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Rebuilding the Chevy 3

By Doug Anderson

The Chevy 191/3.1L is the bigger version of GM’s 60° V6 family that began life as the 173/2.8L engine back in 1980. When the 3.1 was introduced in 1988, it had the same bore (3.503"/84.0 mm) as the 2.8L, but it came with a 3.312"/84.0 mm stroke instead of the 2.992" crank that was used in the 2.8L. Although it wouldn’t seem like that would be enough to make a whole lot of difference, the additional 19 cubic inches increased the torque by 20 ft. lbs. and gave the 3.1L another 10 hp. Since then, GM Powertrain has tweaked and tuned the 3.1L with the addition of better heads, sequential port fuel injection, a roller cam, and roller rockers, along with lightweight pistons and low tension rings to get more power and lower emissions while maintaining the same fuel economy.

In fact, the latest 3.1L makes 195 lbs. of torque and 175 hp, up from 175 lbs. of torque and 120 hp in 1988. That’s an amazing 45% increase in horsepower out of the same basic engine! During this same time, the FWD car blocks were reinforced both internally and externally and supported with a structural aluminum pan that was crossbolted to the main caps to reduce noise, vibration and harshness (NVH), so the 3.1L has evolved into a good, dependable, state-of-the-art, pushrod engine today.

GM has used the 3.1L in many applications, including both RWD cars and trucks along with FWD cars and minivans, so there have been several blocks and heads used over the years with some subtle variations that can catch you by surprise if you’re not paying careful attention. The easiest way to understand the differences is to follow each application through from beginning to end, so let’s begin with the RWD cars and walk each one through its evolution.

RWD cars (and Isuzu Rodeo?)
The 3.1L was used as the base engine in the RWD Camaro and Firebird in ‘90 and ‘91 until it was replaced by an even bigger 3.4L version of this same engine family in 1992. There are reports that this RWD engine was installed in some of the Isuzu Rodeos in ’90 and 1991, so rebuilders should be aware of the possibility when selling a 3.1L for a Rodeo.

Blocks
•The 1990-‘91 RWD blocks were unique because they had the starter on the passenger side and three bolt bosses for the motor mounts in an "L" shaped pattern on both sides of the block. The "L" is reversed on the passenger side and upside down on the driver’s side.

• There was an angled pad adjacent to the hole for the distributor, but it didn’t have a threaded hole in it because the RWD cars didn’t come with a second oil pressure sender.

• There were three blocks used for the 1990-’91 RWD cars: They were either 14065457, 14100695 or 10065457 castings.

Heads
The ‘90-‘91 RWD cars had the same big valve, cast iron heads that were originally used on the 2.8L H.O. engines beginning back in 1981. They can be readily identified by the small, cast triangle that’s located on top of the bolt boss on one end of the head, right where the flange for the rocker cover goes around the corner.

There were three head castings used, the 14054884, 14031327 and 14054879.

Internal engine parts
Pistons: The RWD engines came with the cast iron heads that had the big chambers, so they all used the pistons with the shallow dish (about .850").

Cam: The 2.8L H.O. cam that had more lift and duration was installed in all of the RWD engines.

Oil pumps: There were two different oil pumps on the 3.1L. The first design 1990 pump had a 5/8" (15.9 mm) pickup tube and the second design ‘90 and later pump had a 3/4" (19.0 mm) tube. Be sure to provide the right pump for the customer’s application. Or, better yet, send the matching pickup tube along with the pump to avoid any possible problems.

Crankshafts: There was only one crank (c/n 981) used in 1990 and ‘91, but it came with and without the notches cut for direct ignition (crank-triggered spark). The RWD engines all had distributors, so the cranks didn’t need the notches, but most of them came with the notched cranks anyway. Either one will work in this application.

FWD minivans and RWD Isuzu Troopers
There was another version of the 3.1L engine that came in the FWD minivans with the long noses, including the Chevy Lumina APV, the Pontiac Transport and the Olds Silhouette, from 1990 through ‘95. This same engine was also installed in the RWD Isuzu Troopers and Rodeos from 1990 through ‘92. It was dropped from the Trooper and Rodeo after 1992 and replaced by the larger 3.4L engine in 1996 when the minivans were updated.

