other way around. That at least is good to know. I thought I fixed the sync loss. Its random and I thought I had the sensor wires to close to the ignition wire so I made sure I have good separation and also closed the distance between it and the wheel. I have not had a sync loss in over a week until now. Now I am not sure what to do. The crank sensor I am using is the one sold from DIYautotune.com. Its the plastic one. So should R56 be turned back 6 clicks or left turned counter clockwise 12 turns? Not sure if I turned R56 back 6 clicks or left it but not really sure which one I should do. Everything between hall effect with pull up and a VR sensor are the exact same configuration on the MS except for the pots.

[dragbug]

Any advice on the above?

[Matt Cramer]

See if you can capture the sync loss in a tooth log.

[dragbug]

I can certainly try but it's pretty random and I can't always have my laptop in the car.

[dragbug]

Matt here is a tooth log I did this morning on my way to work. I didn't experience any sync loss that I know of. But maybe something in the log will show a potential issue? I will keep trying to get a log of the sync loss in the mean time though.

[Matt Cramer]

Sorry, but it looks like you've captured 230 pages of no problems with the sensor input whatsoever.

[dragbug]

Ya, there was actually over 570 pages but I had to cut some out so I could post it. Even zipped up the full log was to big.

OK so no problem in the log. Could the pots be the issue? Would they cause a really random problem or would it be more consistent? Just looking for ideas right now while I work at getting a log that shows the sync loss.

[Matt Cramer]

It MAY be possible to fix the issue with adjusting the trim pots. But for me to offer a systematic diagnosis instead of a wild guess, I'd prefer to see a tooth log that captures the problem.

[dragbug]

Understood. However, the one question I kind of asked but never got an answer for was how to set the pots up correctly for my setup. Do I leave both pots turned 12 or more turns counter clockwise or do I turn R56 6 turns clockwise? Which is the correct way for the DIYautotune hall effect sensor that is I assume using the VR conditioner built into the MS3?