: CEL came on: codes

11-29-09, 08:49 PM
My CEL came on again. This is after 160 deg stat, pulley and tune. Once again, it happened on a warmish day sitting in traffic. The temp ususally reads two notches below 220, but when sitting, it goes up to around one line below (shouldn't the fans be set to prevent that?). Anyway, the codes were:

P0171 and P0174 (rich or lean a/f ratio condition, both engine banks).

Both times I have been at idle when the light came on. Last time, the light went off after a few days.

Here is what I'm thinking: The CEL only comes on when it is warmish outside,and the coolant temp goes up. If it were a lean condition (from the pulley and resultant increase in air intake), wouldn't it be less likely in warm, moist air (less O2 per unit volume)? Maybe it's a rich condition. Don't engines run a little rich at idle anyway? Is it possible that the fans weren't set to come on at the right times when I had it the pulley and stat installed? With the fan they have in front of the car during dyno pulls/tuning, the fans probably never needed to come on. So the tuning was done with the coolant temp at about 180. When it goes above that (at idle on a warm day), the conditions deviate from those that applied during the tune.

All of this is speculation, but I'm just trying to find an explanation. In any case, the car runs great. Of course, in the summer, the situation may change.

Maybe I should just buy a HP tuner or AFI live to change the fan speed thresholds myself (assuming that is what is causing this). Then, if I ever get new wheels/tires, I can change those ratios as well. Honestly, it's just nice when things are done right the first time.

Any ideas?

11-29-09, 10:39 PM
Dirty mass air meter, or bad plugs are the only reason for it to throw those lean/rich codes in stock form...

Is it my tune that is doing this??? If so, ill just resend you another tune again for your exact mods...

I just dont change anything in the tune that would effect rich or lean conditions at non wot...

11-29-09, 11:44 PM

I had the work done at Hennessey. I have intake, AR headers, 9" crank pulley, 160 t-stat, and tune. They told me I didn't need to change the plugs.

Thanks for the input.

Gary Wells
11-30-09, 06:02 AM
I feel confident that Hennessey would at least like to know about it so that they can make whatever changes necessary to prevent it from happening to future customers, and I would think that it would be in their best interest to at least offer to square it away for you if it is their responsibility.
Have you talked to Hennessey yet about the issue recurring?

11-30-09, 06:47 AM
The fuel trim numbers have to be pretty huge for a code to set.
Minor changes due to ambient temps should be compensated for by the engine computer.

Usually one of two things cause lean codes to set.

1. Induction system leaks. This could be a loose clamp or pulled loose boot after the MAF or a poorly sealing intake gasket or other seal that is either not installed right or damaged.
2. MAF errors. Some of our MAF sensors are very sensitive to changes in the induction system. Aftermarket air filters or CAI kits can change airflow through the induction tube causing turbulence and or airflow that is not in a direction the sensor can compensate for. So many people whined about the MAF screens that GM started taking them out which makes them even more sensitive to changes.

You could have calibration issues but I'd examine those two things first.

11-30-09, 07:40 AM
Its probably the Hennessey intake, it threw such codes on my friend's ride.

11-30-09, 09:53 AM
I guess it could be the intake, but I've had that on for 6 months. The CEL came on after changing the pulley and the stat. I have emailed Hennessey about it.

The car runs great, without a change in the idle quality. Could it still be a vacuum leak?

11-30-09, 09:57 AM
On the intake is your problem. On the inside of the tube, there is a lip near the maf sensor that is creating turbulance. You just need to grind down that lip flush and then it will work just fine. It makes an airsplatter effect.. If that is a word. lol where at certain rpms it just makes the maf frequency jump all over the place untill it goes past that airflow amount.. Once you grind down that block inside the tube, it will be smoother airflow and the meter will report itself correctly again..

The issues, also as with alot of those aftermarket intakes is the change in reported torque to the transmission. Alot of people dont understand that the tcm sets itself up based on load values, and the maf is in charge of most of that.. So if you mess with the accuracy of the maf, "to effect maf readings leaner in most cases" you are applying invalid line pressures to the transmission.. To low of pressure could result in bad things...

Im not saying that the tcm cant try to fix itself, but there is always things that effect other things...

12-12-09, 04:00 PM
The CEL came on again, and I took the car to the dealer.

The car had a vacuum leak. One of the intake tubes (at the "T" on the Hennessey intake) was disconnected. Unfortunately, this is something I should have seen myself. Their opinion is that it is a poorly designed system, and that the tube is too short. With throttle, the engine twists a little and pulls the tube off the intake side. I think it's more likey that the tube didn't get snapped completely on to begin with (and was probably disconnected during the pulley/tstat install). Time will tell. Anyway, they charged me $79 for the diagnostic, which is fair, since it had nothing to do with them. Much better than shipping my ECU (would have accomplished nothing) or going back to Houston.

12-12-09, 05:13 PM
Glad to hear it was something so simple and relatively inexpensive.

12-12-09, 06:05 PM
Thank goodness it was a simple problem. So are you going to have the Hennessey crew send you a longer tube?