Hey guys well 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. So I'm kind of at a loss here any suggestions?

 

bumpppp... just as a side note guys, I had to drive the car last night and surprisingly it ran like a champ I only got on it once ecause I didnt want to hurt anything and was prepared to back off rather quickly but to my surprise it went right past 5k and shifted great at 6500. So I'm at a loss the fact that its intermittent leads me towards a sensor or fuel problem of some type any suggestions?

 

check all the grounds especially on the back of the heads

 

I checked all the grounds I could find and they all appear tight. As I said it only has this problem intermittently so I'm really shying away from it being a fuel problem but I am still going to check the psi when I can. Some people have mentioned it being a crank sensor since the tach stopped working for a minute but I thought the car would run bad all the time if that were the case and I also thought the cam sensor controlled the tach but I may be wrong sorry for the long paragraph typing from my phone

 

I remember I fixed my buddys car when his cam pos sensor wires were cut and he had no tach. I fixed the wires and it ran no different. Not sure if the crank pos sensor will have an effect.

Did you physically reach around and feel the backs of the heads? There should be like 1 on the passenger side and 2-3 on the driver side. I had my car on the dyno and the grounds were not tight on the driver side head and it was breaking up intermittently.

 

to the best of my knowledge there are grounds on the back of the drivers head, one going from the alternator to the frame then 2-3 more on the under side of the block under the drivers side and the others near the tsrter on the passenger side i dont think theres any of the passenger head but if someone has a ground diagram for the engine i'd like to see it just to double check

 

hey Rdbird,
I had sort of the same problem yesterday. I've been driving the car around a little more lately since finally getting it aligned after cmoly k-member and QA1 install. Yesterday was the first time I really gave it a good whack and the car spun a bit then seemed to break up just a bit before the motor shut off . After a very loud backfire I threw it into neutral and coasted a bit to "listen" before deciding to fire it back up and it ran fine. After some conversing w/ my bretheren we were convinced it had to be a ground that was loosened by the car taking off. The car seems to idle a bit sluggish as well which I'm hoping is the ground just barely connected to where it should be.

Long story long...I started a thread listing where ALL the grounds are on our cars in case you we're interested.

 

Whenever I have this problem it either ends up being the Spark Plugs, Wires, or Knock sensor cutting power

 

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.

 

Makes sense. That's weird that it'd be exactly at some point in the RPM band. Keep us posted. GL

 

It sounds like the crank is walking due to worn crank thrust bearing surface;

when the crank walks the CKP sensor can't pick up the pulses from the reluctor wheel and you get no spark and no injector on both banks simultaneously;

yes, check all those things you said, and also you might want to check crankshaft end-play, hopefully it is not this.

 

I'm not really sure how to check this without tearing the whole motor apart ut the motor only has about 10k on it since new so i highly doubt the crank is walking lol

 

^ Yeah... let's hope it's not that. This is a good and interesting read though.. I'll be lurking..

 

One other thing I should add is that its not necessarily losing fuel at that rpm because the injectors are still pulsing and working its almost like its just shutting off spark for a split second then coming back on I'd think if it had crank end play the problem wouldnt be so concise. For crank end play to happen I'd think the problem would occur at varying rpms and the dips in power would vary. Whereas this happens exactly at 5900 every time and dips 10hp before coming back up then falls off 10hp then comes back up etc. The crank walking I think would be more intermittent and unpredicatble

 

^ I'm thinking the controllers in the PCM may be the issue as well. Unless it may be a crank sensor itself. If the sensor is "Tired" it may get to that certain frequency and just send a solid signal instead of the sine wave or just break up all together.

Don't quote me on that though, that's an expensive guess. I'd see what else your tuner/shop finds and trust their decision. Maybe have him show the different sensors in a Tech2, log them and see if one of the crank or cam sensors is breaking up. If they're not failing, then it may be the PCM.


-=EDIT=-
Do you have a tach installed on the car, other than the stocker?? Just wondering if the connection could have corroded.

And page 20 is the PCM Pinout if you want it.
http://cs.chevroletclub.net/graphics...g.diagrams.pdf

 

Nope the car still has the stock tach in it and good thinking on the sensor were going to try swapping pcm's and see what happens since they have a spare around. Do pcms really ever go bad I have to say I've never really heard of it lol.

This stuff just kills me because I normally do all of my own work including swapping the the ls6 into the car over the summer but I simply dont have the diagnostic tools, like a dyno, widebands tech 2 etc to diagnose this so I have to trust the shop which I do completely as they are well respected in south fl and are upstanding guys so i guess its just the waiting game at this point lol