Discussion
Bit of back story, have a MK2.5 MX-5 2004 with 125000 miles on it. So in the beginning of 2023 hada CEL come on with p0171, p0420 and another one to do with the MAF sensor.
cleaned the maf sensor and that code hasnt come on since.
After a couple times of resetting the CEL the p0420 code went away and hasn't come back.
Now the p0171 code is causing me issues. I noticed when first checked the CEL my LTET was at +20.3 at idle, every time I reset the code it would come back in a few days. Since having the car was using E5 fuel, then at the time couldn't get hold of some so switched to E10 which was at the beginning of summer 2023 and then the CEL didn't come back until around the end of September 2023, only coming up with p0171. While the CEL didn't come on throughout the summer my LTFT was still at +20.3 The car doesn't misfire, starts every time, idle can be a bit rough sometimes and it drinks a lot of fuel (obviously with +20.3 adjustment)
I've narrowed down the issue quite a bit think, I've replaced the cheaper parts that could cause the issue but didn't want to sink money into the more expensive parts unless I actually knew what the problem was (no point wasting money on things that don't need replacing yknow) So what I've done so far is replace the fuel filte, spark plugs, engine temperature sensor, I've swapped out my pipercross panel filter with an OEM one which made no difference, new pcv valve, cleaned out the throttle body, did a fuel pressure test in January of 2024 which was a little low so swapped it out for a new one; LTFT remains very high but the CEL hasn't been on since.
The obvious problem would be vacuum leak so I've done a smoke test and the whole intake side hold pressure with no leak, the exhaust has small vacuum leaks at almost every flange even the one between the upstream and downstream 02 sensors (I've ordered a new centre pipe and a full set of gaskets which will be done in a couple days) also have some used and serviced injectors arriving soon so can completely rule out the fuel system.
I've been going over my live data for a while. MAF g/s hovers around 2 at idle and increases with rpm, upstream 02 sensors oscillates between.1-.9 volts, a potential issue is the downstream 02 sensor which reads 0.035 volts at idle and when slowing down but hovers around .4 volts while accelerating. STFT bounces between 0-11% at idle.
I've reset fuel trims in the past and while it was adjusting the car would hesitate as if it would hit a blank patch where it would hover at the same speed for a second even with rpm increasing.
Did the smoke test the other day and since then it's feeling like it's hesitating again so had a look at fuel trims but they weren't reset. While driving under different loads my LTFT would drop down to +17-18% when start accelerating, go back up to +20% after a couple seconds then back down to +17-18% the faster went. Also noticed whenever slowed down or stopped the LTFT would go back to +20%. MAF g/s would increase with rpm. Also noticed when start accelerating or floor it my downstream 02 sensor would go from 0.035V up to around .4V which is when my LTFT would drop then when i slowed down or stopped the voltage would go back down to around 0.035. would also add that sometimes when floored it my LTFT would drop to 0, car feels like it hesitates, stft would go jump to +20.3% the the LTFT would jump back up
Sorry for the long post it's just been stressing me out a lot. I'm just a driveway mechanic that loves to learn so in no way a professional, any help would be great guys appreciate it
cleaned the maf sensor and that code hasnt come on since.
After a couple times of resetting the CEL the p0420 code went away and hasn't come back.
Now the p0171 code is causing me issues. I noticed when first checked the CEL my LTET was at +20.3 at idle, every time I reset the code it would come back in a few days. Since having the car was using E5 fuel, then at the time couldn't get hold of some so switched to E10 which was at the beginning of summer 2023 and then the CEL didn't come back until around the end of September 2023, only coming up with p0171. While the CEL didn't come on throughout the summer my LTFT was still at +20.3 The car doesn't misfire, starts every time, idle can be a bit rough sometimes and it drinks a lot of fuel (obviously with +20.3 adjustment)
I've narrowed down the issue quite a bit think, I've replaced the cheaper parts that could cause the issue but didn't want to sink money into the more expensive parts unless I actually knew what the problem was (no point wasting money on things that don't need replacing yknow) So what I've done so far is replace the fuel filte, spark plugs, engine temperature sensor, I've swapped out my pipercross panel filter with an OEM one which made no difference, new pcv valve, cleaned out the throttle body, did a fuel pressure test in January of 2024 which was a little low so swapped it out for a new one; LTFT remains very high but the CEL hasn't been on since.
