need help

[neverlate]

I have a 99 c5 vette and is coding p0300 and p0420. it is showing misfire on 1,3,5,7. with low efficiency bank 1 on the cat. the o2's are reading so I am assuming they are ok. It will misfire for some time and then will smooth out but return a short time later. It sometimes will not do this again for weeks. Now it is happening with more frequency. I pulled the plugs after all the misfiring and they did not show any fouling problems. Would the misfire be caused by the bad cat or is the cat failing because of the misfire? I would love to drive this car but my first six months of ownership has not been a pleasant experience. Had to replace engine after the first 30 days and it just seems to continue.AAA is going to start sending me a membership check just not to call them.It is a nice garage ornament though.

[ninobrn99]

thats the whole left side of the motor. Id look at the wiring harness and the coil packs.

[neverlate]

thanks I am trying to see the wiring behind the left rear side of the head. I believe that there is a clip that can cause chafing on the harness supplying power to the fuel injectors.This might explain the clean plugs with a misfire as no fuel would be introduced to the cylinders but would misfire just the same. Spark with no fuel would equal misfire I would think. The only thing I don't understand is that it doesnt show any injector codes or harness voltage problems.This one could take awhile.Thanks for replying I have this listed on 2 forums and no one has responded until yours.I will post results when I get it all sorted out.

[oneBADDz]

It can actually be caused by the O2 and/or the cat also. check the wires to the O2 and maybe switch it to the other side and see if the codes move with it

[GatorSS]

... Had to replace engine after the first 30 days and it just seems to continue...

Ouch! Previous owner(s) must have really ragged it out. Who replaced it?

[neverlate]

I replaced the engine with a brand new ls1 direct from GM It came with everything new right down to the spark plugs. The engine install went well but now I have developed multiple random misfires and now one whole bank is showing misfire. What is strange is that it will smooth out and run fine for some time then when you least expect it flat on its face it goes.This did happen when i first bought it with the old engine so I would think it is more of an electrical problem. I pulled the Gm history on this car for warranty claims and it has had the pcm replaced 3 times and the bcm 1time. I have the car at Revxtreme in Tampa Fl. as of this writing.I don't know much about them but they were listed on the Corvette forum.I hope they are good at this because it is wearing me out.

[ITP]

Definitely sounds like a wiring/connection issue.

[jimmyblue]

I'd look to the driver's front O2 sensor first, and see if
you can log its voltage (or even just read it with a DMM)
when it is misbehaving. But this seems like an unlikely
thing to go bad, so early on a fresh motor. Unless the
sensor was reused and a connector pin was for sh!t, or
something.

You might want someone to inspect the PCM and see
it there is any sign or prior meddling, codes turned off
that might have been helpful in diagnosing etc.

[neverlate]

I had the car to REVXTREME in Tampa FL. and the cats were bad and replaced, the o2 sensors are reading within specs. The intake was pulled to check the harness to the rear of the engine and that shows ok. I picked up the car and drove it for a day and it started all over with a terrible miss. I was by a chevy dealer and they cleared all the codes so I could get it home.
It showed the following: 10pcm po300

40bcm u1016

60ipc u1016

ao-ldcm u1064 , u1255 , u1016, u1096
a1-rdcm same as above

28tcs u1016 ,c1276
etc etc
what is baffling everyone is that it will run just fine and then it goes into limp home mode.It is showing a lot of loss of communications with pcm and serial data line malfunction codes. The guys at the shop have spent alot of time on it and it will run fine while there It just seems to be a hit and miss thing but it is doing it more frequently now. I have used up all my AAA tows and the shop is 50 miles from home and it is tough walking or waiting in the heat.

Any thoughts would be appreciated .

[Perthss]

Talk about problems! Hi guys im from perth Weatern Australia,ive got a VX SS not sure if you guys know what that is but its like your GTO from OZ but with 4 doors.lol. Ive got a different problem with my gen 3.From idel to a bout 2500rpm it misses like crazy,GM told me for some reason its poring fuel in both banks but there not sure why.I replaced MAP sensors an all sorts of things but cant find the problem, and according to the ECU everything is running just fine!dont think so. Im using about a 20 Ltrs to do 35 mile hows that for good fuel usage. Any one ever heard of this sort of problem any help would be so helpful???

[m.j.lecki]

good morning just read your situation..sound like the same prob ihave now..
set of headers and some bolt ins and...supersmog the air rich situation..
no codes no help from gm.did u everfix yours..if so how...thanks

[jimmyblue]

The 'vettes (and other drive by wire) have a whole
separate throttle angle controller deal and if you
have an unreliable bus that's probably going to throw
a fit and put you to a "safe" mode. Chasing that kind
of thing down, ought to be fun. With so much messing-
around been done in that engine bay, there's a good
chance of electrical mishap. You just have to inspect
for physical damage, especially around the obvious
areas that people would have been working (headers,
whatever). Also check that the ground strap to the
rear of the head is intact and tight, as well as any
other sheet metal - engine block grounds. Of course
you can't very well check something that went and
got lost. Might put the voltmeter from block to sheet
metal, verify that there is no difference (try a few
combinations of chassis and engine accessory loads,
A/C, lights, boom-boom etc. just for yucks). Even a
few hundred mV can jack some of the sensors around
that don't have their reference ground sourced by
wire straight from the PCM. Of course the fun thing
about intermittents is that they only misbehave when
you take the test gear away....