[CraigG.]

Hey Everybody, Please forgive me for I must sin - I have a tow vehicle question.

How would you know if the Cat convertor is clogged or partially clogged?

I have a 1988 Chebbby C2500 with a TBI 350 with over 150K.

It runs well to drive - but gets only 9 MPG WITHOUT towing.

It has been tuned fairly well with traditional parts.

Any ideas?

Thanks Again and Blessings, Craig

[Pontirag]

remove the CAT.....for the purpose of testing and evaluation. When you are done testing and evaluation...........reinstal it.

Dont misplace that cat during the Test and evaluation period. It must be reinstalled when you are done testing. Also in some states, if you sell a used vehicle that has had its polution controles removed, a court of law can lawfully compel you to reinstall those missing components at your cost. Colorado is one of those states. I'm betting California and new jersey are others.

[sleepy]

Did you get it used or new. What kind of mileage WAS it getting?

[CraigG.]

Just bought it at 150K - so I don't know about mileage when new.
Craig

[1stGen Brawler]

I'm not advocating any illegal activity here.....but depending on what state you....I mean the hypothetical person....lives in....I think I'd cut off that hypothetical cat and I would hypothetically not lose one bit of sleep at night.

[CraigG.]

I am talking 9 to 10 MPG on the highway.

Thanks, Craig

[sleepy]

Originally Posted by 1stGen Brawler

I'm not advocating any illegal activity here.....but depending on what state you....I mean the hypothetical person....lives in....I think I'd cut off that hypothetical cat and I would hypothetically not lose one bit of sleep at night.

Speaking "hypothetically" Take the cat off and look inside for melted guts. Then pound the guts out with a "hypothetical" steel rod. Re install the "hypothetically" empty shell.

Those years(mid to late 80s) 2500s were not known for good mileage. If it has a tach What RPMs are you running at speed? 2 or 4 wheel drive? Auto locking hubs?

[NBF823]

I agree with the others that your going to have to remove it to investigate. I'm not sure what kind of cat is under there, it may be difficult to visually see if it's ok. Driving it with a test pipe would offer the awnswer though.

There's alot of aftermarket hi-performance cats on the market now that flow well and don't cost alot. Short of cams, I don't see why the newer ones would bother anyone to run. I use a three way on a car that was built long before three ways existed. It's a nice cat, it can see right through the honeycomb - flows pretty good, I have performance mods, and I still pass smog. Given a choice, I wouldn't run one, but I'm more comfortable with it, and following the rules with my daily driver. My T/A is another story though - it's probably going to be a fire hazard with my cam selection.

As far as telling you the symptoms of a clogged cat - I think there's to many variables; how clogged it is, and the type of motor you got. When mine clogged I had more power to 2k then power would just drop, you could feel the motor struggle, and didn't have the power to spin to redline in any gear but first. That was a really clogged cat.

[Pontirag]

I'm curious what would have clogged it. its totally irrelevent but the symptoms might indicate Looming problems.

some clogged up because of a product called Molly which was molibdinum (di?)sulfide or some jibberish like that. It was used alot and do all end all solution to everything untill it was discovered that It messed up polution controls including CAT's. there were and are alot of snake oils used to extend the life or tired engines. Also you might want to pull a valve cover and observe rocker arm action. that indicates cam wear.

does not matter. also consider that exuaste systems can collapse internally and cause alot of restriction.

[screamingchief]

Too much fuel will ruin any cat in short order,they'll go nuclear I kid you not.

They'll get so F***ing hot they glow red and they'll catch the carpet inside the car on fire sometimes.

Ask me how I know about that.

Coolant is another big killer of cats,just about any car/truck that has had the headgaskets go bad,and as such has dumped a bunch of coolant into the exhaust system,will need new cats @ least 75% of the time,based on my years as a ford line tech.

Anyhow,yeah the quick & dirty test is to unhook the exhaust ahead of the cat and see if it runs better,sure it will be loud,but if it's really plugged up you'll know pretty quickly,if it's only slightly plugged then that test might not be enough to tell you one way or the other,then you gotta try something like the test pipes mentioned by the others.

Good luck with it.

Bret P.

[poncho in a can]

I've heard of guys using various low PSI gauges (oil pres.?) to check for back pressure.
I guess checking before and after the cat?? I dunno, just thinking out loud........

[Pontirag]

yea that wil work but its a bit involved

[stimpy]

you can buy a back pressure gauge for cheap now days ( $50) , I have replaced my cats on my truck ( 96 chev c3500 7.4L 150K miles ) from just age as the monolithe in it deteriorates over time , ( I have to take a biannual sniffer test here and the Hc rates where climbing slowly) before the one side of the motors cat failed( true duals) it was getting progressively worse milage and running hotter and finally started to make a whistling/ husshing sound thru the exaust , when we took them off the one bank was plugged so you couldn't see thru it , the other was partially melted ( I believe from a bad injector ) all the back pressure kit involves is a electric drill to drill a hole in the pipe ,and it supplys the hose and fittings . when your done you just plug the pipe with the supplied plug , alot of the mechs I know just remove the o2 sensor temporarly install the fitting with the hose barb there , then reset the computer( disconnect the battery for a few minutes) after they are done with the test ( it will throw a code for bad O2 )

[CraigG.]

Ok - Thanks guys you have been very helpful!

Thanks Again and Blessings, Craig

[andyleonard]

The standard improvements on an 88 3/4 ton with a tired small block are new plug wires, headers and a cold air hose from the grill to the air cleaner. Almost for sure you are only getting fresh air up from the exhaust manifold, as the thermostatic air flap is broken closed. You probably have misfire from rotten wires. Plugged cat is a maybe, but if you have big tires and a big right foot, you'll never get much over 12/13 mpg with that setup.