vbb

Today I swapped in an 05 ECU (not my original one) and got my keys reprogrammed at my local Honda dealership to match. No problems intially, the programming went fine, but almost as soon as I pulled out of the dealership lot I noticed that the yellow CEL light in the dash came on (malfunction indicator lamp). When I came to my first stop, the idle would drop all the way down to maybe 500rpms and my car would almost shut off, but then would "catch" right at the last second and keep idling. The rest of my ride home was all highway and/or green lights, so I had no problems after that, except the light never went off.

Got the car back in my garage at home a few minutes ago and tested it out. Same issue happens every time... it'll idle at around 900-1000rpms just fine, but then as soon as I give it a little rev, the rpms will go up, then when I take my foot off the gas, the rpms dip all the way down to about 500rpms, the car shudders a bit like it's about to shut off, then it "catches" and the rpms rise back up to 1000rpms or so and it keeps on idling. I repeated this at least 5x turning the car off and on in between, and it's the same thing every time.

Update!!:
I reset my ECU and let the car idle for about 15mins and that seems to have smoothed things out a lot, and I don't have the CEL anymore. No more jumping idle... nothing. Test drive was going along fine, so I decided to do a WOT run. When I hit VTEC, the CEL light flashed on me. I immediately backed off and the light went away. The car was smooth after that and the light didn't come back. The car is now parked safely in my garage. When I hit VTEC at partial throttle, or when I was at WOT under ~6Krpms, I got no CEL.

When the EMS was running my car, there were no problems at all with the idle (or with any warning lights). Now that I'm back to an 05 AP2 ECU, anyone have an idea of what could be causing this issue? The car used to be supercharged before I got it, and still has the Vortech FMU attached to the car (and maybe the Walbro fuel pump, although I can't swear to that). Niether of those should cause the problem though I wouldn't think. I have the K&N shorty intake and a Megan header, but the stock cat and AP2 exhaust on the car.

akirhol

While I doubt I could actually assist in resolving the source of the issue, I can say for certain that your post is missing one critical piece of information.

I'm assuming you mean the CEL, you should go to your local Autozone and have them pull the code or rent a scanner for the day if you have another car to get you there. Having that code is likely to help narrow down the issue.

vbb

Well, I honestly couldn't remember what the CEL looks like on the S2000, but this light didn't say "check engine" or "CEL" or anything like that... it's just a small picture of an engine, which in my daily driver (BMW) means something different than the CEL. According to the manual, this is the "malfunction indicator lamp."

Either way, I just pulled the negative battery cable for about 2mins then reconnected it. The engine light is off (for now) but the idle issue is still present.

pastoli

That FMU would be a good start to go back to stock and the fuel pump too. The FMU runs off of vacuum off of the car, so its possible it might be getting too much fuel? You would have to see what your A/F ratio is. Go get that CEL checked out and go from there.

siriljitsch

You also need to give the ECU time to learn. Which means that you should have let the car idling for 15-20min after you installed it. Otherwise the ECU will use generic stock values. Since your engine seems to be modified these values do not apply, which could cause your rough idle.

I would pull the code, then reset the ECU and then give it time to learn...

vbb

Thanks for the advice. I did exactly that... reset the ECU by pulling the negative battery terminal for a few minutes, then I started the car and just let it idle for about 15mins. The idle was a little rough at first, but then smoothed out a bit. The CEL was gone.

I then took it on a test drive and it seems to have fixed the idle issue. But now, at the VTEC switchover, the car was a little rough for a second and I got the blinking CEL light. It went away just as soon as I let my foot off the gas.

So now... any ideas about the flashing CEL at VTEC? Is it going too lean? The light went off about a second after I let up, and I drove the car back home without any other problem.

akirhol

Don't know about how much the ECU changes this, but typically a flashing CEL indicates a critical and potentially engine damaging issue. Don't know if you can pull codes and it show what caused the flashing CEL (maybe as a pending?), even with it off right now, but may be worth a try. I would personally be hesitant to take it into VTEC at all if it causes a flashing CEL.

FWIW, the terms MIL (Malfunction Indicator Lamp) and CEL (Check Engine Light) are interchangeable. And you should avoid resetting the ECU before pulling codes.

vbb

Thanks for the heads up. I won't reset the ECU again before getting the codes. I'm thinking (guessing again) that the reason I got the flashing light when I hit VTEC at WOT was because the car was running lean. As soon as I let off the gas, the flash went away, and didn't do it at any other point (wot under ~6K rpms, or partial throttle over ~6K rpms).

SgtB

Resetting the ECU does ZERO. Our ECU is constantly learning and adjusting unlike some ECUs. Your idle problem is a result of the ecu not knowing your engines idle characteristics. Resetting it will only delay it learning them. Get the code and let us know what it is. If it was a brief flash that went away while at or near WOT, I'm betting it was a random misfire storing the 1st part of the two part code. It should sort itself out as the ECU adjusts its long term fuel trim setting to match your vehicle.

spectacle

My bet is the previous owner modified the wiring to accomadate whatever EMS he was running. Verify that there isn't any wires left loose and tapped into that font need to be. And get rid of that FMU. It may not be causing the issue, but you don't need it if you're NA and it's one less thing to worry about.