The vehicle is a '91 Volvo 740 2.3 inline 4, Turbo automatic with 250K miles.

Several months ago I was driving the car 35mph, the engine developed a terrible vibration for about 30 seconds. I stopped at a light waiting to get somewhere to pull off. While waiting at the light the vibration went away. I accelerated and immediately a strong knock developed, related directly to engine speed. I drove the car home and parked it, assuming a bearing had spun. I would start the engine from time to time to let it idle for a minute and the knock was always there. I had a buddy hear the knock and he also thought it was rod knock. Fast forward to today. I've removed all bottom end bearing caps expecting to find damage indicative of a spun bearing (scoring, heat marking, bent bearing tangs, etc). I've found nothing. Not a scratch on the crank journals, not a scratch on the bearings, front or back, no compressed bearing tangs. The bearing caps have no scoring on the bearing side which would indicate a spun bearing. I'm stumped. the engine runs fine other than the loud knock. The bearings in this engine look freakin' brand new. 250K miles and not even a spec of copper is showing! Amazing. Really a testament to Volvo design and quality.

On the off-chance this was a really suck lifter (mechnical lifters on this engine) I checked all lifters by hand and they all spin freely in their bores.

I'm stumped.

I'm planning to plastigage the old bearings. If everything is in spec, I'll put in new bearings while I'm in there and close it up.

Any other ideas?

[This message has been edited by Rainman (edited 07-28-2007).]

tranny?

I thought about the tranny, and asked a buddy (mechanical engineer) about that as I know nothing about automatics. He said there isn't really anything transmission related that he could think of to produce that knocking. I thought about the input shaft possible banging about, but this is a really solid sounding knock, which I wouldn't expect from an input shaft just spinning being able to produce.

I'm auto ignorant though. Is there anything in particular in the tranny that could cause this?

What about the pistons...could it be caused by a wrist pin problem at the other end of the connecting rods?

EDIT: Better to tear down the whole thing and find the problem, than trust to luck and have it grenade and possibly destroy the block.

[This message has been edited by Vonov (edited 07-28-2007).]

Just went out and checked for seized wrist-pins. Everything felt fine. I don't have the head off so the pistons haven't been removed, but just by feel, the wrist pins allow the rod to rotate freely and none felt any more slack than the rest.

I spun the transmission input shaft by hand as fast as I could (with the torque converter in place) and no noise or knocking was felt/heard.

I'm beginning to question if it was a belt driven accessory causing the noise, be it the water pump, power steering, or a/c. They all worked as they should, but I haven't found anything in the engine to pin-point the noise.

I'm on a time crunch with this project. I gave this car to my little sister last year, she drove it to Kansas City and lived there while she had the car, driving it for about a year. She then left it with me here in Ohio to store for her while she stayed with our parents (on the west coast) for the summer. Now with college starting back up, she will be flying into town in several weeks and then driving the car to Florida for college once again. I'm in Ohio so I want to have this issue not rear its head 1000 miles away from me or else she'll be scrapping it down there probably, and I love that car and would rather see it again.

[This message has been edited by Rainman (edited 07-28-2007).]

quote Originally posted by Vonov: What about the pistons...could it be caused by a wrist pin problem at the other end of the connecting rods? EDIT: Better to tear down the whole thing and find the problem, than trust to luck and have it grenade and possibly destroy the block.

I'd agree, wrist pin. Can be hard to detect until the piston assembly is out. Even slight wear can cause a knock. Got a 2.8 out of an 85 Fiero in the barn with a bad pin, sounds like the engine is huffed but it was just a few thousands of an inch wear on a pin in a motor with only 53,000 miles. New pins, rings and bearings (rings and bearings because it was apart not because they were bad) and back together it goes

[This message has been edited by tutnkmn (edited 07-28-2007).]

That 30 sec shudder could have been a weight being thrown on the flywheel.

quote Originally posted by Toddster: tranny?

ditto...

got milk, er
fluid?

