OK, driving to work fri. morning, half way there I notice a stumble at part throttle while accelerating.Hmmm, back to a cruise, feels fine. Next time I accellerate, I see the check engine light go off. But it didnt just go off, its flashing.So I know this deal already,P300. Pick up the predator and pull the codes, still cruising at 70. Multi. misfires, and both banks rich.WTF? I call my tech buddy and he has no clue what could have happened while cruising on the highway to cause this.. It barely catches an idle at the toll booth, and accelerates like ****. Get to work and inspect for obvious stuff like vacuum lines and plug wires. Nothing looks amiss. Finally get a techII on it an perform a cylinder power balance test and found no change at idle by disabling #s 3,5,and 7. WTF? So today I changed the fuel rail AND injectors to a set off my buddys vette that ran great when these parts came off, and I still have the same problem and can unplug injectors 3,5,and 7 while running with no change. I would appreciate any input cause I dont want to drive it 40 miles to work tomorrow on 5 cylinders, cant be good. Thanks...

 

Wow, nothing? never seen a thread go this long with no replies... OK, checked the plugs in question, and the are firing.Pulled the valve cover and everything looks in proper order. As I was pullind the valve cover and removing the PCV valve and tube, it came detatched from the back of the intake, almost looks broken. Vacuum leak??WTF??

 

If the plugs are firing the next thing you need to check is the injectors circuits. You should be able to do this with a test light, but a noid light would be best. it is possible that the injector drivers in the PCM are bad. If the injector drivers pass your next step is a compression test.

 

Thanks for the input Jimmy. OK, I redid the vacuum line from the PCV valve that goes behind the intake, no change. It had to be leaking, yet was not the cuase of my problem. All of the plugs in the 3 cyls. in question smell like fuel, and as I stated earlier, we changed the injectors and rail yesterday. Im getting no codes while idling, yet the motor is obviously running on 5 cyls. I plan to get a techII over here this afternoon, but Im stuck at home lookin at this thing till then. Can I test the drivers in the PCM with a TechII? If not Ill source a light from someone. Thanks again for any input anyone has...

 

ANY input anyone has....

 

unplug the harness from the injectors and test the voltage, if you have like a haynes book or someone else knows...see if it is putting out the voltage it's sposed to. I would also check the resistance u get in the harness ends too.

 

Good info, thanks. Anyone know what that voltage should be? The car wont idle now and Im goin nuts, missed 2 days of work now,WTF!! Anyone ever seen 3 cylinders just die while cruising on a relatively low mile LS1(50,000).The heads are just as new and there was no noise, till I tried to drive it to work this morning...
I can get another set of pushrods, springs, hell even heads if I need them, but I still have no idea what is wrong?!?

 

have you tried a compression leakdown test for the piston rings? someone over on PT.net lost 4 pistons at the same time, all on one side of the engine. needless to say he didnt have much power coming out after that. this is a really weird problem you are having. mult. misfires and the solution not being the plugs or plug wires...

 

Yeah, been trying to avoid that I guess... Gonna try to get a guage over here today, hate to think this relatively new motor is done.

 

A couple of things to think about:
what do the spark plugs look like?
Do you have spark?
what did the comp check look like?
Are they stock injectors? if not do you have enough fuel pressure?
Check That the positive battery connection is tight?
Check to see that you are getting 14.0 volts at idle?
How old is the battery?
hope this helps

 

Unfortunately all that checks out except I havent done a comp. test yet...
As I said before, it was running great until all of the sudden all three cylinders just died at once.

 

OK, now I have a broken valvespring, from driving it this morning, yet the pushrod is fine! What is going on in this LS1??? All of the other springs/pushrods look fine, yet I still have three misfiring/dead cylinders. Broken valvespring as an effect of what???
Someone help please....

 

Even if I have no compression in three cylinders, would that cause a valvespring failure?? I dont get it.!!!

 

do you have stock valvesprings? aftermarket springs are stiffer so after miles and miles and abuse theyll just give. i cant tell what springs you have from your sig unless they are stock...you can also bust a spring is you abuse it cold. like ifyou just start the engine, WOT with soe comp918's they wont hold up long.

 

No, springs were from sdpc, sold as a kit with Manley ti retainers. Not sure what brand the springs were, but not heavy duty at all(.600 max lift) with light installed pressures. Maybe someone knows these springs? Like I said though, this spring wasnt broken for the past three days while all three cylinders have been missing/dead.
Thanks for all the input guys, keep it coming...

 

Alright, the springs on the offending cyls. are 'fatigued'. I can push the valves off the seat by hand. On the #1 cyl., I cant move the springs...
So the theory now is that these valves were either hanging open or actually floating(?) causing the "dead cylinder" situation. Will replace soon, probably with cheap, durable LS6 springs, hopefully this fixes my problem!

 

hopefully this will fix your problem! goodluck