Long and frustrating troubleshooting - 1993 Z28 still running badly/rich?
 

EDIT AFTER ORIGINALLY POSTING: I do want to mention that I've been digging through a bunch of the "running rich" threads here and haven't found anything that fits what I'm seeing. Also, to the best of my knowledge the car is fully stock. Thanks!

Hello all, a friend on one of the other Camaro boards suggested I drop by, apparently this forum has a great deal of Gen 4 knowledge. I am at my wits' end with my 1993 Z28 (6-speed.) Any guidance would be greatly appreciated. Here's the history...

I bought the car a little over a year ago (August 2024) and the next day it flat-out died. Towed to a shop and they replaced the MSD optispark with one from World Power Systems, the water pump and ICM. I suspect the optispark was fine, but I have no way of confirming but the way the car just died is apparently not typical of an optispark failure. Anyway, the car ran fine after that but I didn't drive it much. I took it in for service in May 2025 and on the way to the shop it was stumbling a bit. The shop replaced the plugs (with Iridium) and wires and things suddenly got much worse... Hard to start, stumbling much more (especially accelerating from anything under 2500 RPM). I ran heet, octane boost, injector cleaner through it to no avail. Over the next few weeks the car got much worse, bogging down badly under acceleration to the point that it wasn't drivable. Eventually figured out the cat was completely clogged, got that replaced. Then we were back to poor starting, stumbling under acceleration, but at least drivable. The shop tried their best, but they don't know these cars at all. Eventually they put in copper plugs which were a little better, and they told me that six of eight cylinders were running rich.

I had collected data with TunerPro RT a couple of times and noticed that it would constantly show a red ERROR for both injector banks. I sent the ECM off to G7 Computers and they "fixed" it (though I don't know what they did.) I haven't had time to hook up the TunerPro to see if the error is gone, but the car is actually running somewhat better. Now it simply feels as if it's missing at least 100 horsepower. It'll cruise decently on the highway, but full throttle gets a pretty weak response. At least there's very little surging/stumbling now, just... No power. At the moment I'm running a couple of bottles of cat/O2 cleaner through the car to see if that helps.

Here's what I know from my mechanic, my own troubleshooting, and looking at TunerPro:

• No codes thrown at all
• Fuel pressure is fine at idle, 36 lbs or 44 lbs if I pull the vacuum line off the regulator (can't test under load as the hose on my gauge isn't long enough)
• Temp sensor is sending reasonable data (around 190F once warmed up)
• Oxygen sensors are sending data that seems reasonable (jumps rapidly between maybe 200mv and 900mv)
• MAP seems to be sending reasonable data too
• EGR valve holds a vacuum when tested with a hand pump
• The short term/long term fuel compensation values (can't remember what they're called) seem reasonable
• Mechanic says that from the black on the spark plugs, six cylinders are running rich
• I pulled the injectors and cleaned and tested them, they seem fine but I am a total amateur
• The behavior is roughly the same in closed loop vs. open loop

What has been replaced on the car so far:

• Battery
• Optispark
• ICM
• Sparkplugs
• Sparkplug wires
• Ignition coil
• Fuel filter
• Water pump
• ECM refreshed by G7 Computers

At this point I am very close to just selling the car for whatever I can get, but I thought I'd see first if anyone here has insight or perhaps can refer me to a decent mechanic in the Seattle area that can help me for less than the $5000+ I've already dumped into this problem.

Thanks in advance for any help you can provide, and apologies for grumpiness in the above post, I truly am at my wits' end.

first make sure the spark plug wires go to the right cylinder. its easy to cross a couple and have the car still run half ***. it will not throw a code. also be a good idea to check spark at plug with spark checker. best to pull 02 sensors and see what they look like. fouled 02 sensors can cause rich condition. if you have a acetylene torch you may be able to burn off the black buildup and get some miles out of them.

Thanks for the suggestions. On the last round the shop did check the spark and re-verify the wires were on the correct plugs. Also confirmed compression is 150 on all cylinders, for what it's worth. I can def pull the O2 sensors and take a look, I'm betting they are gummed up after having the engine run rich enough, long enough to full clog the cat with soot, though the car runs the same closed-loop vs. open-loop, so I'm not sure the O2 sensors are the problem. Still... worth a check, thanks!

P.S. you seem to be Washington state... Do you happen to know if a good shop around Seattle that can dig into this with me?

make sure they used the correct lt1 wiring. there are two different orders, there's a different order on the iron head lt1 that I have seen guys use thinking all are the same. I did all my work myself but now too old so I been sorta looking around here for a helper myself. I live on the coast so dont know any shops in sound area.
https://cimg2.ibsrv.net/gimg/camarof...a4f20096d7.jpg

Thanks, I will check this out as soon as I get a chance.

Well it was a good thought, but all eight wires are going to the correct cylinders per that diagram you shared. I'm fresh out of ideas at this point.

I know it's a bit of a drive but the best tuner in the Pac NW is Tommy Wong in Vancouver, WA.
If he can't get you sorted out nobody can.
Welcome to Wong's Performance Engineering

Thanks, I'll hold Tommy Wong in mind as an option. Really appreciate it!

I've just realized that the car doesn't smell rich anymore. I did get the ECM worked over by G7 computers and it occurs to me that they may have actually fixed my rich condition and that I'm perhaps now facing something entirely different. So back to square one with TunerPro to see if anything obvious comes up.

you can test to see if cylinders are firing and burning fuel. wet down exhaust manifold start engine and see if they dry off quickly in the exhaust manifold area close to the head. if one takes a little longer then others to dry then you know something is going on with that cylinder. if it stays wet its not firing at all. an infrared thermometer works great if you have one.

Wow. That is... Absolutely brilliant. I'll give it a shot, thank you!