Poor idle when radiator fan comes on?

**Larry** · 12-12-2007, 09:21 AM

I have an 08 automatic Corvette. What would cause idle misfire when the radiator fan comes on about 50% or more?

**sunsetorangess** · 12-12-2007, 10:34 PM

Are you sure its a misfire? Whats your voltage at when you say its misfiring.
J

**Larry** · 12-13-2007, 02:57 AM

Originally Posted by sunsetorangess

Are you sure its a misfire? Whats your voltage at when you say its misfiring.
J

02 voltage drops down to double digit values. I called it a misfire because that's what it feels like but believe it's going lean actually. Site won't let me post the log or I would.

**0rion** · 12-16-2007, 09:17 PM

what mods do you have? Tuned?

**Larry** · 12-17-2007, 03:39 AM

Orion,

I have headers and and intake. I have HP Tuners. This misfire occurs with the stock file and with tuning. Tuning seems to make no difference.

**0rion** · 12-17-2007, 05:46 AM

first off you ran an 11.8@118 with headers and intake? That's impressive. Secondly, have you logged misfires with hptuners to see if it's cylinder specific or if it's random? Is there a certain time when it does it? Such as start up or does it do it all the time? Did it just start out of the blue or after headers/intake were done or something along those lines?

**Larry** · 12-17-2007, 07:42 AM

Originally Posted by 0rion

first off you ran an 11.8@118 with headers and intake? That's impressive. Secondly, have you logged misfires with hptuners to see if it's cylinder specific or if it's random? Is there a certain time when it does it? Such as start up or does it do it all the time? Did it just start out of the blue or after headers/intake were done or something along those lines?

I have a log of the misfires. They are random, all cylinders misfire. I believe it's a lean problem as my 02s go down to double digit when the radiator fan comes on around 60-70%.

Had the poor idle from the day I drove away from the dealer. I read posts that the 08s idle wasn't as good as previous so unfortunately I didn't take it to the dealer prior to my headers and intake. They are not the problem in any case. Had I got in traffic and had the fan came on at a high % then I would have immedicately taken it back to the dealer. But it wasn't until after the headers that I found the problem with the fan/idle and in traffic.

In closed loop I get misfires consistently at idle and lower rpm with low load. With load it goes away. WOT is unaffected. In open loop idle quality is much better, even acceptable until the fan comes on. Totally unacceptable with fan. Even with engine cold if I turn the fan on via VCM controls idle goes bad.

11.802 is pretty impressive and it will even do better in cooler air. It was 77* on that pass with IAT temps even higher. If I could only get idle quality to an acceptable level I could enjoy the car.

**mrr23** · 12-17-2007, 07:53 AM

in your file, is there a way to increase the airflow number for the fan? in mine, i can change the airflow number for the fans to compensate for the drain on the alternator. i increased mine from .1 g/cyl to 1.1 g/cyl. now, the idle doesn't drop.

**Frost** · 12-17-2007, 08:46 AM

Something just to keep in the back of your mind... It may not be lean at all. Misfires (when real) send unburnt fuel AND air out of the tailpipe. That fresh air/fuel mix hits the O2s (or wideband) and shows up as a false lean.

**Larry** · 12-17-2007, 11:06 AM

mrr23,

There are some fan relay controls. State vs. desired % and Output control vs. state. Not sure what those do. Idle speed doesn't seem to be a problem. Very rough idle but rpm is fine.

Frost,

Definitely could be. I really don't know for sure whether it's lean or too rich. Don't see any black smoke or smell anything so I would believe lean vs. rich.

**mrr23** · 12-17-2007, 11:14 AM

Originally Posted by Larry

mrr23,

There are some fan relay controls. State vs. desired % and Output control vs. state. Not sure what those do. Idle speed doesn't seem to be a problem. Very rough idle but rpm is fine.

in my case, by adding more airflow to the equation, the PCM knows to bump the idle up and add more fuel, if needed.

tell you what, email me your file, so i can have a look at it. mrrjjdh03@aol.com

**Larry** · 12-20-2007, 11:11 AM

I checked my voltage from my battery to ground and had an approx 1/2 volt drop. I jumped from the ground of my battery to the fan ground. Fan makes no difference in idle quality now. Half my problem solved! Yea! Still poor idle in Closed loop however. I think maybe my main ground is bad. Haven't been able to find it so far.

At least the engine doesn't almost die now with the fan on!