The Neglected 3.9 V6 Dodge Truck Engine
The 3.9 V6 was a close relative of the 318 V8 — with the same bore and stroke — created simply because they needed a V6 engine for the new Dodge Dakota. Developing a new V6 from scratch would have been expensive and time consuming.
The 3.9 was only used in the Dakota (base motor), Ram 1500 (base motor), and Ram 1500 van (optional; a 318 V8 was standard). The engine was a stopgap but it served its purpose well in 1988; despite upgrades, by 2001 it had been outclassed and was replaced by a 3.7 liter version of the 4.7 liter "Next Generation" engine which had better gas mileage and more power.
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. I don't remember how they fixed it but I do remember it was acceptable in the end."
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 2-barrel 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.
The fuel injection from 1988 to 1991 was single-point, central fuel injection; gas was injected into the throttle body. This system was relatively inefficient, but still easier to control and maintain than a carburetor; replacing it with a modern multiple-port injection system in 1992 resulted in a power boost (though since the heads and other parts were upgraded at the same time, partly for more interchangeability with the 318, it's hard to isolate the effect of the better injection system).
In 1987, 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, far exceeding the single-barrel-carbureted 225-cube 1987 slant six’s 95 hp and 170 lb-ft of torque. Fuel injection did not change the power rating but greatly improved driveability. The engine was given lower-friction roller hydraulic tappets, which were used on the 318 as well starting in 1987.
The Magnum versions, with multiple-port injection, were rated at 180 hp. Torque jumped from 195 to 220 lb-ft as well. Gas mileage was roughly 2 mpg better than the similarly set up 318.
In 1994, 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. In 1996, EGR was eliminated.
In 1997, the 3.9 received sequential multiple-port fuel injection, where each injector fires as you'd normally expect anyway - as the cylinder is drawing in air (in standard multiple-port injection, the injector often fires against a closed valve). That and a larger spark plug gap helped to increase responsiveness, though horsepower and torque ratings remained the same as in 1996 - at 175 hp @ 4,800 rpm and 225 lb-feet of torque @ 3,200 rpm. At the time, the 318 was producing 230 hp @ 4,400 and 300 lb-ft @ 3,200.
Willem Weertman, the head engine designer, remembered, “They wanted to have an upgrade power plant from the 4-cylinder so 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 in order 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.”'
Engineer Pete Hagenbuch remembered, “The 3.9 which was built in Mound Road, on the Mound Road machining equipment which included a 90 degree bank angle and 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.”
|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
1997-end: Sequential, returnless multiple-point
|Construction||Cast iron block and heads|
|Compression ratio||9.1:1 in 1997|
|Fuel||87 octane unleaded regular|
|Oil, coolant||4 quarts oil, 14 quarts coolant|
|Emissions||3-way catalyst, two heated oxygen sensors|
|16 city, 22 highway (2WD, manual transmission)
15 city, 18 highway (4WD, automatic transmission)
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, “The 3.9L V-6 was known to have had a vacuum leak at the 'belly pan gasket' on the intake manifold. This may be causing this issue. Another indicator of this problem is oil consumption. Look down the throttle body with a light and see if there is black residue, either wet or dry, at the bottom of the intake. Since it is okay on a cold start, I would not suspect a low battery, but you can have that load-tested, too. And since it doesn't idle slowly and stall right away after a hot start, I would not suspect the TPS.”
“ImperialCrown” pointed out that it may run well when cold or under full throttle because “Oxygen sensor mixture information isn't used by the PCM when the engine is cold or under wide open throttle.”
“Whitevanguy” wrote, “Also clean the battery to engine ground and engine to frame ground. If no engine to frame ground exists, add one. Try wiggling the connectors at the PCM and see if you can induce the problem...These vans are notorious for bad/weak/intermittent grounds causing weird electrical issues. Chasing down electrical issues without eliminating the grounds as culprits is foolhardy.” Bad MAP sensors can also cause similar problems.
Eventually a computer code was finally set, for “bad oxygen sensor,” and Dean replaced both oxygen sensors, resulting in trouble-free driving for a while. Numerous codes appeared later, including oxygen sensor codes, but replacing the MAP sensor fixed these and any remaining idle problems.