devilinblack

Well, my ordeal is at an end (at least for now). I haven't posted any real updates or a conclusion to my torque tube bearing vibration because I could not get the damn car to run well enough to even make it around the block.

I had recently checked and replaced vacuum lines and unblocked the EGR valve that some PO had blocked off. Shortly after this it developed a slight hesitation at about 2500 RPM that would go away above 3000 RPM I attributed this to a mixture problem but wasn't driving it enough to worry about it too much. About this time is when I started getting horrible noises from the clutch area that demanded my attention. With the great help from the folks on this board I isolated that to a torque tube bearing that had migrated back in the tube. Once this was repaired I put the car back together only to find that the poor performance above idle had gotten much worse.

Now, it would start and idle, but the idle was low and it could not be accelerated above 1000 RPM. All it would do is make some booming noises from the intake. I checked the fuel pump flow and pressure, air plate movement, etc. Once my CIS gauge arrived I checked the pressure to the WUR and that was fine. Just in case I replaced the green wire, and then as a last ditch effort I replaced the WUR with a rebuilt unit from 928Intl. No change.

As a last ditch effort I dropped the exhaust. I seemed to recall the engine revving freely the one time I started it before installing the exhaust after the torque tube work, but because of the noise this only lasted for a few seconds. Sure enough, it started up and even though my idle mixture was out of whack from my troubleshooting efforts, it revved all the way to red-line with no problems!

I removed the section with the cat and with the help of a 3lb hammer and a length of angle iron I cleaned out all of the material inside, reinstalled it and it started and ran great. Woohoo!

After letting it warm up and adjusting the idle I went for a short test drive and all I can say is wow! I was pleased with the performance of the car since I bought it in August even though it had a driveline vibration above 4000 RPM from the torque tube problem. Wow. So this is what these cars are supposed to drive like! Apparently this cat, while allowing me to pass smog, was not flowing even remotely freely. Now, not only can I accelerate all the way to red-line with no vibration, but the acceleration is so much better than before. It's like a totally different car.

Anyway, just wanted to thank the people on and off the board that helped me with this and to put this up here in case someone else is having a similar problem. Dropping the exhaust only takes a minute and I wish I'd have tried that in my troubleshooting long before today.

SeanR

Very nice. Amazing what a bit of breathing will do for the engine. Enjoy.

Mrmerlin

its like plugging in a V8 when you only had a 4 cylinder

Shark Attack

Now, Why did the cat clog?

Charley B

Carnac sees a tire replacement issue in the near future.

dr bob

Cats 'plug' as a result of poor combustion. The ceramic substrate is a honeycomb, with the exhaust gasses passing through the holes in said honeycomb end to end. When your engine misses or the mixture is way off, the catalyst heats up to the point where the ceramic degrades and the holes close up.

Think you might have a plugged or even a partially plugged exhaust? You can diagnose with a vacuum gauge, attached to manifold vacuum. That's the line that runs to the fuel pressure regulator/dampers on later cars, by the way. Anyway, manifold pressure is low at idle and lowest when the engine is being dragged at higher RPM with a closed throttle. Think 'engine braking'. Conversely, manifold pressure is highest when the engine is not running, or the throttle is wide open and no longer restricts airflow to the engine. Most obvious at lower RPM's with throttle wide open. With the exhaust plugged, the manifold pressure is high (little or no vacuum) under even moderate load and RPM's. Normal operation would allow you to set the throttle at some mid position, causing an increase in pressure until the car accelerates and the RPM's come up, at which time manifold pressure would drop again at a steady speed. Restricted exhaust would cause the manifold pressure to rise and stay relatively high as you find that load where the exhuast restricts as much as the throttle does.

Keep in mind that this description says "pressure rising", which is the same as "less vacuum", and "pressure dropping" which means more vacuum. 18" of vacuum might be a typical idle reading, dropping to 0" as you accelerate, going to 30" on a trailing throttle.

