Engines! We expect them to do so much these days. We want them to fling us around on our errands for hundreds of thousands of miles, sipping the bare minimum fuel all the while. That’s simply the bare minimum these days, but some engines fail to meet these demands in a grand fashion. This leads us to our question today—what’s your most hated engine design flaw?
Try as they might, sometimes automotive engineers get it wrong—and an engine ends up with a fatal flaw. Months or years down the track, a poorly designed part lets go, or wear hits some critical point. Suddenly, dealerships are getting bombarded with calls, forums are melting down with rageposts, and the automaker is left to arrange repairs via an expensive recall campaign. Or, hell, even buybacks!
You would think that automakers would catch these problems in testing. After all, new engines and new vehicles typically go through millions of miles of testing before they ever hit the road. And yet, somehow, this still happens every few years, even to automakers with the best reputations for reliability.
Earlier this week, we were talking about a controversial example—that of the Cummins 6.7-liter diesel. Listen to some, and they’ll tell you the engine will gladly ingest a poorly-placed nut, munching itself to destruction in the process. Listen to others, and they’ll tell you it’s an incredibly rare ocurrance. It might have never led to a recall, but it’s a compelling tale nonetheless.
Other situations are more cut and dried – for example, when Oldsmobile engineers tried to make a diesel out of an old gasoline V8. That engine had drastically underrated head studs that meant head gasket failures weren’t just a possibility, they were a near certainty. Because of that decision, many car buyers saw diesel as a devil fuel in America for the best part of a decade afterwards.
You might also tip your hat to Lancia. The company famously built an engine so fragile that you could destroy it with a simple turn of the steering wheel. Properly Italian motoring, right there.
We love digging into weird engineering topics, and often it’s the quirky little failures that excite us—and you!—the most. So do tell us—what engine is your bugbear, and what is its fatal flaw that should never have made it to production? We’re absolutely dying to know what has your motor dying. Sound off below!
Credit: GM, Cummins, BMW
Bad engines?
I’d say a tie between the Life F35, W 12, and that Subaru 1235, flat 12.
The fact that most people have never heard of these F1 engines in spite of millions spent is an indication of just how bad.
MEL Ford engines,
Cons:
Odd design doesn’t share parts with other Ford V8 platforms
Zero support from parts companies then and still
Nylon coated timing gears that break off and cause timing to go off
Power steering on crankshaft..
Water diverters inside the block that wear out and cause all kinds of issues
Will bend a pushrod if a sparrow farts sideways in Montana.
Pros:
The Ford Modular (4.6/5.4/6.8) exhaust manifold bolt breaking problem. This one seems simple, but it plagued the Modular engine for its entire production run. These engines use dinky M8 exhaust bolts/studs and CONSTANTLY break them. Even the tiny little 2.4L 4-cylinder in my Tacoma has stainless M10 exhaust studs! Countless F-150s, Super Dutys, Vans and Explorers have the iconic Ford ticking from exhaust manifold leaks. Ford issued a number of TSBs throughout the 2000s, changing the material to stainless, using different nuts, etc.. None of it really worked, and they still break studs.
For whatever reason, Ford just refused to redesign the heads to accept larger studs. The Modular V8’s replacements, the 5.0 Coyote and 6.2 Truck engine, both had much larger M10 exhaust manifold studs. The Modular V10 soldiered on in their medium duty trucks for another 7-8 years after the 5.0/6.2 were introduced, still with small M8 exhaust studs, still breaking them regularly.
While this one might not seem that “egregious” at face value, my argument is that it is egregious because of how prevalent it is, and how Ford knew it was a problem, knew what the problem was, fixed it in subsequent engine families, but absolutely refused to fix it for the Modular family that was still in production.
I just can’t get with this. I currently own two modular Ford vehicles both well over 200k and both have perfect head studs. One even being the V10 which gets driven hard, over the years I’ve probably owned, and pulled 50+ mod motors and out of all of those maybe have had a handful of studs break… And still easily removed.. now my LS engines on the other hand have all broken multipul studs well before 100k
These engines are absolutely infamous for breaking exhaust manifold studs/bolts. Multiple TSBs, constant complaints online. I swear every other explorer or super duty I see ticks loudly the moment they hit the gas.
A few Mercedes-Benz engines…
M116 with aluminium block (1979–1983): it had a single timing chain that broke too easily and too often. For 1984 model year, the engine was redesigned to accept dual timing chains as found in larger M117.
