I've got a 1987 C4, automatic. I lost power (very noticeable hesitation) near the house the other day and after pulling it into the garage I noticed a little banging noise that could be heard while listening from the side of the car or under it. Please note, very little can be heard when listening over the top of the motor.

So I started googling and checking the car. The harmonic balancer was starting to slip backwards, so I figured I might as well go ahead and replace it before it does any damage. The timing mark was a full 60 degrees off its "home" position, and since I did my timing 6 months ago I figured that would need to be checked. Got the new balancer on and found the timing to be spot on. However, I still have the knocking sound and loss of power. So I start listening with a stethoscope. I checked:

• Each exhaust outlet at the manifold
• Intake side of the head near each cylinder
• Valve covers
• Oil pan on both sides
• Pre-cats (before and after)
• Main catalytic converter
• Bellhousing

I couldn't find the source. For fun and because I like wasting my time, I dropped the oil pan (much easier than what the FSM says) and checked for play by hand, kind of hoping something would be obvious so I'd have an idea of what it was. Nothing. Rods have a little play forward and backward (expected) but I couldn't find anything moving that shouldn't. No metal in the pan either...

This should have been done before the pan drop - but I followed that with a temp check on either end of the pre-cats and main cat. It was difficult to get some consistent readings, but it appears to be that the main cat was almost 200 degrees on the front, and 110 on the rear. That seemed off a bit so I hooked a vacuum gauge up to the upper plenum by the FPR and checked for vacuum. It sits near 20 inches when warmed up, goes up a bit as I increase the RPMs to 2k, and then spikes and settles back down when I let off quickly. It's not clear to me if that completely rules out the main cat or not.

I'm out of ideas. Worse, I sold my other car to make this my DD - and I move in under two weeks.

If a video would help, I'll put one together.

Thoughts?

 

With the car being a Automatic before the pan came down yes chase the things you did but first the Torque Converter should have been unbolted to take Trans Noises out, And a Brief listen with Drive belts off also. Once you had taken the time to pull the pan and you gave a shake and look you should have at least done a basic Clearance check with some plastigauge or if that far in do a bottom end rebuild if needed.

http://www.plastigaugeusa.com/
It's available at most parts store on request if not always online, ever time I used it we used the Green Packs

 

I went back under the car and tried to find the noise again. I heard something on the front of the main catalytic converter, nothing on the rear of it, and nothing at the rear of the pre-cats. So I dropped the exhaust (never realized 7 bolts would do it) and found this:



Knowing I live in an apartment complex with two apartments on the other side of the wall and ceiling to my garage, I went ahead and started the car up without the cat/mufflers on. She sounded much better and the knocking noise is gone. Tomorrow I'm going to pull the front pipe and knock the pre-cats out. After that I can take it for a spin and ensure everything is fine. If it is, I'll have to look at replacements, if not, I'll need to leave her be for another week or two while I move. I'd really like to drive it around though. :-)

 

looks like your cars,or any cars, way of having kidney stones... never saw that b4.

 

I have well done, I am surprised that you didn't fall rite on top of that in your first look's that had to make a Racket.

Now on a more Serious Note, Be Careful with that stuff it's full of Heavy Metals. So make sure NOT to Breath it or Suck up dust with Shop Vac. because that would really throw the dust around.

 

Yeah, I didn't hear anything my first go around and only barely heard it on the second attempt.

I dropped the front and knocked out the pre-cats. The passenger side is the one that broke apart, leaving only half of the material left in the pre-cat. I buttoned it all back up and started her up. It *seems* like it's running good, though there's a noticeable pop when letting off the throttle (in park at least). When it dries up outside I'm going to take it for a quick spin to see if there's a loss in performance still. Hopefully this has solved the problem and it'll be good to go.

Assuming that goes well, I should have a new front pipe and catalytic converter on Wednesday. If things don't go well, it'll have to wait until after the move...