ClarenceT

Last week vette starts to idle really rough, and idle drops off to 700, and i start hearing clanking. I'm not sure if the clanking is ping, or that weird gear rattle everyone is always talking about, but it sounds pretty bad.

I pulled all my plugs, and put new ones in because it was time anyway. Also found that my distributor cap had busted points, so I replaced that with a new rotor as well. This made the car run better above idle, but still idles low and clanks/rattles.

1. What is that rattle? It's driving me nuts, and it sounds bad for the motor. Shouldn't the ECU pull the timing or something?

2. What's causing this? It stops clanking and runs normally with a little gas, generally 900+ RPM = fine. No knock/etc @ WOT either. When I fire it up cold, no clanging, no rattles.

I was thinking O2 sensor? After it warms up = rattles/crappy idle = closed loop = bad o2? Does that make sense?

Thanks for the help guys, I've been using the vette as my daily so I'd like to get this resolved as soon as I can

C4Ownr24

Do you have a cam or something in your vette? even 700RPM seems like a high idle to me. The FSM calls for 450RPM in D........Later changed to 550RPM.

ClarenceT

Not as far as I know anyway. I put this motor in from another vette that was wrecked with about 60k miles. I have a manual, so if that changes anything.. I said "high" because normally it idles around 800, and everything used to be fine. I never reset the idle. Checked the TPS once, it was well in range, so I left it alone.

MilehighZ51

I had a similar rattle at idle on my '86. Turned out to be a bad pre-catalytic converter on the left exhaust pipe, just after the manifold.

ClarenceT

Huh! I was actually just thinking about that.

Do you mean it was bad as in, it clogged and caused the rough idle - or - it physically rattled at normal idle?

MilehighZ51

Specifically, the element inside the converter had broken loose from the housing and was banging around inside. It was audible mostly at idle.

I can't say it affected the idle quality much, however after replacing the pipes the engine does start easier. FYI, I replaced the front pipe assembly with Eckler's p/n 34198, which is a 49 state legal front Y-pipe/converter setup.

ClarenceT

Hmmm... Man too bad you didn't get a nice WAV file of that for me to listen to haha.. You know now that I think about it, I guess it might be. Although mine is a deep clunk, like it's something hitting the block from the inside. It's scary. Goes away the second I have it in gear and take off.

SupaVette

Is it a manual? maybe the clutch fork is not aligned properly? does the noise go away if you put the clutch in?

MilehighZ51

Yes, mine sounded very similar to a rod knock- I was worried at first. But there were inconsistencies to a rod knock. For example, the rattle was in phase with the engine speed up until 1000rpm or so. After that, it became inconsistent, and often disappeared. In theory, it is possible that the element inside the converter had a large mass, hence it's resonate frequency was low, probably close to the frequency of the exhaust pulsing at idle. When the engine increased beyond this, the resonance stopped because the excitation frequency exceeded the resonate frequency of the element. (Perhaps there were some elements of turbulence involved as well, but that's the theory, anyway.)

Good luck,
Dave

ClarenceT

Oh ok, late update. Turned out it was the pre-cats got rid of 'em, problem solved. While I was there, I got a new cat too.

Now I have an 'occasional' "clack" Happens when the engine temp is hot (220f), and I suspect it's the DMF.....