TTAC’s analyst extraordinaire, Ronnie Schreiber writes:
My daughter has a Volvo 940. The local Volvo mechanic said that the noise it’s making is the #1 piston hitting the head because of a sloppy big end bearing, which explains the low oil pressure at idle. I was thinking that 60 or 70 years ago, a mechanic might have fixed just the bad bearing, without rebuilding the engine. I was thinking just to do a quick fix, I could drop the oil pan, take off the offending rod cap, mic the crankshaft journal, buy the best fitting bearing per the crank measurement, and bolt everything together with the new bearing. So, Sajeev, with this true piston slap question, is it worth it to try a cheap engine repair like this?
Sajeev replies:
I’d run with that plan, though a small part of me insists on pulling the motor and replacing all bearings. And gaskets, since you’re going in there. Or, conversely, getting another motor if you have over 200k on the clock. That said, let’s bring in TTAC’s resident Volvo-guru, Alex Dykes:
Alex replies:
Ronnie, sounds like you’re on the right track to me. Given the age of the 940 I’m assuming you’ve probably put on more than 100,000 miles already? Since it sounds like you’re a handy guy, I’d give this fix a try before I did anything else. If this doesn’t fix it, then I’d just run out and grab an engine of the same displacement from any Volvo 740, 780, 940 or 960. The 700 and 900 series are often considered the last of the “bullet proof” Volvos so overall I’d say this fix, or even a used engine would be a worthwhile repair. Be sure to check out prices on used 940s, 960s and S90s before you explore a used engine, with the recent trends in used car values, you might be able to pick up a whole car cheaper.
[Send your queries to mehta@ttac.com]
I’d say it’s a maybe. It depends on whether you can get the oil pan off and get access to the offending journal with the engine intact. I did see a mechanic actually do this same thing back in 1974 on a ’73 Chevy Caprice with a 454 engine. I have no idea if the Volvo will be as accommodating.
I don’t like doing major lower-end work with the engine still in the car. Assuming that the block doesn’t need to get pulled because of damage, it’s still not usually a time-saver. The time spent removing the engine is usually not the majority of such a project. Yank the motor, put it on a proper stand, and do the project correctly. This car is worth doing the job right.
If the piston is slapping the head, then it means that the big-end bearing shell is completely gone. That would mean that the crank is also destroyed. And every time I’ve seen a piston smack the bottom of the head, it’s collapsed the ring lands, which then results in the rings gouging the bore. Now, I don’t really think this has happened, because the engine typically loses oil pressure at this point and runs just long enough to tear up every single internal component.
I agree with the verdict, but…
The 960 and 760 have a completely different engine than the 240, 740, and 940. The red-brick B234 engines tend to be as cheap as dirt. So if the bearing repair doesn’t work out, I would see about getting a replacement from Craigslist for a couple hundred bucks and putting it in for maybe $300.
I would also check compression before doing this as well. If it’s too low, you may just be better off running this one to the ground and just R&R this engine.
One site that may be worth your while…
http://www.car-part.com
On most engines the rod brg. should have been making loud knocking noises long before the piston would hit the head. Had you heard other knocking noises before the one you now hear? Main brgs. affect oil pressure much more than rod brgs, so if your oil pressure is real low, all the brgs. are probable worn. That being said if you have the time and the desire you nothing to lose by having a look.
Depending on how long this has been going on, you may find that the rod journal is no longer round and is more of an oval shape. If that’s the case then it’s pretty much done.
I have a hard time believing the piston is hitting the head and the car still drives. As others have mentioned, the crank bearing noise would have been going on for some time previously. A dented piston, crunched valves and leaking head gasket are additional problems that just fixing the crank bearing won’t cure.
Twotone
I agree with the assessment that if the piston was smashing into the cylinder head the car would not drive anymore. At least it would miss that cylinder completely due to crushing the spark plug and destroying the valves. Then it would continue to self destruct further due to no oil pressure, maybe the bent valves would interfere with the operation of the cam (or is this a pushrod engine?)
