2.7 can't idle properly - Miata Turbo Forum - Boost cars, acquire cats.

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Old 06-06-2012, 10:48 PM   #1
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Default 2.7 can't idle properly

The car has never really run properly, ever. Life has been happening, and with a spare car I've never sat down to fix it. However, I've just now become annoyed with it enough that I need to take care of this. Part it out and buy a 911 or 944T, figure out how to fix it, sell all the parts and V8 swap it... SOMETHING to get myself on the road again with RWD.

When the car has been left for a day or so and is at an ambient temperature, lately that's around 90F, it will start, rev to 2000-2400 RPM, start to settle, then die with no attempt to catch itself over the course of about two to three seconds after firing. Cranking it again for the second try will almost always get it to run, but it will rev high, start to settle, almost die (600rpm), then rev to around 1800rpm and very slowly come down as it warms up.

When idling, it periodically hunts from 850-1000 RPM (idle target is 952 RPM). My AFR is around 16 -- I can't find a single place that recommends a 'good' idle AFR.

If I blip the throttle anywhere from 1% to 100% real quick, the engine stutters, drops around 200rpm, then revs cleanly. The time it takes the engine to respond to throttle from idle is reliably 250ms.

Above 2000rpm the car seems to drive perfectly fine.


I've got the following:
- RC 750cc injectors, low imp
- BP engine w/1mm oversize Wiseco pistons
- GT2860RS turbo
- Forge BPV (atmosphere, for now)
- FM base map for RC750 + potato
- Lightweight flywheel (11lb iirc)
- EGR tube delete. Manifold is secured with an aluminum block-off plate.


Troubleshooting thus far:

Re-calibrated the WBO2 and TPS with the battery at 12.4v (jumped to the subaru).
Replaced the throttle body and IAC with a used unit off an engine that idled fine -- cleaned them prior to installation just to be sure.
Checked the following vacuum connections on the intake manifold, verifying they are secure and in good condition:
-Charcoal canister nipple (repurposed to MAP)
-Cruise(?) nipple (repurposed to BPV+WGA)
-FPR nipple
-The three vac tubes associated with the big diaphragm in back (EGR?) -- one red stripe, one blue stripe, and one black that just goes to a sensor near the passenger fender. The black one is the only one that plugs straight into the head.
-Brake booster tube
Pinched off each vacuum line and looked for a change to the idle -- none.

IAC tests:
If I screw the idle valve in all the way then remove the idle control solenoid lead, the car still runs. It's obviously taxed, and runs around 800-900 RPM, but it wavers slightly less. Blipping the throttle nets pretty much the same results as 'normal' -- slight stutter, rev, return to below idle, catch, idle.

If I plug the IAC back in with the valve all the way screwed in, the IAC does its business and the car idles around 850-1000RPM. No change from 'normal'.

If I have the IAC plugged in and and start to turn the idle screw outward, the ECU will bottom out the IAC duty cycle (19.7%) very quickly, like two turns out.

Regardless of IAC/screw conditions, my idle vacuum seems to hover around 500mmHg (19.6 in Hg). I've read somewhere that desired vacuum in a Miata is 20-22 in Hg.


I'm at a loss here. I'd call Jeremy at FM for help, but I don't wanna get treated like the retard I probably am. :\

Is there any other information I can give to make troubleshooting this easier?
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Old 06-07-2012, 09:24 PM   #2
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A 16:1 AFR at idle is too lean. Adjust your fuel so you are at 14.7:1 at idle and see how it goes. You may even have to go richer than 14.7:1 since not all injectors can idle 14.7:1 nicely. When you adjust your fuel make sure you do it in all the surrounding cells as well since they are also used to interpolate the fuel.

If after doing the above it still dies just after firing, you probably also need to increase your post start enrichment.

My Hydra 2.5 also hunts during idle and running it richer at idle helps. I also find that if I don't run it closed loop (narrow band) during idle it behaves better. However, I'd prefer to run closed loop with the narrow band during idle to get 14.7:1 which is better for my CAT.
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Old 06-07-2012, 09:39 PM   #3
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Interesting, I was under the impression that if anything it was too rich at 16, given that there's no load.

Though thinking about it, the loss of initial power at idle when I hit the gas could definitely be it starting to die from not enough fuel.

Thanks for the tip, I'll try it out and report back!
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Old 06-07-2012, 10:12 PM   #4
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Well, idle's a lot smoother now.

bumped the idle cell up to .701 from .548 -- cells around there I did the same thing to. This is definitely on the right track.

I disabled short term fuel trim, the target was set to 14.7 (it was adding the max amount of fuel before to no avail). As I write this, she's purring at 14.3 AFR. It catches idle better after stabbing the throttle if I richen it up even more into the 13.7 range.

Also, this entire ordeal is increasing my idle vacuum substantially. I went from ~498 to ~540.

Does this all sound right?
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Old 06-07-2012, 10:22 PM   #5
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Yep, it all sounds right. No injector will want to idle nicely at 16:1, and you have big, old-tech injectors, so they aren't going to idle nicely at 14.7:1 either.
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Old 06-07-2012, 10:29 PM   #6
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Just noticed my IAC is bottoming itself out. Turns out my previous calibration of the throttle plate close stop was bad. Fixed that, IAC works as intended now -- I'm 3 or so full turns out on the bypass screw and it's at ~34% ! It also seems to be catching itself rather perfectly right now, too. Nice. 8)
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Old 06-07-2012, 10:31 PM   #7
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Quote:
Originally Posted by Savington View Post
Yep, it all sounds right. No injector will want to idle nicely at 16:1, and you have big, old-tech injectors, so they aren't going to idle nicely at 14.7:1 either.
tyvm for the confirmation and explanation why! This augmenting VanMSM's post makes me feel a lot better about the whole situation.
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Old 06-08-2012, 03:43 AM   #8
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Good to hear it's running better for you now. If you want to make your life easier you could buy some ID1000's (Injector Dynamics) since FM sells them and thus Jeremy should have a solid base map. Though you were using their RC750 base map and it wasn't treating you too well.

I have ID725's and they will happily idle at 14.7:1. The ID1000's should also easily do the same.
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Old 06-08-2012, 11:57 AM   #9
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I thought about that briefly once I bumped the idle AFR to the high 13s. Dropping another $400 just to idle slightly leaner isn't worth it to me right now. :\

Another update, my entire map is lean. Did a test pull from 3k to 7k and saw 12.1-13.0 AFR under boost. Gotta fix that, pronto. Short term trim response time seems to be absolutely useless when it comes to being under boost.. looks like it resets the fuel trim % as soon as any major change happens. I didn't pay extra for the long term either, thinking STT would suffice for immediate changes. Ah well.

When I started it this morning, she fired up just fine and didn't hunt at all. Though, until fully warmed up, the throttle was a bit sketchy. I didn't recalibrate the TPS, so I bet I have a dead spot from 0-1% or so. I'll update after TPS recalibration. :]
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Old 06-09-2012, 01:30 AM   #10
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TPS recalibrated, virtually no change. Idle will dip to around 450-500rpm if I clutch in above 2k and let it drop. I'm guessing I have to mess with the IAC anticipation settings, since I have lightweight internals and a light flywheel.

I think this is pretty much resolved. I definitely have a lot of info to work with now. I'll post back if something else goes awry. Thanks again!
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