Blocks
1990: Casting numbers 14092219, 10065459 and 10118618
The 1990 FWD minivans and RWD Troopers shared a common block casting with the FWD cars, but there were some important differences, too. They did have the starter on the driver’s side and the threaded, blind hole for a detonation sensor on the passenger side, just like the FWD cars, and they even shared the same accessory mounts with the four hole rectangular mount on the right and the three hole, triangular mount on the left, but they were not interchangeable with the blocks used in the FWD cars.

These blocks were unique to these two applications, because of several features:

• The boss for the crank sensor was cast on the passenger side of the block, but the hole for the sensor wasn’t drilled because both the minivans and the Troopers used a distributor instead of direct ignition.

• These blocks all had the angled pad back by the distributor, but the hole wasn’t drilled for the second oil pressure sender.

• There was no bypass hole drilled in the front of the block below the inlet for the water pump.

1991-‘93: Casting number 10118653 or 10110519
The 1991 FWD minivan/Trooper blocks were similar to the ‘90 blocks, but there were some notable changes, too.

• The machined pad for the fuel pump was removed to save weight, but the two bolt holes that were located on the top and the bottom of the pad were still there along with an additional hole that was added in the front corner of the block. It was slightly below the deck surface and just above and to the left of the existing hole on the top.

• There was another threaded blind hole added directly over the oil filter mounting pad, probably for a detonation sensor.

• There were three pairs of raised "X" reinforcing ribs added on each side. They tied the external walls of the block to the deck to improve the integrity of the whole casting and reduce NVH.

• Two structural bars were added diagonally across the lifter valley. Some were cast almost as high as the end rails and then machined down lower while others were cast low enough to clear the bottom of the intake manifolds without machining. They have to be .435" below the height of the end rails to fit these applications.

1994-‘95: Casting number 10240934
There were more changes made to the FWD minivan block in 1994:

• The cam plug on the back of the engine was changed from the thick, bolt-on plate to a round, press-fit design.

• There was another hole added for an oil sender on the driver’s side of the block. It was located 4-1/2" in front of the bell housing and 2-1/2" above the pan rail.

• All of these blocks came with the low structural bars (.435" below the end rails) in the valley, so they cleared the intake manifolds used on the minivans.

We have seen some of these 10240934 blocks with the crank sensor hole drilled and the oil sender hole on the driver’s side not drilled; this is backward from what’s required for the minivans, so there appears to be some other application for this block. We suspect it may have been used to build replacement engines for the 1991-‘94 FWD VIN "T" cars, but this configuration wouldn’t work unless the water pump bypass hole wasn’t used for the FWD cars, so it’s not a sure thing. Just make sure that the crank sensor hole is plugged and the oil sender hole is drilled before building one of these blocks for a minivan.

Heads
The 1990-‘95 FWD minivans and the 1990-‘92 Troopers used the same, cast iron H.O. heads that came on the 3.1L RWD cars. They can be identified by the small triangle on the top of the front bolt boss.

Look for a 14054884, a 14031327 or a 14054879 casting number.

Internal engine parts
Cranks: The FWD minivans all had distributors, so they didn’t need the notches cut for the crank sensor, but most of them actually came with the notched crank. You can use the 981 casting with or without the notches.

Pistons: These engines came with the cast iron heads with the big chambers, so they used the pistons with the shallow (.850") dish.

Cam: GM used the original 2.8L, non-H.O. cam with less lift and overlap for "improved idle stability" in the minivans. Don’t use the H.O. cam in these engines.

Oil pumps: The first design oil pump with the 5/8" pickup tube was used in the first part of ‘90 before GM switched to the second design pump with the 3/4" pickup tube in mid-year.

FWD CARS
The engines in the FWD cars are a little more complicated. There have been five different blocks and four different heads used since 1988. Some of the castings are similar to the ones used for the FWD minivans, although they’re not quite the same, and others are totally different.

Blocks
1988-‘90: The FWD cars originally shared a common block with the FWD minivans and the RWD Isuzu Troopers as was previously noted. The same 14092219, 10065459 and 10118618 castings were used, but they were machined a little differently for the FWD cars.

