High Idle with injector cut.

So here is my problem. I start the car, instant 1800-2500rpm. Drops to 1500 immediately, then jumps to 1800-2000. At this point the wideband flashes "lean" and the injector pulse width (monitored through MMCD) drops to 0. Then it jumps back to 1800 RPM or so dropping to 1500 RPM , then the wideband shows lean again and the PW is 0. This happens approx 1x a second. When the car is at operating temp the symptoms sometimes go away. When it is not doing this it idles at 1350 90% of the time. 10% of the time it idles at 850. When the AC is on it idles at 850 40% of the time.

Turning on the AC during warm up drops the idle to approx 1400 and (sometimes) stops the surge.

I have checked the ISC, it appears to be a new/reman/not original unit. It ohms out at 40 across each coil (matching the revision spec). The ect ohms out correctly at the correct temperatures, and the connections are good. The FMIA is deleted. I have not performed a BLT test due to lack of equipment, but have checked all the vac lines (all two or three of them....) and snugged up all the charge piping. I have read many of the other posts relating to idle surge, and haven't seen anything matching my symptoms. That of course does not mean there isn't a post. I have also checked the vfaq, and many of the pages relating to idle surge are dead, as is the 1000 answered questions.

I am running a stock auto ecu, 2G Maf in an Injen intake, SAFC II, and stock boost on a 16g. This problem started after I replaced the ECU that was in it with a known good one. The old ECU ran so bad that this problem may have been concealed by it.

 

You've set the BISS?
Ground the timing check connector when the engine warms up then set the idle as per the underhood label (750-850 i think, maybe higher for a/t).

Or just quickly turn it in a bit. Either way, there is too much air getting past the throttle plate causing the idle to do this. Closing the BISS should fix it.

 

[toybreaker]

Quoting 687OF2000:

I am running a stock auto ecu,



That may be part of the problem.

The auto ecu is comped for smaller injectors running at a different fuel pressure, (if it's still got the stock chip in it).

 

Interesting. Well, I have a new ECU hopefully being shipped to Terry for capping etc by another member as we speak.

As far as setting the BISS I have avoided that. As I said, the car occasionally idles at the correct speed. If I turn the BISS down 500 or so RPM, that would make the low idle 300, and it would likely die. At least that has been my thinking. I guess I should just give it a try and see what happens. Thank you both for your input, I will report back on the BISS setting.

 

If the car isn't an auto the mapping will be different as stated. That might throw things out of whack a bit, but you have a SAFC to compensate somewhat. The problem with that is by leaning out the car with the SAFC (since the ECU might be over fueling by thinking the injectors should be smaller) your ignition timing will be advanced. Not a big problem with light boost...

Anyway, the BISS is super easy to adjust and adjust back if the problem changes or goes away later.

 

...Actually I am putting in 20%ish fuel at all rpm (high throttle) to get the Wideband to show stoich. Of course it is a PLX wideband, and according to a test I just saw against a calibration medium, it could be off as much as 1 AFR!! Which does not make me happy. Of course the test was not nice enough to tell me which direction it was off... When I get the chip in the new ECU I am going to talk to Keydiver about exactly what map the car needs.

 

That would be removing ignition timing then. Might have something to do with the FPR, but you'd think 450/390 would mean you'd be rich. The A/T FPR likely gives more pressure which could help you out.