M156 was first complete engine to be developed by AMG rather than basing off the Mercedes-Benz engines. It has a long list of issues, including the poorly designed cylinder head bolts that broke off too easily.
OM612 DE 30 LA, the only diesel engine modified by AMG for W203 C 30 CDI AMG. The reliability was so bad that Mercedes-Benz terminated this engine much earlier than planned.
As a breed, 100% of all interference engines with timing belts.
I can accept the necessary evil of interference + belt, however I wish replacing the belt was less of an ordeal or expense.
The Toyota 2uzfe engine disagrees with you. That engine will outlive us all. Agreed the timing belt every 90k is not optimal, but for longevity you can do no finer.
Modern BMW engines (n20/26, n54, n55, n63, etc.): too many issues too list
Ford powerstroke 6.4: ditto
Toyota 7mgte: silly headbolt torque specs
Honda 3.5 with VCM: cylinder deactivation is such a bad idea that even Honda can’t make it work and the fix was even dumber. My wife had a pilot with one and we brought it in for excessive oil consumption and misfires. They tore it down and replaced the piston rings on one cylinder. Same amount of oil consumption and misfires after. Shes had Lexus’s after that and no issues.
I thank the goddess regularly that I didn’t buy the 135i with the N54 I wanted.
To be fair the n54 is excellent if you go single turbo, plug the direct injection to go port, and basically replace every crap bmw plastic part, especially the cooling system. They can handle a lot of hp before they self ventilate the block
Those all sound like very trivial changes to do to an engine 😀
Lancia Thema 8.32.
A front wheel drive Thema (same platform as the Saab 9000) with a Ferrari. V8.
So far so good, except the engine has to be lowered for a spark plug replacement, as the rearmost pair of spark plugs are somewhere under the windshield.
I vote for a future winner of which it’s a bit too early to talk yet: The new Mazda inline six found in the CX90/70.
All in all nothing to complain about yet, it’s too new, except that it’s flipped the wrong way – the timing chain is towards the cabin.
Peripherals such as the water pump (which I believe is still mechanical) are for some reason also there, so a water pump replacement is an engine out job, just as will be some other jobs that would otherwise not have been that difficult.
A pitty, given that it has a non-hybrid standard version, with no intermediate electric motors and whatnot, which could have become a base for a vehicle that would last quite long. I loved absolutely everything about that car and was considering it as a future purchase.
PS: Spark plug replacement on this engine is every 40k miles, per Mazda. I would expect the accessibility to the rear ones has been planned ahead.
That sounds like the Mazda version of the Audi B6/B7 S4 4.2L V8 with the infamous timing chain guide failures. It’s set up the same way. The timing chain guides regularly fail on the earlier engines, and it’s an obscenely expensive repair even if the engine doesn’t eat a valve, since the whole block has to come out.
I’m surprised this Audi engine hasn’t been mentioned yet. I’m pretty sure it was the subject of a Jalopnik article once upon a time.
The BMW N63. The entire engine. Case closed.
N54 high pressure fuel pump was special.
Yeah but at least the rest of the N54 is okay for the most part, whereas N63’s catastrophically grenade with little to no warning. Just quoted out a brand new N63R (one of the newer revisions) because at 53k miles it spun a bearing and sent metal everywhere, seizing the motor.
I’d say it’s BMW making brittle plastic the primary choice of material in their engine bay.
And setting the engines to constantly run at borderline overheating under light loads to improve fuel economy, thus increasing the temp in the engine bay.
Word is the diesels have less issues with the same plastic parts all around.
I would say GMC/Chevrolet AFM/DFM. These are generally weak spots of otherwise excellent engines (yes call me out all you want…I will NOT care), and usually die due to NEGLECT or ABUSE. I am not including the bearing issues ….as those are supplier problems (happening in MY22 and early-mid MY24. Any insults will not be tolerated.
The killer dowel pin in 5.9 Cummins comes to mind as well.
The Nissan VK56 had catalytic converters/exhaust manifolds break and tear the engine apart….
The 5.7 3UR-FE had quite a few actually. One is the secondary air injection pump. Once it went bad….truck would fly into limp mode and fixes would be thousands in terms of repairs. Toyota NEVER recalled the engines for this, which is pathetic really and left owners high and dry (also, this only happened to versions of the 3UR built in the US- not the ones in the Land Cruiser/LX strangely enough). The air pump is also an issue for the 4.6L as well.
