My wife's car (1995 BMW 525i) has been slowly losing power and having reduced fuel economy over the last 18 months. I rarely drive the car. 2 years ago, it got about 32mpg on highway trips. Yesterday, I drove it for about 150 miles and only got 25-26mph.

It is down on power and very occasionally smells like a morning-after Guiness-fart. I am 90% sure that the cat is shot. I priced OEM replacement cats and every source I checked was in the $485-$500 range (yikes!). There is also the obvious choice of getting a universal cat and making it fit...

How can I be absolutely sure that this cat is bad? I'd like to be completely sure before I commit the time and potential effort on this.

Thanks

When you floor it and you can't get past 3k in first. Or atlast thats how we figured it out in my friends T/A and my old Camaro when they went out. Normally they don't throw a code when they are backed up, but mine would throw a misfire everynow and then because of the backpressure im guessing. But normally when they go, they go much quicker than that so its hard to say. Whats the milage on the car, and are there any visable dents in the converter?

Unfortunately for this situation, the car has variable valve timing, so at around 3,300 the motor spools up nicely, even now.

The car has 80,000 soccer-mom miles on it. There are no visible dents on the cat.

Beings thats its been a slow process I would do a good tuneup on the car and go from there. Plugs/wires fuel filter, 02's possible, you know the regular stuff and run some top engine cleaner through it and then take it out and flow the crap outa it. Cars that get babied around all day are prone to carbon buildup, so that could also be some of the problem. Beings that its been such a slow loss of power, I really doubt its the converter. Because once stuff starts to break down inside one, it usually goes really quick.

When a cat is shot, does it restrict flow? What really happened when a cat's shot? I'm just guessing here, but if it restricts flow, see if you can feel lack of flow from the muffler when you gun the gas pedal.

seafoam it?

Yeah, I thought of the plugs and wires. I was planning on starting there when my wife mentioned the car sometimes smelled like a Guiness-fart. (She would know, being my wife. ) Last week I smelled it myself.

Up until now, we've always let the BMW dealer do the scheduled maintenance on the car (my wife didn't want me touching her car - smart girl)... I have to check - we did one of the more expensive maintenance trips recently - the car might have already received new plugs and wires. I need to check that.

If the O2 sensor had gone mad, I assumed the car would be throwing codes. That's why I ruled that out (I could be wrong).

As far as the amount of flow out of the exhaust is concerned, that would be a valid test if I knew how the thing felt before...

What is seafoam?

seafoam is...............seafoam. i believe i just might be the wrong person to ask, but it was recommended to me and my gf to do to her 94 formula. its supposed to clean out the engine of buildup and the like. i think we did some searching on camaroz28.com for other people that had done it, and had done some internet searching about it also. for her formula, we disconnected the tube to her brake booster, and after turning on the car, let it suck 3/4 or a full can of seafoam (the tube has to be able to intake both the seafoam and air or the car will shut off). when you're almost out of seafoam you're supposed to completely submerge the tube so that right before you run out of seafoam the car will shut itself off. from here i'm pretty sure you hook up the brake booster back up, can't remember if we were suppsed to let the car sit for a certain amount of time or not....all i remember is that you turn the car back on, let it run, give it a few revs, and a huge pillow of white smoke emits from your exhaust. after your car is done being a chimney, you take it out and drive it like you stole it for a good 10 minutes or so. as far as i can remember, seemed to help...car just seemed to liven up some. so yeah, since i know i didn't provide enough info. to make anyone comfortable about the stuff, i'd do some research. but for a car that might be consistently seeming more sluggish....seafoam might be a favorable option. hope this helped at least a little.

Never used the seafoam, always used the GM top engine cleaner. But thats only because my dad is a GM employee and he brings that stuff home by the truckload...along w/ brake cleaner. But its does the same basic thing. Just shoot the stuff down the throttlebody while you rev it up and use the entire can of it. Then take her out and flog it aroung w/ it locked down in a low gear to maintain higher RPMs, then change the oil when your done and hold off on the plugs until after you do it and you will be fine. Start w/ the cheap maint. stuff before you go dropping the big bucks on a "shot in the dark". BTW 500bones is about what GM wanted for a SINGLE converter on my 96 Camaro....

I thought you were pulling my leg talking about "seafoam". I searched and found this:
http://members.nuvox.net/~on.roz/cars/z28/seafoam.html

I guess it's real. Never heard of it.

For some reason, I find this process scary. Even so, maybe I'll try this or something like it. I had been occasionally putting in BG injector cleaner in the car, but it didn't seem to make a difference.

How does one know if a catalytic convertor is shot?

How about when you shake it and something is rattling around inside.
My truck's cat is doing just that ....