The obvious problem would be vacuum leak so I've done a smoke test and the whole intake side hold pressure with no leak, the exhaust has small vacuum leaks at almost every flange even the one between the upstream and downstream 02 sensors (I've ordered a new centre pipe and a full set of gaskets which will be done in a couple days) also have some used and serviced injectors arriving soon so can completely rule out the fuel system.
I've been going over my live data for a while. MAF g/s hovers around 2 at idle and increases with rpm, upstream 02 sensors oscillates between.1-.9 volts, a potential issue is the downstream 02 sensor which reads 0.035 volts at idle and when slowing down but hovers around .4 volts while accelerating. STFT bounces between 0-11% at idle.
I've reset fuel trims in the past and while it was adjusting the car would hesitate as if it would hit a blank patch where it would hover at the same speed for a second even with rpm increasing.
Did the smoke test the other day and since then it's feeling like it's hesitating again so had a look at fuel trims but they weren't reset. While driving under different loads my LTFT would drop down to +17-18% when start accelerating, go back up to +20% after a couple seconds then back down to +17-18% the faster went. Also noticed whenever slowed down or stopped the LTFT would go back to +20%. MAF g/s would increase with rpm. Also noticed when start accelerating or floor it my downstream 02 sensor would go from 0.035V up to around .4V which is when my LTFT would drop then when i slowed down or stopped the voltage would go back down to around 0.035. would also add that sometimes when floored it my LTFT would drop to 0, car feels like it hesitates, stft would go jump to +20.3% the the LTFT would jump back up
Sorry for the long post it's just been stressing me out a lot. I'm just a driveway mechanic that loves to learn so in no way a professional, any help would be great guys appreciate it
Do you mean LTFT (long term fuel trim)?
Can you also see the short term trim? This would give you quicker feedback when you're checking for problems.
I guess that +20% means the ECU thinks the engine is running lean and added fuel to correct that.
You need to know whether it was actually running lean and is not running correctly, or was previously running correctly and is now running rich.
If you can find anyone with a wideband EGO sensor that would be one way to check.
One potential cause for the ECU thinking the engine needs extra fuel is an exhaust leak upstream of the first EGO sensor. If you can see live readings via an OBD reader or similar, you can try clearing the adaptive settings and then restrict the exhaust outlet very slightly to give a slight back pressure. This discourages air from leaking in through any exhaust.leaks. If the EGO readings look normal when you do this and then start correcting once you remove the backpressure, that suggests the air leak is causing the correction.
Can you also see the short term trim? This would give you quicker feedback when you're checking for problems.
I guess that +20% means the ECU thinks the engine is running lean and added fuel to correct that.
You need to know whether it was actually running lean and is not running correctly, or was previously running correctly and is now running rich.
If you can find anyone with a wideband EGO sensor that would be one way to check.
One potential cause for the ECU thinking the engine needs extra fuel is an exhaust leak upstream of the first EGO sensor. If you can see live readings via an OBD reader or similar, you can try clearing the adaptive settings and then restrict the exhaust outlet very slightly to give a slight back pressure. This discourages air from leaking in through any exhaust.leaks. If the EGO readings look normal when you do this and then start correcting once you remove the backpressure, that suggests the air leak is causing the correction.
Thank you I'll have a look at that before fitting the new exhaust midpipe and gaskets, i can check the stuff with my obd scanner. There is an exhaust leak right after the upstream 02 sensor though not a crazy one, then one right after the downstream O2 sensor also not a crazy one. I'll reset any adjustments and try out restricting the flow and post when I do it with what I find
Going by that rule and from what I've observed over time the maf seems to be fine then. I believe it does have egr, I've checked this by blocking off the exhaust and using soapy water and nothing came from there during the smoke test but I'm sure there's more efficient ways of testing it, any recommendations?
Edit: I've checked around the pipe that connects the exhaust to the egr using those methods but not the solenoid itself, how would I go about checking the solenoid ?
Edit: I've checked around the pipe that connects the exhaust to the egr using those methods but not the solenoid itself, how would I go about checking the solenoid ?
Edited by Mx5Reen on Sunday 3rd March 19:29
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