By on June 20, 2012

 

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.

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54 Comments on “Piston Slap: The Folly of the 90-degree V6?...”


  • avatar
    racer-esq.

    I can understand Mercedes, GM and Chrysler using 90 degree V6 engines out of penny pinching cheapness, and I can understand Maserati and the Peugeot/Renault/Volvo collaboration using a 90 degree V6 out of European quirkyness.

    But I was surprised to see on the Wikipedia link that the first Honda V6, the one used in the Legend, NSX and first V6 Accord, was a 90 degree V6. Honda has no V8, and is not the kind of company, like Mercedes, that would use an inferior design just to allow for shared tooling. Apparently Honda initially went with a 90 degree V6 for the marginally lower height. It has switched to 60 degree V6 engines.

    • 0 avatar
      CJinSD

      Honda was obsessed with low hood heights in the mid ’80s. Even their front suspension was initially billed as being designed to allow for a lower hood. Other properties seem to have been secondary.

      • 0 avatar
        Marvin McConoughey

        Two of the many considerations in engine choice are 1) the inherent vibration and noise of whatever engine design is chosen and 2) the engineering measures taken to conceal vibration and noise from the driver and passengers. Honda and, more recently, Audi, have used the 90-degree v6 engine very successfully because they have invested sufficient money in engine mounts, body structure tuning, steering wheel vibration damping, and acoustic insulation to largely isolate the engine characteristics from the cabin. The engine itself can be made smoother by reduced reciprocating weight, longer connecting rods, higher precision, and so on. The wide cylinder angle is a bit lower, at least potentially, and provides more room for mounting equipment such as turbochargers. The 60 degree cylinder angle remains a better choice in my view.

  • avatar
    dts187

    I’ve had Dakotas with the V6 and the V8. As mentioned, the 6 will always have a slightly “off” idle character. In normal operation, I never really considered the V6 “rough”. Just kinda choppy. Over time it did develop the tendency to idle at higher RPMs and be rough enough to shake the truck. If this is happening, clean the IAC with some throttle body or carb cleaner. It should smooth out after a day or two of driving. If that doesn’t do it or the roughness returns after a bit just replace the IAC.

  • avatar
    nickoo

    The GM v6s to come out in the late 70s were mostly just v8s with 2 cylinders chopped off. Why GM didn’t just keep the v8 design and downside the bore, or just stick with the already in production straight 6, is a question that seems to have no good answer, the short lived early 80s 267 v8 was an attempt at such, but in execution (I owned one) failed miserably–All because of the absolutely terrible carb put on that engine. Once you pulled off the emissions controls and put an edelbrock intake and small 4 barrel on it, it ran as good as any other small block chevy.

    • 0 avatar
      noxioux

      There are a couple of reasons to abandon the 250 straight six in favor of the 4.3. Production on the 4.3 was cheaper, as it shared more than a few parts with it’s bigger SBC brother. Probably the most important factor was that the V6 is a much more compact package, which was absolutely necessary for the downsizing happening in the post-malaise era.

      Downsizing the bore on what would essentially be just another small block V8 would make little sense, as it wouldn’t have any net effect on the size or weight of the engine. I very much doubt that 267 would hold a candle to a decent 4.3 Vortec in the same vehicle, even with the better induction.

      • 0 avatar
        nickoo

        You have to look at 16 valves vs 12 valves, also the thickness of the block on the 267 plus it’s long stroke make it a decent long wearing engine with good torque. I agree the weight/packing issues are valid in a FWD car, but pretty much every single one of GM’s RWD cars could have all fit the straight six up until they ended production.

      • 0 avatar
        indi500fan

        I just rebuilt a 250 straight six from my Camaro.
        FWIW the pistons, rings, rods, bearings, and valvetrain are all common with SBC variants.

      • 0 avatar
        Pig_Iron

        My 267 V8 was a nightmare: one of the rockers disintegrated (amongst other things). My tech couldn’t beleive his eyes. He said he’d never seen anything like it.

        In any case, I still adore the Buick V6, but time moves on…

        How can GM take a mongrel like the Buick to such heights, and yet so badly botch other power plants at the sam time? It’s baffling to me.

  • avatar
    ajla

    Optimum or inherent balance is vastly over-rated. The occasional vibration lets you know the engine is there.

    Despite Sajeev’s OBVIOUS RAGING ENVY against ingenuity and illustrious history of the 90-degree V6, I do not think you should accept your problem as a design flaw.

