• Park vehicle on level surface.
• Remove radiator cap (best to do this on a cold engine).
  • Slowly rotate cap counterclockwise 1/4 turn and stop.
  • Wait until hissing stops, indicating remaining pressure has been relieved.
  • After all hissing ceases, continue rotating cap to remove.
• Open radiator drain valve Note: catch and dispose of used coolant properly.
• Open bleeder screws 2 or 3 turns (protect the optispark from coolant).
• Remove block drain plug on lower LH side of engine and knock sensor from RH side.
• Allow system to drain completely. Alternate: You can put a garden hose in the radiator neck if you want to force clean water through the radiator. This may not force the water through the block if the thermostat is not open (probably won't be with cold water present). You can remove the thermostat to allow the water to more easily reach the block. Allow to drain after flushing.
  

• Close radiator drain valve.
• Install block drain plug and knock sensor (if removed during drain procedure).
• If you did the alternate flushing above and have removed the thermostat, you might want to fill the block with your coolant mix through the top of the water pump before placing the thermostat back in. Some people report that this helps to lessen air pockets (I've never done this and never had a problem). Fill until the water pump remains full, install thermostat and water neck, then continue with the next item.
• Using a 50/50 mix of coolant to water, fill the system through the radiator neck (bleeder valves open). You should hear a hiss of air from the bleeder valves as you pour in the coolant.
• LT1 coolant capacities for the 4th gen F-body:
  • With Manual Transmission - 15.3 quarts (14.5 L)
    
  • With Automatic Transmission - 15.1 quarts (14.3 L)
• Close bleeder screws when bubbles disappear and only coolant is visible.
• Fill the coolant recovery reservoir to the COLD fill mark. This will provide the extra coolant required to replace the air left in the system upon the first couple of thermocycles.
• Install coolant recovery cap.
• Block wheels and run engine in Park or Neutral with the radiator cap off until thermostat opens (you should see coolant circulating in the radiator tubes).
• With the engine running, add coolant to the radiator until the level is as high as you can get it. This may be tricky if you have an electric pump because coolant may try to gush out-be careful. Install cap sooner if coolant gushes out.
• Install radiator cap.
• Check for leaks after the engine is up to running temperature.
• Monitor engine temperature. If the temperature goes up into the red zone, turn off engine and allow to cool. After it has cooled, check the level in the remote reservoir and correct if necessary. Open the radiator cap and check the level there, too. If it needs much coolant, open the bleed screws as before when filling. Run engine again until the thermostat is open and adjust level as needed. When the system is under pressure (thermostat open, engine running), you can check for air at the bleed screws, if you like.

Note: It is possible that the LOW COOLANT lamp may come on after this procedure. It should go out after the engine has gone through several heat and cool cycles. Make sure that the remote reservoir is kept at the proper level.

edited 5/23/2013

The fans are programmed to come on when the a/c is turned on. A/c Pressure monitoring sensors feed the PCM info and depending on the situation, the PCM may command the fans off for brief periods. If your a/c is broken or operation inhibited because of low refrigerant, the fans will not come on when the a/c is turned on (but will still operate by engine temp). Also, when the car reaches sustained higher speeds, the fans may be commanded off so incoming air can flow through the radiator unimpeded and provide the cooling needed.

Fans will come on when field service mode is initiated and also when the SES lamp comes on. The PCM does this when certain (most) DTCs are detected to protect the engine from a situation where it may overheat.


There are two versions of the dual fan configuration:

Here are some fairly simple things to check for various complaints:

~Low coolant lamp on~

The

If the light seems to come and go, make sure the level in the remote reservoir is proper. Normal operation of the cooling system often causes coolant from the radiator to overflow into the remote reservoir. As the engine cools down, the radiator creates a vacuum and pulls this coolant back into the radiator. The piping from the neck of the radiator to the reservoir must be air tight for this to occur. Since these cars are getting older, it is not uncommon to get a small leak in the pipe that goes under the battery. Acid wears away at the pipe until it makes a hole. Even a small hole is enough to cause problems. A telltale sign is a small amount of coolant under the right front of the car after it is parked a while. Usually, only taking out the battery will reveal where it is coming from, because it slowly drips on the splash panel underneath and may travel along to another area to drip off.

If the lamp is coming on for no apparent reason (you have verified coolant level is fine-that is, checked the level in a cold radiator and verified you have the proper level in the remote reservoir), you have a few choices:

The thermostat only has control of opening temp to allow coolant flow, after that it does nothing but cause a predetermined amount of restriction in the flow. To make the most of the lower temperature thermostat, it should be accompanied by reprogramming of the fans, so that they will come on at a lower temperature. This will help to maintain a lower overall temperature in all driving conditions (especially when stopped in traffic). It is not mandatory that you do this and a 160º can be installed by itself with no other modification.

