'86 coupe, tapping under car
 

I have an '86 automatic coupe. There is a light tapping sound coming from underneath the car... seems to be from about where the catalytic converter is. It is definitely RPM related - not vehicle speed related. At 1500 RPM, it's most noticeable regardless of what gear the tranny is in. (It sound like hold a card in the spokes of a spinning bicycle wheel). You can hear it from inside the car, but not outside. It seems to be worse when the car is cold. There is no vibration associated with this sound.
My first reaction was a cat going bad. The previous owner said that the cat is hollowed out. Although I'll be going under the car this weekend to investigate, does anybody on the list have any ideas of what it might be?
Thanks,
Ed

RE: '86 coupe, tapping under car
 

Here's a wild guess: could it be a very small exhaust leak? The card in the bicycle spoke example you gave made that jump right to my mind.....

RE: '86 coupe, tapping under car
 

I agree, sounds like exhaust leak @ the manifold to cylinder head. That is exactly the characteristics it exhibits because cold engine manifold is shrunk, heated it expands. [8D]

RE: '86 coupe, tapping under car
 

Well, I got under the car and listened for the source of the tapping. Near as I can tell, it's coming from the fuel lines that run under the rocker on the right side of the car. I touched the lines, and one of them was throbbing in association with the tapping sound.
Any idea why a fuel line would do this? It is very noticeable when inside the car with the doors and windows closed. I realize fuel pumps make a little sound, but this is loud enough to be heard over the grumble of the engine and hollowed out cat/loud exhaust.
Thanks,
Ed

RE: '86 coupe, tapping under car
 

Are the fuel lines secure? All the clamps present? If they are, you need to install fuel pressure gauge and see if pressure is pulsating. Should be consistant pressure. If not, probably time to replace fuel pump. [8D]

RE: '86 coupe, tapping under car
 

The A.I.R tube from the engine to the Catalytic converter might be vibrating.

RE: '86 coupe, tapping under car
 

Thanks for the headsup - what brand of clutch & pressure plate would you recommend and did you go back with a new flywheel?
Thanks

RE: '86 coupe, tapping under car
 

Since it now sounds like it's coming from the fuel lines, I'll borrow a fuel pressure tester this weekend and check out the fuel pressure.
In the meantime, I did some internet research on TSBs for the 'vette.
Does anybody know anything about TSB #86234, issued April 87? It apparently addresses a Fuel Return Pipe Rattle/Knock - almost sounds like what ails my car...

Ed

RE: '86 coupe, tapping under car
 

I looked through TSB's on my Direct Hit website and nothing yet. I will try and look through Alldata @ my shop when I have time and see if any are listed in there. :)

RE: '86 coupe, tapping under car
 

Found this:

RE: '86 coupe, tapping under car
 

Thanks! I'm still going to check the fuel pressure this weekend. If that checks out OK, then I'll see about getting the part described in the TSB. Hopefully, then, that'll take care of it.
Take care,
Ed

RE: '86 coupe, tapping under car
 

Well, it look like Chevrolet discontinued the "filter-accumulator" kit for the 'vette. This according to two dealerships I called up.
Back to the drawing board...

Ed

RE: '86 coupe, tapping under car
 

Sorry, but I could have probably told you that. If it is over 10 years old, GM discontinues a lot. They don't want you to fix it, they want you to buy a new one.[:@]

RE: '86 coupe, tapping under car
 

If you call Summit or JEGS, they may have something that might work. I would suggest a new A/C Delco fuel pump. I have one in my 86 and have no pulsation probs with it. [8D]

RE: '86 coupe, tapping under car
 

mech were did you find that. what site..... sounds like a informive site:)

RE: '86 coupe, tapping under car
 

The Direct Hit site is a pay service. Monthly subscription and the rest came from my Alldata at my shop.

RE: '86 coupe, tapping under car
 

I had a similar problem...but...it turned out to be a 10 year old raccoon. Apparently chevy thought that was a nice option. I now have to go find a newer one to replace the old onehttps://www.corvetteforums.com/micons/m16.gif

RE: '86 coupe, tapping under car
 

:D HA! Did you contact the ASPCA?:eek:

RE: '86 coupe, tapping under car
 

So I finally hook up a fuel pressure gauge to the car.The carhas an adjustable fuel pressure regulator, and the fuel pressure reads 47 psi. The needle bounces back and forth rapidly between 46-48psi. This *might* be the pressure gauge. I think I'm going to reduce the fuel pressure at the regulator and see what that does for me, although I doubt that that is the cause. I'm leaning towards a new fuel pump and "pulsator"...
Ed

RE: '86 coupe, tapping under car
 

Stock fuel pressure spec is 32 - 35 lbs @ ilde. If you have stock fuel injectors, pressure is to high. Maybe why you have pulsations.

RE: '86 coupe, tapping under car
 

Here's an update on the "tapping" problem. I replaced the fuel pump, pulsator, and strainer. Still have the tapping noise.
Two questions: I bought a new fuel pump relay. The Chilton manual doesn't tell me where the relay is and I really don't want to dig around looking for it. So: can anyone tell me where the fuel pump relay is located on the '86?
Second question: I am going to set up a pulse dampner for the fuel return line. Does anyone out there have a car that had had TSB #86234 done to their car?I'm looking for more info/photos about how Chevrolet fixed the problem. Also, does anyone know of a generic pulse dampner that might fit this type application. Jeg's and Summit don't seem to have anything like this.
Thanks,
Ed

RE: '86 coupe, tapping under car
 

Fuel pump relay is one of the relays next to the master cylinder. Still haven't cured your tapping noise? Does it do it at idle? Can you put it on a lift and listen while it is running?

RE: '86 coupe, tapping under car
 

Did you govern your fuel pressure down a bit?

RE: '86 coupe, tapping under car
 

Found the relay - thanks.
I did reduce the fuel pressure prior to installing the new pump and pulsator. It didn't make a difference.
I drove the car quite a bit for the last two days. When cold, the tapping is still there. It goes away when it warms up.
I think I'm going to try to do my own TSB on the car. Just need to find the right filter/accumulator.
I'll keep y'all updated...
Ed

RE: '86 coupe, tapping under car
 

I'm far from being an expert but it still sounds like a bad exhaust gasket (if they even exist today) they were nick named "donuts" they would make a ticking sound until the car or manifold warmed up. Think about it all the fuel line stuff you checked, shouldn'tbe effectedby warming up.

RE: '86 coupe, tapping under car
 

Read farther back up this thread, we already discussed that. But thanks for the input. :D

RE: '86 coupe, tapping under car
 

I have an 86, and it does the same thing. Someone told me once that it's the pulsator in the tank, but It wasn't worth the effort to pull the pump and see.
another person told me to put a fuel filter in the return line at the tank, and it would go away. haven't done that.
It only happens in winter until the car is warmed up to approx. closed loop about 160 degrees. it's annoying, but it goes away!