    Before you give up on the 90-degree design, try to clean your throttle body and clean or replace your IAC (http://www.youtube.com/watch?v=b-xU6kHaabk&feature=related).

    • 0 avatar
      nickoo

      Balance isn’t over-rated at all, there’s a reason a straight 6 is one of the longest lasting engine designs–due to harmonic damping. Also, I don’t know about Chrysler, but to combat balance issues destroying the car, GM used soft engine mounts, step on the gas and the engine starts moving before the car…If you drove one of the chevy 3.8L (not the buick) or early 3.3L v-6s you would know just how horrible they were.

      • 0 avatar
        ajla

        I wrote “inherent balance” not “balance” in general. Maybe I should say “optimum balance” instead.

        Just because late 1970s Chevrolet cheaped out on NVH doesn’t mean everyone should throw the design out the window.

      • 0 avatar
        Marvin McConoughey

        I drove an early Buick V6 with the wide angle V6. It was acceptably smooth, thanks to Buick noise and vibration engineering. I doubt that the engine itself was smooth, but the resulting vibration was well isolated from the driver. It also had very unimpressive power.

  • avatar
    Gardiner Westbound

    I have an Acura with the C35A, SOHC, 90°, V6 engine. It idles at 650 RPM. There is no trace of vibration or roughness.

    • 0 avatar
      racer-esq.

      Not that Honda didn’t do a really good job with its 90 degree V6, but it means something that Honda has abandoned that design for a 60 degree V6.

      • 0 avatar
        TEXN3

        I have the same smooth and torquey engine in my TL with 142k miles. The reason Honda switched was two-fold: the 60 degree engine fit better transversely (Legend and TL had longitudinal setups) and used less space, because of that it allowed for more use as we see today. The C-series is a great motor but the J sings even better but without the same low end feel.

      • 0 avatar
        CJinSD

        The first C series was in the first generation Legend, which had a transverse engine. So did many other cars that carried C series engines, like 1995 and later Accords and NSXs.

        The change in V-angle reflected that Honda had moved on from advertising the lowest hood-lines in the industry. One almost might think that short engines would be important again to compensate for nitwit pedestrian impact regulations, but apparently not. Or it could be that Honda knows they won’t sell any V6 powered cars in Europe, so the tests only have to be passed with the 4s.

    • 0 avatar
      Pig_Iron

      Do the coin test and tell us the results. Take a Loonie, put on edge vertically on the engine, then take it up to redline and back (slowly – don’t gun it) and see if the coin tips. There is a legend Jag V12s could do this with a thrup’ny bit.

      Since it’s a V6, which is half a V12, if you made it half way to redline, I’d say you did pretty darn good. Sorry, I can’t think of any American coins that with flats on the edge.

  • avatar
    Mark MacInnis

    As I am certain Bertel will confirm, being married to a Japanese woman, the Japanese loathe N-V-H…people from Japan cherish harmony and quietness above all things…it is a national trait. So it really should surprise no one that an American 90-degree V-6 and a japanese 90-degree v-6 would have extremely different N-V-H characteristics.

  • avatar
    Educator(of teachers)Dan

    Least smooth engines, V8s chopped down to V6 and I4s that were made by chopping a V8 in half… I’m looking at you Iron Duke.

    • 0 avatar
      nickoo

      The worst part is, GM still uses the 4.3L in the base model work trucks. That engine should have died 15 years ago, I can’t imagine the LS motors are that much more expensive to produce.

      • 0 avatar
        Educator(of teachers)Dan

        Absolutely… I thought the 4.3 was going to finally die when the Atlas I6 was released but the 4.3 has now outlived the Atlas. The 4.3s only saving grace is the fact that usually when driving one I forget that its not a 305V8 SBC.

      • 0 avatar
        noxioux

        I like the 4.3 Vortec, but for the life of me, I can’t understand why GM didn’t use the 4200 six in the Colorado/Canyon. Any savings in production cost had to have been immediately lost in the overall perception of quality/value.

      • 0 avatar
        CJinSD

        The Colorado/Canyon were designed to exclude the Atlas-6 so that marketing wouldn’t be able to force its offering at a later date. GM did this because they wanted to use the small trucks to offset half ton trucks and big SUVs on their CAFE average. Never mind that a 5-cylinder 4WD GM pickup used as much fuel as anyone else’s most powerful V6s while being slow and nasty. The customers were supposed to take one on the chin so GM wouldn’t pay fines. It turns out that the LS fit in its place, so marketing eventually got their way, but not before the small truck lines took on a stench of mediocrity, and that is being generous.

