I guess great minds do really think alike, LOL. That is just what I was thinking right after I read initial post about the O2 sensor and posted my question about the "kegger mod". I figured I would at least see how bad that O2 sensor was to get at and give your suggestion a whirl before getting too many variables thrown into the mix. I already put $150 bucks into the sensors, cap and rotor to no avail and didn't even gain much (any?) diagnostic information. Frankly I was skeptical as I didn't think that even an almost totally plugged cat could cause the thumping and knocking I was hearing, but the downside risk / time investment was minimal. I was a professional mechanic for 25 years before closing my last shop 10 years ago and I have seen many plugged cats in my career, this just didn't "feel" like what was happening here. Anyway, checked out the sensor location, grabbed the right wrench and frankly it took longer to get the harness disconnected than it did to unscrew the O2 sensor. After I washed my hands I reconnected the battery and turned the key. The engine fired immediately and while it wasn't idling perfectly (6 month old gas), it was running. AND NO KNOCKING or THUMPING!! I believe my exact words were... "Well I'll be dipped in ****!". I have to admit, pulling the O2 sensor was a great suggestion. I would never have believed it if I hadn't seen it with my own eyes. That cat must be plugged tighter than a bulls *** in fly season. Now while the engine is not running perfectly, just from the difference pulling the sensor made I am pretty confident that this is the the key piece in the puzzle. Thinking back and remembering all the blown plenum gasket posts especially noting that some of the symptoms are: excessive fuel AND oil consumption...both of which I had noted getting (slightly) worse over the last 10 K of driving... Duh! I would be willing to bet that when I tear it down I will find a slight plenum leak which caused the clogged cat issue. Came on so gradually I just chalked it up to high mileage until the day it finally shut down or melted totally. Anyway, thanks again and Kudos to all that replied, I think we just might win this one.

So at this point my game plan is to resolve the cat issue, then if the results are still positive pull the intake and do the "kegger" modification and change the timing chain. At the same time I will do the pan gasket and rear main seal as I have a small leak there and it will make the timing cover gasket easier. I will try and remember to get some photos of the teardown to post and I will let you all know how it ends up. My sincerest thanks again!

 

Sometimes a cat is partially clogged, and at any given time, can full (or nearly fully) clog. The knocking can be from the backpressure in exhaust pushing back to combustion chamber as valves are trying to operate. Removing the O2 sensor allows some of the exhaust to escape, but not nearly enough to make it run great.

I suspect plenum is more than slightly blown....and this will take out the cat.

Glad you were able to get this diagnosed. If you look into the intake through TB with a light and mirror, you will likely see more evidence of plenum leak.

 

Oh by the way, any suggestions as to what you guys have been doing on the cat issue? In the old days people were known to knock the honeycomb out of the shell and run a gutted converter but I wonder what this will do to the mixture mapping? Plus this thing is like a pipefitters nightmare, with all the bends in the piping I don't think I could get a bar up in it do clean it out completely. I have a great exhaust guy that could put a high flow aftermarket cat in but that would require having the truck towed 25 miles to his shop. Is there a decent, reasonably priced OEM fit replacement or performance upgrade cat that you guys would recommend as a bolt in? This is an Arizona truck so the original exhaust system is in great shape, no rust. I already had half the converter unbolted before it got late and I quit for the night. Thanks again, DACH in Cave Creek.

 

Man, I haven't been to Cave Creek in years....

You can knock it out, but O2 sensors will not function correctly. There is a way to trick the downstream to keep from throwing codes, but .....

There are a number of high flow replacements. Several threads here about what types others have used. I picked a a Walker about 2 weeks agofor my Yota, but haven't installed it yet due to cold wet weather. This will be the first cat I have ever replaced (truck has over 270,000 miles on it). The PO of my Ram had just replaced the cat (twice) before I got it. Found out what the cause for that here on DF.

Someone else that has changed theirs may be by here shortly with a recommendation.

