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The Neglected 3.9 V6 Dodge Truck Engine

The 3.9 V6 was a close relative of the 318 V8, even keeping the bore and stroke, created because the upcoming Dodge Dakota needed a V6 engine. Creating a new V6 would have taken too much time and money.

dodge dakota V6

The 3.9 was used in the Dakota (base motor), Ram 1500 (base motor), and Ram 1500 van (optional; a 318 V8 was standard). Launched in 1986 (for the 1987 Dakota), the engine was a stopgap, but it was good enough to last 13 years. It was finally replaced in the 2001 trucks, ironically by a 3.7 liter version of the 4.7 liter “Next Generation” V8 engine. (Also ironically, a closely related engine — the 360 V8 — had two cylinders added to make a V10 for trucks and Vipers.)

Creating a new engine from an old one

Willem Weertman, the head engine designer, said, “They wanted to have an upgrade power plant from the 4-cylinder; the V-6 was designed as a way of furnishing a V-6 for the least possible tooling costs. Because of capital investment, we didn’t want to get into a whole new engine. We just wanted see what we could do with what we had and that caused us to look at the V-6 version of the Mound Road Engine.  That was the way it was done. 

“The engine had to be shorter than the V-8 to fit into the compartment.   It was only in later years that enough space was found in order to be able to put the V-8 into place. ... We had a challenge on the V-6 because the crank-pins had to be split in order to get away from the very unequal firing if we had only 3 crankpins, each crankpin having two of the connecting rods as is V-8 practice.  The reason is that the engine would be rather badly out of balance and would have not been acceptable even in a truck engine. So we had to do some redesigning of the bottom end in order to split the crank pins and make the firing order a little more uniform and it seemed to have worked out ok.”

machining V8 engines

Head of engine tuning Pete Hagenbuch remembered, “The 3.9 which was built in Mound Road, on the machining equipment which included a 90 degree bank angle. It was another one of those boom-boom, boom-boom type engines. I had two of them, both automatics, and it didn’t bother me a bit; but the manuals were awful, especially if you lugged them down in speed.. It set off all kinds of sympathetic vibrations, just an awful way to build an engine. And Chrysler at that time had principles; we didn’t build engines that way.

“I’m confident that it went through the top and came back down with the message that ‘no way, you can’t do that — it’s not commercial.’ Then years later we had already admitted that automatic Tempests and F85s were acceptable as long as you didn’t watch them idle under the hood, where they were just thrashing around. With the vibration absorption you get in a torque converter, they were okay as far as driving was concerned.”

The 3.9 in use

In 1987, its first year, the 3.9 used Holley 6280 two-barrel feedback carburetors. This setup may have lasted into 1988 on Dakotas, but then Dodge slapped an EFI intake and induction on the 318 and the 3.9. They shared a Holley throttle body, until the Magnum versions, and used a non-vacuum advance distributor. One engineer wrote, “I remember they had an awful time with the fuel/air distribution. We were all asked to help out if we had a glimmer.”

From then until the 1991 models, they used single-point fuel injection— relatively inefficient, but easier to control and maintain than a carburetor. The 1992 trucks used a multiple-port injection system instead; the heads and other parts were upgraded at the same time, with more parts interchanging with the 318. The result was a hefty power boost.

In the 1987 Dakota, the 239-cubic-inch V6 pushed out 125 horsepower at 4,000 rpm, and 195 lb-ft of torque at 2,000 rpm, with a 9.2:1 compression ratio. That was far above the 1987 slant six’s 95 hp and 170 lb-ft of torque. Both the 318 and 3.9 gained lower-friction roller hydraulic tappets with this model year.

The Magnum versions, with many changes (see the 318 page), leaped up to 180 hp, and torque jumped from 195 to 220 lb-ft. Gas mileage was roughly 2 mpg better than the Magnum 318.

3.9 liter V6 power

On the 1994 Dakota and Ram, the exhaust manifolds were shrunk to 1 5/8" and the exhaust was reduced to 2.5" from 3", eliminating 5 horsepower but no doubt saving some money. The greater efficiency of the system led engineers to drop EGR on the 1996 trucks.

In 1997, the 3.9 received sequential multiple-port fuel injection, where each injector fires as the cylinder is drawing in air (in the older multiple-port injection systems, the injector often fired against a closed valve). Aided by a larger spark plug gap, the system increased responsiveness, though horsepower and torque ratings remained the same as in 1996: 175 hp @ 4,800 rpm and 225 lb-feet of torque @ 3,200 rpm. At the time, the 318 was producing 230 hp and 300 lb-ft of torque.

Displacement 239 cubic inches / 3906 cc
Bore and stroke 3.91 x 3.31 (90 degree V6)
Valves Overhead - 12 valves, roller followers, hydraulic lifters
Fuel injection 1987: None, Holley carburetor
1988-1991: Single-point electronic
1992-1996: Multiple-point
1997-end: Sequential, returnless multiple-point
Construction Cast iron block and heads
Compression ratio 9.1:1 (1997)
Redline 5,250 rpm (1997)
Fuel 87 octane unleaded regular
Oil, coolant 4 quarts oil, 14 quarts coolant
Emissions 3-way catalyst, two heated oxygen sensors (1997)
EPA mileage
(1997 Dakota)
16 city, 22 highway (2WD, manual transmission)
15 city, 18 highway (4WD, automatic transmission)

Common repairs

Allpar forum member Dean C. found that his 2002 3.9 van would stumble and idle poorly, a few minutes after a warm restart. When the throttle was blipped, it popped; when given a lot of throttle, it revved and ran normally. The stumbling lasted up to a couple of minutes; if driven, it would hesitate, sputter, and pop for several minutes unless given full throttle. After a couple minutes, it would be fine until the van was shut off for 5-15 minutes. Cold starts were normal.

Bob Lincoln wrote that the the 3.9L V-6 often had a vacuum leak at the “belly pan gasket” on the intake manifold, which could cause stumbling and poor idle after a warm restart, and “pops” when the throttle is “blipped.”  Bob wrote that another indicator of this problem is oil consumption; the owner can check for black residue, either wet or dry, at the bottom of the throttle body.

“Whitevanguy” added that there can be problems with weak or intermittent grounds causing “weird electrical issues.” Bad MAP sensors can also cause similar problems.

(repair tips | performance tips | the men behind the 3.9 V6: Willem Weertman | Pete Hagenbuch)


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