Technical Please help me identify this engine noise

[Ster1]

I just got my 1980 spider running the other day, and the engine is making a strange noise. It seems to start and run fine. I don't have a radiator in it yet, so I can't get it hot. The noise seems to correlate more with crankshaft rotation than camshaft. It seems to be coming from the left side of the engine. It's only on deceleration also. Please give it a listen, and let me know what you think.



Thanks for any help!

 

[F123C]

Sounds to me like a timing belt that is way too tight.
I watched your previous video and note what work you've already done including replacing the t/belt.

As a 1st step, I'd remove the V-belt driving the water pump, alternator etc. to eliminate these as a possible cause of the noise you demonstrated.

My thinking on why the noise, on engine slowing down, might be caused by the t/belt tension being too tight :-

When the engine is accelerated the 2 belt runs between the crankshaft and inlet cam and between inlet cam and exhaust cam are pulled tight, the remainder of the belt, between exhaust cam and tensioner and tensioner and crankshaft is effectively the slack side as it's not under as much load. But when the engine is accelerated and then allowed to slow down, as you've been trying in your video, I reckon the belt loading is increased on the tensioner side - if the belt is over-tensioned initially, it will be way over-tensioned when it's slowing down, causing the noise you're experiencing.

I also noticed some marks on the back/flat side of the timing belt. (I'm assuming you fitted a new belt). These marks remind me of those seen when the belt has been run too tight or has been in service for a long time.

Maybe try turning the engine so that the valve timing marks align, prise off the belt using your fingers (should be possible if the belt is tensioned correctly) and without loosening the tensioner (this way you can refit the belt if it and the tensioner are not the problem). With the belt removed, check the tensioner bearing carefully for wear or roughness - you may already have done this before fitting the new t/belt.

Correct belt tension (cold) without using a gauge, is that you can just about twist the longest belt run (i.e. between crank and inlet cam pulley) by 1/4 turn, with your fingers.

Another thing that I recall happening on some of these twincam engines is that the camshaft belt pulleys were made from some sort of plastic? material with steel centres. These steel centres could come loose, the pulleys could develop cracks radiating from the centre - both of these issues could cause a noise similar to what you're experiencing. (the earlier twincam engines used all steel belt pulleys).

I also noticed in the last part of your video @c. 21secs something seemed to be fluttering in the vicinity of the t/belt between the tensioner and crank pulley - might be trick of the light or a shadow?

In summary, I reckon the noise is being caused by either a too-tight timing belt or a deteriorating belt pulley (centre coming loose), or both.

Hth,

Al.

 

[Ster1]

Again, thank you!

You're making some good points, and it got me thinking. I believe the tension is ok. I did the 1/4 turn test as you mentioned, and according to that, I'm fine. I watched the pulley closely as I accelerate and decelerate the engine, and it doesn't move or seem to be making noise, but I did not specifically check it while it was apart, so definitely a possibility.

Side story - I had a truck once that would randomly throw the serpentine belt for no apparent reason. That thing drove me crazy with checking pulley alignments, bearings, etc. Until a guy on a forum, just like this, told me to replace the tensioner. I cried "BS" on him because I was young and thought I knew everything. Guess what... as soon as I replaced that tensioner, the belt stayed on. I, of course, had to go back and eat crow, and admit he was right. So, your comments about the tensioner certainly carry weight with me!

Anyhow, back to this - I believe the tension is ok, and I believe it's more or less consistent throughout the running cycle, you got me thinking... every engine has bit of timing lash between the crank and the cams as the engine is running. I work on mostly diesels (which almost all have gears, so the lash is fairly minimal, and no flex at all), but this engine has a timing belt. Honestly, I don't work on very many timing belt engines. I have to believe there is some lash, and possibly some flex as the engine is accelerating and decelerating. In other words, the valve timing seems like it would wander a bit. Possibly only by a fraction of a degree on either side, but some wandering. Assuming that's true, as the engine is accelerating, and the intake side of that belt is under tension, and possibly stretching a little, the valves retard just a bit. Conversely, as the engine is decelerating, the exhaust side of the belt is under tension, and the timing advances a bit. Again, possibly by only fractions of a degree. If I have the valve timing too far advanced in my static timing, it is possible that while the engine is accelerating, the belt is allowing the valves to go into the proper (or closer to proper) timing. When the engine decelerates, the belt then allows the valves to go too far in advance, and we are hearing piston interference! That correlates a bit with some observations I made. One being that the noise seemed to correspond with crankshaft speed, not camshaft. The other being the sound itself sounded like a valve slamming shut. It was probably the piston slamming the valve shut.