They can be recognized as FWD blocks by the following features:

• The starter mount is on the driver’s side, just like it is on the FWD minivans;

• There’s a blind hole between the first and second cylinder on the passenger side for the detonation sensor; and

• There’s a four hole, rectangular mount on the passenger side and a three hole, triangular accessory mount on the driver’s side, just like the ones on the blocks for the FWD minivans.

But, these blocks were unique to this application because:

• The hole for the crank sensor was drilled in the machined pad midway back on the passenger side because the FWD cars all had direct ignition; and

• The hole for the second oil pressure sender was drilled and tapped in the angled pad back by the distributor because some of the FWD cars came with a real oil pressure gauge in addition to the idiot light.

1991-‘94 VIN "T" without SEFI
GM made some significant changes in the FWD car block in 1991. The new castings were either a 10110519 or a 10137086.

• There were three pairs of "X" shaped ribs added on both sides of the block. These stiffened the block and tied the upper deck surface to the external walls so the block was stronger and quieter.

• Two diagonal, structural bars were added in the lifter valley. Some of them were almost as high as the end rails (they were actually about .100" lower), but it appears that most of them were cast or machined down lower (about .435" below the end rails), even though the intake manifolds for the FWD cars cleared the tall ones.

• The angled pad back by the distributor was eliminated.

• The machined pad for the fuel pump was removed, but the two threaded holes that were at the top and bottom of the pad were still there along with another bolt boss that was added to the left front corner of the block, just below the deck surface.

• These blocks also have an additional threaded boss directly above the oil filter mounting pad.

1993-‘94 VIN "T" with SEFI
GM built some of the ‘93 and ‘94 FWD "A" and "W" cars with the NB2 California emissions that came with sequential fuel injection. Likewise, the ‘94 Cavaliers and Sunbirds that came with the 3.1L engine had SEFI, so some of the 10110519 castings had an additional hole machined in the top of the block, just behind the timing chain, for the cam position sensor that enabled the computer to sequence the fuel injectors correctly each time the engine was started.

1993-‘99 VIN "M" "Enhanced 3100"
The "Enhanced" FWD car engine was completely revised in late 1993. There have been several "Enhanced" castings since then, including 10137093, 10191737, 24504089, 24504150 and 10224227.

• The main caps were made of powered metal. Three of them were machined flat on both sides and then drilled and tapped so they could be cross-bolted to the structural aluminum pan. The front of the block was reinforced on both sides to reduce NVH.

• There was no longer a blind hole for the detonation sensor on the passenger side, but the one on the driver’s side was still there.

• There was another hole added for an oil pressure sender 4-1/2" in front of the bell housing and 2-1/2" up from the pan rail.

• These blocks still had six bolt bosses for the A/C compressor mount on the left side, but they were arranged in a completely different pattern than the one that was found on the previous FWD blocks. Make sure all six holes are drilled and tapped. The lower hole in the front isn’t always drilled on some of the 10191737 castings, so it apparently isn’t used all the time; but it’s better to drill it and avoid any possible problems in the field.

Heads
The FWD cars all came with aluminum heads, but there were four different versions.

1988-‘90 Vin "T": The 10048651, 10048696 and 14089834 castings were originally used. They had a small, heart-shaped chamber with a very prominent divider that stuck part way into the chamber between the intake and exhaust valves.

1991-‘94 Vin "T"
The 10087511 casting was used from 1991 through ‘94. The chamber was very similar to the one found in the earlier head, but the divider between the valves was removed. Note: Some rebuilders interchange the 1988-‘90 and 1991-‘94 heads in pairs without any apparent problems.

1993-‘95 Vin "M" "Enhanced 3100"
The 10154743 head castings had "D" shaped exhaust ports and conical valve springs with small retainers. The revised chambers look almost identical to the original design with the divider that sticks part way into the chamber in between the valves.

1996-‘99 Vin "M"
The 24503769 casting is identical to the previous 10154743 casting except for two differences: 1) It doesn’t have the hole in one end for the temp sender; and 2) there’s a slot machined across the middle of the rocker pedestal to hold the roller rockers in a fixed position. This eliminated the need for pushrod guide plates.