 

Lots of folks use magnaflow converters. That is what I have. Have only had it about a year, but have had no problems yet. It'd be nice if you could temporarily remove or gut the cat and see how it ran. Just to make sure. Is what I'd want to do anyway. Then do the plenum repair, etc. I'd prefer replacing the cat and/or plugs last if I was going to use combustion chamber cleaner through the engine after the plenum repair if the plenum was leaking badly.

 

Might run seafoam through it before putting cat back on, but as plugged as this one seems to be, it may not flow enough. Don't know for sure..

 

Excellent! Thanks for the follow-up report.

About converters: I'm running a Magnaflow Direct-Fit and am very happy with it. I could get away with no cat at all here because no one's going to look for it, but I figure that if I'm going to spend money to improve my low-end power I might just as well spend some on the cat (to keep scavenging efficiency up) rather than on working around its absence.

 

I saw a scotty kilmer video on youtube where he states he puts a gallon of laquer thinner in with a half tank of fuel and runs it at about 2,500 rpm for 30 minutes to "clean" out the plugged cat... he stated he had been doing this with great success for like 25 years.. he also said it can actually be washed in a bucket of detergent and water and a spray hose..(no im not kidding). I replaced my cat setup with a magnaflow part# 23285 for $186 bucks shipped from ebay...it includes the pipes to the manifolds/header and to muffler... it was a bolt on for me... my old cat was gutted and fuel consuption was higher and and I noticed a bit more of low end power with new cat...

 

I got on the net looking up direct fit converters for this thing and after quite a bit of searching, researching and calling Magnaflow I ended up ordering a Magnaflow direct fit #45285 from a seller off of Ebay. Man, even from Ebay this thing was pricey ($400), thank goodness I could use the 49 state one. I found one of the Eastern converters on Rock Auto that would have only cost me about 260 delivered and was very tempted but after talking to Magnaflow and seeing how many of you guys are running them with good results I decided to bite the bullet and go with the more expensive one. A couple of points I found while searching for this converter that I wanted to pass on. First, MF technical services said that certain Dodge trucks (like mine) were designed for larger capacity converters. This came about due to me finding some Ebay sellers listing direct fit Magnaflow converters for my 1998 5.2 in the $ 250 range (23285) and then others for 500. According to Magnaflow, the same converter will not work for the 3.9 and 5.2-5.7 even if the sellers say they will and that this might be why some people are putting on a new converter and getting O2 codes and less performance than they should. They might fit, but they have less flow and capacity. This might apply to other brands of converters as well.

It is also noteworthy that the MF converter I bought has a two piece y pipe that needs to be welded after fitting it up. I was hoping for a total bolt on but since this is the only MF cat that tech services says is right for my truck I will deal with it. My thought is to assemble and bolt up the Y pipe / converter assy, put 3 or 4 tacks on it with the MIG and then drop the unit and weld fully. MF says they don't recommend a clamp on the Y pipe joint.

I am going to have a friend give me a quote on water jetting an aluminum plenum repair plate for the kegger using the gasket as a pattern, will see how this pans out. If I do have him do it I will save the DXF file so it can be replicated easily. Well hopefully I will get this thing stripped down this week and the engine work done (plenum repair, cloyes timing chain and rear main seal) and be waiting on the cat to arrive. I will post results / pics when I get this thing back up and running.

 

I bought a Walker cat for 1/2 the price of Magnaflow for my Yota a couple of weks ago. The Walker replacement for a Ram is also much cheaper than MF. Since my trucks are stock, I did not think the extra $ was worth it. I physically comapred the Walker and MF and saw NO difference other than the name embossed on the heat shield.

There is a seller on ebay that sells the plenum plate with screws for about $50. http://www.ebay.com/itm/DODGE-JEEP-A...821fe0&vxp=mtr

I bought mine from there and it fit perfectly. Will need the fel pro gasket from local parts house.