98 5.7 rough idle and awful smelling exhaust

[hfdrick5488]

Looking for suggestions…

I acquired a 1998 5.7L Suburban that has a remanufactured engine with 20,000 miles on it. An issue that I need help to get around is a rough idle and awful-smelling exhaust. There is no CEL illuminated. The exhaust sticks on your clothes similar to when you use a lawn mower for extensive periods. Not like raw gas. No loss of power. The timing is dead on.

I have replaced the following:
-Plugs
-Cap
-Rotor
-Wires
-Temp sending unit
-Cleaned throttle body
-New throttle position sensor
-New idle air control
-Upgraded spider with regulator
-New throttle body and upper intake gasket

I have access to a reader so I can get live data, I just don’t know what is within the normal range on certain things.

Thank you in advance.

 

[Schurkey]

Rough idle: Connect scan tool, start already-warmed-up engine, look for misfire counts for each individual cylinder. You replaced the upper intake gasket, shame you didn't do the lowers when you had it apart.

Stinky exhaust: Catalytic converters probably murdered by misfire. If you fix the misfire, MAYBE the catalyst(s) will come back to life. Don't count on it.

Does the CEL come on during bulb-test?

 

[hfdrick5488]

Rough idle: Connect scan tool, start already-warmed-up engine, look for misfire counts for each individual cylinder. You replaced the upper intake gasket, shame you didn't do the lowers when you had it apart.

Stinky exhaust: Catalytic converters probably murdered by misfire. If you fix the misfire, MAYBE the catalyst(s) will come back to life. Don't count on it.

Does the CEL come on during bulb-test?

I connected the scan tool and got a misfire in Cylinder 3 and 4 multiple times or pretty much every time I noticed a stutter which was a lot. I’ve had the upper intake torn off multiple times due to 3 bad spiders so if the issue lies there, it's no problem for me to do it again. Yes it did have the “stutter” before I replaced them.

In my state you don't need to pass emissions testing due to the age of the veh. If the converters are dead would it affect how the engine runs? Or would stinky exhaust just be the issue?

No, the actual light is burned out and I have plans on fixing it but have higher priorities. What I meant by the no CEL was the reader will tell me if there is one. It’s the Bluetooth kind that connects to my phone and can always be connected.

 

[Schurkey]

The misfire has to be tracked-down and fixed. Maybe it's the lower intake gaskets, maybe it's something else...but the misfires have to be corrected.

The CEL bulb needs to go higher in the priority list.

Converters can melt and plug the exhaust system. Misfire makes this more likely. They can clog with carbon, and plug the exhaust system. Either way, the engine won't run if it's bad enough. I plugged a catalyst on my Trailblazer, by the time I found an exit off the interstate, I was going under 5 mph as my "top speed". If you're lucky, your catalyst is merely "wounded" and could come back when the misfire is corrected.

 

[hfdrick5488]

The misfire has to be tracked-down and fixed. Maybe it's the lower intake gaskets, maybe it's something else...but the misfires have to be corrected.

The CEL bulb needs to go higher in the priority list.

Converters can melt and plug the exhaust system. Misfire makes this more likely. They can clog with carbon, and plug the exhaust system. Either way, the engine won't run if it's bad enough. I plugged a catalyst on my Trailblazer, by the time I found an exit off the interstate, I was going under 5 mph as my "top speed". If you're lucky, your catalyst is merely "wounded" and could come back when the misfire is corrected.

Would spraying carb cleaner around the intake gaskets give me a better idea if they need to be done ? Or in this case would that not work?

Alright, tomorrow I will pull apart the dash and replace the bulbs.

Ah wonderful… I suppose only time will tell if they come back to life.

 

[Schurkey]

Would spraying carb cleaner around the intake gaskets give me a better idea if they need to be done ? Or in this case would that not work?

By all means, give that a try.

I thought GM--and the aftermarket--improved the lower gaskets and made them reliable. The originals were troublesome. But I'm hearing that the lower gaskets are still a problem; I'm thinking that in my driveway, they'd get replaced "just because".

Best process would be to try to find a vacuum leak or other source of misfire (ignition, fuel, compression) before disassembling stuff; but if you can't find anything...cram in some fresh lower gaskets, and verify that the manifold isn't cracked.

Alright, tomorrow I will pull apart the dash and replace the bulbs.

Good plan.

Ah wonderful… I suppose only time will tell if they come back to life.

Yup.

 

[Casey916]

Cell

The misfire has to be tracked-down and fixed. Maybe it's the lower intake gaskets, maybe it's something else...but the misfires have to be corrected.

The CEL bulb needs to go higher in the priority list.

Converters can melt and plug the exhaust system. Misfire makes this more likely. They can clog with carbon, and plug the exhaust system. Either way, the engine won't run if it's bad enough. I plugged a catalyst on my Trailblazer, by the time I found an exit off the interstate, I was going under 5 mph as my "top speed". If you're lucky, your catalyst is merely "wounded" and could come back when the misfire is corrected.

Cel bulb what's that probably a dumb question but my question stands

 

[Schurkey]

CEL = Check Engine Light
SES = Service Engine Soon light

Both of those are more-properly called "MIL" Malfunction Indicator Light, the Society of Automotive Engineers preferred term.

 

[2drXmobb]

Looking for suggestions…

I acquired a 1998 5.7L Suburban that has a remanufactured engine with 20,000 miles on it. An issue that I need help to get around is a rough idle and awful-smelling exhaust. There is no CEL illuminated. The exhaust sticks on your clothes similar to when you use a lawn mower for extensive periods. Not like raw gas. No loss of power. The timing is dead on.

I have replaced the following:
-Plugs
-Cap
-Rotor
-Wires
-Temp sending unit
-Cleaned throttle body
-New throttle position sensor
-New idle air control
-Upgraded spider with regulator
-New throttle body and upper intake gasket

I have access to a reader so I can get live data, I just don’t know what is within the normal range on certain things.

Thank you in advance.

Do a smoke test and look for vacuum leaks. One of my hoses came loose and I couldn't figure out why it was running so rich. Do you have emissions like egr etc?

 

[2drXmobb]

I connected the scan tool and got a misfire in Cylinder 3 and 4 multiple times or pretty much every time I noticed a stutter which was a lot. I’ve had the upper intake torn off multiple times due to 3 bad spiders so if the issue lies there, it's no problem for me to do it again. Yes it did have the “stutter” before I replaced them.

In my state you don't need to pass emissions testing due to the age of the veh. If the converters are dead would it affect how the engine runs? Or would stinky exhaust just be the issue?

No, the actual light is burned out and I have plans on fixing it but have higher priorities. What I meant by the no CEL was the reader will tell me if there is one. It’s the Bluetooth kind that connects to my phone and can always be connected.

If your cats are bad it'll throw off your O2 readings. Since you don't need emissions you can tune them out. And 3 bad spiders is unusual. Are you installing them correctly I've seen people bend the connectors to the spiders and have it misaligned. And throw off everything from just the bent pins.