The LT1 engine family was short lived, only spanning 6 years, between 1992-97, before being replaced by the Gen III (LS family of engines in 1997). It featured reverse flow cooling, a distributor mounted on the front that was driven by a short shaft from the cam, high flow exhaust manifolds, a short runner intake. The cylinder heads were also revised to provide better distribution and the design was such that it will not interchange on the traditional small block pattern. The timing chain cover, intake, accessory brackets, block is all unique to the LT1 engine. The connecting rods will work in the earlier small block and are reportedly stronger than the famed "pink rods".
There was also a smaller version of the LT1 that displaced only 4.3L, known as RPO L99. Be careful when searching for one of these blocks. It will have a 4.3 cast into the top of the block just ahead of the bellhousing flange on the back side of the block.
There are several variations over its brief history.
1992- First introduced in the Corvette, rated at 300 bhp, iron block, aluminum heads, 4 bolt main block. Non-vented distributor. Non computer (4L60) controlled automatic transmission, still uses TV cable to control transmission line pressure.
1993- Introduced in the F body (Camaro/Firebird), rated at 275 bhp, iron block, aluminum heads, 2 bolt main block. Non-vented distributor. Non computer (4L60) controlled automatic transmission, still uses TV cable to control transmission line pressure.
Batch fired injection on the 1992-93, speed density (MAP), removable prom.
1994- Sequential fuel injection, MAF sensor added in conjunction with MAP sensor. LT1 and L99 introduced in the Buick Roadmaster and Chevrolet Caprice, Cadillac Fleetwood, Chevrolet Impala SS. The "B" body cars has iron heads instead of aluminum and the engine was rated at 265 bhp. First year of computer controlled automatic transmission (4L60E).
The computer prom is not removable, flash prom was used. Non-vented distributor.
1995- Vented distributor was introduced and the engagement spline is not interchangeable with the 1992-94 models. Electrical connectors and ignition coil was also changed and will not work with the earlier models. Last year of OBD I.
1996- First year of OBD II computer control which adds a crankshaft sensor to refine timing. Also has two extra O2 sensors mounted downstream behind the catalytic converters. Last year of the LT1 in the Corvette. LT4 option was introduced, rated at 330 bhp. Last year of the LT1 in the GM "B" body cars.
1997- Last year of the LT1 engine, available only in the Camaro/Firebird.
LT1 Electrical Page
The first picture shows the ignition coil and control module used on the 1992-94 LT1 engines. The top arrow shows the 2 connector/4 pin coil that was used on many 4, 6 and 8 cylinder cars from 1984 on up. They are fairly common. The bottom arrow shows the ignition control module. This is held on with two 5 mm screws and there is a heat transfer compound between the module and the heatsink. When you remove or replace the module, be sure to put a thin coat of the heat transfer compound on the module before installation or you will destroy a $80 part very quickly. The draws enough power to get hot, so let it cool before removal.
The picture on the right is the 1995-96 LT1 engines but it uses the same ignition control module.
First picture shows 1995 and later distributor with vent tube. The LT1 engines used two different distributors, the early version 92-94 has a much smaller diameter bushing for the distributor drive, while the 1995 and later is larger. They will interchange. Also the early design (1992-94) did not have the ozone vent tube (arrow on first picture) that was connected to a controlled vacuum source to pull fresh air through the distributor. The second picture shows the inside of the distributor which follows the conventional firing order, but the tracts inside go to the appropriate side of the engine for the plug wires. Third picture shows the rotor and the two small screws that hold it in place. To pull the distributor cap off requires using a special external Torx tool or in a pinch you can use a 5/64" nut driver obtainable from Radio Shack. It barely fits on, and you will have to tap on it gently to get a bite and remove carefully.
Once you remove the rotor and outer plastic shield and then the metal RFI shield you will find the optical module. This rarely requires removal unless you have determined the module is bad. The second picture shows the optical disk. You will notice a set of very fine slots on the outer edge and eight more slots in the inside of these. The outer ring has 360 evenly spaced slots, one for every degree of rotation, this is used to control spark timing, and since the camshaft rotates once per two revolution of the crankshaft, it is in reality seeing 720 pulses. The inner ring slots is used for cylinder identification. You will noticed four of the slots are not the same length, while the other four evenly spaced slots are. Since the length of the slots are converted into degrees, the ecm times the time the optical module sees the slot "open" thus the cylinder it pertains to. This is used to measure engine rpm, provide injector pulses, calculate spark timing and energize the fuel pump. Like the outer ring, the ecm "sees" 16 pulses for each revolution of the camshaft, the ecm then determines if that cylinder is on the compression or exhaust stroke.