Anyway, a vacuum gauge is an old-school tool that really does have a few new-tech uses if you know what to look for.

jon928se

jeez Dr Bob - that was hard work, I had to read that a few times to deal with absolute manifold pressure as opposed to "vacuum" - something most of us who still have vacuum gauges are more used to.

Where you chairing a protest committee recently and had to be precise/specific about the interpretation of the rules ??

For anybody else scratching your heads.

Atmospheric pressure at sea level is 15psi or thereabouts.
The pressure gauge on your tyre or on your air compressor or on your fuel rail always reads 15psi wrong it is measuring the pressure in the tyre/tank/fuel relative to atmospheric pressure, not to zero psi - only totally achievable in space. So when your fuel rail says 42psi it actually means 57psi but there is 15psi of atmosperic pressure pushing the other way.

"Vacuum" gauges read the pressure differential between atmosphere and the intake thus when you have a big vacuum (throttle closed and engine being driven by the wheels thus engine sucking hard and no air can get in past the closed throttle) they read a higher number.

WallyP

Jon's post explains why the correct terminology is usually "psig", not "psi"...

devilinblack

Thanks for the info Dr. Bob. With the way the car runs now it's obvious that this has been a problem since I bought the car. I do have a vacuum gauge but aside from what I was going to be testing on the WUR, it didn't occour to me that this could have been diagnosed that way. Good info for future use though certainly.

I put about 150 miles on it today and it was such a pleasure to drive that the last six weeks of work and troubleshooting now seem totally worth it. It pulls smoothly all the way up to redline with no hesitation at all and the power increase is simply amazing. I guess it also helps that I can now rev over 4000 rpm without the horrible vibration from the torque tube. It really was great to be able to spend the day tooling around the back roads and enjoying the car the way it was meant to be.

dr bob

Gosh, Jon!

I'm sure that when it comes to architecture and structural stuff, you stick to terminology that's specific to the task at hand. Other words and terms aren't quite as good. In my spare time I do a lot of process and performance analysis for power plants. In that slice of the real world there's no such thing as vacuum. Everything is represented as pressure since vacuum is relative for the most part. Using pressure allows one to easily predict performance, while vacuum requires an inversion and a reference to mean anything. Using vacuum is a lot like using grams to define molecular weight, IMHO. Go with whatever works for you, though.

LT Texan

Reminds me of an old college roommate lesurely expaining fluid dynamics and how "rocks" just flow slower than, say, water.

I got Dr. Bob's post.

Oh, and Stephen, on the 3lb (kg's pls, or were you near a massive body when you measured this, such as pluto?)?) sledge and angle iron method, was there any tearing up of the cat casing involved? Did you ram the angle iron down the cat's throat with the sledge?

devilinblack

It's a 3 pound "drilling hammer" like so:


I stood the cat on end (rear end up) and used the length of angle iron to break up all of the catalyst and dump it out. There's also a couple of what appear to be wire gaskets and some padding in there that were a little more tricky to get out but the whole cat body is clean now and there was no need to damage the casing.

Here's the pile of stuff that came out:

dr bob

Problem is obvious from the picture. See that little grey piece, far left side and half-way up? Meltdown started there.


HTH!

6mil928

Bob can I just get a simple description for testing for a clogged cat?

WallyP

- Hook a vacuum gauge to a manifold vacuum port. Don't use one with a boost venturi (as the connection to the brake booster) or a check valve (as the connection to the HVAC).

- If you have the engine at idle, you should see something like 16 inches of vacuum.
- When you stab the throttle, the outside air falls in, and the vacuum drops to near zero.
- As you hold the throttle steady (without redlining the engine, please!), the vacuum should very quickly rise to perhaps 20 - 25 inches as the RPM stabilizes at perhaps 3000 RPM.

- If all is well, the vacuum will stay steady as long as you hold the throttle steady.

- If the exhaust is plugged, the vacuum will start dropping, even though you are holding the throttle steady. If the exhaust is really plugged, it is possible to hold WOT with zero vacuum without redlining the engine.

If the exhaust is only slightly restricted, a novice probably can't tell. An old, experienced guy like dr. bob or me might be able to tell.