[NA] Rough idle after NB swap with EUDM manifold, 99 injectors, BP5A cam, Headers - Miata Turbo Forum - Boost cars, acquire cats.

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Old 04-22-2014, 11:32 PM   #1
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Default [NA] Rough idle after NB swap with EUDM manifold, 99 injectors, BP5A cam, Headers

Hi everyone,

I completed swapping a 99 NB motor into my 97 NA a couple of weeks ago and am currently preparing it to get dyno tuned by a local tuner. Ever since the swap, the car has been running on my stock 97 ECU with the following modifications:

99 NB Motor
99 Injectors (NA fuel rail with FPR)
BP5A cam
EUDM Intake Manifold
Hondata IM Gasket
Racing Beat 4-1 Headers

Above 1000RPM or so, the car sounds, accelerates, and runs perfectly fine. It is only when the car is at a standstill the idle either hesitates and dips around 650-800rpm, or constantly goes back and forth between 1200-1500 RPM. The car is currently on the stock 97 ECU until I get my MSPNP ready to go. Does this sound like a simple closed-loop tuning issue common with the modifications until it is tuned? Or an issue with either a vacuum or a valve?

This is my first swap so I'm not sure if the car should idle just like my original motor did, even with the listed modifications on the stock 97 ECU. I figured it simply needed to be tuned, but if it is supposed to idle like my original motor despite the different injectors, much larger intake manifold, headers, and BP5A cam, I'd need to get it sorted out before my dyno session next Monday.

Thanks,
Kris
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Old 04-27-2014, 11:43 AM   #2
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Vacuum might be an issue, the combination of the MSM intake cam and the square top IM might be too much for the stock ECU to handle.
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Old 04-27-2014, 12:49 PM   #3
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Did you exchange throttle bodies? I "think" the 94-97 TB (and related idle control valve) is different from the NB TB. If you just plugged the 97 ECU into an NB TB, it might not work.
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Old 04-27-2014, 01:12 PM   #4
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I would not expect it to really run well on a stock ECU. My 97 with 01 pistons, 99 head, and other breathing mods runs roughly and way down on power compared to how it runs on megasquirt. It runs just well enough on stock ECU to pass emissions.
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Old 04-27-2014, 04:36 PM   #5
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Quote:
Originally Posted by hornetball View Post
Did you exchange throttle bodies? I "think" the 94-97 TB (and related idle control valve) is different from the NB TB. If you just plugged the 97 ECU into an NB TB, it might not work.
+1
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Old 04-27-2014, 09:22 PM   #6
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Yes, I exchanged throttle bodies. It's the squaretop manifold mated to my 97' throttle body, which actually leads me to my next question -

I purchased the K&N Intake off 949racing that is for the NB. According to emilio, its the same intake used on the 95R. I was able to utilize the IAT sensor bung for my MAF delete/IAT install from my MSPNP, however there is no nipple that I can connect the 3/4" hose from the bottom of the 97' throttle body.

I found instructions from Revlimiter that involves drilling a hole into the intake and fitting in a 90 degree elbow/grommet to attach the IAC hose:

revlimiter.net - Intake Isolator Box

However, considering i'm deleting my MAF, running an IAT with a MSPNP, are there any other options I can do with this hose before I start drilling a hole in my brand new intake?

Thanks again.
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Old 04-28-2014, 01:29 AM   #7
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That's why you're running badly on the stock ECU. You've got unmetered air entering the engine at idle. You likely won't have any issue once you switch back to speed/density with the MS.

You can just put a breather filter on the bottom of the '97 TB and call it a day. That's what I'm doing on the Red Car. If you've got boost, this would result in a boost leak, but normally aspirated it's no problem.
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Old 04-28-2014, 02:17 AM   #8
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Quote:
Originally Posted by hornetball View Post
That's why you're running badly on the stock ECU. You've got unmetered air entering the engine at idle. You likely won't have any issue once you switch back to speed/density with the MS. You can just put a breather filter on the bottom of the '97 TB and call it a day. That's what I'm doing on the Red Car. If you've got boost, this would result in a boost leak, but normally aspirated it's no problem.
To clarify, the K&N intake isn't installed yet on the car. The symptoms described in the OP are with the stock 97' intake installed on the car.

I pulled two codes today - one was the EGR purge valve (i deleted the Charcoal canister) and the other was the crankshaft position sensor. Would that be a cause, or a result of these symptoms? Do these sound like a timing belt issue? I forgot to mention that I changed that was well. However, even at 1000 rpm the car runs fine.
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Old 04-28-2014, 11:25 AM   #9
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Did you plug the purge valve? If not, it will be a vacuum leak.

Doesn't hurt to verify TB alignment. Crankshaft position sensor code can't be good.
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Old 04-28-2014, 03:45 PM   #10
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Crank position sensor shouldn't matter on the 97 ecu. It was only there on the 96-97 for misfire detection for emission reasons. Did nothing for performance. I'm guessing he's running off the 97 cas bolted to the 99 head.
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Old 04-29-2014, 12:18 AM   #11
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Quote:
Originally Posted by hornetball View Post
Did you plug the purge valve? If not, it will be a vacuum leak. Doesn't hurt to verify TB alignment. Crankshaft position sensor code can't be good.
Nope, didn't plug it. I haven't gone for emissions yet, you mind elaborating on that? Do i simply plug the two nipples on the purge valve and the code will go away?


Quote:
Originally Posted by ryanr View Post
Crank position sensor shouldn't matter on the 97 ecu. It was only there on the 96-97 for misfire detection for emission reasons. Did nothing for performance. I'm guessing he's running off the 97 cas bolted to the 99 head.
Thats exactly what im doing, 97 CAS on 99 head. So is that to be expected then? No need to worry?
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Old 04-29-2014, 01:31 AM   #12
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Not worried about the code. I'm talking about the rough idle. An open purge valve is an unmetered vacuum leak. Either plug the nipples on the purge valve or plug the nipple on the IM that goes to the purge valve.
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