I checked my timing marks one more time, and I found something. The timing indicator down at the crank has 3 "teeth" on it. When I took the old belt off, the timing aligned with the middle tooth, so that's the one I used with the new belt. According to my manual, I am to use the top tooth. So, if that's true, and everything I wrote above is true, my valve timing is too far advanced, and would cause severe engine damage if I took it out for a drive. I'm going to reset the timing to that top tooth, and report back. This is fascinating stuff. I can't wait to test these theories.

 

[Ster1]

Well, no dice so far. I took the V belt off, I tried retarding the timing one tooth, and advancing the timing one tooth, and the noise is still there. Tension seems good, tensioner seems perfect. The pulleys are metal. I thought I found a crack, but it was just a loose spark plug. I may have stumbled across the reason it was scrapped in the first place. The noise sounds like it’s coming from the top end, and my only option may be to pull the head.

[ame]https://youtu.be/zTt8ShU3GZM[/ame]

 

[F123C]

It's about 25 years since I last worked on one of these older Fiat twincams (used to be a Fiat dealership Mechanic), so relying on memory...

Watching your latest video and particularly the 'new' valve timing marks you're using reminded me of something.

Your crank and cam marks are fine but your auxiliary shaft position might not be. This is something that is usually covered in the shop manual. There should be a timing mark in the form of a small hole on the auxiliary shaft pulley - this is supposed to be positioned iirc 34 degrees to the right of vertical (again, iirc, that's about 3 teeth to the right). The reason for this precise positioning of the aux. driveshaft (on your engine it only drives the oil pump?) is that there is an eccentric on this shaft inside the engine that was used to operate a block mounted mechanical fuel pump on the older models. This eccentric could collide with the rod bearing of No. 2? cylinder if not timed correctly. (I don't know if Fiat deleted this eccentric on the 2 litre fuel-injected engines).

Maybe this is what is happening on your engine i.e. this eccentric is just kissing the adjacent rod bearing cap when slowing down? If I understand your video correctly, this aux. pulley has been set the same way on both your advanced and now correct valve timing procedures.

You make some very good points about the possible slight variation in valve timing when a timing belt is used. If you watch the long side of the belt (i.e. between the inlet cam and the crank) when you transition from accelerating to decelerating the engine, you can clearly see a type of slackness appear - this much surely translate into a valve timing inaccuracy at this time.They used to say that these belts have strengthening fibres that prevent stretch but I reckon that the rubber teeth must 'give' slightly when the load direction is changed, so maybe contributing to another very slight timing inaccuracy.

But I don't think these slight variations could cause the pistons to lightly touch the valves - you had your valve timing out by 5* at the crank and no apparent touching. Back in the day, we used to alter the valve timing on these engines from standard - advanced the inlet cam by 1 tooth to improve pick-up, retard the inlet by 1 tooth to cure jerky acceleration/poor idle or vice-versa on the exhaust cam - never had a problem, bearing in mind that even if nothing touches when the engine is turned over by hand it doesn't follow that at high revs everything will clear due to valve float, rod stretch etc. A buddy mechanic used to advance the single cam on Fiat 128 and early X1/9 by 2, sometimes 3 teeth to 'improve' performance after everything else had been checked/adjusted - never bent a valve, these problem cars then went like stink.

I assume that the cam pulleys are secure on their respective cams, no missing/damaged locating dowels etc.

The confusing thing about your engine noise is that it's only apparent when the engine is slowing down. Bearing noise is usually there all the time but louder on acceleration. Had a loud noise on a twincam once that sounded like a bad rod bearing but disappeared when the engine was slowing down or I backed off the throttle - turned out to be a piece of the piston skirt had broken off, allowing the piston to rock so that it struck the cylinder head. Had a noise like a bad rod bearing on an Alfa Romeo Alfasud (flat 4 sohc per bank), disappeared on the overrun, oil pressure was perfect - turned out the weld on one of the exhaust header pipes to it's mounting flange had broken - sure didn't sound like an exhaust blow.

If you have an inspection camera, you could maybe take a look down each plughole to see if anything has been touching. Any time we had a noise on Fiat engines after servicing, we had a look down the cyl. bores, wasn't unusual for one of the little 8mm nuts holding the air cleaner onto the carb. to 'accidentally' get into a cylinder - had work 'buddies' who thought doing this on someone else's job while their back was turned was funny. :bang:

If anything else comes to mind I'll post again, hopefully before you remove the head....

