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Normal idle characteristics


Richard123

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My 1994 S4 has the back pressure control valve removed as well as the vacuum throttle bump disabled.  When I start the car from cold, idle sits around 1200 rpm and is fairly rough.  After a minute it revs itself up to a much smoother 1800 rpm and stays there for a few minutes.  Warm idle drops to an eventual 950 rpm and the car runs fine.  

 

My question is:  Is this how it should be expected to behave?  Does the ECU not have an initial "high idle" built into it as it expects the BPCV/throttle bump system to give it that high idle when first started?  I need to know if I actually have an ECU based issue to chase down, or if reactivating the vacuum throttle bump (at a reduced level) is the way to achieve a fast idle on start up.  

 

(Stock ECU, no Freescan info)

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Think about...

25 minutes ago, Richard123 said:

vacuum throttle bump disabled

And, IIRC, that's an 'ON-OFF' type thing. The vacuum capsule jacks up idle, or not.  (That idle IS adjustable via a setscrew on the actuator rod)

 

+++++++++++

 

That said, don't fix a symptom. You MAY have a partially clogged or 'corded'  injector, or a duff sparkplug. Why not start the car, but after 45 seconds, shut 'er down. Pull the spark plugs, and post a picture of them for diagnosis.

 

When was the last time that valve clearances were checked? Too-tight exhausts can make cold idle poor, but not affect idle once warmed up...

Atwell Haines

'88 Esprit

Succasunna, NJ USA

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Yes, the vacuum capsule is on/off and currently "off".  Valve clearances are fine, checked a short while ago.

 

The car definitely runs rich at first idle.  I can hold the pedal down a tad (to say 1800 rpm) to simulate the throttle jack and the car is smooth.  The question is whether or not the ECU should be giving me the 1800 rpm (via the IACV) on its own, or if the logic is that the throttle jack gives high idle for the first minute, then IACV takes over after that.  I know lots of guys have the BPCV removed and vacuum throttle jack disabled and wonder what their start up characteristics are.    

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From memory, my engine starts and runs at a high idle of about 1,400 rpm from cold, in the open loop mode. At no point does it ever climb above that, and idle speed drops as it warms, until it settles at about 950 rpm. I have no EBPV or throttle jack fitted, they weren't there when I bought the car.

Seems strange if it climbs up to 1,800 rpm.

Margate Exotics.

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Quoted for truth. I've removed my EBPV and the throttle jack too, and this is exactly how my car behaves.

When I bought the car though the EBPV had been wired open but the throttle jack was still active and sometimes the throttle cable snagged on the linkage causing a high idle like you describe. However, if you've already removed it properly....

Is your IAC operating within spec? I remember when I messed around with the minimum air bleed screw for kicks once the car would go crazy after the first throttle press (moving the car forwards) - my idle could go really high. Setting the IAC counts back between 30-40 put things right again. 

Vanya Stanisavljevic '91 Esprit SE | '97 XK8

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Hmmmm.....ok.  So it seems something is up.  The throttle jack is still there, just no vacuum line to the actuator.  It may well be that air screw/IACV setting.  I was hoping not to have to try to get Freescan running again.  I did it years ago and it was beyond me then.  And I'm not getting any younger!

 

Probably will dive into it as the idle is pretty rough cold (something isn't right) and the 1800 rpm is a bit much with the loud (Larini) exhaust until it starts to warm up.  I'll probably be posting for Freescan/driver/emulator/Windows/baud rate help next.  

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On 9/6/2017 at 14:58, ian29gte said:

Richard, there's plenty of very capable guys on here who can walk and 'talk' you through the Freescan process, so it shouldn't be too painful an operation. And, as has been suggested, a Freescan log is always a good first port of call.

Believe it or not, I got Freescan to work!  (I might be getting younger!).  

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On 9/9/2017 at 17:06, Giniw said:

I think I read somewhere the cold idling speed may be risen when the weather is very cold. Maybe a temperature sensor could be faulty?

You may be spot on.  At first glance, the water temp sensor voltage and the coolant temp don't seem to jive.  But desired rpm is never indicated up near 1800 rpm.  RPM also rises and falls without throttle position or IACV count changing.  I will record a proper start-up and drive tomorrow.  (I was too lazy to do it for this morning's commute!)      

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Well I suppose that if the desired idle is not 1800 RPM it's not revving because the ECU thinks the weather is very cold.

Maybe that could be a leak in the intake that leaks sometimes depending on the vibrations of the engine when it's cold, and become air tight because of the dilatation when the engine is warm? It's just a guess, I don't know.

If for some reason the advance is not correct (and not constant) it could alter the idling too, maybe it could be a problem with your top dead centre sensor? I have no idea why it would be only with a cold engine, though ... That's just another guess.

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I think I figured it out my wild idle via the attached Freescan file.

 

On startup (line 130) idle is low so car sets desired rpm high (2387 rpm) and IAC goes full open (240), but car is WAY WAY rich on the map and has rough and low idle.

At line 726 the ECU closes loop and immediately starts to cut fuel (pulsewidth drops from 3-2 which very slightly improves idle).  At line 848 pulsewidth drops further to 1 as ECU continues to cut more fuel and idle continues to rise.  (IAC still wide open, O2 sensor still rich). 

Finally at line 896 ECU has cut enough fuel to have O2 go lean and idle rpm is up around 1650-1700 rpm.  It stays this way until 1338 when I lose patience and drive away.

 

In my opinion, the problem is either that the fuel pressure reg is failed (unlikely as the car doesn't seem to overfuel at idle except at start-up) or more likely this is caused by the "oversized RC injectors" which the previous owner advertised the car as having.  This is made more severe by us being at fairly high altitude. 

 

Does the diagnosis make sense to you Freescan experts??  

start up sept 12.csv

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I think I got it sorted.  

Started with TPS of 0.73V and IACV of 31 when hot.  Turned the TPS to get it lower and got 0.51V but IACV was in the 50's when hot.  I turned the air bypass (which is just the throttle stop) a 1/4 turn and now TPS is 0.53V but IACV is back down to around 29 hot.

 

The car now starts with a brief burst of RPM (like 1500) and settles to a relatively smooth 1350 or 1400 with a request in that area.  The car is no longer requesting the high idle (+2000) so the IACV isn't pinning open (240 counts) and the ECU isn't over fuelling, so the car isn't nearly as rough.

 

I may give it another 1/8 or 1/4 turn to get the the IACV a bit lower yet, but so far so good.  Mornings still above freezing, but not for long.    

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