Mystery engine noise

#1

I had to replace my engine recently, and true to form I went with the risky option and installed an engine that had been lying around a shop a while unused. It came out of a car that was hit from behind, but the guy who sold it to me did not pull it himself or know anything about the car other than it was a 325 (mileage is anyone’s guess). I pulled the valve cover off to take a look at the head and the engine turned over with just the starter and sounded healthy.

I got it running two days ago. There was a lot of coolant in the exhaust system from the previous engine (number 5 intake valve came off the stem and punched through its seat) and after about half an hour the smoke abated. No problems so far.

After about fourty minutes driving, the engine was all the way up to temp and an intermittant knock happend at idle. It first started when I was sitting at a red light. The engine was idling at about 700 RPM. It started tapping once per revolution, and not every revolution. When I brought the idle up to 1000 RPM the noise went away. I could not hear it at all when the car was in gear and moving, even during engine braking or constant throttle.

I put 5W-30 non-syn after the motor swap. My intent was to run about 200 miles and change the oil. I intend to go to a little heavier oil when I change. I did not add any ATF yet.

I am wondering if this could be one cylinder striking a sticky valve, or something related to the oil pressure and valve train. I do not have an external oil guage, but the idiot light never came on. It sounds a bit like a diesel when it’s happening, and I’ve been keeping the throttle at 1000 RPM in the mean time to avoid it. I’m going to do a leak down test tomorrow. Any one have an idea what’s going on? Thanks in advance…

#2

That oil is pretty thin.

You need to work to localize the sound. First see if you can figure out which cylinder it is coming from by pulling plug wires one-by-one and seeing when the sound goes away. Second, spend some time figuring out if the sound is coming from the head or the block.

Check your valve gaps. If they are ok, worry about a rod bearing. Checking your rod bearings is pretty aggressive so don’t do that unless you really have to. Lift the engine up a couple inches, pull the oil pan off and start pulling off rod bearings starting from the back. If #'s 6-4 are ok, then it’s probably not that.

So you did not take the head off of this replacement engine?

Possible causes:
–Piston slap. Probably not because piston slap should go away when the engine gets warm.
–Worn rod bearing. Could be. Hot thin oil would exacerbate. Sound shouldn’t go away at 1k rpm tho.
–Valves out of adjustment. Probably not because this should get better when the engine gets warm.

#3

Are you sure it’s a knock? It’s common for the transmission/clutch to “rattle” at low engine speeds. The rattle noise is randomly timed, but a knock will happen at consistant timings. In a high milegeage motor with unknown bearing conditions, thicker oil might be a better solution than 5W30.

#4

I am planning on going with 10W-40 for the next oil change. I live in a moderate climate (Pennsylvania). Advice for oil weight is welcome.

I hope the leak down will give me clues as to which cylinder is affected. If the valve is sticking open at low RPM, it may be open slightly at zero RPM. Otherwise I will try the “plug pull” method. I will borrow a stethescope as well to see if head or block.

I did not pull the head on the new engine, but I did adjust the valves (.010" cold). I am getting “normal” valve tick at all times. When this knock happens, it’s noticably louder. I can imagine the cylinder tapping a valve closed, but I hope it’s only my imagination.

It is defilitly tied to engine RPM and only happens at idle in neutral or with the clutch in. The motor came with a transmission attached, and like the motor, I have no knowlege of the clutch or tranny history (but I do know that $250 is a good deal for an assembled working drivtrain with wiring harness installed).

I am getting intermittant oil level warning light, though the oil level is constant and correct.

Is the oil a working fuid in the valve train; does the valve train depend upon a minimum oil pressure to function? I don’t know if this head has hydraulic lifters. If so, would a lower oil pressure due to low viscosity cause the lifter to malfunction? My thinking here is that increased engine RPM will generate more flow from the pump (or more pressure) fixing the problem.

I am considering adding ATF prior to the oil change to break up any gunk in the oil passages. I have been babying it, keeping the revs below 4k mostly. Thoughts?

Thanks for your suggestions.

#5

No on the ATF. The solution to cleaning out the inside of the engine is quality oil and regular oil changes.

Re. oil choices. Do a search in this forum for “oil”. The best cheap and readily available Summer solution is Valvoline VR1 20W50. Also cheap but harder to find is Brad Penn 20W50. For Winter the best cheap solution is Brad Penn 10W40. Oil is like religion. There’s lots of opinions but few facts. Brad Penn has terrific High Temp Shear (HTHS) #'s, VR1 not quite as good, but still better then Mobil1 15W50 which is popular and easy to find.

AMSOIL and Redline make great products, but aren’t cheap.

The valve train is mechanical.

A sticky valve would not be at the top of my guesses. We have pretty strong valve springs and our valve guides get loose over time, not tight. Not impossible, but not at the top either.

I like a worn rod bearing. It could be that it’s knocking is being covered up by other engine noise at higher rpm, because normally you’d be able to hear it at rpms higher then idle. The low pressure light is another indicator that makes worn bearings more likely, although there’s a number of other possible causes for low oil pressure that don’t cause knocking.

On the other hand, if I understand you correctly, the sound would go away if you put the clutch out and drove down the street at idle or nearly so. I don’t think rod knock would go away under those conditions, so take heart from that.

IMO the smartest E30 guys are at E30tech. We have our charms, but they are more technically oriented then we are. I’d ask there.

#6

I will check out E30tech. Would rod knock be subdued during cold engine operation? The “Engine Oil” light on the headliner comes on, but the light on the dash never does. I was under the impression the one on the headliner was for level and on the dash was pressure. I’ll get some 50wt.

Thanks.

#7

[quote=“scookson” post=57140]I will check out E30tech. Would rod knock be subdued during cold engine operation? The “Engine Oil” light on the headliner comes on, but the light on the dash never does. I was under the impression the one on the headliner was for level and on the dash was pressure. I’ll get some 50wt.

Thanks.[/quote]

Rod knock is subdued when engine is cold because thick cold oil is able to maintain the oil “wedge” in the worn bearing. But once the oil gets warm and thins out, the wedge can’t be maintained because the bearing gap is too large.

You are correct about the engine oil lights. The light on the dash is connected to a 7psi switch. When oil flow restriction is getting low because something is worn, at idle with hot oil the 7psi switch can come on. The fact that it is apparently not coming on is good news.

#8

Good news; I’ve isolated the noise to the clutch/trans. I noticed the noise on my commute again today at a stop light while in neutral. When I depress the clutch, the noise goes away. I think my observation about RPM was skewed by other engine noise; when I revved up to 1000 RPM the clank was hard to distinguish from other noise. So, in neutral with the clutch out, it clanks. At all other times, it doea not.

I have the old trans and clutch pack from the previous motor. I’ll throw those on in a couple weeks (traveling next week through Memorial Day) and see if that fixes it. From the reading I did on E30tech, other people have had a similar noise coming from the trans shaft.

Thanks to FishMan for the tip on the clutch/trans and Ranger for the E30tech tip. Having trouble finding 50wt oil locally…

#9

The noise is normal; drive the car and forget about. I wouldn’t swap in another used clutch or trans. A new clutch may quiet things up a bit.

#10

I installed a brand new clutch and throwout bearing when I swapped my tranny. I still get a decent amount of throw out bearing noise. Just ignore it.