MX5 Surging - Bad O2 waveforms
Discussion
Hi All!
A friend of mine has a MX5 MK2.5 with rough/surging engine issue and throwing precar 02 codes. Been to many garages, all replaced either the precat 02 sensor or MAF, no change. I hoped my limited knowledge and the great minds of this forum could help out a poor man trying to get keep his pride and joy on the road =)
I've plugged in my OBD and following:
-Fuel trims show ECU adding and taking away fuel constantly causing surging, especially when idling
-Precat O2 shows a 'decent' waveform when on a drive under hard acceleration/heavy load
-Postcat O2 is hardly reading, stick at 0V when idling
Idling:
Crusing
Accelerating
I suspect a blocked CAT or exhaust issue... but I'm only guessing. Could anymore please point us in the right direction!
A friend of mine has a MX5 MK2.5 with rough/surging engine issue and throwing precar 02 codes. Been to many garages, all replaced either the precat 02 sensor or MAF, no change. I hoped my limited knowledge and the great minds of this forum could help out a poor man trying to get keep his pride and joy on the road =)
I've plugged in my OBD and following:
-Fuel trims show ECU adding and taking away fuel constantly causing surging, especially when idling
-Precat O2 shows a 'decent' waveform when on a drive under hard acceleration/heavy load
-Postcat O2 is hardly reading, stick at 0V when idling
Idling:
Crusing
Accelerating
I suspect a blocked CAT or exhaust issue... but I'm only guessing. Could anymore please point us in the right direction!
Precat sensor is the one which controls the closed loop fuelling.
The post cat sensor is only looking at making sure the catalyst is correctly bringing the emissions levels down.
(Now - Caveat - the following definitely applices to NA/NB and one might presume the NB2 with OBD functionality but I cannot 100% guarantee)
The engine ECU uses closed loop fuelling at part throttle and within a restrcited RPM range - off idle to approximately 4000rpm. Over 4000rpm, the ECU goes into 'open loop' where it is not referencing the Lambda values at all - it simply uses the lookup tables and x-refs against RPM, throttle, and intake temp.
I would first test the car by driving it, when warm, and checking how it runs up at 5, 6, 7 thousand RPM. If the car pulls well, and smoothly, in the upper reaches this again suggests the problem is with the closed loop lambda values and operation.
Otherwise, I'd go down the route of physically checking the Catalyst as suggested.
The post cat sensor is only looking at making sure the catalyst is correctly bringing the emissions levels down.
(Now - Caveat - the following definitely applices to NA/NB and one might presume the NB2 with OBD functionality but I cannot 100% guarantee)
The engine ECU uses closed loop fuelling at part throttle and within a restrcited RPM range - off idle to approximately 4000rpm. Over 4000rpm, the ECU goes into 'open loop' where it is not referencing the Lambda values at all - it simply uses the lookup tables and x-refs against RPM, throttle, and intake temp.
I would first test the car by driving it, when warm, and checking how it runs up at 5, 6, 7 thousand RPM. If the car pulls well, and smoothly, in the upper reaches this again suggests the problem is with the closed loop lambda values and operation.
Otherwise, I'd go down the route of physically checking the Catalyst as suggested.
snotrag said:
Precat sensor is the one which controls the closed loop fuelling.
The post cat sensor is only looking at making sure the catalyst is correctly bringing the emissions levels down.
(Now - Caveat - the following definitely applices to NA/NB and one might presume the NB2 with OBD functionality but I cannot 100% guarantee)
The engine ECU uses closed loop fuelling at part throttle and within a restrcited RPM range - off idle to approximately 4000rpm. Over 4000rpm, the ECU goes into 'open loop' where it is not referencing the Lambda values at all - it simply uses the lookup tables and x-refs against RPM, throttle, and intake temp.
I would first test the car by driving it, when warm, and checking how it runs up at 5, 6, 7 thousand RPM. If the car pulls well, and smoothly, in the upper reaches this again suggests the problem is with the closed loop lambda values and operation.
Otherwise, I'd go down the route of physically checking the Catalyst as suggested.
Hi and thanks for your reply,The post cat sensor is only looking at making sure the catalyst is correctly bringing the emissions levels down.
(Now - Caveat - the following definitely applices to NA/NB and one might presume the NB2 with OBD functionality but I cannot 100% guarantee)
The engine ECU uses closed loop fuelling at part throttle and within a restrcited RPM range - off idle to approximately 4000rpm. Over 4000rpm, the ECU goes into 'open loop' where it is not referencing the Lambda values at all - it simply uses the lookup tables and x-refs against RPM, throttle, and intake temp.
I would first test the car by driving it, when warm, and checking how it runs up at 5, 6, 7 thousand RPM. If the car pulls well, and smoothly, in the upper reaches this again suggests the problem is with the closed loop lambda values and operation.
Otherwise, I'd go down the route of physically checking the Catalyst as suggested.
Car pulls well in higher RPMS, no problem there. Only seems to be lower Revs (closed loop) and idling the engine keep adjusting fueling making the engine surge and nearly stall. Precast lambda seems to be stuck around 0.45V when idling, post Cat 0V.
ECU keeps reducing fuel (negative fuel trims) however this doesn't seem to affect lambda readings much, also like an exhaust leak? Had a good look at exhaust but nothing obvious. May fuel pressure or stuck injector?
Belle427 said:
What were the codes?
Hi and thanks for reply,Yes it keeps throwing PreCat O2 sensor fault, can't remeber exact code but think it was P0130.
Only seems to throw the code because negative fuel trims go to -15.
Had 2 new sensors fitted and looked at by 2 garages. Wiring tested by myself ok.
Hello, first time ever commenting, I've been looking all over forums and found this thread,
I have a 2004 miata all stock, and since we bought it, when the car is warmed up and the outside temp gets over 90 the car will start to surge hard up an down from 2k to 3k rpm about every second. but if I push past to about 4k it will start pulling ok. And when surging the faster i go an higher rpms from 3k to 4k the surge gets longer the closer to 4k I get. Like it goes from 1 second surges to about 2 seconds, then when it hits 4k it stops. But as soon as I let off the gas an drop below 4k it does it again. And if I park it for 30 mins and start it back up it's gone till the car gets hot again. But it never overheats and no obd codes and oil pressure stable the whole time. Any help would be appreciated. I'm not a mechanic but I do have decent understanding of cars and have worked on quite a few before.
I have a 2004 miata all stock, and since we bought it, when the car is warmed up and the outside temp gets over 90 the car will start to surge hard up an down from 2k to 3k rpm about every second. but if I push past to about 4k it will start pulling ok. And when surging the faster i go an higher rpms from 3k to 4k the surge gets longer the closer to 4k I get. Like it goes from 1 second surges to about 2 seconds, then when it hits 4k it stops. But as soon as I let off the gas an drop below 4k it does it again. And if I park it for 30 mins and start it back up it's gone till the car gets hot again. But it never overheats and no obd codes and oil pressure stable the whole time. Any help would be appreciated. I'm not a mechanic but I do have decent understanding of cars and have worked on quite a few before.
Hella post bump but if you see this - read my earlier post up there.
These cars only use closed loop fuelling under about 4k rpm - above that, its just using the mapped tables/values for fuelling.
Thus, your issue suggest the problem is with the closed loop reference system, ie the lambda or MAF sensors, or a vacuum leak.
These cars only use closed loop fuelling under about 4k rpm - above that, its just using the mapped tables/values for fuelling.
Thus, your issue suggest the problem is with the closed loop reference system, ie the lambda or MAF sensors, or a vacuum leak.
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