Decel / engine braking.

[78 GMC Jimmy]

Hey, so I have a 400 CID motor with a manul trans.

I noticed that if I get up to about 4K on the tach, and then rapidly let off the gas, it makes a lot of noise on decel. Almost like a Jake Brake. Its much more pronounced when the engine is cold, which I assume is because the automatic choke is still partially engaged. Once its warm, the decel sound is considerably quieter.

So is decel from a high RPM with the choke on bad for the motor? Or just a waste of fuel? Or is there no side effects other than the increased noise level?

 

[Blue85]

Does it have the A.I.R. pump? If so, I would expect that to making the burbling sound. Maybe you could try unplugging the diverter valve or something to verify.

If not, could it be a bad U-joint or noise from the rear end?

 

[Wes Harden]

Air injection check valve if bad will make noise on decel. Quick check pinch the hose closed and try to recreate noise

 

[6872xtc]

I wouldn't rev an engine that far without having it warmed up enough to have the choke completely opened. Could be just my old ways and preference.

It could be that it's the unburnt fuel getting ignited in the exhaust which you are hearing. Of its more of a popping, then I definitely would say unburnt fuel.

 

[Wes Harden]

yeah revving on cold oil is wear happening at an accelerated rate