    • 0 avatar
      Pig_Iron

      Um, I think the Iron Duke was 2/3s of a Pontiac I-6. As the was the 153 Chev. But the Pontiac was a crossflow head IIRC.

      The ChryCo/Mopar A4 midget racing engine was half an LA v8, but I don’t know if it’s made anymore.
      http://www.bpemopar.com/MIDGET.htm

    • 0 avatar
      Robert Gordon

      “Least smooth engines, V8s chopped down to V6 and I4s that were made by chopping a V8 in half…”

      I take it then you’ve never driven a vehicle with a Ford Essex V4 then…

  • avatar
    stuntmonkey

    The 90 degree V6 is all about lowering the center of gravity baby, long live the original NSX.

    No v-6 engine has perfect primary or secondary balance, but with the 60 degree engines (fires every 120 degrees), the crankpins are spaced evenly apart so the firing order sounds ‘smoother’… however, with the 90 degree engines, counterweights and balancing shafts and splayed crankpins, it can actually produce a ‘smoother’ engine than a 60degree engine with no balancing shaft.

    If the pins aren’t splayed, a 90 degree V6 fires every 150 and 90 degrees. I don’t think anybody makes an engine like this anymore.

    With a rough idle, though, I’d look elsewhere, too many other simpler things to do with engine smoothness.

  • avatar
    krhodes1

    The 90 degree PRV V-6 was supposed to be a V-8, the gas crisis of the early 70′s downsized it. It was also supposed to have balance shafts in all versions, but in the end only the Peugeot version got them. That engine is smooth as glass and tough as old nails. Shame the Volvo version was niether.

  • avatar
    Juniper

    As many have commented, The 90 degree V6 has good primary balance but uneven firing order. The firing order gives you the “roughness”. So they offset crankpins etc. to achieve a compromise between the two. The only good solution is a balance shaft. The 60 degree has the opposite situation. Even firing, bad balance. The inline 6 has inherent good mechanical balance and even firing. The only reason for a V6 is package size.

    • 0 avatar
      stuntmonkey

      > The only reason for a V6 is package size.

      Well, theoretically, you could get more power from a V6 than from either a straight six or a flat six. The crankshaft and cam shafts are shorter, reducing rotating mass, so all things being equal, you might be able to get a higher rpm ceiling. The problem with the boxer is that the pistons are self-cancelling… at TDC and BDC the counteracting pistons don’t exert a force on each other… theoretically, boxer engines need heavier flywheels to keep engine momentum up.

      The only semi-comparable engines I can guess at are the 7th generation Honda 3.0l v6 versus the e46 BMW 330 with 3.0l v6 versus Mercedes C320 with 3.2 v6. Similar era, similar horsepower levels, but the Honda gets there using 87 octane (60 degree v6, less inertial), and the Mercedes needs a little bit more displacement (90 degree engine, extra shafts and balancing?.

      • 0 avatar
        Jason Lombard

        “versus the e46 BMW 330 with 3.0l v6″

        I keep reading about the mythical BMW V6 in dealership ads and on Auto Trader. It must be a real unicorn, as I’ve never seen one in real life! Joking aside (and you probably know this), the e46 330 did have a 3.0l motor, but it was an inline configuration, not a Vee.

      • 0 avatar
        stuntmonkey

        > e46 330 did have a 3.0l motor

        Good catch, my bad.

  • avatar
    wstarvingteacher

    Have one of the 4.3 engines in an old S10 (1991), with a 700r4. Can’t say I’m very happy with it at smog control time or with the idle characteristics. What I am happy with is the 7 foot bed which will carry a round bale of hay and the overall strength of the vehicle.

    20mpg isn’t all that much to brag about but it will do it full of cargo. I guess that life is just full of compromises. I agree that the compromises would be a lot less with a straight six. Wake me up if they ever make an engine better than the ford 300. In the meantime this will just have to do and if I can keep a smog sticker on it, it will.

    • 0 avatar
      nickoo

      You’d have to go with a cumins inline 6 to find a better engine than the ford 300. Straight 6 is still the best engine design and should have never been abandoned in RWD vehicles.

