bad missing, backfiring.. took soem scans.. heres what i found! any help appreciated!

[lcvette]

Hello guys, been having a problem with my wifes 95 LT1 M6 vette. has been running perfect with the mods on it for about a year: PP heads PP intake 219/219 .560/.560 112 cam LT headers 52mm TB 30 LB injectors no cats no EGR no AIR programmed by Ed Wright symptoms... car will sometimes run perfectly for a day or two then without warning, it begins to miss stutter backfire shake and sometimes die when coming to a stop. I ran PCMCOMM tonight for about 20 mile trip where the car ran perfect for the first half even after reaching normal operating temps for about 15-20 minutes. while this was running right, the Integrator L abnd Integrator R were both sitting between 126-130 fluctuating back and forth staying together, all other readings looked great. then the problems started and i noticed that the Integrator R went from the 126-130 to 195 and stayed there. this is when the car starts running terrible. I have replaced almost everything including injectors trying to hunt the issue down, and am now at my wits end.. can anyone give me any advise? also, i would like to do some real time scanning recording with datamaster and have used it on the past on my old laptop but cannot find the demo version anywhere any more! any one know where to DL any more? any help on this will be appreciated! Chris

[tjwong]

Chris, what did your O2 sensor voltages look like? It sounds like the O2 sensor died and the voltage went low. So in this case the PCM thinks it is lean and tries to add fuel, and it does that by cranking up the fuel trims and if it continues to see a lean condition it tries to add fuel until the trims peg at their high limit. Does a lot of black smoke puke out of your tail pipes? I had that very same thing happen to me sometime ago, and it was a dead O2 sensor that was the culprit. If thick black smoke is coming from one side of the exhaust I would definitely suspect a O2 sensor.

[lcvette]

TJ,

yes thick black smoke poors from the pipe under acceleration when the car is acting up... the 02's have about 300 miles on them.. i replaced them while trying to figure out the problem... they were bouncing back and forth though from 100-800 back and forth and closer to 300-600 while cruising when running good. I felt under the car for an exhaust leak and found a big one where the header collector mates to the catback.. and guess what my 02 sensors are on the catback side... i guess this could really cause some issues huh! the only thing that kills me is that the car will sometimes just run perfect with no warning and run perfect for a while.. i mean it smooths out clean power, no black smoke.. mpg's take an instant turn for the better and reach 23-24mpg on the highway.. when it acts up it will drop FAST down to about 12-13 mpg and stay there until it clears itself up. ughh.. i will move the 02's to the coated header collector and see if i can get the leak fixed.. the collectors and catback were welded a little off and i think the fact that they were constantly under a bind has destroyed the collector gasket.. this would explain why the leak came about so soon. i recently swapped from a 3" SLP exhaust that had aquired a bad leak here.. i just welded up soem collectors to my stock LT4 exhaust system and ran it without cats.. really quieted the car down alot much more pleasant to drive like this but now the LEAK is back.. tomorrow or the next day i will cut the pipes and reweld them a litle better or even might look for soem of the sure seal beveled style collector couplers and weld them on.. just want the car quiet and running smooth for my wife as her daily driver!

let me know what ya think!

Chris


[Modified by lcvette, 11:51 PM 2/10/2004]

[tjwong]

Chris if you have exhaust leaks near the O2 sensors, that can cause the O2s to see a lean condition. Even though it maybe rich with every exhaust pulse there is a low pressure pulse that actually induces a vacuum which sucks air into the system at that point of the leak. Of course the sensors would see this as free oxygen which in its mind means a lean condition. How is the wiring on the sensors are they OK, meaning has the O2 sensor harness been disturbed or modified. The sensors "breath" throught the wire harness, and if it can't breath it will do the same thing as giving a low voltage output because it has no reference O2 at the sensor.

[ncChris]

the sensors have been modified, but they were clipped about 1" from the plug to add extention wires to run to the front O2 harness near the oil pan. I am going to put everything on hold until i can get the leaks sealed up and then reset the PCM and see if this helps straighten it out. I am also going to put the O2 sensros closer to the collector gathering where the 4 primaries are brought together.. i always felt the O2's were too far from the primaries to give the most accurate readings. I will let ya know what i find.

thanks again!

Chris

[tjwong]

If you clipped the wires at the sensors wiring between its connector that could be your problem. If I was you I would get some new sensors and get a set of sensor extension cables from Caspers Electronics under their part number 109010, these are 12" extension cables that plug in direct to the O2 sensor connector. Splicing the wires at the sensor can cause problems with the sensors ability to breath through the wire. The sensor actually gets its reference air via the wires insulation as a conduit for the reference air.

[lcvette]

Well, the car ran great for about 2 years with the sensors spliced this way. BLM's stayed great and she was reading dead balls on. wasn't until recently she started acting up. this is killing me!

Chris

[tjwong]

Chris I would still be looking at the O2 voltage when the problem occurs. There are only a couple things that can cause the trims to peg at 195 and one major factor that controls this is the O2 sensors. In closed loop the ECM runs from the VE tables if your car is a 92 to 93 LT1 or in case of a later 94/95 car the PCM relies on the MAF sensor. But in either case the ECM or PCM will trim the fuel based on the O2 feedback. And if the O2 sensor goes whacko and pegs itself at 100 to 150mV I can assure you that the ECM or PCM will peg the fuel trims to 180 and up to its limit in order to correct for what it thinks as an extremely lean condition. And it will do so to the extent of pouring fuel in to the point that the engine is puking black smoke and runs like crap.

The other device that could cause excessive fuel is the coolant temp sensor. If it goes open circuit the ECM or PCM would think its -40* outside and will try to correct for that situation. And if the opposit happens the ECM or PCM thinks its 250* and will lean it out. When this problem occurs does the black smoke only comes from one side of the engine? Your 95 is SFI controlled whick means that each O2 sensor only affects the bank that is connected to. If black smoke only pukes out of one bank, then I would would really be looking at that O2 sensor, because that is the only thing short of a leaking injector that would cause excessive fueling on only one bank.


[Modified by tjwong, 8:57 PM 2/12/2004]