There is a pair of LED diodes that send light through each set of slots (one low, one high resolution), as the disk rotates through the firing order. A pair of photosensors on the other side pick up the light beam (or lack of ) and transmit this data to the ecm and the ignition module as pulses to be interpreted into usable data for the fuel and spark timing management.
The low resolution signal is the eight slots on the inner ring. When the light signal is blocked, the cylinder is at top dead center and provides the crankshaft rpm and position.
The width of the slots is used for cylinder identification. The slots for cylinders 8, 3, 5, and 2 (in the firing order) are not he same size. They increase in width, and the computer actually measures the time the light passes through during engine operation.
Cylinder # 8: 7º
Cylinder #3: 12º
Cylinder #5: 17º
Cylinder #2: 22º
The other cylinders are identified because the follow the larger slots on the firing order. The width of these pulses is 2º. In total, the low resolution sends out 16 pulses, eight on and eight off. Besides providing top dead center and rpm information, they also assist in providing injector pulse, calculate spark timing and energize the fuel pump.
TECH NOTE: If the ecm does not see the pulses from the inner ring, the engine will crank but will not run. Check for a loose or faulty pigtail connection to the distributor. A broken locking tab on either connector end may cause it vibrate loose and result in missing, poor performance or no start condition.
The outer ring, which consists of 360 slots, provide a total of 720 signals to the control module to regulate spark timing. This is equivalent to about 1º of rotation.
Handle the optical disk very carefully. Bending it may cause the signal to the ecm to become distorted resulting in poor performance or not running at all. Also be sure to make note what side is up before removal, a small mark on it from a Sharpie pen will eliminate the guessing and keep you from the frustration of having to remove the distributor to correct the problem.
The picture above shows the correct positioning as you visualize the disk rotating in the clockwise direction.
There are two different pigtails used on the Optispark distributor. The outer pigtail has a short connector that plugs into the distributor, this is used on the 1992-93 module years. The inner pigtail has a longer connector to the distributor for the 1994-96 LT1.
1992-93 LT1 Ignition Circuit
1994-95 LT1 Ignition Circuit
Troubleshooting the Optispark Ignition System
1996-97 OBD II LT1 Ignition System
The 1992-93 model years use ecm # 16159278. It is similar to the 1990-91 Corvette weatherpak ecm. The connectors plug in the same way. The 1992-93 LT1 engines are MAP systems with batched fire injection. It also has a removable prom chip. The later years do not.
The 1994 through 1996 module years uses the similar style ecm, using a 4 connector terminal for the ecm. The 1994-95 ecm numbers are either 16181333 or 16188051. This is the OBD I ecm's, using flash proms instead of the conventional removable EPROM of the 1992-93 LT1. In 1996 the ecm numbers changes to either 16214399 or 16242921, this is the first true year of the OBD II systems.
The 1994 and later LT1 engines are sequential injection and use a MAF system with MAP as a back up. The 1996 differs as it also includes a crankshaft position for tighter fuel and emission control.
The 1994-96 ecms also have knock modules that can be replaced. The knock sensor module numbers will also depend on the number of knock sensors your LT1 engine has.
|Year and ecm #||Single knock sensor module part #||Dual knock sensor module part #|
|1994-95 ecm #'s 16181333 or 16188051
1996 ecm part #'s 16214399 or 16242921
|16177700||16177690, 16188309, 16188709|
The 1992-93 Diagnostic can be retrieved either by jumping the A-B terminals of the ALDL connector or by using a scanner.
In 1994 this changes with the introduction of a 16 pin connector and the phase in of future OBD II diagnostics. Jumping terminals will not work as in previous years, it will require the use of at least a handheld scanner.
The OBD II style diagnostic connector
Hand held scanners such as this one from Actron will allow you to pull trouble codes, check real time sensor data and allow you to erase the trouble codes when the problem is corrected. Some models (CP 9145) such as shown above will allow you to scan and diagnosis the earlier OBD I systems as well.