Friend's car. I have not seen or heard it yet since issue occurred. 2015 Stingray automatic with 104K miles. Has a whirring noise in auto and manual and a thump in auto only. Vette tech at local dealer says it has a sticking valve issue between 4 and 8 cyl modes. Diagnostics points to VLOM. No CEL popped up. Another tech says one or more lifters may be stuck or sticking. Car seems to run fine other than the thump in auto. If it had a broken valve spring would it be evident noise wise? It seems to be ok in manual mode. At idle in park and neutral all is quiet. Repair estimates are between $2K and $5K. Any input on what I should look for when I go look at it for myself will be greatly appreciated. Owner is not a mechanic. I'm not a pro but have been twisting wrenches for decades on my own cars. Thanks.

 

IF my V4 valves and junk went into crap mode, I think I'd just junk the entire system and set the car up old school and have an 8 banger all the time. WHat the heck do you gain? 1 or 2 miles per gallon "maybe"?

A broken valve spring would express itself quite well with noise to eliminate any confusion.

Elmer

 

From this statement, I would be looking at the transmission pump and torque converter. Nothing in the AFM system would make a whirring noise.

 

That was my first thought. The torque convertor was replaced at some point in this car's life. I hope to get with the owner this weekend.

 

I had a chance to listen to the car today. Under hood was quiet except for a high pitched whistle from the front of the engine. Goes away once warm. The part that has me baffled is this:

In D when it goes into 4 cylinder mode there is a pop and then a thop, thop, thop continuously until it returns to 8 cylinder mode. I can't pinpoint the noise location but sounds like it's in the middle of the car. It runs like crap in 4cyl and has no power. 8 cylinder runs smoothly.

I have a 2017 Z06 with auto and you cannot tell when it switches from 8 to 4. This car you can tell without even looking. You can feel it as well as hear it.

The aforementioned whirring is the fuel pump. It sounds like an impeller or bearing going bad. It also stays on when the ignition is in run but the engine is not running.

The car otherwise runs great if it's in 8 cylinder only. Driving it in M using paddles the car pulls like it should and smooths out nicely at highway speed. In 4 cylinder mode it's crap. The transmission shifts fine in 4 or 8. The thop only occurs in 4 and sounds like it's driving with a flat tire.

Diagnosis from the dealer points to VLOM and the estimate to repair is 3 to 5K. Parts not available until June.

If anyone has an idea what to look for I'd appreciate it. Thanks.