You can fix a SAIP with a bypass, but there is NOTHING you can do to prevent either cam tower leaks, or valve spring failure. Cam towers have been an issue THROUGHOUT the 5.7’s lifespan…and repairs are extremely costly depending on how bad they are. Valve springs are less common, but are FATAL when they happen. Usually engines cost between 11k-17k….if based on dealerships cost (but cheaper options do exist). BOTH of these issues occured in Land Cruisers/LXs and in Tundras/Seqouias.
Let us not forget the alternator and the starter placement……
The other engine with a known flaw that I somehow missed was the Jeep 225 odd fire v-6 with the nylon gears in the timing/cam area. The motor is otherwise durable.
Any modern car with no way to adjust the idle speed other than buying a computer and learning to code.
Considering there is no reason to adjust the idle speed for 99.9% of drivers, I wouldn’t really consider that to be an egregious flaw.
Well you’re entitled to your opinion.
Well, it’s an opinion shared by every OEM on Earth. And it’s coming from someone who actually drives an EFI car that he has adjusted the idle speed on.
If the stock idle is no longer working for you (and you don’t have an aftermarket cam, etc.), then you need to fix the problem with the engine. Adjusting the idle speed to cover for whatever mechanical problem you’re ignoring is just lazy.
Its an electrical/software issue according to my investigations and forum research and yeah, good luck trying to fix it without dealer only software.
Genuinely curious why you would want to adjust idle speed apart from some very very slight vibration adjustment
Poor factory proramming. The stock idle was fine when new but as the engine ages it dips into violent shaking territory under certain conditions (no, it’s not an air leak it’s definitely electrical, possibly a marginal electrical contact). The idle was set at the absolute minimum level to meet emissions and speculation on the forums is the air screw was removed by design to keep DIYers from making adjustments.
The ECU is SUPPOSED to autocorrect for such things but it dosen’t. No codes are thrown, not even a hidden one. A simple turn of the air screw could probably fix it but because the air screw doesn’t exist it’s find a CARB certified tune that does it track down the intermittent electrical gremlin. So $$$ vs a free 10 second turn of the screw.
I call that a design flaw.
Maybe it’s just time to clean the throttle body. I’ve seen more than a few cars where people forget that’s an actual maintenance item and the car has corrected more and more over time for build-up, but then eventually runs out of adjustment.
That was the second thing I tried. Nope. It was pretty clean to begin with
That’s pretty weird. I’ve seen plenty of cars that were compensating for a ton of issues (usually carbon build-up) with automatic idle adjustment, but I don’t think I’ve ever seen one that was clean and idled too low – at least not after given some time to correct itself.
My buddy had a 78 Mercury with the 302 and idle too low with AC on, but that was because he “lost” the step-up booster. If you don’t mind a stupid high idle, a screw turn can “fix” that one.
“What Is The Most Egregious Engine Design Flaw You Know Of?“
I know a guy who designed an engine block and got the rear face (the bit that mounts to the gearbox) a fraction of a degree off of perpendicular to the crank bore. It’s CAD, you really have to screw up to accidentally misalign something. And another guy who changed the bore spacing on the block without telling the guys doing the crank or the cylinder head. These were both caught in CAD.
Then there was a guy who got mm and inches mixed up on his CAD machine while sending data out for prototyping, so one of the brackets for a doodad came back bigger than the engine.
A more subtle one was the guy who forgot to put oil drains in the cylinder head. They only spotted that a few minutes in to the first engine run when oil burst through the cam cover seals. I got the job of fixing that one.
Then there are my design flaws. Sometimes it’s a mistake: on a research engine I put the cam belt tensioner on the drive side of the belt, so adjusting the tension changed the cam timing. That got spotted after it was built but before it was run. The guy who found that one will never let me forget it. I’ll never forget it.
Then there the flaws you have to design in. There is a production engine out there that you can only fully strip using a “crows foot” instead of a socket because there wasn’t a way of bolting those parts together with the right tool access. Not without changing production tooling on existing parts, which would have cost hundreds of thousands of dollars. That’s the mistake I Google for, but so far I’ve not found anyone on line complaining. It’s either a super reliable engine or cheap enough that people scrap them instead of fixing them. It’s been out of production for a while now, so I think I’ve got away with it.
My worst design flaw caused a recall. When the warranty claims started coming in I was still working at the same place, so they came to me and asked why the doodad I designed was failing. That’s a horrible feeling. Luckily the fix was easy: I still had a copy of the original design that had been rejected by purchasing because the stronger material was too expensive. The cheap version made it through accelerated lifetime testing, but that’s just cycles, not actual time. I fight a lot harder on that stuff now. A lot harder. Also that place changed their validation tests to include a test for that failure.