Find a better mechanic. One who isn’t just making shiznit up.
I’m not trying to sass but if you follow through with your plan all I can say is Welcome to thrownrodsville – Population: You!
I had a similar experience on son Edward’s first car, an ancient RWD Mazda 626. Like your mechanic, a mechanic also thought the noise might be a bad big-end bearing, and I managed to get the pan off and check them. They were perfect. It was almost certainly a bad/worn piston pin. We just lived with it, since it wasn’t that bad.
I tend to agree with the other commentators that if the big end bearing was that bad, very likely the crank is damaged. How bad is the sound? Only on acceleration? On the other hand, the low oil pressure does tend to confirm the diagnosis. Our Mazda had good pressure. Good luck.
Having recently unsuccessfully attempted this same ‘quickie’ repair on a Buick 3.8L, I’d suggest rebuilding or replacing the entire engine. If only 1 bearing is bad, even that has an ominous cause (such as debris blocking the oil passages), and the others will fail soon after.
Any piston actually striking the head means the crankshaft is probably toast, with heavy scoring and lots of diametral wear, and most likely unsalvageable with a simple bearing change. The combustion chamber clearance is much greater than normal bearing wear limits.
Once upon a time I had a very loud metal knock on cyl #1 in Neon 2.0L SOHC engine. Chrysler mechanics tried to sell me on the “piston slap” theory. It turned out that the valve adjuster seized up. It has a little piston that’s driven by the engine oil. There’s no chance that your Volvo engine has the same problem, however the point is that engines have other components that produce these scary metal-on-metal knocks. I also think a piston being so off that it hits the head is so major that dropping the pan will not save you, not a chance.
Personally I’ve found that attempted repairs like this are always like openning up a can of worms. It could be just one thing, but almost guaranteed you’ll find something else has gone wrong somewhere.
When it came to a repair on an engine I once had to do on a Ford Fiesta it actually worked out cheaper (and far less hassle as it turned out) swapping out the whole engine and box.
If you look hard enough, you’ll find plenty of suitable donor vehicles for very little cash – I’ve seen a sub 100k miles 740 in mechanically sound condition (the body didn’t have a straight line on it) sell for £150 at auction… so I’m sure you’ll find something.
mmm Volvos. i’m tempted by a 1990 960 for sale locally with 130000 miles. however i know nothing about Volvos… may be best to stay away
You might consider looking inside the cylinder. I think they call the tool a borescope. Mechanic should have one. Goes in spark plug hole and rotates around.
In the olden days you would get to put eye against a tiny lense, like a microscope. Thats the $200 version I see at famous cheap tool seller online.
Now they make them where you see it on a screen and can record or put into electronic file the video, price is times ten.
For medical use its times a hundred.
I agree that if the piston is hitting the cylinder, you have more issues than just a new bearing shell down below.
Just a hack mechanic , not a Volvo hack mechanic. If the rod bearing is so bad that it allows the piston to collide with the head, it would have thrown the rod by now. A rod knock is not a long term condition. They rapidly worsen to a terminal state. Chances are the crank journal is toast. Further investigation is needed. If it is indeed a rod, it will manifest itself in short order. However a slapping piston, a bad main bearing, or a loose wristpin will make a heck of a noise for 1000s of miles without affecting overall engine performance. Turn up the radio if the noise bothers you.
I’m very skeptical that a simply worn main bearing is causing piston to head contact. The bottom end of a Volvo red block engine is so overbuilt it isn’t funny. Pull one apart and it looks like the bottom end belongs in a 500 c.i. Cadillac, not a measly little 140 c.i. four cylinder.
Me, I would pull the engine, disassemble and rebuild it. No junk yard 15+ year old who-knows? engines for me.
Finding a decent engine to drop in would be difficult. Part it out.
Probably a good chance its not a rod brg. at all. Find another mechanic or just drive it. Sajeev– have you thought about a follow up article once some answers become known? I do enjoy this part of TTAC.
