I need your help identifying a noise please. I have a 1986 Corvette with an automatic transmission that's acting different now that it's gotten colder out. I started it up the first Saturday in November and I heard a strange oscillating thump sound coming from either the middle of the car or towards the back. The sound continued while letting it warm up in my driveway for 3-4 minutes. I put the car in reverse and backed out of the driveway and the noise seemed to go away after shifting out of park. I stop once I'm in the street and I shift the car into drive and the noise is not there. Twice the car has shifted hard from reverse to drive once I'm in the street, which causes the engine RPMs to drop to about 200 and it almost causes the engine to die, but the RPM's bounce back quickly. The car seems to drive fine with no noise once I get it out of the driveway. This has happened three different times now. This occurred two Saturdays in a row when I first started the car in the morning around 9:00 AM. I started it again on the third Saturday and it was fine. I started it again this past weekend and the noise was there again, but not as loud. I was thinking it could be the converter, but I would think the sound would appear to come closer from the engine and the noise seems to occur from the seats back. I have a 1998 Dodge Ram 1500 that made a similar noise when the catalytic converter went out, but that noise didn't go away. I would really appreciate it if you could please listen to my video below and let me know if you have any ideas what it could be. The noise is a little more subtle in the attached video and was louder the first two times it occurred. It may sound more like a rattle to you. I'm afraid somethings getting ready to break.

 

So, remember... when things warm up, metal expands, and everything changes. It's definitely exhaust related.

It COULD be a small hole in the exhaust somewhere, but more than likely, the exhaust is "just" starting to tap onto the body or frame somewhere. Do you feel a vibration when you hear the noise?

As for it shifting hard from reverse into drive... are you doing this when the car is cold? Most GM cars from the 80s will idle high as they are warming up. I think with an 86, you have a TPI... they didn't use ThermAC anymore, but the engine will still run a higher RPM while it's warming up. If you put it into drive from reverse when cold, it'll do that. My recommendation is to go from Reverse, put it into neutral, and just blip the throttle a little bit. This will usually cause the engine to drop the RPMs slightly because now it's no longer in "open loop" mode. E.g., there's a quick rush of hot exhaust gasses that heat up the oxygen sensor enough that it finally starts to do something, and the engine revers to closed loop mode... and drops the idle.

GM cars weren't designed with heated O2 sensors back in those days, but there are several kits to upgrade them. This is the kit that I used: Heated O2 Sensor Retro Kit Early 1 Wire GM

... it connects to the existing wiring harness (no modification needed), and then has one wire that needs to be grounded (to literally anything nearby that's steel), and a second wire that goes to "switched power," which can be your ignition coil in your HEI distributor.


I found this thread, kind of worth reading, but most of the responses are incorrect: Heated O2 Sensor upgrade on a 86 - CorvetteForum - Chevrolet Corvette Forum Discussion
In the end, this guy was talked out of it because the old people told him he was being dumb. But... he wasn't. A heated O2 sensor is a simple upgrade that can be done to any pre-heated O2 GM vehicle. Having a heated O2 sensor brings the sensor up to temperature more quickly, and allows it to exit open loop mode more quickly, and begin actually feeding information back to the ECM. GM eventually put heated O2 sensors in all of their vehicles (I think maybe 93 or 94), and it's standard now in literally every car being sold.

Here's another link from here where they mention the kit I link above: Converting to 3 wire O2 sensor - CorvetteForum - Chevrolet Corvette Forum Discussion

In addition to more quickly lowering the rpms at idle on cold starts, it also ensures you use less fuel and improves emissions during that time as well (which is usually the first 30-60 seconds of warmup). It does nothing differently once the car is warmed up. The hard shift from reverse to drive isn't going to destroy your transmission, but it's not great for it.


Back on the noise though... honestly, the only thing you can really do is put the car on jack stands, or ramps (safely), start it, and try to locate the sound. My guess is it's something simple though...

 

Thank you for your comments 82-T/A. I'll look closer at the exhaust and the other items you bring up in your comment. There was a slight vibration sound coming from the floor. I wasn't sure if the RPM drop once I shifted into drive was related or not, but I went ahead and added it to make sure everyone had all of the facts. I hope it's something minor like you're saying.

