MegaSquirt II A/C Idle Up Modification

[Drive Stainless]

A probably unrelated issue I have is that on some hot starts the car will start, then immediately stall. It always starts up the second time. The RPMs don't fly up above 1200 or whatever it does on a normal start, but rather it gets up to 700-1000, then stalls out. Starting it a second time works fine. I usually go from "off" to "start" without pausing for fuel pump priming in "run", but I don't know if that's relevant, and honestly it shouldn't matter.

This can be related to heat soak. Check your IAT temp (in TunerStudio) prior to a hot start next time; it may surprise you. Where is your IAT sensor located?

[jangell]

This can be related to heat soak. Check your IAT temp (in TunerStudio) prior to a hot start next time; it may surprise you. Where is your IAT sensor located?

I have a MAT, which is mounted on the throttle body below the throttle plates. I labeled a picture when I got my box of parts from Josh:

https://images.squarespace-cdn.com/c...g?format=2500w

The temp does get up there -- it gets up to 120 C after about 20 minutes of highway driving, and seems to stay at that level. I have no idea if that's a reasonable temperature or not.

But I haven't looked at it before a hot start. I see what you mean -- without the air circulating the the MAT would be hotter. I'll check that next time. Not sure what to do about it, exactly... Thanks!

-- Joe

[FABombjoy]

It's telling me I have too big of a throttle body, you mean?

Huh? No, that was Owen's issue causing off-throttle bog Your TB is probably fine.

This is what stock, turbo B28F, and LT1 throttle data looks like (I grabbed the first log I could find from SpittyBug's setup here):
https://imgur.com/a/migSZrG
The non-linearity is because the first 2 TPS setups measure the cable spool angle and not the actual butterfly valve angle.

-0-25% doesn't seem to do as much as the 25-50% but it makes it easier to control at low loads.
-The turbo car reads a larger spread because the turbo adds a 3rd variable component to load but the same basic shape exists in both.
-The LT1 throttle reads almost 80kPa at 25% TPS. At 10% throttle, MAP is anywhere between 40 and 62, and at 11.5% it's 42-76. This would make it challenging to tune and control engine output.

I checked my last log and I see around record 6607 that it dropped to 640 RPM, and the AFR went up to 15.7 soon after. I see how the IAC is opening to try to recover from the low idle as well. Based on what you're saying above, I need more fuel in the ~650 cells to compensate for that?

With the # of steps on throttle left, your engine is likely starving for air which is the principle reason, but lean low-load cells aren't helping.

I guess the problem I always had with the low cells is that the only time I would get into that position was by mistake, not intentionally, although the R-N-D cycle seems to make it happen reliably enough that I might be able to adjust the fuel properly.

Just set your closed-loop target RPM lower, then tune that area. Or stuff a towel in the intake

[jangell]

Huh? No, that was Owen's issue causing off-throttle bog Your TB is probably fine.

Ah; got the threads confused.

With the # of steps on throttle left, your engine is likely starving for air which is the principle reason, but lean low-load cells aren't helping.

Which I'm reading as open the throttle plate a little so that the IAC isn't the only source of air, right? And add fuel as needed.

Just set your closed-loop target RPM lower, then tune that area. Or stuff a towel in the intake

I don't know why I didn't think of lowering the closed loop target. Probably because I was concerned with getting that right. I'll give it a shot. Or get a towel.

Thanks!

-- Joe

[Josh]

your throttle body is off a 4.0 I6 Jeep
the injectors are from a mustang.

Both are sized properly for the engine.

There was a comment about running a 5.7L lt1 throttle body, that is way to big for a prv.
I also noticed some heat soak with the iat mounted in the throttle body elbow.

SOmewhat apples to oranges comparison, but i have my IAT sensor (MAF) in the pontoon with my LS setup and heat soak has never been an issue.

[jangell]

Ah, thanks -- I got the Mustang and Jeep thing confused. I was pretty sure that you'd know what sizes to get.

I could look into moving the MAT to the pontoon, if the heat soak issue is just that the sensor isn't reading what MegaSquirt is expecting, and not that I have to change some settings in TunerStudio to compensate. It should be easy enough to tap a hole in the aluminum plate that holds the air cleaner. Not that it's a big deal to have to start the engine twice when it's hot or anything. It's not like I'm stranded and unable to start the car.

-- Joe

[jangell]

I played with the idle a bit this evening. I opened the throttle screw to bring the IAC down to the tens range, but I can't do much more fine tuning until I get the idle stable.

Currently it's seeking between the 600s and 1200s. Sometimes it narrows down to 700-900 range, but other times it shoots up. I re-interpolated the ignition table low-end RPM columns to be the same as the VE table columns (so, 600, 740 and 840) so that the cells line up, on the theory that it would make adjustments easier. Pretty much everything I do doesn't affect the seeking. The AFR wants to stay in the 13s at and below the target idle of 790, presumably because it's rising and falling as it seeks instead of holding position.

I'm wondering if it makes sense to turn of closed loop idle for the purposes of tuning these low-end cells so that I can force a specific idle RPM. That would remove the IAC from the equation and presumably make it easier to get this set up. Then I can turn closed loop back on. Does that make sense, or would I just be confusing myself even further?

No logs, since I was constantly messing with the ignition and VE tables (mostly the latter), so they wouldn't be terribly useful.

Thanks again

-- Joe

[Spittybug]

Turn it off for tuning. Set AFR fuel authority to zero as well. Don't let MS do anything other than what you want for tuning purposes.......

[opethmike]

Has no skin in the game. Stays in the game anyway.

[Spittybug]

Has no skin in the game. Stays in the game anyway.

I do it for the love....that and this group needs more gurus.