HI Guys.
I recently bought a 1992 C4. Lt1 engine. I purchased the vehicle with a faulty waterpump . I renewed this and while testing found the engine miss firing under load. If the revs go up slowly, its fine.
I renewed the plugs and fitted a set of MSD leads. Still missfires..
Any thoughts to help?????

 

If the water pump was leaking, or coolant poured out as you replaced it, the optispark probably got damaged.

 

No offense, but I don't think you are in any position to be giving technical advise on an engine you know nothing about. How about focusing on getting yours to the track or dyno?

While there is a possibility that the opti could be going you will generally get a check engine light. To start off pull the codes and report back, if you get codes 16 or 36 the probability of the opti needing to be replaced is higher. Before replacing the opti I would however exhaust all other options, pull the ICM and have it tested, when you reinstall make sure to clean mounting surface and apply heat sink paste. If the coil is original and has a lot of miles it wouldn't hurt to replace it.

 

I have ordered a new MSD coil . problem is, I'm in George, South Africa, so getting parts here takes some time. And support is far apar. I also have a 93 pontiac firhawk, so I can swop out parts if necessarry, but dont want to go there.
The problem started just after I replaced the waterpump, so I am also thinking some water might of got into the dizzy .

 

And coolant very well could have gotten into it. But I'm assuming that the car is together again now, might as well exhaust all options before tearing it apart again

 

This is a chronic issue with these cars and I dealt with it more than once over the 14 year period during which I owned my 1995. You can find threads here in the archives that go on for hundreds of posts without resolution. I recall some guy that virtually replaced dozens of things and never nailed what was causing the problem. Could be something as simple as a bad plug or plug wire.

 

Fault code. 0012 and 0016 then 0023 - 0027.
if this helps.
still missing.

 

Weak spark or lack of fuel.

Now.. which is it and whats causing it. If it ran ok before the WP.. then I'd highly suspect the opti-spark.

Other but less likely causes.. timing way off, big vacuum leak, plugged fuel filter (starving under load), excessive resistance in plugs or plug wires.

 

When the engine is cold, start her up and run it for 30 seconds and shut her down. Now feel the exhaust at each cylinder and look for the cold cylinder, now its easy you know what cylinder is causing the problem so remove the spark plug. A jumper lead is good connect that to the negative on the battery and the other end to the spark plug metal body, if it sparks as someone crankls the engine it is a fuel problem.
You can easilly unplug the connector from the fuel injector next to the bad one, simply swap them over and see if the fault is gone or transferered to the other cylinder.
If there is no spark check with another spark plug and test the ignition leads with a multimeter then if they are good it points to the opti.
Only after doing these simple checks can you determine the fault, be it a bad optispark or a fuel injector (do not forget the wiring to the injector connector, if the connectors are pulled of with the leads not the correct way of releasing the plug you can damage the connector)
That happened to me had a bad missfire would accelerate like a pig the engine would shake but at a steady cruise would run fine, it was a bad injector electrical connector on the harness replaced it and was running as it should. Took me a long time to pin point as testing the connector with a multimeter and having someone crank the engine i could see the 12 volt pulses, however it would not cary any load at the strip of metal in the plug was cracked. I found this after having an expert look with a diagnostic scanner, no codes and was stumped.
I woke up one morning and though to check the header temps, found the cylinder that was down and swapped the injector lead from the one next to it and transferred the fault. Was a simple fix to fit another injector connector.

 

1992 fault codes in the range you listed:

13 Oxygen sensor voltage stays between 0.35 and 0.55 volts for 60 seconds. Engine was at normal operating temperature and had been running for at least 2 minutes after start with a throttle angle above 5%.
14 Coolant temperature sensor signal indicates a temperature of over 266? F.
15 Coolant temperature sensor signal indicates a temperature colder than -56? F.
16 A low resolution pulse was not detected by ECM within 720 high resolution timing pulses.
21 TPS voltage was above 2.5 volts for 3 seconds when the engine was running, or when the TPS was above 4.8 volts at any time.
22 TPS voltage was under 0.23 volts when the engine was running.
23 Inlet air temperature sensor signal showed an air temperature below -31? F for 60 seconds after the engine has been running for 4 minutes.
24 Vehicle speed was 0 MPH when engine speed was between 900 and 3000 RPM and throttle position angle was less than 2% for 5 seconds.
25 Inlet air temperature sensor signal showed an air temperature over 302? F for 12 seconds after the engine has been running for 4 minutes.
26 ECM detected the wrong potential voltage at the #1 quad-driver module circuit for 20 seconds.
27 ECM detected the wrong potential voltage at the #2 quad-driver module circuit for 3 seconds.