For a while now, Ive been troubleshooting a problem where the car hesitates when accelerating in a higher gear. The car would hesitate, and then accelerate after the pedal had been depressed for a second, occasionally accompanied by a mild back fire.

I took the car back to my tuner last week and we drove around for about 30 minutes while he monitored everything. What he showed me was that the car was going extremely lean when the accelerator was not being depressed. As soon as you even tickled the gas, everything jumped back to normal.

What he said was that there was probably either a leak upstream of the MAF or a leak in the exhaust prior to the O2 sensors which resulted in fresh outside air being registered by the O2 sensor.

According to him, the most likely cause was that my LSX intake had cracked due to rubbing on the factory valley pan bolts and outside air was being drawn in through the bottom of the intake which hadn't been registered by the MAF. So this afternoon I pulled the intake off, and while there were definitely wear marks from the factory valley pan bolts (which Ive since replaced with button head bolts) there aren't any cracks in the intake.

So now Im back to square 1. I saw quite clearly that the car goes very lean when coasting, so the question is, what would cause that condition? Any thoughts or input is greatly appreciated!

Thanks!

 

Either way, those bolts need to be replaced. They should not be contacting the intake, and you do not know how it is influencing the sealing when torqued down.

 

agreed....bolts have been replaced as of 10 minutes ago

 

While I don't disagree with there being an unmetered airleak somewhere causing the hesitation, the car is designed to go lean during coasting, it's called Deceleration Fuel Cutoff (DFCO) in Hp tuners. DFCO is one of the many things in the ECM operating system that helps the Vette get such great fuel mileage.

Hope this helps.

Jim

 

agreed....however (and please keep in mind I know next to nothing about tuning) the value that he was showing me as I was driving, and yes I should have paid more attention but I thought we had found our problem, was between 0 and 0.1 when the accelerator was stepped on, however when coasting, it would quickly get as high as 25 and remain there until getting back into the gas. Again, please excuse my ignorance with regards to this topic.

The value he was point to on the lap top I assumed was air to fuel ratio, with 1:1 being a normal condition and 25:1 being a very lean condition, but again, that was an assumption that may have been very wrong.

 

Ok. Based on your tuner's experienced, what does he think the AFR should be during decel (no throttle input) condition?

 

honestly couldnt tell you....heading back on the way to work tomorrow to talk to them, now that there is no readily identifiable air leak from a cracked intake

 

Just a thought..but give your PCV system a good once-over...I had "lean stall" problems every time I came off the throttle (after installing an LS7 valley cover and catch-can)...turns out it was my PCV hoses. Check your lines while the engine is running...squeeze and if the engine tries to stall out...there's your culprit.

Hope this helps...

 

I'm by no means an expert tuner, and the only tuning program that I am familiar with is HP Tuners. Would be curious to know what program your tuner us using. The only way your tuner could show you a 25:1 AFR is by using a wide band O2 sensor. The stock narrow band O2 sensors only know 14.7:1 and if the AFR is less than or greater than 14.7:1. With HP Tuners and narrow band O2 sensors the only way I know to tell if DFCO has engaged is too watch the O2 sensor voltages, they will flat line lean, but still won't be able to tell you what the AFR is.

Again hope this helps,

Jim

 

That may....I did a bunch of stuff at once to include aftermarket valve covers that required fabbing up a line to use as the PCV to the intake. I also added a catch can at the same time. Ill get the intake back on tomorrow and start messing with it. Thanks for the tip!

 

Thanks Jim, Ill know more when I go back and talk to them tomorrow

 

Also... look at the wiring harness on the back of the intake (drivers side) @ the back of the fuel rail.. There is a "blade type" piece that the harness rubs on. It actually rubs though the harness,wire causing it to ground out... Do a search.. It is quite common.

 

just checked, harness looks good, no rubbing. Anyone else have any bright ideas as to where an unmetered air leak would be likely to occur?

 

he might have been showing you the AFR in lambda since 14.7 AFR = 1 lambda, although it would be out of normal range for a lambda number of 25 as lambda = actual ARF divided by AFR stoich (14.7 for gas)

 

ok, ill have to sit down with the guy and get a better idea of exactly what he was talking about. I got anxious when I thought we had found the problem

 

I have a similar problem. Pulling a little unmetered air from somewhere. Misfires at idle but goes away by 1100-1200 rpm. HPTuners shows a pretty even distribution of misfires accross all 8 cylinders. A few misfires are normal but if I let it sit there and idle (particulary when its cold) it'll get enough misfires to set a P0300- at least it did before I desensitized the misfire count. Anyway, I can completely smooth out the misfires by spraying starting fluid under the throttle body, towards the underside of the intake. I need to get off my but and look into this but its not bad enough to motivate me right now.

Good luck to you though.

 

thanks.....mine isnt even so rough as to cause any misfire codes but it certainly does hesitate and then surge when you floor it after coasting for a minute. with the HPTuners up, it looked like it would go lean while coasting, and then if you floored it, it would suddenly dump in a crap ton of gas to compensate for the lean condition which resulted in the hesitation and the occasional exhaust backfire

 

This may help, although it is not exactly the same situation as yours. I have a hesitation due to the tune I got when my headers were installed. It only happens when going from overnight cold to around 115 to 120 degrees coolant temperature, in other words, about 2 minutes on the way to full engine heat. I found that it will mostly go away by thoroughly cleaning the MAF and throttle body of all deposits, and retightening all clamps on the air intake locations. It's as if my car is right on the ragged edge of a too lean tune, but I live with it because it runs so well once I pass through this threshold. Like I said, maybe not even close to fixing your issue, but if you haven't gone in and looked at your throttle body or MAF screen in a while, it can only be good for you to do the maintenance.

 

yup, MAF has been cleaned, TB has been cleaned, all clamps on intake have been tightened. Thanks for the tip!

 

I don't recall the DFCO being set to work on my car, but I do believe it was going open loop and lean in a certain range while decelerating. I don't recall under what conditions though I believe it was higher in the RPM range and would happen when downshifting while decelerating. It's been a while since I was tuning it.

Peter