Well, road test was not helpful........ still a soft 'thump' or 'whump'
only heard only at idle, and way after closed loop has occurred. Strange that it is not heard at first start-up when the electric pump should be active.

Following the narrative (below) by SunCr, I checked for leaks at the valve connections. The only scenario would apply if air was being diverted overboard, but I don't know how that happens, or where it happens.

Anyway, thank you all for helping...... fortunately, the 'noise' is faint, and no codes are evident. Vette runs perfectly.

From a prior post by SunCr:
"As Technology developed, the O2 became heated (CATs more efficient) and the need for a belt driven Pump became less to non-existent. Those that continue to use them, only did so at startup, they became electric, still through the Headers, but more so just to get the CAT up to temp and to help pre-burn all the startup HC's. After a timed run, it's shutdown until the next startup.

Thump noises at the Checkvalves could be a broken valve or Diverter Switch (if used) or maybe an Electric Pump that's still cranking long after it should be off. There will definitely be a thump or pulse if the hoses aren't connected or if they're leaking, or if it's diverting air overboard (generally only heard at idle). Scanning to check fuel delivery will tell you if it's Lean; even if there's no Code."

 

not familiar with the electric drive pump so bear with my question.

can you see or hear when the pump is running, or do you have to put a test light or ohm meter on it to see if it has voltage gong to it?

 

Gary, my pump makes no noise. I have not tested voltage, only placed my hand on the pump to sense any vibration. Of course,
my engine was running at the time, so any vibration I felt may have been transmitted from the engine. So, to answer your question, no, I can't see or hear if the pump is running.

This was copied from a link in this thread:
On the 92 LT1, the AIR pump operates after start-up any time coolant is above 59 degrees F. It runs for a maximium of 150 seconds or until the system enters closed loop. It also runs for 25 seconds if coolant temp is above 149 degrees F at startup or until Closed Loop starts. The ECM will shut the pump off once the system enters Closed Loop.

If true, my pump is not operating full time, and the 'noise' I hear does not fit within the parameters of pump operation. The 'noise' is more discernible in the wheelhouse than listening outside with the hood raised.

Fortunately, my wife's hearing is more acute than mine, so I have a second opinion source.

The 'noise' has to be from an electric motor as it is not rpm related, and it is not coming from the ac fan.

By-the-way....... did you solve your 'noise' problem with the new bracket?

 

yes that bracket fixed mine

 

I'm going to run it without the belt for a minute or two and see if the noise goes away. Will report back.

 

Well, it would seem obvious that if you eliminated it all together you could rule it out. Easier on the Electrics because you're not taking out the WP when you unplug it. Assuming it's still there with it disconnected, I'd be suspicious of an a/c system that's slugging liquid to the compressor. Of course that usually wipes out the compressor.

 

Don i noticed the same sound from my 93. Its more like the chuffing sound that someone else described. About a hundred years ago an old mechanic showed me how to listen with a welding rod held up to my ear and it worked. Cause now i have a fancy mechanics stephascope (spelling?) I had a 56 chevy that made so much racket id only get worried when it wasnt noisey! Dave

 

OK, Dave, did you just accept the noise, or did you find a remedy?
Replacing the valve seems to have lessened the noise, but I don't have a scientific comparison between the old and new valve.

Thanks for suggesting a 'stethoscope' method for diagnostics, but the damn noise won't cooperate when I'm ready to test.

 

IF? the noise is the same as what i hear and i think it is im just living with it. I just got car back last night from shop. The mechanic who i trust looked everthing over and under the car. I had rear shocks and brakes put in. Also needed the right caliper replaced. Im doing front shocks this week end. I gave him a list of specific things to check closely while he had car. I have done this in the past with him,check u joints,jack shafts,exhaust etc. I also had him listen for ANY engine noise. The only thing he brought to my attention was the injectors clicking. He said at first he thought they were valves tapping until he popped the hood. He never said a word about that chuffing sound. When i got in car to leave he walked out so i opened the hood and yeah you guessed it not a single chuff or huff to be heard. The new Bocsh injectors are doing what they are suppose to do and the car runs great! Dave

 

"I don't believe my vette could run any better."

