I have a 98 V6 Camaro and I have misfire and low power when I accelerate. Idle is sometimes erratic and with a missing cylinder, sometimes not.
Fuel filter, O2 sensors and spark plugs are new. Always the same trouble.
Sometimes, when misfires occurs, the tachometer drop.
Sometimes the engine works well, sometimes not.
My tech said there is no DTC code with his scanner. He would like to try new spark plugs wires and new coils.
Thank you very much for your help.

 

Normally I am a "Don't throw parts at it guy" but new wires and coils on a 25+ year old car is a good thing.

I know you said no codes but no check engine light either? Normally with a ignition miss you will get a cylinder miss fire, In many cases it may be a random miss fire. With low power and a miss fire the engine could be going lean. Scan tool while driving and watch the O2s. Does the miss happen under wide open throttle? Have you done a tap test around the MAF sensor? If you disconnect the MAF sensor and drive the car does it run better? Do not do this unless you have a way to clear codes.

If the car runs better under WOT that means something is causing a lean condition. WOT runs about 20% richer then normal.

The tap test on the MAF sensor is design to see if your element in the sensor is broker. Think of it as a head light that when you slap it, it starts working. A tap test failure is any stubble or change in how the engine runs. It is done with light taps with the handle of a screw driver.

Disconnecting the MAF sensor puts the entire system in Limp Home Mode. This is a predefined timing and fuel mapping based on average conditions. It is there to get the car to a mechanic if something fails. If the engine runs better in LHM that means there is a sensor or wire that is causing bad data to be sent to the PCM. Like its 80 degs out and your air temp sensor says it is -20 degs. LHM will use a fuel/spark mapping that just assumes it is 72 degs out. Note: this does not mean your MAF sensor or air temp sensors are bad. This just points to the next path to follow.

None of this is the proper way to follow a manual on fixing this issue. It is just some quick tests that may help you figure out the area of the issue.

 

Thank you very much for your help.

No codes recorded with the scan tool but the check engine light is blinking sometimes during a few seconds .

Nothing is changing when I tap or disconnect the MAF sensor. After disconnecting the MAF sensor, I had to erase the code "low voltage" for this sensor.

The miss happens not only for a wide open throttle but when I accelerate. WOT is not better and may be worse. The only better condition is driving on a flat road at a constant speed.

The O2 sensors has been changed for new and the problem is still there.

 

So things are pointing to spark. Next I would use a spark tester. The tests will raise the voltage requirement and may show which cylinder is failing. As you add pressure to a cylinder you require more voltages to fire a plug. As you add more charge to a cylinder you add a small amount of pressure but when you add a load to the crank (try to stop the motor) the pressures goes up very fast. On a car that idles without a miss but then misses under lot we find out the a part of the ignition system that can handle the idle voltage can not handle higher voltages needed under load. Commonly this is a secondary ignition system problem but it can be the primary. Without a Oscilloscope it is hard to tell what is going on.

I like to use a AC ST125 spark tester because it will stress the system. Many spark tests do not add resistance. I have seen them that let you change the gap the spark must jump. These you can keep increasing the resistance. The issue with that is if you increase the resistance too high you can cause problems that where not there before.

The thing I don't understand is the lack error codes. The check engine lite blinks when damaging something is possible. The two things I am aware of is an engine running too rich or a dead miss. Both can result in too much fuel entering the CAT and both will set a code. The only way I know of that will not happen is if someone has tuned the car not to record codes.

 

Thank you one more time.
I have a scope and I built a module to get spark signal.

But the problem with this V6 engine is to get signal on the spark plugs. It's easy to have it from the coils but not at the end of the wires because it's under the exhaust pipes and the only access is under the car.


I don't think the car is tuned not to record codes because I bought it new in 1998.

About idle, it is sometimes good and sometimes not with misses and erratic speed.

 

It should not matter where on the wire you hook up the scope. Since it is all one circuit a a defect anywhere in the circuit will show all along the circuit. There is a finger print that a secondary circuit makes. Even at the coil you can tell if a cylinder is lean or rich. How long the plug fires and the amount of power it takes to fire a cylinder can tell you a lot about the circuit. In your case since you do not have the training you just need to see a difference from other cylinder. If the scope you are using is not designed for car you will need to get it adjusted so you can see the proper wave form.

I have a china made scope that hooks to a lap top that works good but I did not trust the digital pattern so I hook an old scope I used to tune antennas. Getting the setting right on the standard scope was kind of a pain. I think I found something online to guide me.

Something is wrong a blinking lite is big deal not having a code after a blinking light could indicate a PCM issue. You will need a manual.

Higher end scan tools will track miss fires.

 

My scope is an old Tektronix 453 and is working very well to repair antic radios and TVs. My trouble is not with the scope but with the wire to pick up the signal. I don't know if I need to put a coax or a single wire. With the single wire hooked up, the curve is full of parasites and with the clip taken from a strob the signal is low voltage.

 

I drop my car at my mechanic a week ago and he doesn't find the problem ! Do you think a possibility could be a little too much oil in the crank case ?

 

Here is the result :
Spark plug number 5 is destroyed after only 8000 km ! (AC DELCO 41-921 Iridium)
The reason could be the defect of the coil. The oxygen sensor detects the defect and reduces the richness. The temperature rises and the spark plug is destroyed.
With the new spark plugs and the new wires the car was still broken down. With the new coil, everything is OK.