SC Odd Engine Sound Under Load
 

After a successful valve adjustment and replacing the spark plugs, wires (Beru), distributor cap / rotor, oil change my SC pulls better than it ever has and runs beautifully with a small exception:

When I am driving and floor it, in any gear, under load the car starts making a very odd thudding noise. It sounds like I left a screw driver in the engine bay (which I did not). It sounds like it eminates from the driver's side of the engine and I cannot replicate it while just sitting in my garage revving in the engine.

I did re-hook up oxygen sensor, which may be the culprit. Not sure. If the timing is off by a few degrees, retarded, does anyone know what sounds would appear? I have to readjust my timing after all this, I'm sure.

And what are the consequences if I disconnect the Oxygen sensor, if any? Other than bad gas mileage?

I feel like a knowledge virgin.

That sound could be pre-ignition or "knock" which typically shows up under load. No way to be sure from your description, but check for that anyway as it is deadly to your engine. What octane gasoline are you using and when was the last time you checked your timing? Knock can easily appear with a too advanced timing.

Most likely it has nothing to do with the O2 sensor. Pinging on an SC is a high pitched rattling sound that starts as you pass 3500 rpm under WOT. It is not a thudding sound. I would start looking at the engine and trans mounts and the exhaust. The O2 sensor has many advantages on an SC. The first thing I would do is hook up an analog dwell meter to the green white wire in the test plug and let it tell you what is going on with the CIS system.

I'd also check for broken head studs.

Did you have the engine sound-under-load before the work that you did?

I was chasing something similar on my targa. It sounded like an exhaust leak (like a marble rolling around the engine), but it would only occur under hard acceleration and with the engine fully warmed up. You couldn't hear anything amiss at idle and by throttling it up and listening with a stethoscope.

What it ended up being was the exhaust valve on No. 5 was adjusted too tight. You might need to go back and check your work.

Brian

I am running Shell now... and it didn't occur prior to my valve adjustment, or I didnt hear it. The car is significantly quieter now then ever before. I am going to check the timing this week to see what's going on.

I pray I dont have to tear it all back apart and recheck the valves. If I do, it will be my third attempt. Although, I am getting rather efficient in the process.

This is why I love these cars, never a dull moment.

I will post more as I continue to solve this problem.

Quick update.... I checked the timing. As I feared, it is spot on. It looks like I may have to dig back in and adjust the valves, again! I am thinking my inability has cost me nearly $100 in oil alone. Learning is an expensive proposition.

Next stop, more oil and valve cover gaskets. On a good note, my have solved my oil dripping problems once and for all!

Make sure the spark plugs are the correct heat range. They should be Bosch
W5DC. Any hotter plug may cause pinging or knock.

How are you checking your timing? Make sure you are pulling the vacuum and retard lines off if the dizzy. If you are only checking timing at idle, the retard line will pull your timing back by about 5 degrees. So you could be advancing it too much - 5 btdc with retard connected may actually equal 10 degrees. I'm on my phone so I can't see your sig to determine the year of your SC.

What about this? My 80 SC had a noise under load in low RPM's in gear 3/4/5. I have the engine out to fix the tranny (no bad bearings) and may have had a cheezy tensioner. Is the noise at all RPMs or only the low ones? My guess is that low RPM noise could be the chain slapping at specific harmonics.

Larry

i would unplug the O2 sensor and go back to the original setup before jumping back into the valves.