redbird555

iTrader: (14)

I had this in another section but I didnt get too many hits so maybe it will be better here.

Anyway I've got some kind of gremlin going on here... Last week I drove back up to college (200 mile drive) and I got into the throttle a couple times and the car shifted fine at 6500 no issue. Fast forward 6 days later, now I dont drive the car much up at school and when I do I rarely get on it.

Last night I jumped on it for a couple secs and it ran great I didnt look at the tach to see where it shifted but it hauled *** and shifted fine, today I take the car out and decided to do a wot pull, i had the shifter in 1 (not the brightest idea i know) and hit it from 20 to eliminate the downshift lag well when it hit about 5k the car started to break up pretty bad and had no power.

At first I thought the tach was just slow and I was hitting the limiter so I tried it again but left it in D the car downshifted fine but at 5k it started to break up very badly again and I just let off at that point the tach instantly dropped to 0 but everything else was still running fine with no weird noises. It stayed that way until I got a block from my appt and then the tach suddenly came back up and worked ok. I got back, popped the hood, and theres no weird noises coming from the engine, it drives fine at low rpm as best as I can tell, no codes either. I also revved the car to 6k in the driveway and it didnt breakup at all.

When I saw the tach drop to 0 I immediately thought of a bad cam sensor, but I've had one go bad before and in that case my tach never started working again and the car had a hard time starting due to it not knowing where the cam was. The car also has a brand new ractetronix fuel pump in it and ran great yesterday so i doubt its that.

A couple nights back i took the car out and to my surprise it ran great with no issues and shifted at 6500 without a hitch. A couple people have mentioned grounds but I've checked all the ones around the head and frame. If someone knows or could post a diagram of where alll the grounds are I'd love to see it.
Thanks for the help

Frost

iTrader: (45)

Keep an eye out for crank-sensor related codes.

redbird555

iTrader: (14)

thanks frost, its not currently throwing any codes but when I get home I am going to put a fuel pressure gauge on it just to check the pump again and check for immature codes in the pcm is there anyway to check the crank/cam sensor with hptuners? also could a weak coil cause it to rev fine in park but missfire under wot?
i installed the bigger alt upgrade about 2 weeks ago also which is just the 140 amp alt from a truck, direct bolt in and now my voltage never drops below 13 but i dont really feel this has anything to do with it because the car ran fine after the install

chrysler kid

iTrader: (14)

Bad maf or 02 sensor on the way out

redbird555

iTrader: (14)

I thought about the maf sensor but wouldnt a bad maf cause hesitation and poor driveability everywhere? and one other thing that happened to me last night but i'm fairly sure its interdependent of this issueis Over the past couple weeks I've had a couple small instances where my car would not crank, like flat out I would turn the key and nothing would happen but on the 2nd try it would crank up just fine. I thought no big deal I'll look at it over thanksgiving break well tonight it didnt start and left me stranded. The security light is not flashing so I doubt its the VATS system but again I cant be sure I also do not hear a "click" from the starter solenoid like its even getting power. I feel its probably unrelated to the hesitation issue as this has been going on for a little longer but I'm still at a loss here

blade2222

Disconnect the maf sensor and drive it and see if that solves your problem.

chrysler kid

iTrader: (14)

Mag sensor is used to start the car and to measure air volume at wot.

In this order
Check your fuel pressure
If thats good replace your front 02s
If that doesn't solve it try a new maf

redbird555

iTrader: (14)

well I can rule out the maf because I unplugge it and the problem persists an the o2's are not used at wot throttle when you hammer it the compute ignores the o2's and goes off the tune in the engine so theres no issue there because the car runs great at part throttle, the fuel psi is also fine at wot along with a new racetronix pump installed

blade2222

O2 sensors are not used at WOT.

ae13291

iTrader: (8)

i had a similar problem to yours, but it was very faint, and misfired slightly across the powerband. mine turned out to be a cracked ceramic on the spark plug, mine was #4 cylinder.

chrysler kid

iTrader: (14)

I know it sounds like a pain but you need to check the fuel pressure. Get a cheap pressure gauge and check the pressure on the schrader valve.

My formula had a similar problem and the hose that connected to the fuel pump in the tank had rotten out and my car only had 18psi at idle instead of 52psi like it should have

redbird555

iTrader: (14)

thanks guys I had to have the car towed home 200 miles from college (thank god for AAA membership) and its currently at lashway motorsports down there since I dont really have a place to fix it getting looked at they said its been running fine for them?

They are putting it on the dyno though tonight to log fuel pressure so we'll see what they say.

redbird555

iTrader: (14)

just an update guys but im afraid its not very good. The car is over at an ls shop where they have been looking at it on the dyno for a few days. What we have gathered is that the car will misfire every time at 5900 rpm the graph dips down about 10 hp with each misfire but will then rebound up to its previous reading before it misfires again.

The odd thing is that when the widebands are plugged into both sides of the engine, both banks go lean the same amount, misfire at the exact same time, and at the exact same rpm every time. The problem never fluctuates in the rpm band. So we're thinking that the chances of it being a bad coil, plug, or wire if very slim seeing that it happens all the time at the same time. Were going to try swapping coils with a known good set just to be sure and the fuel pressure was checked and holding good at 59psi up top.

They're thinking maybe a bad pcm since the problem is so exact, if it were a fubared ground or power wire or an ignition problem I'd think that it would be having issues at varying rpm points or on one bank.

crainholio

iTrader: (2)

There is no "Mag" sensor on F-bodies.

If you meant MAF sensor, you're completely wrong as to its function and use by the PCM with stock firmware.

Stock PCM setup ignores the MAF after 4,000RPM.

Good advice on the fuel pressure. The other two steps are simply throwing parts at the car, which rarely gets results.

redbird555

iTrader: (14)

yes and i verified the maf sensor was working by putting it in open loop and still having issues along with monitoring the o2 sensors with a scanner