Engine Missfire?

[TransAm95]

Hey everyone, well the other day while driving mr car I was in 6th going like 60 mph, and hit the gas to accelerate a little and my car started to shake alittle bit, and my shifter would move. So when i got off the highway I decicded to experement a bit, i went to 3rd at about 30 and hit the gas pretty hard and everthing seemed to jump, the shifter handle moved and then soon as i got to about 43 it was fine. I did the same this time in 5th going aobut 40 and it did it again untill 50 or so. I rembered my friend (96camaro) having the same problem a couple months ago, but he had about 101k on his lt1 at the time and i have about 68k on mine currently. But I took him for a ride today and he said it's doing exactly what his did. He changed the plugs and wires and it fixed him right up... Has any one else had expierence with this problem?? Does any one agree disagree with it being plugs and wires. Or possibly the opti going bad at 68k? Thanks for any input/advice/comments

[blackthunder50]

There is a good chance it may be the opti showing signs of death, we have all seen them fail at all different times. Before you go and tear it apart and replace it, try the small things first, those metal plug boot covers on 1 or 2 of the spark plug boots can cause misfires. Check for loose wires and all connections. Last I would have some one put it on the scanner and see which cylinder if any is misfiring. Good luck, and remember to check shbox.com because he has GM parts guys that will be able to get optis for a little less money

[Injuneer]

His 95 is OBD-I. There's is no misfire detection in OBD-I.

It could be sprark, it could be fuel.

Fuel:

Check the fuel pressure. Put a gauge on the Schrader valve. Should get 42-47psi at idle with the FPR vacuum line disconnected (plug line to prevent vacuum leak). Reattach vacuum line. Fuel pressure should drop proportional to intake manifold vacuum... 6 to 8psi drop with a stock cam. If its all OK so far, take it out on the highway, and measure fuel pressure at peak load.... WOT/5,000+ RPM. Should hold at least 40psi under these conditions.

Could be a weak fuel pump or dirty filter.

Spark:

If you know someone that has a 'scope and knows how to use it, you can track down the weak component. If not, it becomes a little more hit or miss. Sounds like the ignition may be breaking down under heavy load. At 68K miles on an 11-year old car, could be plug or wire problems. The stock AC/Delco plugs lose the little platinum pucks off the electrodes over time, and you end up with a huge gap (0.070-0.100") that misfires under heavy load.

Wires could be experiencing insulation breakdown, or the ends may be corroded. Pull a couple wires, look for corrosion..... particularly in the Opti terminals. Pull the coil wire, and look for corrosion. Check the Opti harness connectors - there are two, one on the Opti, and another on the passenger side of the intake manifold (gray connector on the black bracket that also holds the EVAP purge solenoid. Check all grounds for looseness or corrosion.

Replacing plugs and wires at this point would be a good idea.

Then look for weak components.... IC Module (gets heat soaked, can be spaced off the head for cooler operation) and coil. Last resort would be the Opti. Check for DTC codes. The optical sensor will set codes for problems with the high and low resolutiuon pulse patterns. There are no codes for problems with the high voltage section (cap/rotor). 'scope really helps here.

Other causes:

Check for a leaking EGR valve. It should be closed over 3,500rpm, and when you go WOT. If its leaking, could be a problem.

Faulty O2 sensors - get a scanner on there and look at the O2 sensor behaviour. Look at the the long term fuel corrections (BLM's) for both banks of the engine, checking for corrections of more than 5%, or significantly different (split BLM's) on the two banks of the engine.

Faulty TPS or MAP sensor. Scanner will tell you how these are responding.