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Erratic Startup Idle - Could Also Cause a Misfire


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So I'm 99% sure my IACV is giving me grief but can't get my head around exactly what's wrong with it.

 

Car is an NA-T manual running AEM EMS v2 standalone.

 

When I start the car, the idle is wildly erratic quickly rev'ing up and down between 1.5-2.5k and then once the startup map times out the car settles right back down and behaves fine. The same thing happens from a stone cold start with the only difference being the idle moving much more slowly and going on for several minutes until the car warms up.

 

If I disconnect the IACV, the erratic idle goes away and the car sits pretty happy on a warm startup. It will then run lean while driving around and cold start is generally a big struggle which are both understandable as there is now no air control for cold start. Now I guess I could have the map tweaked to just operate without the IACV but that'll probably set me back a full mapping fee and I'd like to know what's causing it in the first place.

 

I've switched the IACV out with another one and the problem remains. I'm going to dismantle my one tonight and give it a good clean but don't think that's the issue as it almost seems electrical? Maybe the ECU can't sense the position of the IACV and is wildly searching for the position?

 

I'm also suffering horrendous ignition breakup at the top of the rev range on WOT from 3rd onward. Would this IACV issue contribute to a misfire? I've been through most of the ignition system and it hasn't solved the problem. So I'm hoping for 2 birds/1 stone here. AFR's are fine as it pulls through the gear (low 11's iirc).

 

Any pointers would be appreciated, I've been chasing this for going on a year now and with the weather starting to pick up again it's really starting to wear my patience :(

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Have these problems been issues since it was last mapped?

 

The ECU doesn't sense the position of the IACV - it is a stepper motor and is just opened more or less depending on the value of the duty map and the error between the rpm and the target (in closed loop).

 

I am not familiar with the AEM software but you should have maps on your ecu called something like IACV duty vs temperature, afterstart enrichment vs time and target RPM vs temp. In closed loop the ECU uses these as a base point and adjusts the air flow within a set range of duties. As your car idles fine when it is upto temperature it suggests either the low temperature part of these maps are wrong or the afterstart enrichment is too high.

 

I would guess your ignition breakup problem is an unrelated problem. When was the last time you changed your plugs - what temperature rating are they? Are you using a distributor, stock coil, HT leads or coil packs? How much boost are you running?

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Have these problems been issues since it was last mapped?

 

The ECU doesn't sense the position of the IACV - it is a stepper motor and is just opened more or less depending on the value of the duty map and the error between the rpm and the target (in closed loop).

 

I am not familiar with the AEM software but you should have maps on your ecu called something like IACV duty vs temperature, afterstart enrichment vs time and target RPM vs temp. In closed loop the ECU uses these as a base point and adjusts the air flow within a set range of duties. As your car idles fine when it is upto temperature it suggests either the low temperature part of these maps are wrong or the afterstart enrichment is too high.

 

I would guess your ignition breakup problem is an unrelated problem. When was the last time you changed your plugs - what temperature rating are they? Are you using a distributor, stock coil, HT leads or coil packs? How much boost are you running?

 

It was pretty much immediately after it was mapped, everything was fine at the garage when I picked it up. Car was on and off a few times while I was there. I drove it home, immediately blew off a boost pipe on the first pull and it's been doing this ever since (including the ignition breakup). It just seems weird that it was fine one minute and then acting up after an intercooler pipe came loose. I've checked in the area around the pipe and nothing seems to have been damaged as a result of it. It's as if something has changed or failed but I can't figure out exactly what. Maybe the ECU is playing up? I can always try and plug it in to my laptop and see what shows up.

 

The plugs were changed when the car was mapped so they are brand new NGK BKR7EIX's. Exactly the same as before when the car was running fine. It's about the only thing I myself haven't changed so i may have to do that just to rule out the chance of a faulty batch. Other than that, stock distributor, new HT leads and new stock coil. Boost is around 12psi.

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It was pretty much immediately after it was mapped, everything was fine at the garage when I picked it up. Car was on and off a few times while I was there. I drove it home, immediately blew off a boost pipe on the first pull and it's been doing this ever since (including the ignition breakup). It just seems weird that it was fine one minute and then acting up after an intercooler pipe came loose. I've checked in the area around the pipe and nothing seems to have been damaged as a result of it. It's as if something has changed or failed but I can't figure out exactly what. Maybe the ECU is playing up? I can always try and plug it in to my laptop and see what shows up.

 

The plugs were changed when the car was mapped so they are brand new NGK BKR7EIX's. Exactly the same as before when the car was running fine. It's about the only thing I myself haven't changed so i may have to do that just to rule out the chance of a faulty batch. Other than that, stock distributor, new HT leads and new stock coil. Boost is around 12psi.

 

Presumably when you collected it from the garage it wasn't a proper cold start, hence why it may have run ok? Is there any reason you havent taken it back to them? Have you looked at the AEM logs?

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Presumably when you collected it from the garage it wasn't a proper cold start, hence why it may have run ok? Is there any reason you havent taken it back to them? Have you looked at the AEM logs?

 

No it was a full cold start as I collected the morning after they'd finished the work. We went out to the car, started it and let it warm up while generally gassing. Car was then turned off while I went in to square them up and was fine as I started it and left. It doesn't hold idle even when warm either, it's actually more noticeable when warm (idle hunts more aggressively). I took it back and got all the ignition components from them to work through and try sort the misfire issue. We suspected a vacuum leak with the idling issue at that point as well. I've then ruled that out and traced the problem back to the IACV.

 

If I can figure out how to connect my laptop to it, I can try and see what the logs read. I'd rather get a good idea of what's wrong with it before taking it back as there needs to be something that's failed to cause these issues. It'll be a silly fix like a loose wire or something, I just know it :D

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Could you have had a boost leak during mapping which you essentially fixed when you reinstalled the intercooler hose? The added restriction of the intake at idle may cause it to hunt.

 

It's MAP sensor based so I wouldn't have thought so? Hmmm

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  • 2 weeks later...

So I plugged into the ECU to try and see what's going on during the startup part of the map. All sensors look to be reading okay from what I can see. It looks to me like the revs are freely running away and as soon as it hits the upper target RPM set in the start map, it alters the timing to drive the rpm back into range again. As soon as it's back within target, the timing levels back out and the cycle starts again with the revs freely climbing. The whole car kind of shakes when the revs are driven back down, presumably from the timing swing. I've taken a video of the hot start. Same thing happens from stone cold but on a much slower and wider rev range scale.

 

 

The only thing I can think of at this point is that maybe the car was mapped with a boost leak that has now been fixed? More air than the car is expecting could be what's driving the idle up? Any input would be great as other than that, I'm pretty lost

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