1999 VIN "J"
There was a 1999 VIN "J" engine with NC1 emissions installed in Chevy Malibu and Oldsmobile Cutlass models sold in California and some of the Northeastern states that required lower emissions. These engines came with an AIR pump that blew into the exhaust manifolds and the 10218170 head casting that had slightly different intake ports and a revised intake gasket. The hole for the alignment peg by the middle port was moved up from the right side of the port (facing the intake) to the left side so the gaskets couldn’t be mixed up.

Internal engine parts
Cranks: The FWD cars all had direct ignition, so the cranks had to have the notches cut for the crank position sensor. The original crank used from 1988-‘95 was the 981 casting. It was replaced by the 268 casting in 1996; both are interchangeable.

Pistons: These engines all used the pistons with the deep (about .150") dish because the aluminum heads always had the small chambers.

Cams: There have been three different cams used in the FWD car engines:

•The 1988-‘94 VIN "T" engines without SEFI came with the 2.8L H.O. cam.

•The 1993-‘94 VIN "T" engines with NB2 California emissions; the M13 four speed automatic transmission and SEFI came with the flat tappet, H.O. cam, but it had an additional tab located right behind the front journal that was used to trigger the cam position sensor. GM lists it as a p/n 10166324 and has superseded all of its flat tappet cams for the FWD cars to this part number; but it’s a little bit pricey. We know of at least one aftermarket supplier who offers it now at a much better price.

•The 1994-‘99 VIN "M" and 1999 VIN "J" engines used a hollow, assembled roller cam that GM characterized as having an "all new, higher lift, low overlap" design. The original 1994-‘95 roller cam was a p/n 10166337 that had "6337" imprinted on the barrel in front of the first lobe. It was superseded by p/n 24505674 in 1996 that has "5674" imprinted on the barrel in front of the first lobe. Both can be used interchangeably. They can be easily identified because:

• They have a notch for the cam position sensor cut in the ring right behind the front journal;

• The cam gear for the roller cam is bolted on with one big, center bolt instead of the three small ones that held the timing gear onto the flat tappet cams; and

• The VIN "M" and "J" roller cam was changed in model year 2000. We haven’t checked it out to see if it’s interchangeable with the earlier cams, so it’s best to keep it separate for now. It should have "7450" etched on the barrel.

OIL PUMPS
The original 2.8L oil pump with the 5/8" pickup tube was used from 1988 through the first half of ‘90 (first design 1990). The 1990 pump with the 3/4" pickup tube has been used ever since then.

More information that you need to know
That’s the story on the blocks, heads and cranks along with most of the big pieces. But, there’s more you need to know about the 3.1L before rebuilding it.

• All of the 3.1L pistons have very short skirts and tight tolerances. Most piston suppliers call for .0014" to .0015" clearance and they mean it. Honing the bores +/- .0005" will cause either scuffed pistons or noisy ones.

• The original cam plate was bolted onto the block and sealed with a small dab of RTV that was supposed to fill the groove in the block and seal on the back of the plate. Getting a used plate to seal with RTV is pretty difficult, so it’s better to use one of the cork gaskets with the steel load limiters that are available in the aftermarket.

• The 1993 and up VIN "M" and the 1994 and up VIN "D" engines came with a special, stamped steel, round cam plug. It’s available from GM under p/n 10241154.

• The FWD cars with direct ignition use a stub shaft in a short housing to drive the oil pump. Oil will leak up around it if the hard, flattened o-ring is reused, or if the new o-ring is nicked during installation, and you will get blamed for a rear seal leak because the back of the pan ends up getting oily.

We recommend installing a regular paper Chevy distributor gasket in addition to the new o-ring, just for cheap insurance, because the entire top of the engine and the intake manifold must be removed to replace the o-ring if it leaks oil around the stub shaft. We also recommend a new o-ring and the gasket for the RWD cars and the FWD minivans, because a leaking distributor seal will usually be misdiagnosed as a leaking rear seal on these engines, too.

• The 3.1L uses a special, shorter roller lifter that’s only 2.200" long. It’s available from GM under p/n 17120070 or as a VL138 in the aftermarket. It’s unique to the 3.1L and the 2.2L/134 Chevy four cylinder.