Al.

 

[Ster1]

Fair point about the aux shaft. When I advanced / retarded the crank and kept the aux shaft the same, that was a test of not only the valves but that shaft. Since there was no change, no matter what I did, it can't be that lobe. Or at least, in a timing related way. Yes, there is still apparently a shaft with a lobe in the 2.0 FI engines. Some folks have devised a method to delete the lobe, and plug the oil passage, so the shaft is just a dumb shaft and has no bearing on anything, except the belt itself.

I have a camera just for this, but it's not the greatest quality. I doubt I'll see anything, but you bet it's worth a shot.

 

[Ster1]

Well, I can certainly say there is nothing rolling around in any of the cylinders.

[ame]https://youtu.be/nLoKmAwJTv0[/ame]

 

[F123C]

^^^ Your cylinders all look ok, just a little carbon build-up and certainly no sign of valves having touched pistons or anything else untoward.

I've listened again, several times, to your videos demonstrating the 'noise' when engine is slowing down. Still no wiser. Engine starts easily, idles well, picks up ok. Can't think of anything inside the cylinder head that might cause this noise. Everything I can think of that might cause any kind of noise will do so at all speeds, not just on slow down. Unless maybe it's a broken valve spring???
Although I've never known these engines to break a valve spring, even when abused - they're double springs so if one broke, the other would still close the valve, so engine might seem ok, but maybe make a noise? (the 2 springs are wound in opposite directions, so shouldn't interfere with each other even if one breaks).

This noise still sounds to me to be something to do with the belt or maybe something in the intake tract possibly being triggered with the change in vacuum?

Did this engine have the 'noise' when running with the old timing belt?
If you still have it, maybe try fitting it (you should be getting quick at setting valve timing now ) just to see if the noise disappears?

Maybe try reducing the belt tension a little to see if the noise changes?

I mentioned trying a mechanic's stethoscope to locate the noise (if you don't have one, press the tip of a large screwdriver against the area to be 'listened' to and hold the handle against your ear). I think I'd check the area of the inlet plenum chamber/throttle body? at the front l/hand corner of this engine. Are there any valves etc. that respond to a change in vacuum in this region, e.g. brake-booster?

It would be a pity to pull the cylinder head and be unable to find anything wrong inside, only to find the cause of the noise was in some external, easily reached ancillary component (BTDT :bang: )

Al.

 

[Ster1]

Once again, your words got me thinking. This time about carbon. Honestly, I don’t know why I hadn’t considered this before, but here we are. I went back out tonight with my little camera, and took special notice of the valve recesses in the pistons

[ame]https://youtu.be/GB78CwiTuSY[/ame]

If it were timing related, my messing around with the timing would have shown something, but it didn’t. Also, there would be more marks on more pistons. This seems isolated to 2 valves. I think it’s carbon built up on a few valves / guides, slowing their closure. I had always thought this sound was a valve slamming shut. The only problem with the theory is, why only on deceleration? If the valve is slow, it’s slow everywhere. It revs up nicely. That part doesn’t jive, but there is certainly interference.

I wonder if I let it run a bit longer to see if temperature makes a difference. Or maybe throw some sea foam in there. There’s only a few gallons in there. One bottle of sea foam ought to have a fairly noticeable impact.

I’ll keep you posted!

 

[F123C]

^^^ you might be onto something here.
Sorry I couldn't see the slight contact between valves and pistons on cyl's 2 & 3 in your earlier video but your latest close-ups definitely show some contact has happened at some point.

You asked why, if the problem is indeed a couple of valves sticking slightly, why the noise is only there on engine slow down - might it be because the cams are opening the valves as per normal but the springs can't close them quickly enough to avoid some slight contact with the piston crowns on cyl's 2 & 3? Perhaps these 2 valves were open all the while the car was out of use.

These Fiat engine use valve stem oil seals, these are so effective that I've wondered how the valves/guides last so long - also means that it takes forever for some oil to make it down the valve guides after a long period of non-use and rubbing surfaces have dried out.
Is it possible that the exposed sections of the valve stems might also have some rust build-up?

Al.

 

[Ster1]

Well, here it is...

[ame]https://youtu.be/jvMKo0xBP1E[/ame]

I was planning on pulling the motor anyway, but I had hoped to drive it just a little first. I got this for a winter project, and it looks like I’m going to get my way. Hell i’m only a few bolts away from removal anyway. I’m going to do engine and transmission.