Something else to consider is that when the engine gets to ~220º (even before the stock fan ON temp of~226º) and you are at MAP loads of 70Kpa or more, the PCM begins to retard the timing. That is one reason why people feel their cars don't run well when they are hot. The GM folks built the retard into the spark tables because when the engine is hotter, there is more chance for spark knock. If you can keep the temperatures from getting up into that range, then you might feel more power when you need it.

Altering the fan ON temps can be done through reprogramming the computer or an aftermarket "fan switch" such as sold by SLP and JET . Manual fan switches can also be wired up to operate the fan relays so that the fans can be operated at any given time the driver wants (like in staging lanes). There are explanations on how to wire the manual switch up on the 'net and there are even a couple of wiring diagrams in the electrical section of my Tech Page. If you look at the fan schematics, you can probably see that there can be several solutions to operating the fans manually (my preference being to control the existing relays).

edited 4/9/2014

T56 Service Manual (pdf format)
Right click if you want to download and save
Left click to view online

When at #1 TDC you can adjust the following valves:

Intake: 1, 2, 5, 7
Exhaust: 1, 3, 4, 8

Rotate the crank one revolution until the pointer is again at 12 o'clock. This will let you adjust the remainder of the valves. If you did #1 the previous time, you should be now at #6 TDC.

When at #6 TDC you can adjust the following valves:

Intake: 3, 4, 6, 8
Exhaust: 2, 5, 6, 7

Continue the above procedure for each cylinder until all valves are adjusted to the same amount of preload. This procedure will work for any hydraulic lifter cam with adjustable rocker arms. Refer to this diagram posted above if you need visual reference. The reason I specify lifter movement in the above is because when you start off with completely loose rockers, there is no valve movement to watch.

Cam timing animated gif
left click to view or right click to download

The opti has two functions in the spark process. The first thing that happens is as the cam turns, the optical section of the optispark picks up the signals by the rotation of the shutter wheel. The pulses are sent to the PCM via the optispark electrical harness. The PCM processes the signals along with other sensor input and determines the proper time for the coil to fire. The PCM sends a signal to the Ignition Control Module (ICM) and it, in turn, causes the coil to fire. The spark from the coil travels through the coil wire back to the secondary ignition section of the optispark (cap and rotor), to be distributed to the proper cylinder.

Remember, if the opti is never sending the signal to the PCM, the PCM will never send a signal to fire the coil or generate any injector pulses.

Here is some testing you can do. Refer to this diagram for 94-95s. 93 and 96-97 ignition system diagrams are slightly different overall, but the ICM connector is the same. Note: Pay attention to the fact that the diagram may show A-B-C-D out of order for ease of diagramming. They are actually in proper order on the physical connector. The connector has the letters embossed on it, so make sure you are testing the correct one.

Disconnect the ICM connector. Leave coil connected (important).

Turn key to ON.

Check for dc voltage with a digital meter at harness terminal "A" to ground and and also "D" to ground. Note: Use a modern digital meter with at least 10 megohm impedance to protect the PCM (in case you measure anything in that direction). Do not use a test light for looking at voltage potentials from electronics! Also make sure your meter leads make good contact and are clean and tight. Check that you get a zero ohms reading by touching the two leads together. If it is not zero, merely adjust the readings you take by the skew. That will ensure accurate resistance readings.

Result should be 10v dc or more on the A and D terminals. If you get no voltage, use the diagram and chase back toward the coil and the ignition fuse. Power for the ICM comes from the ignition fuse and through the coil, so any of that could be bad.
If you have good voltage, switch the meter to ac scale and connect test leads to terminal "B" and to ground. Observe meter while cranking the engine. You should see between 1 and 4 volts ac (those are the pulses that trigger the coil to fire).

If you don't see the proper ac voltage the problem could be the optispark, the harness to the optispark, the PCM (not common) or any of the wiring in between. Visually inspect all the connections for possible poor contact or corrosion.

You should also check "C" of the ICM harness for continuity to ground.

ICM Harness

A = ~10vdc-12vdc (or your system voltage)
B = 1vac-4vac while cranking
C = ~ 0 ohms to ground with key OFF
D = ~10vdc-12vdc (or your system voltage-might be slightly less than reading at "A")

At the end of the optispark harness (disconnected from the opti) with the key ON, you should see:

Optispark Harness

A = ~5vdc
B = ~5vdc
C = ~12vdc (or your system voltage)
D = ~0 - 0.2 ohms ground with key OFF

If you have the means, looking at the low and high resolution signals from the opti to the PCM with an oscilloscope is probably the best way to tell their health.

edited 9/19/2023

1995 PCM Pinouts

1995 PCM Wiring

Put everything back to normal when you are done. Be sure you unblock the throttle!