 

I agree on the exhaust as the source. Since you were sitting still that is the only thing moving (vibrating) towards the rear of the car. Heat shield or broken/missing exhaust hanger bracket

 

Thank you dburgjohn. What confuses me is I warm up the car for a couple of minutes and the noise disappears as soon as I start driving it. I'm going to have to take a look underneath the car when I get some time.

 

When metal gets hot, it expands, so it could be that it's "taking up slack" once it warms up...

 

I KNOW YOUR PROBLEM AND I HAVE THE FIX!!! I chased this "seasonal" issue on my 1986 for 5 years until I came across a thread here that addressed that issue. What you are experiencing is hydraulic knock caused by fuel injector operation when the engine is cold. Once the engine warms, the noise goes away. And yes, the hammering is LOUD.

In 1985-86 cars, there was no accumulator placed in the fuel return line to damper the hydraulic hammering that some cars exhibited in cold weather. GM came up with a fix that was outlined in TSB 86-234:

Service Bulletin Number: 86-234
Bulletin Sequence Number: 043
Date of Bulletin: 8704
NHTSA Item Number: 90911
Make: CHEVROLET
Model: CORVETTE
Year: 1985
Component: FUEL:FUEL INJECTION UNKNOWN TYPE:ACCUMULATOR
Summary: FUEL RETURN PIPE RATTLE/KNOCK-HAMMERING/KNOCKING TYPE NOISE-CAUSED BY PRESSURE PULSATINGS IN FUEL SYSTEM GENERATED BY FUEL INJECTOR OPERATION-REPAIRED BY INSTALLATION OF A FILTER-ACCUMULATOR ASSEMBLY IN FUEL RETURN SYSTEM-SERVICE KITS CONTAINING PARTS AVAILABLE FROM GMSPO-SERVICE KIT P/N-COUPE 10068791-CONVERTIBLE 1006 8797-ALL 1985-86 CHEVROLET CORVETTE MODELS

I bought one of these kits, installed it on my 86 and it FIXED the hammering. My car is as quiet in cold weather as it is in warm weather. The accumulator in the aforementioned kit mounts on the frame just above your fuel filter. Below is what the kit looks like installed after I put it on my 86:



There is one of these kits on ebay now. It's a little pricey, but it will DEFINITELY fix your issue. Guaranteed.

Link--> NOS OEM GM UNIT FUEL KIT 10068791 RARE VITAGE GENUINE CHEVROLET CORVETTE

 

Thank you Red86Z51 ! I'm so thankful that you ran across my post. I don't think I would have figured this one out on my own and not many mechanics in my area are familiar with the C4 anymore. Yes, my sound is like a hammering and the loudness of the sound does seem to corelate with how cold it is. The colder it is the louder the noise is. I also really appreciated the picture. I just purchased the kit off Ebay. I hope you have a wonderful Christmas and thank you for my Christmas gift of knowledge!

 

You're welcome, I'm glad I could help.

I would like to add a tip that will make the job much easier. It's tight trying to drill the holes for the filter (accumulator) bracket, so if you don't have a pneumatic right angle drill, you will want to pick up one of these for your drill (if you don't already have one):

Link ---> WARRIOR 3/8 in. 90° Angle Drill Attachment

Again, glad I could help out a fellow '86 owner. Please post back again once you've got it installed and let us know just how much more you LOVE your '86!

Merry Christmas you you and your family!

 

Did you get a chance to install the kit yet?

 

Unfortunately, no. I'm having problems with regulators that keep burning out on my alternator. I've been working on resolving this problem first. I was too busy over the holidays to get to it, and I came down with some nasty cough virus. I'm hoping to get my alternator issue resolved then maybe work on the kit. Here's the link to my alternator post if you're interested:

1986 C4 Corvette - Alternator Problems: - CorvetteForum - Chevrolet Corvette Forum Discussion

 

Geeze, talk about a full plate, I get why you haven't gotten to it yet. Well, now I have 2 things I look forward to seeing the results on.

Staying tuned, and hope each one goes smoothly for you!