[This message has been edited by DeV8er (edited 07-28-2007).]

Wouldn't a loose rocker arm also cause a similar noise?

A cracked flex plate will cause a knock that sounds exactly like a rod knock. And I have to agree that vibration you felt was probably a weight from the flex plate letting loose.

Carbon knock? Are the pistons or CC's filthy?

May as well go further and check those pins while you have it apart. I'm going to vote for a wrist pin also.

quote Originally posted by fieroluv: A cracked flex plate will cause a knock that sounds exactly like a rod knock. And I have to agree that vibration you felt was probably a weight from the flex plate letting loose.

I sold a great trans am because I thought it was a knock. Guy pulled it and found a broken flex plate.

If the flex plate dings like a bell its good if not there is a crack.

Is it a diesel? You don´t specify...but I know the diesel engines can develop a 'knock' which is injector-related, rather than lower or upper bearings..heck old London diesel taxi-cabs used to sound as if the ends were knocking even when brand new!!! Just wanted to clarify which fuel it uses
Nick

I have also heard a knock from a good flex plate. One of it's retaining bolts had backed out a little thumping on the engine block every revolution.

I've looked the flexplate over and don't see any visual signs that its cracked. I'll give it a wrap with my knuckles and see how it sounds. Flexplate on this car is a solid disk, like a flywheel, but thinner.

Dang it. I did not mean a retaining bolt where it bolts to the crankshaft. I meant a torque convertor mounting bolt.
Good luck.

I bought a old Honda Accord once that had a knock. I tore it down and rebuilt. I started it up and it still had the knock. It turned out to be the overhead cam pully. The keway had enough slop in it to cause a knock. I pleened the cam and some crazy glue fixed the problem. 30,000 miles later I sold it still running strong.

While you have it apart check the motor mounts and cross members.

The motor mounts are soft due to oil leakage contamination, but no external moving parts were rubbing. I've checked the keyways on the crank and cam. The key on the crank is pretty far in the crank with little protruding for the crank gear. Not yet sure if that is the way it is supposed to be or not. I've plastigaged the rotating assembly and it all spec'd exactly as brand new.

I'm wondering if the issue is in the head. I've rotated all belt driven accessories by hand an they all feel/sound correct.

[This message has been edited by Rainman (edited 07-29-2007).]

The timing belt tensioner broke on my Probe once and sounded just like a rod knock.

------------------
1986 SE 350 V8

quote Originally posted by tesmith66: The timing belt tensioner broke on my Probe once and sounded just like a rod knock.

Thanks. The tensioner felt fine when spinning it by hand, but I've bought a replacement anyway since I'm in there.

I'm not that familiar w/Volvos...do they have harmonic balancers?

I've finally got time to report the findings.

Turns out a small "U" shaped clip designed to hold the intake hose clamps in place on the intake hoses dropped into the intake and was sucked into the engine. It explains the momentarily stuck throttle, the engine vibration, and the knock. I'm fairly certain I know how it got into the intake and when, wasn't me or my fault. The piston is ruined, as is the head. The clip is pressed into hte aluminum head, you can see it around the 12 o'clock position on the head. Its going to cost more than looking to restore the engine right now, so its likely I'll sell the car. The car ran/drove perfect before this happened.





Anyone interested in a 740 turbo with all new suspension: poly bushings, Bilstein HD dampers all 'round, IPD sport/lowering springs and new balljoints and tie rods?

[This message has been edited by Rainman (edited 08-03-2007).]

Engine swap.

Try this link.
http://stores.ebay.com/Toms-Foreign-Auto-Parts

http://www.glenwoodauto.com/

http://searchparts.com/foreign-auto-parts.htm

or try your local phone book under used forein auto parts.

let me ask this... I can't tell in the picture, but did the clip scratch or damage any sealing surface? Could you just put it back together?

J.

quote Originally posted by tesmith66: Engine swap.

There's a video on youtube with a Volvo wagon with an LS-1 in it...insanely fast.