If you have issues you need to track further, a leakdown test will be able to tell you whether rings, valves or head gasket might be leaking.

edited 9/28/2010

Normal pressure when the engine is not running and lines have been pressurized is 41-47 psi. This same pressure should be observed at wide open throttle (WOT). Removing the vacuum hose to the regulator at idle "simulates" the same vacuum condition as during WOT (but is not a substitute for WOT testing as noted in the next paragraph). At idle (because of the effect of the vacuum to the regulator) pressure will be less than what you observe with the vacuum line off (less pressure is needed at idle because of the lesser demand). There may be anything from a 3 to 10 psi difference. Note: any indication of fuel in the vacuum line to the regulator, means the regulator is leaking and should be replaced. Check the line for fuel or the smell of fuel.

To fully determine that you don't have a pressure drop off during actual WOT situations, you should tape the gauge to your windshield and take it for a test run. This will tell you if the pump can meet actual fuel flow demands at pressure and not just at a "simulated" WOT condition (as when removing the vacuum to the regulator).

When you have a gauge connected and the pressure looks initially good and then bleeds off quickly when you shut the engine off, you can do a couple of tests to help you figure out where the pressure loss is.

What the factory manual says to temporarily install, is a set of "fuel line shut off adapters" (probably something the normal guy is not going to have available). You remove the fuel lines from the rail and connect these valves in between. This lets you shut off either side of the lines for testing.

You can do the same thing by pinching the flexible lines to shut them off, but risk breaking them. You might be able to do it (your risk) by using a needle nose vise grips and putting some scrap hose as cushions on the jaws. Then use that to clamp off the line just enough to seal it. Obviously, this is not the best way to shut off the lines and could result in breakage. Heat and age can make the hoses brittle. If you don't want to risk it, don't. It's just a suggestion.

You can use the fuel pump prime connector for pressurizing the system (jumper 12v to it to run the pump).

Watch your gauge as you jumper the prime connector. When you have good pressure remove the jumper and clamp off (or use shut off valve) the fuel supply line (3/8 pipe). If pressure holds, you have a leak on the feed line somewhere before it gets to the clamp (or shut off valve) or at the check ball in the pump. If it still goes down, release your clamp (or open shut off valve). Pressurize the system again, then remove the jumper and this time clamp (or shut off) the return line (5/16 line). If pressure holds, then the regulator is faulty. If pressure does not hold, you need to locate leaky injector(s).

If you can't tell a leaky injector from reading the plugs, you can look and see if injectors are leaking by removing the fuel rail screws and pull the rail and all the injectors up, so you can see under them. Leave them over the injector ports. Pressurized the system and look under the injectors to see if any are dripping.

edited 7/11/2010

If you are having a problem not related to the "normal" way the gauge acts as stated above, there are some things you can check.
Locate the pump electrical connector on the rear of the floor pan under the car (above the LH axle tube). It has purple, gray and black wires going toward the fuel tank. Unplug the connector. With a meter set to ohms, read resistance between the black and purple wires going to the tank.

approximate:

To test back to the gauge, ground the purple wire on the body side of the connector back toward the gauge (key ON). The gauge should read empty. With the connector unplugged and no ground applied, the gauge should read full.

edited 3/15/2009

• Depress accelerator slightly
• Start engine, then release accelerator pedal, run engine for 5 seconds
• Turn ignition "OFF" for ten seconds
• Restart engine and check for proper idle operation

  It is NOT recommended to to push or pull on the pintle of an IAC that has been in service. The force required can damage the threads on the worm drive. Also, do not soak the IAC in any liquid cleaner or solvent, as damage may result. When installing a new IAC, you may move the pintle to match the measurement of the old one. The force required to move a NEW valve will not cause damage to it. Use engine oil to lubricate the o-ring. Tighten attaching screws to 27 lb. in.
  
  Note that the 1993 IAC has a square electrical connector and the valve screws into the throttle body, instead of being held on by screws like those of later years.
  
  edited 4/15/04
  
  
  

  
  
  
  
  Restore Airflow to the Dashboard Vents
  
  
  
  This is a common fix for when the air no longer blows out the vents. When there is not enough vacuum to operate the actuators, they will only direct air to the floor and defrost ducts by default. Diagram of HVAC system. You may also notice this happening under hard acceleration or going up a long, steep hill (times when engine load is high and vacuum is low). This might mean you have a small leak.

  An easy way to tell what part of the system may have trouble is to go to the vacuum check valve and test the lines back to the mode selector, intake manifold and to the vacuum tank. The check valve often cracks and is the source of a leak, so be sure to inspect and/or test it. Note: the vacuum check valve is now GM part #15733271. The line to the vacuum tank can often get a hole just under the battery box. It can rub where it passes through the chassis and/or deteriorate from battery acid contamination.

