Strange running habits, need help.

[24 Karat Brown]

94 TBI 350. My engine seems to run strong given its 201k miles, but recently it’s been odd. Most of the time it runs fine, but every once in a while this issue pops up. It’ll start loping, usually on start (only after cooling down from being run before) or when the engine speed drops like when I’m coming to a stop and put in the clutch. It will lope like it wants to be an old Camaro but believe me shes stock other than the classic air cleaner flip. While it’s doing so it puffs out black smoke. But if you stay off the throttle it’ll do it’s thing and fix itself, not without a “SERVICE ENGINE SOON” of course. And now recently I’ve noticed white smoke more often, usually right after you rev it a lil and let off. It’s not a lot, but you can see it, and for sure smell it. Is she finally getting worn out after 201k miles? Or is this some TBI or EGR issue?

 

[Redneckgeriatric]

first thing i would check is ECT. you need it anyway!

 

[RawbDidIt]

94 TBI 350. My engine seems to run strong given its 201k miles, but recently it’s been odd. Most of the time it runs fine, but every once in a while this issue pops up. It’ll start loping, usually on start (only after cooling down from being run before) or when the engine speed drops like when I’m coming to a stop and put in the clutch. It will lope like it wants to be an old Camaro but believe me shes stock other than the classic air cleaner flip. While it’s doing so it puffs out black smoke. But if you stay off the throttle it’ll do it’s thing and fix itself, not without a “SERVICE ENGINE SOON” of course. And now recently I’ve noticed white smoke more often, usually right after you rev it a lil and let off. It’s not a lot, but you can see it, and for sure smell it. Is she finally getting worn out after 201k miles? Or is this some TBI or EGR issue?

Check the basics: fuel pressure, vacuum, compression. The smoke you mentioned has me worried, could be a lot of things, but white or black smoke could be head gasket related, so check your fluids as well, make sure there isn't oil in your coolant, or coolant in your oil. Could also be oil blow-by, a compression test should help you narrow that down, and the vacuum test should help narrow down some issues with the EGR (not all, but a busted diaphragm causing a vacuum leak), and the fuel pressure test will eliminate fuel pressure regulator, fuel filter, and fuel pump.

Best of luck to you, let us know what you find.

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[24 Karat Brown]

first thing i would check is ECT. you need it anyway!

I’m not entirely sure what that is, I know a lot about these trucks but I’m not aware of ECT

 

[24 Karat Brown]

Check the basics: fuel pressure, vacuum, compression. The smoke you mentioned has me worried, could be a lot of things, but white or black smoke could be head gasket related, so check your fluids as well, make sure there isn't oil in your coolant, or coolant in your oil. Could also be oil blow-by, a compression test should help you narrow that down, and the vacuum test should help narrow down some issues with the EGR (not all, but a busted diaphragm causing a vacuum leak), and the fuel pressure test will eliminate fuel pressure regulator, fuel filter, and fuel pump.

Best of luck to you, let us know what you find.

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If it was head gasket related wouldn’t it happen way more often? When it happens it’s extremely apparent. As for fuel pressure same thing, wouldn’t it be more often? I’ve replaced the fuel filter already and made no difference despite the old one being really bad. I believe the white smoke could possibly just be a lot of water in the exhaust which could explain why it’s so rusty (the exhaust) but the smell makes me think otherwise

 

[RawbDidIt]

If it was head gasket related wouldn’t it happen way more often? When it happens it’s extremely apparent. As for fuel pressure same thing, wouldn’t it be more often? I’ve replaced the fuel filter already and made no difference despite the old one being really bad. I believe the white smoke could possibly just be a lot of water in the exhaust which could explain why it’s so rusty (the exhaust) but the smell makes me think otherwise

Great question, a lot of times it would be constant, but it may be just small enough of a leak to only happen after it's warmed up, oozing crap into your heads and cylinders after you shut your engine off and making it hard to start up again. IIRC, the EGR valve can fail in a way that causes a vacuum leak, but not sure if it fails open or shut when the diaphragm fails, either way, it could affect the fuel pressure since the fuel pressure regulator also uses vacuum pressure.
Essentially it's hard to diagnose a problem on a single test, so you run a few, and see what test results match the problem. The most likely culprit is the EGR valve stuck open, without knowing why your engine is throwing the code, the best way to ensure it is without buying a new one is to test the systems that run it, and that it affects.
Another thing that would point to one problem or another, Is the smell foul like rotten eggs, or sweet like coolant?

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[Redneckgeriatric]

I’m not entirely sure what that is, I know a lot about these trucks but I’m not aware of ECT

engine coolant temperature, located next to thermostat housing, cost about $15. notorious for failure and will cause computer to give wacky instructions for fuel and timing, and other things. you cant diagnose problems if ect is wonky and it will not throw a code. two things to always make sure are in spec BEFORE further diagnostics are battery voltage, ECT.
good luck!

 

[24 Karat Brown]

engine coolant temperature, located next to thermostat housing, cost about $15. notorious for failure and will cause computer to give wacky instructions for fuel and timing, and other things. you cant diagnose problems if ect is wonky and it will not throw a code. two things to always make sure are in spec BEFORE further diagnostics are battery voltage, ECT.
good luck!

I replaced that already, no difference

 

[24 Karat Brown]

Great question, a lot of times it would be constant, but it may be just small enough of a leak to only happen after it's warmed up, oozing crap into your heads and cylinders after you shut your engine off and making it hard to start up again. IIRC, the EGR valve can fail in a way that causes a vacuum leak, but not sure if it fails open or shut when the diaphragm fails, either way, it could affect the fuel pressure since the fuel pressure regulator also uses vacuum pressure.
Essentially it's hard to diagnose a problem on a single test, so you run a few, and see what test results match the problem. The most likely culprit is the EGR valve stuck open, without knowing why your engine is throwing the code, the best way to ensure it is without buying a new one is to test the systems that run it, and that it affects.
Another thing that would point to one problem or another, Is the smell foul like rotten eggs, or sweet like coolant?

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It’s a bad smell, there’s no sweetness to it. Would it make more sense to just delete the EGR system? I hear lots of people do it and if it helps her breathe a little better I’m up for it. As for smells when it lopes it smells rich, and when it’s just sitting it smells bad, you scrunch your nose up near it bad.

 

[RawbDidIt]

It’s a bad smell, there’s no sweetness to it. Would it make more sense to just delete the EGR system? I hear lots of people do it and if it helps her breathe a little better I’m up for it. As for smells when it lopes it smells rich, and when it’s just sitting it smells bad, you scrunch your nose up near it bad.

Definitely something with the exhaust system, leaning heavily towards the EGR stuck open. Could be the catalytic converter being clogged or damaged, but not likely if it smells rich at times. Pull the EGR and clean with throttle body cleaner, see if it fixes the issue.
If you don't have emissions testing, I'd remove it and put in a block off plate. I think you can get the computer tuned to remove the MIL to not go off for it as well.

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