Catalytic converter failure anomaly?

Hello first time poster, long time member, I have used this site to aid in troubleshooting but now I think I have a problem that only has expensive solutions. I have a 1989 chevy OBS CC R3500 454 TBI long tube headers, RV Cam, and TH400 with shift kit, 91k miles, about 2 months ago I was pulling my trailer (1850lbs) with 7500 lbs on it with 1500 lbs in the bed, at 60 mph, no problem. Truck is driven on weekends to prevent issues that occur when vehicles sit (learned the hard way) My girlfriends S-10 went down so she was driving the monster to work, called me after a few days saying its running like crap, she made it home, I found front drivers side cylinder (8) spark plug had backed out. Removed plug, cleaned with brass wire brush, reinstalled and truck was running great. After about 20 minutes of driving (empty) on the freeway it started misfiring like a ford and had my foot to the floor trying to keep it going, found if Ieased in throttle it would accelerate, turned around and was able to limp it home. Thought it may be thee cat so I bought a high flow the old one fell out (magic made it fall out, not me) and the new one was installed. Truck runs alot better but still not good, Ive never dealt with a burnt valve but I believe it is my problem but I wanted to see if anyone had a different opinion. I do not believe the problem is fuel related, to get it home from where it started its crap back to my house it disposed of a half of a tank. A parts store guy (all of them are car guru's right?) Said the plugged cat could have had excessive back pressure that pushed out spark plug. My immediate thought was no efging way, but then my second thought was can that really happen?? The plug was covered in black buildup when it was loose but I havent looked at it again. Planning to replace plugs and wires (just changed them almost exactly a year ago) but I appreciate any ideas / thoughts.

Do a compression test on all cylinders, only way to tell if compression is the root of your problem. When changing plugs is the perfect time to do this test.

Here's some of my thoughts:

- excessive back pressure cause the plug to back out, probably not unless it has damaged threads
- compare condition of all spark plugs to the loose one
- do compression check on a few cylinders to check for bad valves
- check for mis-fires by pulling one plug wire at a time and listen for a change or checking individual header temps

If all that checks out come back for a list of new possibilities.

I would expect exhaust restrictions to be described as gutless or no power vs runs like crap. If runs like crap means missing/running rough I would first think ignition or air/fuel related.


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I appreciate your quick responses, Compression test is happening when I get home, plugs and wires are already waiting for me at oreilly's, the cat backing out the plug did seem far fetched but sometimes little/bizarre details can change everything, Which reminds me of something more I should have included, (yeah I know, how could there be more after that monster?) It went from running great after plug cleaning and reinstall, to being on the freeway going fine, then a switch flipped and it went from 300 to 30 HP in about 5 seconds. Now, when cold (i am not letting it run anymore than needed) Engine shakes violently, I also have new motor mounts but want tgis to be fixed before I put them in. The rapid: terrible, great, to terrible . Which is the only reason why my fingers are crossed its not a burnt valve.

For something to switch like that, I'd guess electrical related. What about the ignition control module in the distributor. Although I've only heard of them working or not working, nothing in between. They're known for not lasting forever. And usually die when hot. Go all through the ignition components. You mentioned it burning more gas than usual. Was that because you were pushing harder on the throttle to try to get it to go? I ask to get a sense of whether or not it was starving for gas or running rich. Check the MAP sensor and Throttle Position Sensor as well. I'm assuming no check engine light?


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Me being heavy on the throttle was the cause of it dumping gas out, Ive had the truck for two years and can say with confidence the check engine light is not hooked up, Ive been fixing stupid things I find here and there which is why I threw a High flow cat in, it has a 3 inch single exhaust (Banks power kit for 454 motorhome CAI intake and exhaust kit) previous owner welded in a 2.5 inch cat in right before the muffler, so no matter what It had to fall out anyways. I will look up how to check MAP sensor, or just pick one up if theyre cheap but the only thing I changed to bring it from having a boat anchor attached to hauling ass again was the plug and (again, before cat was replaced) since it was running fine at the time I stopped looking for a problem.. Until another one reared its ugly head.

Okay, got compression test done, couldnt find my compresion tester, used a rental that I hope reads low, because yeah all were pathetic.

8 7
6. 5
4. 3
2. 1

Cyl 1 (the one that had plug back out of that I called 8,)
0 reading. Tried everything, was doing 5 count cranks, removed and reinstalled tried again it would jump up to 130 but never held anything.

Cyl 3
While cranking it hits 80 holds 60

Cyl 5
While cranking hits 70, holds 60

Cyl 7
While cranking hits 70 holds 55

Cyl 2
0 no matter what, also managed to get a spike to 120 out of it but I cant imagine that means ****, needle shot up and down so fast if it does tell me anything it needs rings. (Same scenario as 1)

Cyl 4
Peaked at 85, holds 75

Cyl 6
Peaked at 60, holds 30

Cyl 8
Peaked at 60, holds 30

I imagine the pos gauge is giving a bad number but released all and got matching numbers three times on each cylinder so my deduction is while the actual number is off, its still a good basis for comparison. This truck has never been slow.

Bump

KBZ71 said:

Okay, got compression test done, couldnt find my compresion tester, used a rental that I hope reads low, because yeah all were pathetic.

8 7
6. 5
4. 3
2. 1

Cyl 1 (the one that had plug back out of that I called 8,)
0 reading. Tried everything, was doing 5 count cranks, removed and reinstalled tried again it would jump up to 130 but never held anything.

Cyl 3
While cranking it hits 80 holds 60

Cyl 5
While cranking hits 70, holds 60

Cyl 7
While cranking hits 70 holds 55

Cyl 2
0 no matter what, also managed to get a spike to 120 out of it but I cant imagine that means ****, needle shot up and down so fast if it does tell me anything it needs rings. (Same scenario as 1)

Cyl 4
Peaked at 85, holds 75

Cyl 6
Peaked at 60, holds 30

Cyl 8
Peaked at 60, holds 30

I imagine the pos gauge is giving a bad number but released all and got matching numbers three times on each cylinder so my deduction is while the actual number is off, its still a good basis for comparison. This truck has never been slow.

Click to expand...

Probably bent valve on 1 and 2 pull heads and take them to a machine shop and have them check the heads these trucks being fuel injected tend to not have bad cylinder walls in most cases so I don't think it is rings.

all of those numbers are really low for only having 91k miles on the stock engine. Couple questions/suggestions... did you have all of the plugs out when you were doing the cranking to make the engine spin faster? and is your battery new and strong or does it just barely turn over to get it started?

If either of the above are questionable I would re-do all of them with a strong battery and all of the plugs out. Then for Cyl 1 and 2 (or all of them if you have time) do a wet compression test immediately after your dry test. For this, squirt just a little oil in the spark plug hole and re-do the compression test.if the numbers get better your rings are leaking, if they stay the same you may have a bent valve or head gasket leak.

If you suspect a valve or head gasket there is more you can do to diagnose one or the other but by that time you might have to be pulling the heads anyway so a complete valve job and new head gaskets are on the way anyway.

Good luck!