  • avatar
    jz78817

    anyone who thinks modern (or more recent) 90° V6s are unpleasant really needs to experience an odd-fire Buick V6s. Newer ones with split crankpins and balance shafts are silken in comparison.

    Oh, and there’s also the odd-fire Viper V10…

  • avatar
    red60r

    We had a used Volvo 760 for a while — its wet cylinder liners and poor head seals helped put American Warranty out of business. Probably one of the few cases in which such contracts ever paid off for the consumer. The real pity was that it was a very nice ride when it was working.

  • avatar
    texan01

    The Mopar 3.9 is not a smooth engine. Dad had an 87 Dak and a 94 Dak with the 3.9, and with 200,000 miles or 90,000 miles it idled ‘rough’. About like my friends ’90 S-10 with the 4.3 or the non-balance shaft equipped GM 3.8.

    My 2.8 powered 6000 would idle smooth though, and my OHV 4.0 Explorer even after 300,000 miles idles almost as smooth as a V8 but both are 60 degree designs.

    • 0 avatar
      86er

      The 3.9 in my 2000 Dakota had a rough idle too. Nothing really to recommend it, drank fuel like a 318 with a whopping 225 ft/lb to move around nearly 4000 lb of vehicle.

      Fixed that with another SBC.

      • 0 avatar
        texan01

        The 87 Dak was a 4×4 with the 120hp 3.9. Absolute top speed was 92 mph. 0-60 took 20 seconds and passing anything over 55mph was required planning ahead and dropping back to build up a head of steam to pass. It got 16mpg normally no matter if you were driving 30 or 90mph. I did pull a 5,000 pound trailer with it and it pulled it like it wasnt there though.

        The 94 in comparison was a rocket, and it even broke 20mpg for dad.

        Both trucks hated me though, I couldn’t drive either one without something breaking. It got to the point of if I needed a truck, I’d borrow the neighbors F-150 rather than Dad’s Dakota.

        He traded the 94 off for a brand new 07 GMC Canyon. Biggest complaint I have with the GMC is uncomfortable seats.

  • avatar
    el scotto

    Dude, really? A 10 year old truck is in solid beater truck territory. Rough idle and other quirks just add character to a beater truck.

  • avatar
    benders

    Gen Seal lives on as Conti Seal (Continental bought General).

  • avatar
    nrd515

    I had a 1988 4.3 Blazer that a friend drove until it was literally falling apart with over 400K on it. It shook with 7 miles on it when I bought it. I’ve driven all of the “best” chopped V8′s, and the Mopar was the weakest of them all. They should have made it with the 360, not the 318. A friend bought a Dakota with a 3.9 and it just wasn’t acceptable. Mileage was what a 318 would have gotten anyway, and it was such a slug he was kind of happy when his daughter wrecked it.

  • avatar
    Ian Anderson

    Are your motor mounts any good? I know my 1992 beater Dakota with the 3.9 seemed to idle horribly (and would rip the shifter out of your hand on a 1-2 shift) until I replaced them. After that it’s not a rough motor, it’s better than the 4.3 in my father’s S10 and a balance-shaft 4.3 in a S10 Blazer I drove. As people above mentioned it’s nothing compared to an odd-fire V6.

    Do a tune up an check the mounts (pricey on a 4X4!) and drive it until the POS front suspension gives out.

    For a lightweight truck like my 1992 the 3.9 is fine, since the truck clocks in around 32-3300 pounds with me in it and handles great for what it is. The 1997-04 Dakotas are pigs, though.

  • avatar
    raph

    “Don’t tell the Buick Grand National fanbois that something’s “amiss” with their rides, either.

    Damn skippy, or tell the V10 Ford guys the same thing!

  • avatar
    dolorean

    “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.”

    Love this statement from an Engineer. Had to be the same conversation with the GM engineers in the late 70′s when one of them said, ‘Hellyeah we can take the 350 V-8 and make it a diesel. SEEMS to work okay.’

  • avatar
    probert

    The gm v6 was licensed to jeep when it was part of AMC. I wonder if it’s the same engine in your truck.

  • avatar
    Andy D

    I added a rondel and the kidneys to my 4.0 OHV Ranger to make it a rare BMW v6

  • avatar
    NoGoYo

    I love how loud pushrod V6s can be, the roar of a poorly muffled Cologne V6 upon starting is a sound burned into my brain forever. Somehow, despite being a DOHC design, Toyota’s 3.4 V6 truck engine also has that roar…man, I love that sound.


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