I don’t know anything about Volvo engines in particular, but I have to echo the others’ sentiments that, if the sound you hear was the piston hitting the head, your engine would probably be toast by the time you read this. Your connecting rods will stretch slightly when the engine revs, so if it’s audible at idle, it’s worse when on the highway.
If one of your rod bearings is on the loose side and you have low oil pressure at idle, you may be getting metal-on-metal contact at idle between the crank and rod. You may also have a bad wristpin, which I think someone mentioned. I would pull individual plug wires and see which one causes the noise to stop or lessen.
Alternatively, if your engine has pushrods (not OHC) and hydraulic lifters, the low oil pressure may be allowing one of the lifters to collapse, resulting in slack in the valvetrain. This will also cause a metallic ticking sound.
The low oil pressure may be due to a problem with the oil pump, either worn-out or the regulator is hung-up. if the oil pump is external to the engine it’s probably easy to inspect. If it’s internal then you’re still stuck pulling the oil pan.
mfgreen40 : Probably a good chance its not a rod brg. at all. Find another mechanic or just drive it. Sajeev– have you thought about a follow up article once some answers become known? I do enjoy this part of TTAC.
Maybe a future Piston Slap should be a shout out to the veterans to see how things progressed since getting published.
I have 2-3 right now, I was hoping for a reunion week when I’d publish them in one shot.
Yes, follow ups would be grand. In a good tech thread on my 2 car enthusiast sites the OP will post the resolution. It makes cyber- diagnosis fun.
A “sloppy” bearing would not allow the piston to travel far enough to hit the head. You have a wasted bearing, if that is the problem at all. Assuming that this diagnosis is correct, your connecting rod and piston would almost HAVE to be replaced. The rod has most certainly been stretched out of shape, they are meant to withstand cetrigugal force and compression, but when there is a clearance, it’s like a hammer beating every revolution. Also the Piston if it’s hitting the head the ring lands have surely compressed and will be a problem. You can hand clearance them if you feel the piston is saveable. But my money is this being a bad diag.
A colleague here bought a brand new 240 in 79 and babied it very carefully, but the damned thing kept failing. He took it to the dealer so many times (I got sick of driving him back and forth) that he became convinced that the Dealer was a crook and that the mechanics were SABOTAGING it. He held on to the rustbucket for more than 15 years, but then in 1990 he bought a new MB 300 SEL he still owns and tells me it has been very, very reliable.
Pull the oil pan. If you have a problem with the rod bearing, you should fin gold in the pan. No, you’re not rich. Maybe less rich in a few minutes…
Remove the offending rod cap. Check the crank journal visually and with your hands. Check the bearing for damages.
You should be able to change the 4 or 6 rod bearings by yourself. The mains can be also changed WITHOUT removing the engine, but you have to be skilled before trying that.
But, if your piston is effectively crashing against your hear (which I doubt) your motor by now should be toast.
We did such repair in one of my cars. The rod bearings were fubard. Almost spun one of them.
I’ve personally seen the kind of damage an open (stuck) valve can make to a diesel piston and head, and it’s not pretty.
The days of pouring and scraping babbit and filing bearing caps ended in the early 1950s. Since then bearings are available in standard, .010″ under, .020″ under and so on.
You might get extremely lucky and find a bad bearing but good crank and then a standard size will fix you up. If the crank is significantly worn it will need to be removed and re-ground to the next size.
The piston hitting the head sounds like an imaginative story; I would get a second opinion.
Ronnie, if you’re in Atlanta I have an engine and tranny with 105k, leather seats, and a whole lot of other doo-dads for the same model.
Let me know if you are…
A few quick words since it’s a busy Monday for me.
#1 – Brickboard.com and 700/900 Series FAQ.
#2 – If there are more serious problems, it would almost have to be more cost effective to pull the engine out and swap with a good one from a yard. Don’t know where you’re located, but they should be plentiful enough to find a decent example with mileage somewhere in the 100k range. Access is pretty easy in the engine bay – a little more crowded than a 200 series, but still everythign is there. (currently at 278k mi on my 1990 245DL, eyeballing a ’92)
Best of luck. Z