Ok. So, the materials just broke in pieces? Then Keith, get a mallet and tap on that bad boy. Although, would tapping it break an otherwise fine cat? Never mind, don't pay attention to me...

There are two styles of converters, the older bead style that has many many millions of little BB looking things in there that get really hot and start glowing, and there are the ones that are the long rods and actually look like honeycombes. Your truck probably has the BB style being that its older and I don't thing those pluged up as quickly as the honeycomb ones. The bead ones have a little plug on the bottom where you can replace the beads themselve and reuse the old housing, but its actually easier to just order a new one and slap it on. But regardless of which one you have, they are really tightly packed inside and should not rattle around at all when you shake them. Given the age of your truck, assuming that your talking about the F-somethin or other tow rig, i would either just get a new highflow one or replace it with a straight pipe and be done with it. The old style converters were pretty restricitve compared to the new ones.

I wish it were that easy. Keith, sorry for the hi-jack ...

The cat on my truck also acts as a 2 into 1 Y pipe. The damn cat is 3' long!

I am considering cutting a hole in the top of the cat, just to inspect the internals
and then weld it all back up.

You can also do what we did w/ my friends T/A which is to disconnect it right after the converter and smash up the internals with a big honkin' screwdriver and then turn the car on and blow out all the guts. It takes some time and makes a big mess but gets the job done. Also, make sure and use a METAL catch can because those beads can get up to 1500degrees if I remember right. And then if you want a cat on the truck for emmisions reasons, then just add one on down the pipe somewhere and use the old as a y-pipe.

I know a procedure in one of my gm helm's manual has you place a pressure gauage where the xygen sensor hooks up. It also had specs for how much back pressure was normal. Unfortunately this was for a pickup truck.

That makes some sense to me. I have the Bentley manual for the car. I didn't see that in there, but I'll look again.

The smell could be a bad cat. or could be shitty gasoline.

Other things that happen when cats fail: Mileage suffers, higher RPM performance sucks, since the exhaust is restricted. Car may run warmer than normal if it's REALLY bad.

BTW, I can get you a converter for less than the dealer. Direct fit, the whole deal. FWIW. And the tune-up parts, brakes, shocks.... you get the point.

Pellet style converters haven't been used in about 20 years on anything. Everything uses a monolithic cat now. And they aren't bad in terms of flow or life. Often cars lose power without them. Hollowing one out isn't a good idea (though cheap). It screws the flow up terribly since you have a big 'ol empty room that the air wants to fill. That makes a LOT of turbulence, which isn't good. Besides, Keith lives in Pittsburgh, and they have smog testing there.

Not to mention it gives to a hollow "cow" sound. I never noticed that my car was running hotter, but got the clue when I noticed that I coudlnt even redline it in first any more and then a couple weeks later had to have the car floored in first just to do 25mph....and thats all it had.

Yes, a bad converter can also cause a vehicle to run hotter. I had one go bad, I didn't notice much in normal driving performance or temp wise. However, after a 15 minute session of giving the car a good flogging the temperature skyrocketed. tapped a whole before the cat in the y-pipe and the backpressure was very high. Those numbers should be spec'd in the service manual. If the car was post '95 then your O2 sensors after the cat could have been used to check abnormal readings. Possible BMW's had post cat O2's for '95?. Any kind of software to scan it?
I've gotten into the habit now of scanning all my cars when they run well and then saving the file for later to compare for when something doesn't run right.

....could ALWAYS sell the BigMoneyWaste....

Two things made me realize I will never own a BigMoneyWaste again shortly after dumping our 2000 528....

1. The service manager looked me in the face after having the same problem for 2 years and said..."Have you ever owned a BMW before?.....These are precision automobiles with a lot of high tech systems, and things like this are common..."

2. Right after that he told me how much my next service was gonna cost me...until then we were on the FREE service plan.... ....just about DIED right there on the spot...

I've been asking the same thing about my truck. Here a a link I found that describes checking with a vacuum gauge to determine if the converter is bad. http://www.autoshop101.com/forms/h64.pdf
The procedure is toward the end of the article.

I just did that to mine. It rattled for along time (all the pieces floating around inside) until one day, the car had NO power and it sounded likeit was hissing.

We dropped the cat-back pipe and found a huge piece of cat gut blocking the exhaust pipe. We took a big air chisel and broke all the pieces up, started the car and blew it all out. That fixed it!!!! Sure sounds much better with my Hooker cat-back too!!!! I'm taking it to have it inspected for emissions soon, we'll see if it passes. I understand that sometimes it does, sometimes it doesn't.

When I had my 2000 SS the cat went bad. I tripped a code and it was covered under warranty. Don't have to worry on my 02, no cats.