Thanks for the responses, Dave.

 

I had a 96 olds with the same system of smog pump, and there is a service bulletin to check out the electric pump because they are mounted so low that they take on water during a rainstorm and hold the water in the unit. They rust out some internal metal tabs that hold the motor in the can, and then the squirrel cage rubs on something. In my case, the squirrel cage had a balance weight spot welded to a part of the hub on it and the weight was rubbing on the can enclosure causing a huffing or swooshing sound. I got another one from the corvette wrecking yard, and it was identical. Replaced it, and the shuffling sound went away. Doesn't fix your problem with the noise happening when the pump is supposed to be shut off, but it is something to look for. I drilled a couple small holes in the new unit for water draining.

 

Interesting info, coupeguy....Thank you.

Unfortunately, my 'noise' is intermittent, and only at idle. Coming of the freeway and stopping at a light is when I may hear it.

Being on the edge of deafness makes me a poor candidate to diagnose any hearing related problem.

 

The belt was old so I changed it out. I started it without the belt and the sound was gone. Spun all the pulleys, smooth, no sound, no wobble, slight play in the tensioner pulley. Put the belt back on and the 'slight knock' returned. Used a stethoscope on all ancillaries and nothing stood out as the culprit but I think it must either be the tensioner or the AIR pump. Before I buy a new tensioner, I think I'll gut the AIR pump.

 

Thanks for the feedback..... hope gutting the pump works for you.

 

Ok, on the 86,
Having an 86 coupe for a number of years, I have worn out a lot of stuff. When the tensioner went south, I had the opportunity to take it apart. it has a plastic bump stop/spacer inside where the spring is and that plastic breaks I guess when the belt stretches. When the plastic breaks, it gets in the way of the tensioner working correctly, and causes it to bind. Mine caused a light knock all the time.
Later, the front bearing seized in the smog pump, and the belt just fell off when the shaft on the smog pump broke.......no power steering, no air conditioning, no water pump, no alternator. Lights going on all over the place. I was doing 70, and was able to coast off the freeway onto an off ramp, and onto the lawn of a mortuary that was just starting to receive people for a funeral.
since then I have checked into the rebuilt pumps, and for $145, it seems like a waste. I have procured two used ones, and now have a spare. I am contemplating going with the smog pump eliminator. I made a clone off of a friend's before he installed it.

On the 96, I did not know what that thing was until I traced the hoses on it. It was all rusty, and still had water in it. The guy at the corvette wrecking yard said they sold a lot of them out of the 93-96 cars they dismantled. They had a truck division, and all of the trucks with the electric smog pumps had the hoses and connector dangling on the right side.

In your case, I would listen for a noise coming from the idler / tensioner pulley bearing. My 86 bearing went bad, and the one on my 2001 coupe went bad, and they are replaceable and cost around $18. I replaced the one on my Tahoe, and pressed all of them out using a cut off piece of galvanized pipe and a socket in a vise. pushing the bearing back in, make sure you have the outer bearing race as the part the vice squeezes or the bearing will be destroyed.

 

That must have been exciting for both you and the funeral guests.
Good job of navigation, coupeguy.

 

[QUOTE=coupeguy2001;1584696672]Ok, on the 86,
Having an 86 coupe for a number of years, I have worn out a lot of stuff. When the tensioner went south, I had the opportunity to take it apart. it has a plastic bump stop/spacer inside where the spring is and that plastic breaks I guess when the belt stretches. When the plastic breaks, it gets in the way of the tensioner working correctly, and causes it to bind. Mine caused a light knock all the time."

Yup, that was it, thanks for the advice. Now I can hear the air pump chuffing a bit but it's nothing compared to the light knock the tensioner was making.

 

Is the 'chuffing' constant, or intermittent?

 

It's not the airpump. Still looking.

 

Poking around Service Bulletins this AM. There were 2 that may be related for LT Motors. First is a ticking noise due to loose heat shields on the headers. The second is fuel hammer which needed the fuel lines repostioned and resecured in order to correct it.