maverick1998

Ive got a 90 miata with a MegasquirtPNP running speed density. Naturally aspirated. At idle, my ignition advance will sit at 10 degrees BTDC while warming up as shown by the ECU, and a timing light verifies that it is in fact set to 10 degrees advanced. Once warmed up, at idle the timing will jump between 10 to 20-30 or so back and forth quickly, a few times per second, causing the engine revs to oscillate up and down quickly (think a lawnmower that is still trying to sort out its choke while warming up) (normally over a range 600-100 rpm for a minute or so then eventually die. I have the spark table in tuner studio set at 10 degrees at idle, and I can see it is aiming for this even when surging, so it doesn't seem to be an ECU problem, but I don't know. My AF meter shows right around 14.7, brand new 100 miles ago and properly calibrated. At any engine rev above idle, the engine holds itself just fine with no surging or oscillating, it occurs only at warm idle. I am running a tune done my a reputable tuner, but since that tune I have put a new port and polished head on, a new short block, and temporarily removed my supercharger.
Side note: The car had a high idle since the day I bought it. Never idled below 1200. Now that I have finally replaced the throttle body and IAC, it will idle at proper RPM except for the above issue, leading me to think this issue may have been around before but was hidden by the high idle, or is a result of the new IAC.

I replaced my CAS to try and fix this, and used a timing light to make sure it was set accurately, which did not help. Spark plugs are near brand new (although I am going to install new plugs and coils in a few days to be safe). Wires have maybe 5k miles on them. I can also see my TPS enrichment light flashing on when the timing surges each time (my TPS is calibrated correctly). I also have a clutch sensor bypass (aka some gorilla tape holding the sensor in so I can start the car without pushing in the clutch) so the car doesn't think the clutch is in all the time, something that has cause a similar issue for others.

No major vacuum leaks, just replaced my IAC as old one was beat, brand new 323 PCV valve. What could be causing this? My new IAC? Something else?

I am reinstalling my supercharger this weekend (I took it off while installing a new engine, and have been waiting on a new pulley before reinstalling) so I want to try and fix this before boosting again.

Any ideas are appreciated. I've researched around but cannot find anyone having noticed a timing jump like this at idle (although the timing jump may be a symptom, not a cause), and I know this site is about the best for Megasquirt questions. Thanks.

SpartanSV

You didn't include your tune and a log so we can only guess, but it sounds like your tune needs work.

maverick1998

My apologies, I am new to the forum and am used to Miata.net where you cannot attach files easily. I will post my tune/log this evening. Thank you!

maverick1998

Attatched is a log and map of the car at idle doing the weird thing.

deezums

iTrader: (3)

It's doing exactly what you tell it to do, idle advance.

SpartanSV

Yeah. Turns out changing everything about your engine setup requires tuning changes.

maverick1998

What do you mean? My spark tables are set to be at 10 degrees btdc at idle, is there a separate setting for advance at idle elsewhere? I am new to TunerStudio and the only article I can find about it is from DIYautotune and is far out of date. Id been searching around for a condition in my settings that would cause this but can't find it, and not for lack of trying

maverick1998

Well yeah, I kinda figured that might be part of the cause, the question is what needs to change? Every tuning guide starts with "get your idle worked out first" and so that's what I'm trying to do. My spark tables reference 10BTDC in the range of the engine at idle, and the fuel VE table certainly isn't perfect yet, as I am learning how to do this tuning on my own, but running in the range of 13.5 ish at idle certainly shouldn't cause the current issue, and its kinda hard to be able to adjust my idle well when the car doesn't like to idle for more than a few minutes at a time due to whatever weird thing is happening.

SpartanSV

Oh was that obvious to you? Cause in your first post you mentioned throwing a bunch of parts at the problem. Seemed you didn't think it was tune related at that point.

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maverick1998

I edited my original reply to you as I realized my reaction was over the top, I apologize. I did go through all the parts (the car has 300,000 miles on it, so parts going bad is kidna the norm at this point). I did change the last of the parts I thought it might be, the coil pack and air valve(the car needed em anyway), last night so now I am certain it is a tune. You were correct in my original post I was unsure, and have since eliminated parts as the source of error. Any advice on how/what to change in the tune is greatly appreciated

Side note: I was advised I shouldnt have any problem taking the SC off to drive around town for a few days, id only hit issues with the tune at WOT, so I was thinking it was not a tune issue, but now it clearly is.

hedz0r

deezums already told you where to look. You have idle advance enabled that will add up to 20deg of timing based on the rpm delta of your actual idle rpm and target idle rpm.


I would recommend turning this off for now, tuning your closed loop idle, then reenable the idle advance, then tune that.

EDIT FOR MORE DETAIL:
you can see in the idle advance settings menu that it triggers off of a CLT of 140F and rpm below 900, this is causing the oscillations

maverick1998

THANK YOU!!!!!
This is the setting ive been looking for, I didnt know it existed! I had searched everywhere that seemed logical to try and find idle but i had never found this page! This will almost certainly fix the problem! I didn't realize Deezums was referencing a certain page in TunerStudio. Thank you so much, this is exactly what I needed!

DNMakinson

iTrader: (3)

Note that in the end, you will want this feature. You just don't want it to be fighting your IAC valve movements.

It is very useful in keeping idle steady when loads change (turn lights on, turn steering wheel). It is much faster than IAC to respond.

However, it appears that it is too aggressive at present.. I think mine has almost no negative action, only positive (adding to the advance when delta RPM is negative).

NOW: Post up a meet and greet. Introduce yourself, your car, and your goals.
DNM

maverick1998

Ill post up a meet and greet now! Thanks for the advice!