My best design flaw was a whole assembly of stuff (sorry, I know I’m being vague, but I want to tell the stories without getting lawyers involved) that I knew was going to fail in testing because iteration 1 of anything complex always does. You plan for it and learn from it. When something fails you fix it and carry on testing the rest of the engine (unless it burns or smashed itself to bits). But it passed. Which meant it was heavier and/or more expensive than it could have been, but as it already met both those targets it didn’t get changed. That went in to production (having passed loads more testing). It didn’t get a single warranty claim and in at least the first five years of production no one bought any spares from a dealer (we keep an eye on that to find parts that fail that people don’t fix under warranty).
That reminds me: there was another manufacturer who had an investigation triggered in to one of their engines because one of their service centres kept ordering replacement parts. This sort of spike in demand often shows up a particular flaw related to local conditions (mountains, roads with no speed limits, weirdly fine sand, technicians who break stuff, that sort of thing). However in this case it was just that their assembly was just what we needed for a prototype phase of a new engine, so to save time we sent a guy to the local dealer to buy one at a time and hope no one noticed.
I’m now going to go through all the comments and hope nothing I worked on is mentioned…
I want badly to have many beers with you.
I was just having that exact same thought haha
I will third this statement. We need more of your thoughts shared, Captain.
That’s very kind.
After the second beer I stop worrying about lawyers and name names, which makes the stories more fun.
But after the third beer I make no sense at all.
That’s quite a small window of me being interesting company.
I was trying to guess where and what manufacturer you were working in and thought the mm to inches would lead me directly to the states but rereading it could have been anywhere that used USA parts…
Plastic distributor gear on the 1970s LA V8s. We had a Dart that broke a tooth, and had intermittent issues that took months for my dad to diagnose. Also, that thing ate coolant hoses at least once a year. That was probably a materials issue, but the gear was a design flaw.
First Gen dodge neon. I managed just over 100k on it (still poor) but a friend only got 15k out of his.
Which is a damn shame, because I love these. They were my go to rental for all the years they were available (NYC…no car).
What pisses me off about the Neon, is that almost all of the flaws in the engine are because of the beancounters messing with the materials specs. The fixes for these flaws generally involved going back to what the engineers originally specified, rather than needing to come up with a novel solution.
The entire BMW N63 engine, especially the pre-TU ones. What a disaster. Timing chain issues? Check. Valve stem seal issues? Check. HPFP and fuel injector issues? Check. Turbocharger and wastegate issues? Check. Oil leaks from literally everywhere? Check.
All of these issues would be disqualifying, but then to top it all off, in every chassis, the N63 is crammed into the engine bay, making access to anything on the sides, back, or front extremely difficult. It also has horrible troubleshooting procedures in ISTA. It’s probably the worst engine created in this century so far.
Ooh, here’s another one, that I’m amazed David hasn’t written an article about:
Jeep 4.0 motor mounts. Not sure if this is only in XJs or if it’s in others too, and not sure if year matters. The issue is that some of the motor mounts bracket bolt holes were not tapped quite deep enough for the bolts, so the motor mounts bracket bolts bottom out in the hole before they tighten down on the motor mounts bracket. This means the motor mounts bracket is slightly loose and can rattle around a tiny bit on the side of the block.
This isn’t typically an issue for 100k miles or so, after which the bolts fatigue and break off, and the motor mounts bracket comes off. Causing the engine to fall out. Really.
Not sure how prevalent this issue really was, but you will find lots of instances on forums, and out of the four XJs my family has owned two of them(50%!) have had their engines fall out at some point.
My dad was driving his ’95 Cherokee in town(gently too) when the passenger side motor mount gave up the ghost and the whole engine and transmission flipped to the right. He then limped it all the way home driving that way, with the shifter angled 45° to the right.
I had a similar occurrence in my 1995 Cherokee Sport. I was doing about 50 miles per hour around a fairly sharp curve to the right, when I hit a rough seam or a poorly repaired pothole or something. Bang – broken driver’s side motor mount. I was not only pissed off, I was puzzled – I’d hit bigger bumps harder than that off-roading. I even saw it coming, and it was nothing that would have made me try to avoid it, just a bump in the pavement.
Could this have been resolved with a shorter bolt or even some thick spacer washers? You’d still have inadequate threads, but that would have stopped the push-pull fatigue on the fasteners.
Admittedly, I haven’t read all of the comments, but the inline 5 in my Audi 80 Quattro had a split radiator. Basically 2 very small ones connected by hoses. An ignition fuse is directly under 1 said connection, and is not usually picked up by a scan. So, small drip, fuse shorts, car doesn’t start, $1000.00 in towing and diagnostics for a $35.00 repair.
I put forth to you the Chevrolet/GM 4 cylinder engine put into their Vega in 1971. If ever there was a planned obsolescence failure design, I am not sure. Put a cast iron head on the aluminum block. Add coolant. The coolant electrolytes react with the aluminum block to break down the alloy. The result is a car that used oil 50 miles per quart within a year of service. Oh and don’t forget what the overheating does to the cylinder walls. They sort of fixed it with steel cylinder sleeves. But, it only delayed the inevitable.
It’s probably kind of minor, but when I was wrenching for my daily bread in the 90’s, it seemed every big block Ford that came to me had an exhaust leak at the manifold, usually near the back. The bolts/studs were always rusted stuck, and usually someone before me had broken at least one off. Forget about fixing that with the engine in the truck or the giant Thunderbird it came in.
Idiots at GM who thought the 5.7L vortec spider fuel injection on 1996 to mid 1999 pickups and Tahoes was the way to go. The plastic intake is guaranteed to leak internal and external (caused me a bent rod due to #1 cylinder filling up with water while sitting overnight when an external leak developed). Piece of shit computer only allows 2 degrees of adjustment at the distributor. Distributor cap made so each bank of cylinder is on one side or the other. The cap will crack on the inside with the spider mess of having each cylinder moved around inside of the cap. Lets not forget the stupid plastic timing cover with the crank sensor mounted in it to read the reluctor wheel. Could not find a metal one to run a double roller timing chain when replacing bent rod and don’t even think about adding a minor cam upgrade. One more lesson to the school of hard knocks. Don’t ask. I still have the truck because I can’t afford a new truck at $60K with just the basic options.
Can confirm most of those issues. Spider injector took a shit on my ‘99, thankfully it is possible to upgrade to the newer design. I’ve been through a couple distributor caps on my trucks. Weirdly, the timing cover on my ‘96 is metal, but I haven’t checked my ‘99. I wasn’t aware the intakes were known to leak but good to know.
Didn’t see a mention of the Mazda RX-8 engine, didn’t seriously consider buying one because there were already horror stories by the time I was interested. Also all the cars at the dealer had sunroofs.
You don’t like sunroofs? I thought I was the only one.
Grew up on 20 year old cars, I’ll just deal with the required seals, they are bad enough.
No issues with mine after 80,000. Just change the oil on time and add a bit every once n a while.
Ford modular engines that shoot the spark plugs out of the head. Or have the spark plug breaks in the head when removing. Or the threads strip out.
That one is BY FAR the most egregious design flaw IMO. Ford had been building engines for close to 100 years at that point. Its like they forgot how to screw in a spark plug, something old Henry Ford had figured out on the model T.
The plugs shooting out on 2v Tritons is a design flaw, the spark plugs breaking off in 3v Tritons was just iffy quality control on original parts.
Add in the fact that the 2V Triton achieved peak torque at 2,300 RPM like a damn diesel.
You have to really, really try to make an engine that bad!
Back in the day before everyone thought a truck should drive like a Corolla, a low torque peak was a big selling point. That’s how work gets done without stressing the engine.
A low torque peak wouldn’t be a design flaw, it would be intentional characteristic and I’m struggling to understand why it would be considered a negative characteristic, especially for a truck.
Ford for some reason does not understand the relationship between disimilar metals. Steel spark plugs in aluminum cylinder heads, aluminum fuel pump control modules bolted to steel frames behind wheels that sling salty water directly at them. Im sure theres more examples.
There are so many, The worse one I delt with personally was the DOdge/Mitzu Valve issue on the 3.0 v-6. It hit my young early 20s wallet very hard. Here is the AI summary of the issue:
The 3.0L V6 engine from Chrysler/Mitsubishi is known for having valve guide issues, including loose guides and oil leaks:
Dunno which 3.0 V6 you’re talking about, but the DOHC TT in my Mitsu 3000GT had none of those issues, even though I bought it with 90k miles from a drug addict who didn’t give a shit about maintenance, and I put almost another 90k on it before I got rid of it.
The only issue I had was the lifter tick, likely caused by the prev owner overextending oil changes with the cheapest oil he could find; easy fix by just replacing the lifters with updated ones.
It was the blue smoking dodge/chrysler minivan Mitzubishi 3.0 v-6