351 v6 noisy?

[Troyport]

Hello all, I have a 351 V6 in my 68 GMC. Runs great, but rattles a little at hot idle. Just seems noisy, but can't put a finger on it. The other day while standing about 15 feet behind it, could hear a tinny rapping sound, like when you click your tongue. For fun I put in a bottle of STP oil treatment and it seems quieter. Oil pressure at 700rpm idle before STP was 30. Now about 35. Previous owner used 15w40. Oil was fresh so I haven't changed it yet. Thoughts?

[Funky61]

Hello Troyport;

Perhaps an exhaust leak/manifold or valves need adjusting?

[Troyport]

Maybe... well, definitely have a "ticker" on the driver's side, but that isn't the same sound I hear. And correction, I have 50psi at hot 700rpm idle, and if I drop to 500 I have about 40psi. Vacuum gauge flutters at idle, so probably should adjust the valves before I jump to conclusions.

[AZKen]

Hello Troyport;

Perhaps an exhaust leak/manifold or valves need adjusting?

I second that emotion.

[George Bongert]

Maybe... well, definitely have a "ticker" on the driver's side, but that isn't the same sound I hear. And correction, I have 50psi at hot 700rpm idle, and if I drop to 500 I have about 40psi. Vacuum gauge flutters at idle, so probably should adjust the valves before I jump to conclusions.

Greetings, Troyport!

Given that you have 50 psi oil pressure @ 700 RPM hot, and 40 psi @ 500 RPM hot is well within normal range, and is indicative that you do not have any loose (badly worn) crankshaft or camshaft bearings. I (along with others) suggest checking valve clearances.

[Troyport]

Thanks for the reassuring replies. I will check valve clearances... What viscosity oil would you suggest?

[AZKen]

Everybody has their own idea of what oil brand and viscosity. Read up on it and decide for yourself. You have to get educated by study, not by opinion on certain things. Oil is oil, synthentic is synthetic. Viscosity is about climate and region. I.E. 10W40, W= Winter.

[George Bongert]

Thanks for the reassuring replies. I will check valve clearances... What viscosity oil would you suggest?


Personally, I would recommend a 10 W 30 oil with ZDDP (Zinc) to protect the flat tappet cam that these engines are equipped with. You can use either a good name brand engine oil, or the more expensive synthetic oil in your engine. Personally, I prefer to use the more expensive synthetic oil. It's a matter of personal choice. Synthetic oil lasts longer, and provides for easier and better cold weather starting, since it does not thicken as much as standard crude base oils do in cold weather.

[Prowbar]

One might find this chart handy to select a suitable oil viscosity.

[Troyport]

Adjusted valves, all were fine except exhaust on cyl 1 was very loose. Probably put 3/4 turn on it. I will assume PO adjusted incorrectly and I don't have a cam lobe burning up. I suppose time will tell.

[Troyport]

Update on the noise.. It is exactly the same. Just sounds like it is rattling, with a fairly distinct "tonge-clicking" sound. Goes away with RPM's about 900 or so. Maybe a normal sound? Or masked by higher RPM fan, muffler, etc., ? But it is definitely louder than a SBC V8. It's the one and only V6 BB I have ever heard run, so no way to tell if it is normal. Can anyone point me to a video of one running that I can hear? Thanks.

[FetchMeAPepsi]

For anyone else that needs to know the SPECIAL way to adjust valves on the 305 engines:


dyoTQ0xaLBo

It also shows the running engine.

[LEWISMATKIN]

These engines have an noisy valve train that has a "clackety-clack" noise that is NORMAL and has to be heard for the engine to run properly. the only V6 engines that DO NOT have this sound is ones like Pete Chronis' LD-478 which has a SPECIAL GROUND CAMSHAFT FOR USE WITH HYDRAULIC LIFTERS. If your camshaft hasn't been reground with the profile like Pete's, it must make this noise or you will burn up the valves. I hope this helps with the understanding of the operation of the GMC V6. Also, the video that Fetch provided is a spot on tutorial for running the overhead on the V6 engines.:wub::wub::wub::wub::wub::wub::wub::wub::w ub::wub:

[Troyport]

Thanks for the reply. Yes, for sure, valve clatter is to be expected, I have them adjusted properly. And I have seen this video before. His delivery makes me laugh! Love it! I feel like my noise is lower in the engine, but could be echoing.. It runs nice and has good oil pressure, I have to hit some car shows and hear one run in person to put my mind at ease...

[Prowbar]

If you're willing to make a video (if you can capture the sound) and share it with us, we could possibily offer some input.

[Troyport]

I have 2 videos. One video is at 1000rpm, the other is at 650 rpm where I have the idle set. Note the vacuum gauge, if that provides any info. Flickering a little at idle. However, I can't see where to upload a video. Pictures, yes... but not videos. Please let me know, thanks

[FetchMeAPepsi]

I have 2 videos. One video is at 1000rpm, the other is at 650 rpm where I have the idle set. Note the vacuum gauge, if that provides any info. Flickering a little at idle. However, I can't see where to upload a video. Pictures, yes... but not videos. Please let me know, thanks

I usually just upload them to youtube or some other tube place and post a link. If you do youtube this place can show it but it codes it weird. You code it like this, without the 0s on the ends:

[0YOUTUBE]YOUR-VIDEO-ID[/YOUTUBE0]

[Troyport]

