ChrisGSR

Mystery valvetrain noise here. Wondering if I have a bent cam or a pulley out of balance. Let me tell the whole detective story and maybe someone will be smarter than I am at figuring it out.

Stock B18C1 in a 96 GSR. Just did the timing belt and also did the cam seals. When put back together and started, there was a sharp rattling sound. It seems to come from under the valve cover. Took the cover off and scoped out the situation. Nothing looks severely out of whack.

Here's what I've done trying to localize the problem:

(a) Loose heat shield? The sound of the rattle is the sort of noise that a loose heat shield or a stone in a shield might make. Front HS is off. Catalyst HS is off. Noise is still there.

(b) Did the oil pan get dented, while the engine was on a jack for the belt job, and then block the main oil pickup? That can happen -- though I did use wood between the jack head and the pan. OK, drop the pan to inspect. Looks great, no issues. Also no unusual debris in the pan.

(c) Is it a disintegrating distributor shaft? Pulled the dist and checked. Shaft and bearings look perfect.

(d) Could it be injector noise? B-series injectors can get loud. Yeah, but it really sounds like it's under the valve cover. And the car has never made this noise before in 90K miles. Now it suddenly makes the noise, immediately after the belt/pump/seal job. The injectors weren't touched. I don't think it's them.

OK, let me go deeper into the valvetrain. Cover off. Looks OK. Untorque the cam holders per spec and remove them. They look OK.

Untorque the cam bearing caps and remove them. There is a tiny bit of visible wear on two of the bearing caps for the intake cam -- small pits. For reference, where a human hair is 100 microns in diameter, these are maybe 25 microns in size. They're only noticeable because of the polished background.

Cams come out. The cam rounds look fine. Lobes look good except for one or two tiny pits comparable to the ones on the bearing caps. The #4 primary intake lobe has three small scratches on it, comparable in depth to the pits, maybe 2mm long, right at the very edge of the lobe. Could such small marks make this much noise?

While I have the dist out, I pull the intake rocker shaft and all of the intake rockers (taking intense care to not mix them up). No rocker face damage anywhere. Rocker shaft bores are perfectly clean. Shaft itself gets cleaned and inspected, it is A-OK.

Full reassembly, clean oil applied everywhere. Rocker shaft and rockers go back in. Back out valve adjusters all the way. Cams go back in. Cam bearing caps torqued to spec and checked with a beam torque wrench. Cam holders torqued to spec with the beam TW, in specified progressive sequence, 1/4 turn at a time. Adjust valves on all cylinders and then do a final verification on all of them. OK.

Start up. Rattle shows up two seconds after ignition. Day-um, this thing is persistent.

Did the timing drift due to a bad timing belt tensioner or something? Inspect. No, the engine is in perfect time when checked at #1 TDC.

This is completely weird. OK, my best guess is this: taking the cam gears off in order to change the cam seals might have caused some distortion. I left the cams in the head and put a wrench under each gear arm, wedged against the motor mount, in order to hold the gear against the torque. The torque was enough to leave visible nicks in the gear arms where the holder wrench held them.

Maybe a bent cam, maybe a cam gear slightly out of round, maybe a distorted bearing cap? It's the only stuff I can think of.

Unless anyone has a better idea, I'm going to go out and try to hunt down someone's orphaned GSR cam-and-gear set and swap them in as a test.

Cam swap is pretty easy. But I have no idea what to do about changing bearing caps. Aren't those closely matched to the head?

C

ChrisGSR

Following up here:

I went in yesterday to a local dealer and talked to one of their Acura-factory-certified engine techs.

He said, "Yeah, you obviously forgot to adjust the valves, common mistake."

I said, "No, I did do the valves, and double-checked them, 17/100mm on the intake valves, 19/100mm on the exhausts."

He said, "OK, well, you obviously forgot to adjust the belt tension. A loose belt makes the distributor drive rattle. What you do is to..."

I said, "Put the engine at #1 TDC, back off the tensioner bolt, rotate the crank so that the cams turn three sprocket teeth counterclockwise, torque down the tensioner to book spec."

We went on like this for about five minutes. Finally he said, "Damn! You did everything right. You checked everything I would check."

ChrisGSR

Still trying to puzzle this one out.

I listened with a long screwdriver all over the valve cover with the engine running. The noise has seemed all along to be coming from the intake cam area on the back of the head. Took a listen to the injectors, and the noise is loud there, but no more or less loud than on the cover along the intake cam.

Yanked the injectors anyway. Nothing visibly wrong with them. Screens were all clean.

Wrapped the injectors in socks. It doesn't cut down the noise.

Now that I've calmed down about running the engine with a rattle, I have run it for long enough for the oil to heat up. That drops the idle to normal, and the engine does *not* make the noise below 1500 RPM. In fact, it's very rev-specific once the engine is warm. Needs 1600RPM and not one RPM less before it will make the noise.

dubcac

Are you positive it's not the distributor? I have a very similar rattle, only happens after a similar RPM that you mentioned, and it sounds like it's coming from the distributor...and the distributor is directly linked to the intake cam. Other than that, I'd definitely try a different set of cams and gears, that you know are good, and maybe a distributor if you have a friend with a GSR.

ChrisGSR

Not positive. I did pull the dist, and the shaft looks OK to the eye. I have seen cases where the shaft or bearings were literally disintegrating. This one is visually OK. But it hasn't passed a replacement test.


Sensible advice, but jeebus, what a pain. Diagnostics-by-swap works, but it's always slow and usually expensive. On the other hand, if it's DBS time, at least it's *me* doing it, with whatever secondhand but working parts I can scrounge up. What really hurts is a dealer wrench doing DBS, changing out everything in sight with shrinkwrap goldplated parts at top-dollar shop rates.