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BK02M5

New S14 Idle Challenges

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15 minutes ago, tzei said:

Do this again. I had a similar problem and was hard to pinpoint. Sealing those rubber blocks cured idle.

 

On a side note i did A/B testing with MAF on a dyno. It gave a couple more hp but had some fluctuation in low rpm range so i decided to stay with OE AFM. Maybe some tinkering with resonator tube/volumes up stream could cure that but i never went there.

Thanks.  I plan to disassemble the plenum and intake tubes to check the blocks and O-rings this weekend.   Interesting comment about MAF.  I'll hold off for now. 

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Picked up the car from the shop, and they are really puzzled on this problem.   They finished the AFM adjustment and balancing throttle bodies.  The surging idle is slightly better but still not fixed.  It now surges from 1200 to 1600 rpm at about a 3 second cycle.  I did some troubleshooting today.  Buzzed out the TPS, AFM, ICV, and coolant sensor to the ECU.  All fine and no ground shorts.   

 

When cold, the car idles with no surging for the first ~5 minutes.  It's a little high at 1100rpm but nice and stable initially.   Does this give anyone a clue as to where to look?  

 

Coolant temp sensor was just replaced.  It reads 2.4kOhm cold which I understand is likely okay.   
The elbow on the front of the plenum is solid as is the vacuum port on #4 for the brake booster.  

 

I'm thinking next step is to remove the intake, check the blocks and orings.   I'll pickup some S38 gaskets first.   

 

Appreciate any additional inputs.    

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13 minutes ago, BK02M5 said:

When cold, the car idles with no surging for the first ~5 minutes.  It's a little high at 1100rpm but nice and stable initially.   Does this give anyone a clue as to where to look?  

 

That still says false air to me because when in warm up mode, additional air (ICV) and fuel are added, and the idle is smooth. When the fuel is tapered back as the engine warms up then the engine hunts. So, you may still  have too much air at idle.

 

How does it drive off idle? Stumble off idle on acceleration?

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Yo might also check the O2 sensor output voltage.I think the issue is becoming prominent when the system goes from open loop to closed loop.

 

Cheers,

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13 hours ago, Einspritz said:

 

That still says false air to me because when in warm up mode, additional air (ICV) and fuel are added, and the idle is smooth. When the fuel is tapered back as the engine warms up then the engine hunts. So, you may still  have too much air at idle.

 

How does it drive off idle? Stumble off idle on acceleration?

The car definitely stumbles when accelerating from idle.  Up in the lower powerband, >3500rpm, it does not stumble.  I'm not revving it much above 4500 because the engine rebuild only has 150 miles it.    

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No knowledge about the s14 ecu but I what Ray mentioned reminded me of similar situation I had. Different setup, running electromotive management on itbs (no icv) and was experiencing a similar surge at idle. I ended up turning off the ego authority below 1500 rpm so that 02 stayed in open loop and just referenced the fuel map when near idle. Now idle is super stable. So if 02 is switching between open/closed in that rpm zone and it’s not functioning correctly seems it could definitely be causing issues. 

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Update for today.    Found elbow fitting on #2 for the ICV hose to be slightly loose.  Added a bit of epoxy.   

Checked O2 sensor voltage while running.  I'm using a Fluke DMM which doesn't sample quite fast enough.  It was varying between .09 to .5 Volts.  It may be going higher but I cannot tell with this meter.  


Disconnected the throttle bodies link age just in case they were adjusted too far off of home position.  It made no difference in the surging idle. 

 

Removed Plenum, throttle bodies and blocks to check for sealing.  There are gaskets on either side of the blocks that are fairly thick, ~1/16".   Some type of non hardening liquid sealer was applied on the block side of these gaskets. They look okay to me in the pictures attached. What do you think?   There was no sealer on the face against the head.   I may add some liquid sealer that works for fuel areas.   Pics below show the throttle bodies 1/2 with gasket on and off, as well as throttle bodies 3/4 with gasket on and off.  

 

Next step for me is to reassemble, check it's working the same.  If so, then I'll replace the O2 sensor.  

 

Head - no sealant used here

IMG_3551.JPG.9f67698b5ba706f493cc74d5f2643759.JPG

 

 

IMG_3555.JPG

IMG_3554.JPG

IMG_3553.JPG

IMG_3552.JPG

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Although I see no indication of leakage, I notice that the gaskets are compressed a lot at the bottom, but not on top.  It looks like someone tightened the bottom fasteners very hard first, and then the top.

Did you try to determine if there is air leaking past the injectors? My way of doing this is is to listen with a small diameter hose.

You cannot just measure the output voltage of a narrow band oxygen sensor because it consists of on-of 5 volt pulses. 

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Does the support exist under the air box to hold up that and the throttles?  That could explain the compressed gaskets at the lower half.

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1 hour ago, allbim said:

Although I see no indication of leakage, I notice that the gaskets are compressed a lot at the bottom, but not on top.  It looks like someone tightened the bottom fasteners very hard first, and then the top.

Did you try to determine if there is air leaking past the injectors? My way of doing this is is to listen with a small diameter hose.

You cannot just measure the output voltage of a narrow band oxygen sensor because it consists of on-of 5 volt pulses. 

Are you certain about the pulses? I thought you could measure NB output voltage with a meter...

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1 hour ago, jimk said:

Does the support exist under the air box to hold up that and the throttles?  That could explain the compressed gaskets at the lower half.

Yes, the support is under the airbox affixed with a rubber isolator then the arm goes down to the lower part of the block.  

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3 hours ago, allbim said:

Although I see no indication of leakage, I notice that the gaskets are compressed a lot at the bottom, but not on top.  It looks like someone tightened the bottom fasteners very hard first, and then the top.

Did you try to determine if there is air leaking past the injectors? My way of doing this is is to listen with a small diameter hose.

You cannot just measure the output voltage of a narrow band oxygen sensor because it consists of on-of 5 volt pulses. 

Allbim:  How's this measurement with the hose work to test injector leakage?  

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Allbim:  How's this measurement with the hose work to test injector leakage?  

Use the hose like a stethoscope. Just to clarify, I am talking about air leakage between the injector(s) and the manifold. There should be an o-ring.
Edited by allbim
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1 hour ago, ray_ said:

Are you certain about the pulses? I thought you could measure NB output voltage with a meter...

 

I think it is perceived as a pulse in this case because the ECU constantly adjusts the mixture. You can "back probe" the sensor and get a voltage reading in mV.

 

I routinely use Lambda sensors with just a voltage readout and can say that the voltage can be stable e.g. 500mV. Of course it is for measurement only and not connected to an ECU.

 

On my S14, I have two bungs, one in each downpipe to monitor the mixture via wide band sensors and a voltage bar graph. The center narrow band sensor goes to the ECU.

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