I think I found my noise. In addition to the normal ticking valves, I see I have about
035" play on all 4 most rearward and forward rockers along the rails. Is this normal? The rockers in the middle are kept in place with the springs

[bigblockv6]

These engines have an noisy valve train that has a "clackety-clack" noise that is NORMAL and has to be heard for the engine to run properly. the only V6 engines that DO NOT have this sound is ones like Pete Chronis' LD-478 which has a SPECIAL GROUND CAMSHAFT FOR USE WITH HYDRAULIC LIFTERS. If your camshaft hasn't been reground with the profile like Pete's, it must make this noise or you will burn up the valves. I hope this helps with the understanding of the operation of the GMC V6. Also, the video that Fetch provided is a spot on tutorial for running the overhead on the V6 engines.:wub::wub::wub::wub::wub::wub::wub::wub::w ub::wub:

Lewis, actually the custom ground cam in my 478 remained the solid lifter type so my 478 has plenty of the clackety clack noise going on.

[Troyport]

Update: I found in my service manual that the clearance along the rail for those 4 rockers is .030", which is about what I have. When I slide them back an forth, the "clang" it makes is what I am hearing (pretty sure) At idle, is it common for those to move back and forth?
I also read where they want the valve clearance adjusted with the engine running. That must be interesting to do. Thanks for any additional thoughts.

[Jim A]

I have tried to adjust valves while running on several engines where it is recommended.
I'm not sure any of those efforts gave satisfactory results.
The experiences of others here would be very interesting.

[Ed Snyder]

I have tried to adjust valves while running on several engines where it is recommended.
I'm not sure any of those efforts gave satisfactory results.
The experiences of others here would be very interesting.

I've always adjusted my valves while the engine was running, just like my Dad did on the '65 Suburban he bought new that I own now. I use a breaker bar with a socket. The hinge on the breaker bar makes it easy to hang onto the handle while the socket goes up and down.

[Troyport]

Good tip! How do you handle the feeler gauge and get the gap right? Maybe tighten until the gauge will not slide out, then loosen till it just comes out?

[Ed Snyder]

Good tip! How do you handle the feeler gauge and get the gap right? Maybe tighten until the gauge will not slide out, then loosen till it just comes out?

I use a feeler gauge set that opens up somewhat similar to a pocket knife so that the overall length when open is about 8". Use the socket to tighten or loosen until the gauge slides just easily enough that it's not stuck.

Anyone else have any comments or suggestions?

[Prowbar]

I adjust them with the engine off. I put the no. 1 cyl on powerstroke at TDC, adjust the valves, then do 6-5-4-3-2 and recheck 1. I use a 15/16" socket to turn the engine at the crank pulley. Now this is quite a chore with the big 478, haha. The intake valve closing is something I track on each cylinder.This way I always adjust valves when they are on the base circle of the camshaft.

The deluxe heater on these trucks with the V6 is a big obstruction to work around. Kind of wished mine had the simple heater to make engine work easier.

I love the self locking screws these engines have. Also it doesn't really matter if you adjust the valves hot or cold, the literature mentions the aluminum rocker arm brackets compensating for the valve lash based on the expansion difference on aluminum vs steel. I've checked it a couple of times and it is almost always spot on.

As for the .030" play in the rockers, yep that is a typical thing.

[Clarke]

Like Ed, I adjust the valves while engine is running and at Normal Operating Temperature. I have bent feeler gauges and I constantly slide the the oil lubricated feeler gauge around (while making slight valve adjustments) until I feel a slight drag.

I use a 3/8” ratchet with extension, universal joint and socket. The universal joint helps absorb some of the rocking motion, as I’m not that good of a dancer. ;)

Switching topics slightly:

A while back I hauled a couple loads of gravel with my truck in AR (trailer and gravel weight of 8K-8.5K pounds). Then drove my truck (pulling my camper back to KS) after another warrior weekend in AR, and I noticed a faint slapping noise while pulling the truck into the garage.

The noise reminded of a loose cam chain I found on a motorcycle I was riding in college, which I was able to remedy after making adjustments to the cam chain tensioner.

Back to my truck, I’ve never heard that slapping noise again. It was really hot that day and I was towing my 4K pound camper in the KS winds. I completed another full tuneup and I didn’t find anything out of tolerance.

I often wonder if that noise was my cam chain, as I only have one gear on the cam and when I put all new gaskets on my engine (except head gaskets) about ten years ago, I noticed the cam chain had plenty of slack, but it was in tolerance. I thought about replacing the chain at the time, but had read that a new chain/gear set also has lots of slack.

I later learned that there is a double cam chain/gear set available for our engines, which I plan to incorporate one of these days.

Good luck on your continued noise research. Hopefully, it’s not anything of concern. In the meantime, great opportunity to learn a lot about these unique engines.

[Troyport]

Went at it again with the engine running and hot. I dropped the idle quite a bit, enough that the gen light was flickering. Anyway, pretty easy to do, and didn't take long to get the feel.. If the feeler went in easily, too big of a gap. If I had to force it or it wouldn't go in at all, too small of a gap. Got them all with just a little drag while moving it around. Found 2 just a little tight, but not enough to stay open... and 2 notable loose. I would guess I adjusted them before while just starting to lift. The vacuum gauge seemed steadier, and the 2 that were loose were rapping more than the rest. Now, seems more like all are just clicking. Thanks for all the help!