Hey guys I'm new to the forum and have done a lot of searching but haven't quite for exactly what I'm looking for. I just bought a 95 z28 auto, motor rebuilt about 30k ago with new plugs and wires at that time, opti is 2 yrs old, new tps, original coil (161k). It idles slightly rough and rich, it kinda chugs when you get on the gas below 2k rpms, above 2500 it starts to run great and pull hard. Crusin at 55mph it bucks real bad until i get the rpms up above 2500. I bought this as a spare summer toy and don't want to just start throwing parts at it cuz money is tight. I really want to get it right the first time. oh also the cats are hollowed but have been for 40k. Thanks and look forward to hearing back!

 

Rich and rough at low end and clears above 2500, bucking at 55 MPH (1500-1700 RPM) could be a low end misfire ignition problem, plug, wire, opti rotor/cap, or a fueling problem due to an injector sticking open to long. How old are the O2's? Does it start up rich after sitting a few minutes (leaking injector). Usually I would say pull the plugs and try to isolate it to a cylinder or cylinders, but that's an all day job on these things. Make sure the plug wires are on tight, not touching any metal, not burnt. Check the coil to opti HV wire for corrosion at the terminals, look for sparks arching in the dark. Is the car stock or do you have any mods ?

 

The car is all stock, I'm not sure about the o2. I runs the same all the time, rich, lol. I went through yesterday and unpluged 1 injector at a time to see if I could isolate which cyl had the miss but it ran worse with every pull. That's what pointed me towards a coil. The previous owner (a friend of mine) just started throwing parts at it. Also there is no SES light on so I hate to pay the dealership to scan it and get nothing. For some reason my snap on mt2500 won't communicate with the car.

 

Had the same problem on my 93. Ended up being both the O2's (not a very long shelf life on those). And the plugs were under gapped.

 

My Msd coil just died,but it,s sympton was a no-start cond.& it ran great last fall b/f i stored it for the winter so i doubt it,s your coil.Btw you don,t have to take your car to dealer for a scan;many like Autozone do it for free

 

So far autozone and advanced couldn't do it because it's obd1.5, obd1 cpu and obdII plug.

 

Yep, the '94-'95 OBD1 PCM's are unique in the f-body's and then they stuck an OBD2 connector on the '95 making even more of a one off. Confuses the heck out of people that don't have a intimate knowledge of these cars.. There are a couple of DTC codes for opti and ICM that will not light the SES (16 and 42) so paying for a scan may mot be a complete waist. Any local tuners around that would scan it for less than a dealer ? If you got a laptop and a $65.00 ALDL cable Freescan will do it for you...

 

A long time gm service advisor said he thinks it's a fuel presure regulator. I pulled the vacuum line from it and there wasn't any fuel in there but it did smell like fuel. Any thoughts on that?

 

Next thing would be to put a fuel pressure gauge on it. To see if it's high 30's at idle with vacuum to regulator and jumps about 5 lbs to low 40's w/o vacuum line to regulator (no vac simulates WOT condition). And with key off pressure should hold for at least 5-10 minutes (not leak off) . If there is a fuel smell in the vacuum line that indicates a tear or leak internally in the regulator between the vac switch and fuel lines. It will need replacing then regardless of pressure results. It could be sucking extra fuel into the intake causing your indications at low end, extra fuel in intake becomes less noticeable at higher RPM loads. These things get tricky to trouble shoot w/o the tools available to the dealers...For limited home use a Fuel Pressure tester should run you $25-30.00 it's always handy to have one with these cars.

 

I'm not sure what else may cause a rich condition as such.