• It’s easy to get confused and sell the wrong 3.1L engine. Both the FWD and RWD cars were listed as VIN "T" engines in 1990-‘92. And, all of the 1993-‘94 FWD cars carried VIN code "T" with and without SEFI, in spite of the fact that they were different engines. Ask lots of questions and be sure about the specific application before selling one of these engines.

• The connecting rods for the 3.1L are all basically the same. They may or may not have 574 on the shank of the rod. They are identical to the ones found in the 2.8L except that the ones used in the 3.1L are 0.150" narrower on the small end (0.850" vs. 1.00").

• The 3.1L blocks are all pretty solid and don’t tend to crack except for the 10065459 casting that likes to tear the deck right off the top of the block. You can expect to see around 30% of these blocks that are cracked. Be sure to check carefully for cracks on the outside of the block, just below the deck, and in the cylinders, about 1/2"down from the deck surface. Look around the middle cylinders on both sides first, because they’re usually the worst. Be sure to inspect both sides of the block and all of the cylinders carefully before you invest time and money in the block.

• Both the cast iron and the aluminum heads are pretty good castings. You can expect to junk about 10% of the iron heads and 3% to 4% of the aluminum ones, based on our experience. Watch out for a crack that goes across from the freeze plug hole in the end of the head on over to the head bolt hole on the aluminum heads. It appears to be caused by cocking the head on the dowel or forcing it onto a burred dowel during installation and then pulling it down with an impact wrench. Be sure the head is sitting squarely on the deck before you start torquing it down.

• The 3.1L cranks are not quite as prone to failure as the 2.8L cranks were, but you can still expect to weld or scrap around 20% of them. We find that the #1 or #3 rod journal usually goes down if the crank is bad, so look them over first. It appears that the plastic intake gasket cracks, allows coolant into the oil, and causes the rod bearing to fail.

• The rocker covers are baffled to keep oil out of the PCV system. If too much sludge gets built up in the rocker covers, it prevents airflow to the PCV, pressurizes the pan, and blows oil out past the seals and gaskets. Guess who gets blamed?

• Some of the early FWD cars had two oil pressure senders, one for the idiot light that was also used to turn off the electric fuel pump if the engine lost oil pressure, and a second one for an actual gauge. Drilling the second hole in the angled pad back by the distributor hole guarantees that the engine will work either way for these FWD engines.

• The 3.4L FWD Chevy used the same 24504150 casting as the late 3.1L, but it had a bigger bore. You can identify the 3.1L VIN "M" because the 24503769 head casting is unique to this application, but the 1999 3.1L VIN "J" had the same 10218170 head that was used on the 3.4L FWD engine, so there is no way to tell these two blocks apart until you take the heads off.

• There were some RWD Isuzu Rodeos in 1990-’92 built with the FWD minivan engine that had the starter on the driver’s side. It appears that there were also some built with the RWD car engines that had the starter on the passenger side, so be sure to check casting numbers and starter location before selling a 3.1L engine for a Rodeo.

• GM built a few turbocharged, 3.1L VIN "V" engines for the Grand Prix in 1989 and ‘90. They used the regular FWD car block (14062219/10065459/ 10118618) with the crank sensor hole drilled and the water pump bypass open. The pistons and cam (p/n 90233324) were unique to this application. They all reportedly had the 10087511 aluminum head. Discretion may be the better part of valor here if you get a call for one.

Conclusion
That’s the story on the 3.1L. It’s been around for a long time and it’s still being used as the corporate V6 in some of GM’s best-selling small and mid-sized cars. There have been more than 8 million of them installed over the last 12 years, so there’s a growing market for remanufactured 3.1L engines. And, they’re easy to do, once you know the right combinations.

For additional pictures click here.
For 3.1L Chevy V6 Engine Identification chart click here.

Editor’s Note: A special thanks goes to Roy Berndt at PERA for his help with the research for this article.

Doug Anderson is Vice President of Grooms Engines, Parts, Machining, Inc., Nashville, TN. Anderson is a Board Director for the Production Engine Remanufacturers Association (PERA).