Piston Slap: The Folly of the 90-degree V6?

TTAC commentator Jerszy writes:

Dear Sajeev;

Hopefully you & your fantastic community can help me here.

I recently purchased a 2002 Dodge Dakota Sport 4X4 (3.9 V6, 67k, Auto).

I bought it to replace my 2002 Cougar Sport Package (2.5 V6, 64K, Manual, speed-limited to 139mph) which as you know is not a good suburban truck and can’t really haul things. The Cougar was a fun car, very agile and could haul me around town and being a kitty-car it really did purr. Unfortunately it had to live outside in the rusty north for the last 6 years and was starting to age rapidly. Since I live in a “snow belt” (avg. snowfall ~120 inches a year) it had to be 4 wheel drive.

Now the Dakota is a definitely a truck. Almost as big as the ‘76 Silverado I had 30 years ago and just as four-wheelie as the ‘84 Toyota 4X4 truck I replaced it with. (That Toyota rusted, rusted, rusted so much I had to fabricate a wooden bed for it in 1987!)

Interesting aside with the Toyota, the neighborhood crooks would constantly bust the wing-windows and steal stuff out of the cab, only the first perp got anything and it was nothing but a junky tool-set.

I got so good at replacing the wing window that I could install the window while driving home from the dealership. The tires (which seemed almost magnetic) were also a problem, every screw on the road found a home in my rubber. I bought a set of General Gen-Seal tires at the time (not sure if they make’em anymore) and that cured that problem. Two things stood out about those tires:

· The sound they made when you pulled a screw or nail out, (HSSsssssssssssssss-FIP…) they really worked!

· How the gel on the inside would migrate to the bottom of the tire on a warm day and the out-of-balance thumping down the road for twenty minutes until they rebalanced themselves. (Ahh the memories…)

My only issues with this particular Dakota besides truck-like acceleration and gas mileage (~16mpg) which doesn’t really matter to me as I might drive a whole 3000 miles a year anyway is how rough that engine idles. It runs great otherwise. I have talked to two general mechanics and both thought it was somewhat normal. I am sure for a price they’d look a bit closer but it sure seemed to me that they were serious and didn’t want to pick my pocket.

My question to you is: Is a rough idle normal for this engine?

I can say that it’s only a small problem to me but the fact that runs so well otherwise lends me to believe the mechanics were right. Looking over the 3.9 engines history at Allpar, I find this interesting passage according to Willem Weertman, the head engine designer:

“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.”

Should I pursue this issue more vigorously or just pretend that everything is normal and learn to love this as it is?

Since the chief engineer of this engine weighed in on the issue and deemed it “just OK” in my mind that it probably does idle rough as designed.

Thanks!

Sajeev Answers:

Ah, this takes me back! Back to when I started writing for TTAC, saying things like “the pissed-off 3.8L’s presence at part throttle” in reviews of vehicles with 90-degree V6 engines. And while I enjoyed dancing around technical terms with Farago-like passion and precision, the fact is that these engines are flawed.

You can’t chop off two cylinders from a 90-degree V8 and not suffer a compromise or two. And they will never be the smooth operator you’ve seen in your Duratec-powered Cougar. Or any other modern DOHC V6, for that matter.

While I haven’t driven a 3.9L powered Mopar, your letter (and the Allpar link) suggests it has the same design flaws of the 3.8L GM and Ford products. Wikipedia sums up the problems with this design pretty well.

If you’ve done the basic tune up things, replaced all worn vacuum hoses, etc I suspect you’re stuck with the “charms” of 90-degree V6 engine ownership. Don’t tell the Buick Grand National fanbois that something’s “amiss” with their rides, either.

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry.