noise."Mechanic and Signal Boost Blog: Town and Country leakdown (mechanic) and Silver Sparks (signal boost) · 3:06am Jan 18th, 2022
Today’s a two-for-one special! You get a short mechanic post and a signal boost!
Now you just gotta figure out which one you want first.
Just kidding, it’s a blog post, you don’t get to choose.
The vehicle in question was a early 10s Chrysler Town and Country with about 100k on the odometer and a misfire. The customer had done some interneting, and thought that it might need a cylinder head.
This is one thing that annoys my manager to no end, customers who do their own research and try and diagnose it themselves. This also annoys me sometimes, but mostly because people are bad at research. I like to think I’m pretty okay at it.
I was curious what engine it had (we didn’t have the vehicle yet). I knew that the 4.0L that went in some Caravans/Town and Countrys did have head problems. I think it was a casting problem, or a porous head where the coolant oozed through the aluminum into places it shouldn’t be. Otherwise, a head problem didn’t seem likely; the other engine Chrysler used in these vans was the 3.6L and I haven’t seen any head failures on those.
When it shows up, I get busy diagnosing it. Chrysler’s misfire counter is weird and does weird things, like it’ll count misfires and then forget about them, which makes the test drive interesting. With a GM, I could clear all the misfire counters, drive it while monitoring something else (let’s say fuel trims if I want to rule that out as an issue) and when I get back I can go to the misfire counter and see how many it’s gotten on each cylinder.
I do observe that it’s got occasional misfires on cylinder four (and a code for that), but only at idle or on decel, when the engine really isn’t under any kind of load and the RPM is low. To my mind, that rules out the ignition coil and fuel injector as a fault; most of the time those’ll fail under load but work okay when there’s no load. Or else they fail completely, but that’s much easier to test.
[Still, the fact that it has that fault while idling in park does make testing easier; when something only goes wrong when you’re going 65 it’s a lot harder to test in the shop.]
That leaves a spark plug (unlikely IMHO), intake air leak, or valve problem as the most likely causes. Some years ago we worked on an older Caravan with a 3.8L that had a slight misfire at idle that came and went; ultimately, the problem was a worn lobe on the camshaft and a worn roller lifter; the amount of air the cylinder was able to take in varied depending on whether high or low spots lined up. We finally identified the problem by graphing a compression test on an oscilloscope; the compression varied by about 20psi from peak to trough (should be about the same every time).
Intake leak’s easy to prove on this one; spray brake cleaner at the gasket and if the engine all of a sudden speeds up, you’ve found the leak.
No leak found.
Can’t do much more on this one without taking the intake off. It goes over the ignition coils, so you gotta pull it to get to the spark plugs. That’s a fairly simple task on these.
Plugs are as worn as you’d expect for the originals with 100k on them, but nothing jumps out at me. Plug 4 isn’t loose, the electrode isn’t any more worn than any of the others, and there’s no sign of arcing through the spark plug boot.
Since we’ve got the intake off, it’s no trouble to look down the intake runners at the valves. From the backside, there’s not much to see, but since we’re there it would be foolish to not do a cylinder leakdown test.
Basically, the idea is with the valves closed, the cylinder ought to hold in about as much air as you put in it. It won’t be perfect, but it’ll be most of it. If it doesn’t, you can listen and hear where the air’s going. I think I mentioned this in a previous blog post, but as a refresher, if you hear it coming out the intake, it’s an intake valve; if it comes out the exhaust, it’s an exhaust valve, and if it comes out the oil cap, it’s a ring or piston problem (or cracked block). Potentially you could see it bubbling in the coolant if it was a head gasket problem, although there are easier ways to identify a head gasket problem.
The thing’s got two gauges on it; one tells you how many PSI you’re putting in, and the other shows how much is leaking out. 0% is perfect, 100% is very bad.
This one got an 80%.
In other words, it’s got a big cylinder leak, but when the engine’s turning fast enough (not at idle), the air’s moving in and out fast enough that it doesn’t flag a misfire.
So where’s the air going? I kind of listen at the tailpipe, and don’t hear much. Probably not an exhaust valve, then. So I listen to the engine where the intake should be, and all I can hear is air hissing through the leakdown tester and the leaky air coupling on the shop air hose. The oil cap is right next to that, too, so there’s not much to be learned.
My next idea is the smoke machine. That’s meant to diagnose small leaks in evap systems and doesn’t make much pressure at all (by design), but this is a big leak and maybe I’ll get lucky and see smoke coming out of something.
I saw smoke coming out of something. Behold:
The smoke goes in 4, and it comes out 2! It’s like a terrible magic trick!
So what’s happening? One of the exhaust valves [I think it has two] on number 4 isn’t closed all the way—it’s bent or burned—and because of where the engine is in its rotation, the number two exhaust valves are open on their exhaust stroke, so the smoke takes the easiest path and gets out that way.
Turns out the customer internet was right and the problem likely is in the cylinder head.*
Now for the signal boost!
Y’all remember Silver Sparks, Wizard for Hire?
Silver Sparks - Wizard.
Lost things found, broken things fixed.
Various other spells, just ask!
No love potions or turning lead into gold.
Yes, I do birthday parties!
Well, there’s now a second chapter! yay!
*Depending on what kind of camshaft the vehicle has, it’s possible a lobe has turned on the cam, which would cause the exhaust valve(s) to be open at the wrong time. I don’t know enough about how this engine is designed to know if that’s a likely fault or not.
Thanks for the shout-out!
Love that Izzy pic.
The other CSRs were clearing the lot today from a serious snow event, and managed to get a 20fter stuck while parking it and rolled the rear axle over a snow-ramped parking block, got it stuck, because rear wheel drive. They were trying to power it back out of the oopsie by chaining another 20fter to the front, and running both at full bore. Then a 26-footer. I'm just glad that I wasn't on the lot when it happened.
Might not matter that much, because the one they got stuck was already downed for being a disgusting mess known for leaving oily muck all over the lot. In fact, I dispatched the damn thing last week just after an ice storm, and had to call it back before the customer left the county, because I didn't feel right about them driving it with what looked like at least a quart down, all over my lot.
The GM said that it had been reported before, and the district mechanic had claimed that he couldn't find anything wrong with it. But shit, oil - fucking everywhere. Something's up with it. And now it's both down, and stuck with its rear axle behind a parking block.
Probably should have gotten the AFM guy to hook up his rig and power it over the block with something that's built for it, as opposed to the trucks they were trying earlier. But it didn't occur to me while he was here, dropping off an RV, which is something that always drives the GM up the wall. The AFM and the GM just don't get along. The AFM's too fond of squirreling away garbage trailers w/ bad tires on our lot - the GM says that it's the AFM's job to replace bad tires, not leave them here for the district mechanic to deal with. (AFMs babysit the dealers in a district, we're a center. They're supposed to deal with the minor mechanical issues that crop up at dealers, not shift them to the centers.)
A Pentastar motor with bent/burned valves? Shame. I wonder if the timing is jumped, or rather maybe the chain is stretched just enough to have bent the valves in only one cylinder?
Yeah, but that rules out the fun of going 65 on the highway and having flames shoot out from under the hood
That's .. not so good.
It's letting the magic smoke out!
*Grabs a drink, kicks back in a comfy chair, lights a pipe, and flips open the latest Biscuit Blog*
Ah... It's a good day.
Did you at least show the customer your new magic trick? "Tadaaaaa~!"
5628507
You’re welcome!
She’s adorable, isn’t she? Even if she’s got beans on her head.
5628509
That’s one of those things you want to watch, though, if you can, ‘cause it’s gonna be interesting when the truck comes unstuck, especially if the didn’t have a plan for when it gets loose. (Back when I drove wrecker, I always considered what was going to happen when the stuck vehicle came loose . . . if it was ‘roll over onto the wrecker, I reconsidered how I was going to hook it.)
On the plus side, if it’s stuck it can’t be rented out.
Depending on what kind of truck it is, finding the leak could be miserable without having a hoist to lift it up on.
Back when I worked for U-Haul (long, long ago) I was at one of the locations that had a small shop and did minor repairs on the local trucks. Back then I thought it would be interesting to be one of the roving mechanics, but now I’ve worked on too many cars in gravel driveways to want to do that kind of thing for a living
I bet there’s at least one cursed U-Haul that constantly gets shuffled from lot to lot, never rented, and nobody wants to fix it.
5628524
I find it hard to believe that the chain could stretch in such a way that it would only hit the exhaust valves in number four and nowhere else. I suppose it could happen, but that seems really unlikely.
The customer took the van away, apparently Chrysler has special coverage on the cylinder heads for reasons unknown, and he’s going to try and get it fixed under warranty. If I ever find out more about what went wrong with this thing, I’ll let you know.
5628535
Easy to say when you’re not the one driving the car or trying to diagnose it. There’s nothing like finding out how bad the control arm bushings are on a car when you hit the brakes and the car VIOLENTLY jerks left. (as I recall, the customer didn’t fix that one).
We did have many years back one of the oddest failures I’ve seen. An older guy who owned a Caravan, every now and then it would blow the alternator fuse (the little one), the warning light would come on, he’d bring it to the shop, and we’d put a new fuse in it and it would be fine for months, then the fuse would blow again. The customer wasn’t too concerned about it, since it happened so infrequently.
Eventually, we discovered that on hard acceleration (which the customer almost never did, he’s an old dude), the alternator would short out and blow the fuse. I think you had to get it over 3,000 RPM and then sparks started coming out of the alternator.
Never seen that on anything else.
That’s really, really bad. Almost wish I’d done a compression test on it, too, just to see how much that cylinder had.
It is!
Amusingly enough, just today I got to use the smoke machine again, this time to diagnose an intake leak.
5628745
I didn’t, which is a shame. Although I don’t think the customer would have liked it.
Thanks for the blog post. :)
5629046
You’re welcome!