4 litre V8 poor emissions poor mpg smelly exhaust
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Joined: Jul 2012
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From: New Forest, UK
4 litre V8 poor emissions poor mpg smelly exhaust
Hello Jag-types,
My 2000 MY 4 litre V8 has fairly low miles, drives well, am very happy with her. But fuel economy is lower than expected (maybe recent fuel prices are just making me pay more attention), and the emissions are poor (but good enough to pass vehicle emissions test with advisory on last 2 MoTs). Exhaust is smelly. No engine management light. Suspected O2 sensor was going awry - the upstream heated sensors are apparently less reliable and I did have that fault on my old XJR. But no code and when I looked at the output from each of the four O2 sensors with a code reader they are all behaving normally - going into closed loop, heated upstreams producing variable voltage, basic downstream ones providing consistent signal. Air filter element OK. No audible vacuum leaks in engine bay. So am wondering about spark plugs and/or coils. On an older car with poor emissions I would always start with a basic tune up (plugs, points, condenser, timing, filter), then plug wires, distributor cap and carb setting. Not much help today! Any views on spark plugs? Coils? Maybe I go for a set of 8 plugs and perhaps 2 coils and swap them through in turn. - here I am assuming plug change out is fairly straight forward. Other suggestions? Thanks All. Happy Motoring.
My 2000 MY 4 litre V8 has fairly low miles, drives well, am very happy with her. But fuel economy is lower than expected (maybe recent fuel prices are just making me pay more attention), and the emissions are poor (but good enough to pass vehicle emissions test with advisory on last 2 MoTs). Exhaust is smelly. No engine management light. Suspected O2 sensor was going awry - the upstream heated sensors are apparently less reliable and I did have that fault on my old XJR. But no code and when I looked at the output from each of the four O2 sensors with a code reader they are all behaving normally - going into closed loop, heated upstreams producing variable voltage, basic downstream ones providing consistent signal. Air filter element OK. No audible vacuum leaks in engine bay. So am wondering about spark plugs and/or coils. On an older car with poor emissions I would always start with a basic tune up (plugs, points, condenser, timing, filter), then plug wires, distributor cap and carb setting. Not much help today! Any views on spark plugs? Coils? Maybe I go for a set of 8 plugs and perhaps 2 coils and swap them through in turn. - here I am assuming plug change out is fairly straight forward. Other suggestions? Thanks All. Happy Motoring.
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Joined: Jan 2022
Posts: 556
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From: Glasgow
What gases in your emissions are poor? CO, CO2, O2, or HC?
If your upstream O2 sensors are working fine then you may have a problem with your cat(s). Your downstream sensors should spot this in theory and flag up a fault code. However, if it doesn’t recognise the fault as bad enough to complain about, it may try to adjust the fuelling; theoretically effecting the fuel economy.
Chances are, if all 4 O2 (lambda) sensors are ok but your emissions are iffy then your cat(s) has become contaminated (possibly even clogged) thereby effecting its ability to reduce noxious gases.
If your upstream O2 sensors are working fine then you may have a problem with your cat(s). Your downstream sensors should spot this in theory and flag up a fault code. However, if it doesn’t recognise the fault as bad enough to complain about, it may try to adjust the fuelling; theoretically effecting the fuel economy.
Chances are, if all 4 O2 (lambda) sensors are ok but your emissions are iffy then your cat(s) has become contaminated (possibly even clogged) thereby effecting its ability to reduce noxious gases.
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