My 99 vortec 5.7 STINKS, has a subtle miss but no Codes or CEL

[Schurkey]

isn't the temperature of the cat supposed to be higher than the rest of the system if it's working properly? I'm starting to think that may be at least part of the problems.

The temperature readings make NO sense. The catalyst is colder than the inlet, so either the catalyst is dead, or the thermometer is inaccurate. Neither is a surprise to me, I've been saying the catalyst is likely to be failed since Post #4. The muffler outlet is the hottest part of the whole system, and that's crazy.

I'm guessing half the problem is failed catalyst, half the problem is due to differences in emissivity of the exhaust system.

Nobody wants to talk about the inaccuracy of infra-red thermometers because of emissivity. The thermometer reads infra-red, different surface finishes and different materials emit infra-red at lower or higher levels even at the same temperature. Not many consumer-level infra-red thermometers even have an adjustment for emissivity; and if you have one that's adjustable, you're still just guessing at the emissivity of the object you're measuring--although we know that shiny stuff "reads" colder than it probably is.

https://en.wikipedia.org/wiki/Emissivity

 

[df2x4]

Personally if it were my truck I'd be tempted to either gut the cat or throw on an off road intermediate pipe that doesn't have them and see if that fixed any of the issues. If the rear O2s are already tuned out that's a no-brainer IMO. I don't see the point of running no rear O2s and still running cats.

I wonder if the cat might be clogged up and the extra backpressure is messing with things? Just a thought.

 

[df2x4]

Nobody wants to talk about the inaccuracy of infra-red thermometers because of emissivity. The thermometer reads infra-red, different surface finishes and different materials emit infra-red at lower or higher levels even at the same temperature. Not many consumer-level infra-red thermometers even have an adjustment for emissivity; and if you have one that's adjustable, you're still just guessing at the emissivity of the object you're measuring--although we know that shiny stuff "reads" colder than it probably is.

Should still be good enough for an educated guess though, no?

Emissivity is definitely a thing in regards to IR thermometers, though. I know some of the consumer grade ones come with rolls of black thermal tape that you can put on whatever you're trying to measure. Surely not 100% accurate but it's got to be better than nothing.

 

[brycebba]

Wow thanks for all the great info guys! I didn't know anything about the IR temperature. So I'll see if I can get another catalytic converter. I might need to order it from summit. My current one measures 2.5 in/out and is 11inches in length made by magnaflow. Should I go with that one again?

 

[Schurkey]

FIX THE MISFIRE FIRST.

That's probably what took out the cat to begin with. The failed catalyst is a symptom of the real problem.

If the catalyst was plugged, you'd have problems more at heavy throttle than at idle.

 

[brycebba]

FIX THE MISFIRE FIRST.

That's probably what took out the cat to begin with. The failed catalyst is a symptom of the real problem.

If the catalyst was plugged, you'd have problems more at heavy throttle than at idle.

Oh gotcha, I misunderstood. Hey with that plug wire that arc'd/burned on the header, could it have caused a problem with the distributor cap or anything? Obviously I replaced the wire and the plugs all around are new as of yesterday but it still has that stumble at idle. None of the plugs really stood out from the others as being different Would I see something on that side?

 

[brycebba]

I have some time today to troubleshoot or get data or whatever if there are ideas on what I could check. Thanks!!

 

[Tavi]

I'm guessing half the problem is failed catalyst, half the problem is due to differences in emissivity of the exhaust system.

Nobody wants to talk about the inaccuracy of infra-red thermometers because of emissivity. The thermometer reads infra-red, different surface finishes and different materials emit infra-red at lower or higher levels even at the same temperature. Not many consumer-level infra-red thermometers even have an adjustment for emissivity; and if you have one that's adjustable, you're still just guessing at the emissivity of the object you're measuring--although we know that shiny stuff "reads" colder than it probably is.

https://en.wikipedia.org/wiki/Emissivity

I would hazard a guess that the misfire/excess Fuel in the system is what caused the cat to fail if it has. And I’m not sure it’s a matter of nobody wanting to talk about the inaccuracy; but more a matter of ignorance. So thank you for the insight.

Oh gotcha, I misunderstood. Hey with that plug wire that arc'd/burned on the header, could it have caused a problem with the distributor cap or anything? Obviously I replaced the wire and the plugs all around are new as of yesterday but it still has that stumble at idle. None of the plugs really stood out from the others as being different Would I see something on that side?

From my experience, the arc will not cause damage to the other ignition parts.

Not being there to diagnose. My first suspension would be a leaking injector or leaking spider directly into the lifter valley. I would look at fuel pressure during the stumble.

 

[brycebba]

I would hazard a guess that the misfire/excess Fuel in the system is what caused the cat to fail if it has. And I’m not sure it’s a matter of nobody wanting to talk about the inaccuracy; but more a matter of ignorance. So thank you for the insight.



From my experience, the arc will not cause damage to the other ignition parts.

Not being there to diagnose. My first suspension would be a leaking injector or leaking spider directly into the lifter valley. I would look at fuel pressure during the stumble.

Hey thanks for the reply! The fuel pressure stays totally smooth during the little stumbles. Regarding leaky injector stuff, I don't know if this indicates anything in either direction, the other day when I checked it over a period of time I forgot to take it off overnight and so the fuel pressure gauge was hooked up for about 24 hrs in the end with it settling at 20 psi over 13 hours and stayed at 20 psi until I took it off 11 hours later. I can't remember exactly but I think with KOEO, it was at 55 psi

 

[Tachyon]

Yeah good point. During the rebuild I had the shop install the updated mpfi ACDelco 217-3029 units I hope it wouldn't be having issues suddenly. My fuel pressure gauge has gone from ~60 down to 20psi in 13 hours of being off. Good? Bad?

I'm inclined to agree with 94burbk1500. If I hadn't read where you said you had done the MPFI upgrade, I would have sworn your old CSFI injection was failing. These are the exact symptoms. The injectors fail to close all the way and leak fuel into the cylinders. The stink is the cats cooking. Oil gets thin. damage can ensue. Without the rear O2 sensors, you have no way for the system to detect this condition so you'll see no warnings in your diagnostics. I would guess that your custom tune also eliminates other things that would cause warning signs in order to get the system to run without that other o2 sensor input.

Anyway, I point all this out because it seems like you need to look at what might cause similar issues to a failing CSFI in an MPFI system and the resulting dump of raw fuel into one or more cylinders. Confirm this first by checking your oil. Is it thin? Is it black? Does it smell of gas?
I know getting at the MPFI sucks, but you may end up having to pull it and check every line and injector.