  A hand held vacuum pump (like a Mityvac), makes testing pretty easy (your lungs can be used as an alternate vacuum source ;-) or you can blow through the lines and listen for air escaping). At the check valve, remove the hoses one at a time and apply vacuum to each and see if it holds. The line to the intake manifold will have to be removed at the manifold and plugged back toward the check valve for you to test the line for leaks. If you find a leak on a particular hose, you will have to trace out the cause on that section.

  
  edited 8/31/2019
  
  
  

  
  
  Adjust Steering Rack Bearing Preload
  
  
  
  • Make adjustment with front wheels raised and steering wheel centered. Be sure to check returnability of the steering wheel to center after adjustment.
  • Loosen adjuster plug lock nut (item 15-view A-A) and turn adjuster plug clockwise until it bottoms in the gear assembly (item 30), then back off 50º to 70º (approximately one flat).
  • Tighten the lock nut to 55 lb. ft. while holding adjuster plug stationary.
  
  
  
  
  
  
  Make late model wheels fit on your early model 4th gen
  
  
  Late model 1998 and up axles used a smaller hub center on the rear axles. Therefore the wheels were made with a smaller hub opening on the back. When you try to swap these wheels onto an earlier model 4th gen, the center of the wheel won't fit over the rear axle hub.

  The back side of the hub area on the wheel has to be relieved a small amount to allow it to slip on. You can dry fit the wheel to "mark" where the interference is, then you will be able to see the area that needs attention. Use a Dremel, sandpaper or even a half-round file to take off the little bit that is required. It does not have to be perfect, as the wheels are centered by the conical lug nuts, not the hub and that part of the wheel is never visible. Just relieve it enough to get the wheel on/off easily.

  If you ever intend to rotate your tires, you will want to do this to all four wheels (The fronts will fit without any modification. There are no clearance issues with them). Don't ever try to force the wheel on with the lug nuts. You can strip lug studs and/or get the wheel stuck.
  
  
  
  

  
  
  
  How many catalytic converters and oxygen sensors should my LT1 f-body have
  and is it OBD-I or OBD-II?
  
  
  1993-1995 LT1 f-body cars have one cat, with the exception being California Emissions (RPO NB6) A4 cars having two. All 1995 M6 cars have one cat.

  1996-1997 f-body cars have two cats.

  1993-1995 LT1 f-body cars are all OBD-I (regardless of how many cats they have).

  1993 LT1 f-body cars can flash trouble codes by shorting 2 pins in the DLC and observing the Service Engine Soon lamp (like most earlier model GMs). 1994 and later must use a scan tool. The computers changed from ECM (Engine Control Module) in 1993 to PCM (Powertrain Control Module) in 1994 and later (adding electronic transmission control). Though 1993 and 1994 were both OBD-I, they used different computers. 1993s used the replaceable chip type (PROM), while 1994 and up used flash memory for storing the program (EEPROM).

  1996-1997 LT1 f-body cars are OBD-II.

  1993 LT1 f-body cars have two 1 wire, non-heated O2 sensors. 1994-1997 have two 4 wire, heated O2 sensors (with 1996-97 cars having two additional rear (post cat) O2s to comply with OBD-II). On the f-body, front O2s use a flat connector. When rear O2s are present, they use a square connector.

  The only 1995 f-body engine that is OBD-II is the 3.8L V6 that came out during the model year (replacing the 3.4L V6 that was OBD-I).

  RPO NA5 = Federal Emissions System

  RPO NB6 = California Emissions System

  
  edited 8/13/2010
  
  
  
  

  
  
  
  Requirements for Converting to a Vented Optispark
  

  
  
  If you want to change to a vented from non-vented optispark, you have to do many things. It may not be cost effective unless you are doing a cam swap at the same time.

  Besides the vented optispark itself (GM p/n 1104032), you need:

  • 95 or later timing cover [1995 front cover (includes round seals) GM p/n 12552426-now discontinued]
  • late model or aftermarket cam (or have your cam modified by drilling the hole in the front to the proper size [.500"dia. X 1.0625" deep] and a longer dowel pin added (pin needs to protrude ~.650" from the nose of the cam)
  • 95 and later optispark electrical harness GM p/n 12130319 (cheaper aftermaket harnesses available)
  • optispark vacuum harness GM p/n 12555323
  • vacuum fitting [GM p/n 14082470] or "tee" to provide vacuum for the opti
  • 95 or later timing set components (or at least the cam gear) cam gear (GM p/n 10206039), crank gear (GM p/n 10128346), chain (GM p/n 10128485)
  • timing cover gasket GM p/n 10128293
  • water pump gaskets [2] GM p/n 10128343
  • intake manifold gaskets if you are pulling the cam GM p/n 12524653

  Or you can put an MSD cap/rotor kit on it and essentially have a vented opti. They come with their own similar venting system.

  
  
  edited 1/8/2021
  
  
  
  
  
  
  

  
  
  

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