I believe the timing chain is on its way out in my '75 98's 455, however I'd thought I'd run my thought process by people who've been around longer than I have. The engine is around 100k miles and as far as I'm aware, has never had a rebuild.

The symptoms:

- The engine visibly "shakes" during high and low idle. Not a constant shaking, just an occasional "jerk" every couple of seconds. You can feel this while sitting in the cabin.

- A "tapping" sound during high idle that sounds like a dense piece of metal tapping on a thin piece of metal. The tone also raises and lowers slightly in a rhythmic manner.

- Erratic / rough idle once the engine has warmed up and a slight inclination to sometimes stall at stops. Revving around 1000 RPM smooths it out and eliminates the risk of stalling. Idling in park, the RPM hovers around 800 but occasionally dips to 700 before increasing again.

I know fuel and ignition are both good. I rebuilt the Quadrajet about a year ago haven't had any issues with it. Likewise, the distributor cap and rotor, ignition coils and "bushing" as well as the spark plugs (gapped at the crazy .08 the FSM specified) and wires have all been replaced. I recently (about a week ago) re-set my ignition timing (16° advance at 1100 RPM with the vacuum advance disconnected) and set the idle in drive to 600 RPM. Both idle mixtures screws are 1.75 turns out, as well.

I'm not sure what else it could be. I would imagine a low compression cylinder or valvetrain issues would result in all around poor drivability, but these issues are really only present during idle.

 

With that mileage it's probably due for timing set but, before you dig into that, set the plug gap at .060 and see if things improve.

Olds issued a tech bulletin addressing this in 1976. That .080 gap with HEI and hot plugs was set up to fire lean mixtures but, combined with trying to fire EGR mixtures, it quickly became obvious .080 was taxing the HEI hard. The bulletin recommended resetting the plug gap to .060 the next time the car was in the dealership for service. There was a revised tune-up spec decal that was supposed to be installed when the plug gap was reset.

You might look at the EGR valve too. Possibly needs cleaning.

 

This is the part I would be most worried about.

The symptoms really do not sound like a timing chain / gear.

 

You can use a mechanic's stethoscope or length of vacuum hose to isolate the tapping sound. That way you could at least know which cylinder it's coming from.

 

Thanks for the heads up on the plug gap. I'll try that.

The previous owner "deleted" (unplugged and capped off the the vacuum connection) the EGR valve and from what I saw when I changed the gasket, it was seized shut. I wasn't getting any pinging / premature detonation with it bypassed so I just left it.

ON a related note, they "simplified" the vacuum hose routings by removing the check valve and bypassed the thermal valves on the intake and just connecting the vac advance straight to the vacuum T.

If you're thinking Rod knock, I don't think that's what it is. It sounds far quieter and too "hollow" to be rod knock, I believe. It is somewhere up front. I wonder if the chain has slack and is just slapping the cover. It also disappears (or at least becomes quiet enough that I can't hear it) once it's warmed up and the choke is off.

 

Due to age and mileage on the nylon cam gear teeth replacing the timing chain set would be good long term maintenance but I doubt that it is the source of the problem. If the timing is varying that much you should be able to see it while watching the timing marks with a timing light.

Could also put a timing light on each plug wire and see if the light doesn't flash in sync with the misfire/roughness. If it doesn't flash steadily there is an ignition problem on that lead.

The noise could be an exhaust leak that closes with heat/expansion..Does it stop suddenly or gradually as it warms up?
​​​​

 

I don't think that is possible. Slack in the chain makes it a bit "longer" but wouldn't allow it to bend fore and aft.

 

It could be slapping the side of the timing cover, but that sound could also be a collapsed engine mount letting the fan hit the radiator shroud. Or maybe a bad fan clutch doing the same. Best to isolate the noise precisely before using Joe P's parts cannon on it. Mechanic's stethoscopes are between $15~30 on Amazon. Maybe add 10~15% for getting one at a local parts store.

 

The other thing you can do that is somewhat non-invasive is to remove the fuel pump and use an inspection camera (borescope) to look at the chain and cam gear.

 

Here's another quick way to diagnose a timing chain.
1. Turn the engine over by hand with a breaker bar and a socket on the harmonic balancer bolt.
2. Turn the engine over by hand until the timing marks align at zero.
3. Remove the distributor cap.
4. Turn the engine backwards slowly until you see rotor movement, stop.
5. Note the number of degrees on the timing scale.
6. Four degrees or less is OK.
7. At six degrees it will still run, but poorly.
8. Any more than eight degrees, and it probably won't run.

 

SirV, did you ever get this resolved?

 

I completely forgot to update this. I was in the process of doing so before getting pulled away from it.

I decided to crack it open as the front shaft seal was leaking oil and needed replaced anyways. Not only was there a considerable amount of slack in the chain (had about three inches between the farthest point I could push it in and push it out), but the key for the crank gear was also sticking about half an inch out from the crank and couldn't have been more than an eighth of an inch into the gear.

​​​​The original nylon gear was still in there as well.



I'm honestly surprised this lasted 110k miles without so much as a crack in the teeth. The casting looks to be in worse shape than the nylon.

I'm also doing a few MAW jobs while I'm this far into it. New water pump (someone replaced the original HD style pump with a cheap standard style from Duralast), cleaned the timing cover and painted it and the pump in the correct blue, cleaned and painted the pulleys and fan black. Ordered a new harmonic balancer as well, since the old one started separating while pulling it.

If I remember to I'll post a photo once it's all back together.

 

It took a lot longer than I was anticipating (mainly because I painted and ceramic coated everything I had off) but it was well worth it. Initial timing was way off on first start at 24 degrees advanced (compared to the 12 degrees I read before tearing into it). Set it back to 16 at 1100 and went through the motions of adjusting the Q-Jet and got it to a point where it idles smoothly with less than 30 RPM variation (890-920 in park when warm, 690-710 in drive). A little higher than what the book calls for but definitely smoother and more responsive off idle. Vacuum is also more consistent (and higher) at 22 during idle. No more needle jumping.

All in all well worth the effort, even if it did take me two months.