2002 Trans Am Misfire on 2 cylinders Puzzled

I have been studing scanner danner videos and I believe I have covered the basics here, but now im stuck. I even tried to hire a local mobile mech. paid him $45 to tell me he didnt know.

2002 Trans am LS1 5.7
I rescued the car so I have an unknown history of how it ran before it sat for almost 10 years.

I am fighting a Misfire on cyl 1 and 3.

So Far I have verified it is not The coil, Wire, or Plugs
Also it is not the Fuel injector, I changed it out and still missing.
Compression is 180 on cyl 1 and 155 on cyl 3, 180 on all the rest
The miss goes away when revving the engine.
Changed the Intake gaskets AND even got another ls1 intake to put on and same issue
Vacuum is a steady 25
New Spark Plugs
New Battery
New PCV
Cleaned the MAF with maf cleaner
Cleaned the throttle body (it was really clean already)
Car only has 22k miles on it, was sitting outside for over 10 years
Wire brushed all the grounds

Here is a Video of what I have done. I have a tech 2 scanner, but admittedly it is beyond my skill level to understand all the functions
I only have access to a tech 2, but my neightbor has the most bad to the bone autel scanner, but he doesnt know how to use his either.




Steak dinner to the person who figures this out!! I am lost.

It’s been my experience with misfires that get better off idle are mechanical issues. You performed a compression test but if it were me I’d perform a cylinder leak down test. Also wondering if there’s carbon on the intake valve soaking up the fuel?? What do fuel trims look like?

Also since you have the tech 2 scanner I’d perform a crank variation relearn

Cheryl, Thanks! I will get you a video of fuel trim data tomorrow, Also I did the Crank Relearn. but will do it again as well.

How about the leak down test ?

2 cylinder misfire on adjacent cylinders I always check for a firing order mix up.

I'd also like to know the leak down for #3 compared to a known good cylinder. While you're there, it may be worthwhile to check running compression compared to a known good.

Besides the misfire, how well does it run? A V8 with one cylinder down should still move itself down the road.

For this is an LS1 i think it is impossible to mix up plus wires or coils, are you asking if the tech 2 is wrong? and it is actually cyl 2 misfiring?

arg. did the leak down test incorrectly. i have to wait until tomorrow.

I think you need to dial up the pressure on your leak down tester when you hook it up.
It looks like you've got little air going in, therefore little leakage.
I turn mine up to 40lbs. That's where top dead center becomes important because if the connecting rod isn't straight up and down, the air pressure can push the Piston through the rest of a stroke.
Also I wouldn't remove the rockers or any part of the valve train. You could be masking a problem that could be making the valves hang open.

Here is the Compression and Leak Down test. The compressor is set at 100 psi. These are Harbor Freight gauges and the left gauge is not the pressure going into the cyl.

I took the rockers off because i really wanted to test the sealing of the valves without laboring to find actual TDC

Cheryl,

Here is the Crank Relearn and the Fuel trim data.. Thoughts?

Fuel trims look real good. So was the leak down test done with the air pressure going in the cylinder bumped up like Noah suggested? Second I’d be half tempted to check the misfire counter on your scanner. Example unplug #5 #4 etc injector and see if it’s accurate. Usually gm is accurate in the misfire counters.

Also mind posting video of vacuum gauge reading?

I would like to see you do a running compression test. 150 psi is about 20% less than your known good. Technically its in spec but I would say not good.

Hey Cheryl here you go it is the readings

Any thoughts??!

its only getting misfire information from knock sensors, you've got to verify for sure which cylinder is missing, can you do a old fashioned temp reading on exhaust manifolds from cold t verify which cylinder is missing.
also your fuel trim looked real good so you need to get your o2 sensor data up, need to see if running rich or lean if its switching perfectly you've got fuel issues.

Here are the O2 switching. Would you verify they are working good?

I didn't see this mentioned previously so I figured I would bring it up. The misfire counts on cylinder one are likely just an effect of the cylinder 3 misfire.
Notice how when you disabled other cylinders that another cylinder would start counting up, all though considerably less than the actual misfiring cylinder.
It also appears that the computer is accurate in flagging the missing cylinder based on the results of your testing.
I think it would be safe to focus your efforts on cylinder 3. Your compression test seems to back this up as well.

As for your last post, I don't notice any problem with your oxygen sensors. Looks like maybe b2s2 took a little longer to heat up and become active, but certainly nothing that would contribute to a single cylinder misfire.

Cylinder 3 being low on compression points to a mechanical issue.
The misfire that goes away off idle points more specifically to a valve sealing issue (in my experience).
I trust that you know your tools better than I do, and after reading the instructions for that harbor freight leak down tester I believe you are using it as described, but I don't trust the results of that test/tool.

Personally, I would repeat that test on cylinder 3 at true TDC with the valve train assembled and dial up the pressure or see if you can borrow a different brand tester from your local parts store.

I completely understand why you performed the leak down test like you did, but by removing the rocker arms you're entirely removing the camshaft, pushrods and lifters from the equation.

I just had a Malibu yesterday with a valve sealing issue and I couldn't pin point the source of the leak until I cranked the dial up to 100psi. Then you could hear air coming out the exhaust.

I hope you get it figured out, keep us posted!