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Seeking Thoughts: Stinky Exhaust ++

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Old 10-11-2018, 08:04 PM
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imp
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Default Seeking Thoughts: Stinky Exhaust ++

'94 GT 5.0L HO, 5-Speed, 164K. Runs fine, no CEL, no codes stored. <16 mpg no matter what, mostly rural driving in 5th. gear 75% of the time. Exhaust stinks like unburned hydrocarbons at idle as well as slow driving, when wind blows it back enough into windows.

Don't know if OBD-I can read real-time, my scanner surely cannot. Here's what I've done. Happen to have a brand-new identical engine bought from Ford long ago. Started with new ECT, injectors, Throttle Body, new O2 sensors, replaced thermostat, 190`, new plugs, wires. Eng temp. good, never too hot, even idling with AC full-on, desert heat at 115` +. Fuel pressure runs ~35psi idling.

Wondering about cats. Or, any ideas? Every 5.0HO Fox I've owned, all 5-speeds, did 20+ mpg, even my '93 Cobra. The stink is troubling.......Thanks! imp
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Old 10-11-2018, 11:22 PM
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Can you pull the cats and have a look? I’d start there, and start looking for new ones.
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Old 10-12-2018, 12:53 AM
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Originally Posted by SmokyBC View Post
Can you pull the cats and have a look? Id start there, and start looking for new ones.
Yes, I can. Can you elaborate how/why old, possibly semi-clogged cats might contribute to overly-rich running? I've thought this over and over, but really don't understand it. In my mind, partially blocking the exhaust flow would cause heat build-up, O2s ought to see increased O2 content, due to unburned fuel?

I live where nobody gives a damn about cats. Could bypass them for the hell of seeing what happens. I say this because I've built two EFI conversions using OBD-I, both 5.0HOs, a '79 Ranchero w/ AOD, 1970 Bronco with transfer case using Jeep O/D gears, the big, heavy Ranchero easily got 25mph highway with the 3.25s I threw in the 9-inch; Bronco got less, but not by much, had 3.73s as I recall.

Thanks for your thoughts! Got any more? imp
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Old 10-12-2018, 01:49 AM
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With that kind of miles, I'd suspect an egr bleeding. It would make the o2s think its a bit lean and add fuel.

Last edited by Stepman; 10-12-2018 at 01:51 AM. Reason: Spell check....put ear for egr.
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Old 10-12-2018, 01:52 PM
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Originally Posted by Stepman View Post
With that kind of miles, I'd suspect an egr bleeding. It would make the o2s think its a bit lean and add fuel.
Great idea! Never thought of that. PCM knows position of EGR valve, but not it's condition. Thinks valve is closed, valve could be leaking unwanted exhaust into cyls. That I will check out! Thanks! imp
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Old 10-12-2018, 10:07 PM
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Originally Posted by Stepman View Post
With that kind of miles, I'd suspect an egr bleeding. It would make the o2s think its a bit lean and add fuel.
Well, taking your thought to heed, I removed the EGR valve, blew into it's outlet, valve was seated tightly, no leakage detectable using mouth pressure. Drew vacuum on vacuum inlet, valve moved smoothly, held location with no diaphragm leakage. So, valve appears good. Only other thing I can see related would be if it were constantly demanded "open", and one would think the PCM would relate to no position sensor change, but maybe not!

What I'll do is plug the valve's outlet, put it back, drive it to see what happens. imp
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