Recently had the manifold pipe replaced on my 84 244 (I bought it from FCPGroton - had local mechanic install). Since then there is now a loud rumble and vibration when I accelerate (3000 RPM+), which goes away as soon as the transmission shifts and the RPM's drop back down. The car feels less powerful than before also. Once I've accelerated hard a few times causing the noise and vibration (with warm engine), the car seems to have more power again in normal driving.

The mechanic mentioned having a tough time removing the cat-to-manifold bolts.
Could this be a clogged cat or maybe something entirely unrelated?
Any advice humbly appreciated!

if its abroken cat where the peices are restricting the flow there are two tests...

1. slap the cat (when cold) with the palm of your hand (DONT OVERDO IT!!!) its its broken up you MAY hear it rattle...if it rattles......its toast!

2. buy a motorbike carb balancing kit (just bear with me!) this is 2 (or more) vacume/boost guages wiuth serveral feet of rubber tubing and some screw in adaptors that have m5 screw threads..also get a tap set of the same thread and several feet of plastic 'washer jet' type tubing of the righ diameter to replace the rubber tubes...also score some small (like 10mm or less m5 bolts, smae thread as the adaptors....)

Now (carefully) drill some m4.5 holes in the tubes immediately ahead and behind the cat (not into the CAT!!!!!!!!) and then tap them to accept the adaptors...now screw the adaptors in...now connect two peices of plastic tube top them and route the tubes into the back seat under the door seals (make sure the tubes are NOT crushed closed!)

now connect the ends of the tubes to the presure guages....


now what we have is a means of an assistant in the back seats seeing the presure before and after the cat...and therefore the diferential ACCROSS it...

now go for a drive with the assistant monitoring the guages (please dont pick someone whow gets car sick!) now..you probably wont see much difference untill your really flooring it (like 4.5k rpm and WOT..) and then you may weel get a 'spike reading' as the cat is the main restrictor, then an evening of differential as the mufflers start providing resistance...

with my turbo (ok it flows more air but...) i saw 4.5 PSI!!! spike stabalising to 4.5 PSi prior to the cat 3.5 psi after it...leading me to deduce the cat was toast and the mufflers! (i was right btw!)

come home remove adaptors and plugh holes with bolts (try to cut bolts off so they dont protrude too much into the exhaust...

obviously you can also do similar tests before and after each muffler.....

ok the tools cost you maybe $40 - but they are reusable (eg manafold presure readings, crank case presure readings etc) but it might save you needlessly buying a new cat.....

my guages go from -35" of water to 8psi and seem to be a good range for general use....

hope this helps some....

as always follow the advise at your own risk

If your mechanic really had to bang around the cat to get those bolts to come out he could have damaged it (or more likely, it was on its last legs and he just had the honors of breaking the camel's back).

But before you replace the cat you should make sure it's just not one of the heat shields on the front exhaust pipe that are rattling around. They often come loose and start rattling at high RPMs, or sometimes when you take your foot off of the gas from high RPMs. The solution of choice for most people is just to rip them off. They're not really necessary.

Try listening for the sound in your driveway. Pop the hood up and rev up the engine with the accelerator cable at the throttle body, and listen for the sound. Hopefully the noise will happen at 0 MPH and you will be able to hear just what is making the noise.

I rev it in the driveway and I don't hear the noise. Only when driving.
Transmission? (gulp)

My cat is broken inside.
The ceramic filter rattles around inside the cat, with the car in drive with foot on brake at a light. Sometimes when I really get on it the car losses power(won't reach hi RPM) as the broken parts/chunks restrict the flow.
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Kevin * HONDA spanking,1985 240DL * VOLVO ON!!!!