In the hope it helps, I thought that I'd share something I recently experienced. Recently my rig started surging on prolonged aggressive accelerations. I checked all common things (electrical connections, vacuum connections, intake, exhaust, computer codes, scanner stuff, etc.) and found nothing of note. Then I checked less common things like loose linkages, accelerator cables, transmission cables, fuel filter, fuel pump, etc and everything seemed excellent. The only thing observably different was sporadically slightly leaner AFR's of 11.0 to 11.6 on prolonged aggressive accelerations instead of the typical 10.0 to 10.8. Of extra interest in my mind was that the transition from closed loop to open loop was the typical instant 12.8, then 12.6, then 12.4 etc as always.
I was almost at a loss for a few weeks until one day during traffic the rig shuttered and stalled. I then suspected the Fuel Pump Relay. Luckily the rig started right up. I went to work and then later took her home and cleaned the contacts of the relay with contact cleaner. I wanted to avoid at first the filing trick others have had success with simply because I basically think that the contacts should stay sharp and not be blunted by filing. (Its an OCD thing not to suggest that filing won't work.) Well the contact cleaner helped somewhat but still had surging during aggressive acceleration. I immediately resolved to replace the relay and was almost completely confident that the stalling wouldn't appear again for the few days it took to receive the relay.
Meanwhile I noticed that nearly every time the rig surged faster the wideband was reading 11.0 to 11.6 and that every time the rig surged slower (is that the right way to say something surged?) it read its regular 10.0 to 10.8! After observing this over and over I felt like I stumbled onto something rather significant with a Fuel Pump Relay that was just failing just enough to lean the mix a little at acceleration. The point to this is that this is another example of our rigs running stronger with slightly leaner AFR's.
My understanding from using Toyota texts is that "when the engine is cranked or operated at high speed and/or heavy load, the ECU turns off TR1 closing contact A of the Fuel Pump Control Relay. This allows current to flow directly to the fuel pump, causing it to run at high speed. " whereas "under all other operating conditions, the ECU turns on TR1, which energizes the Fuel Pump Control Relay. This closes relay contact B and forces current to flow through the resistor, causing the pump to run at low speed." I can only conclude that a rapid on-and-off of this cycle caused the surging. Since the default to relay is going through the resistor I am assuming that the ECU rapidly and repeatedly tried to close contact A but the relay could not consistently maintain that mode and then that the default rapidly and repeatedly activated. I'm not sure how to test that assumption but one thing that is clear was that the rig ran stronger for the few seconds it was a little leaner.
Now the next thing on my mind is if its actually possible that with the fuel pump pumping in "slow mode" the rig can really still maintain an AFR of 11.2 !?! That would be absolutely astounding to me. What I think is much more likely is that as soon as contact A closed again the fuel pump fully spun up and rapidly recovered the leaning AFR. I'm not really willing to test this theory but it is something to think through and it is easy enough to test for someone much more daring than me! If there really is enough headroom that the stock fuel pump can provide AFR's of 11.2 on a Forced Induction rig when still in slow mode I'd be absolutely astounded. Even if not, the fact that WOT richens to 10.0 with a stock MAF and 10.8 with the LT MAF and the fact that 10.8 runs stronger than 10.0 and 11.6 runs stronger than 10.8 shows me that these rigs have huge headroom from a fueling perspective, that they like things leaner to a proper point, and that I now need to add an Exhaust Gas Temperature sensor to see if the little leaner operation is increasing exhaust temps too much and then finally to add more air via more boost and continue to test the theory of optimal AFR's for performance and for safety. Stay tuned (pun intended).
I was almost at a loss for a few weeks until one day during traffic the rig shuttered and stalled. I then suspected the Fuel Pump Relay. Luckily the rig started right up. I went to work and then later took her home and cleaned the contacts of the relay with contact cleaner. I wanted to avoid at first the filing trick others have had success with simply because I basically think that the contacts should stay sharp and not be blunted by filing. (Its an OCD thing not to suggest that filing won't work.) Well the contact cleaner helped somewhat but still had surging during aggressive acceleration. I immediately resolved to replace the relay and was almost completely confident that the stalling wouldn't appear again for the few days it took to receive the relay.
Meanwhile I noticed that nearly every time the rig surged faster the wideband was reading 11.0 to 11.6 and that every time the rig surged slower (is that the right way to say something surged?) it read its regular 10.0 to 10.8! After observing this over and over I felt like I stumbled onto something rather significant with a Fuel Pump Relay that was just failing just enough to lean the mix a little at acceleration. The point to this is that this is another example of our rigs running stronger with slightly leaner AFR's.
My understanding from using Toyota texts is that "when the engine is cranked or operated at high speed and/or heavy load, the ECU turns off TR1 closing contact A of the Fuel Pump Control Relay. This allows current to flow directly to the fuel pump, causing it to run at high speed. " whereas "under all other operating conditions, the ECU turns on TR1, which energizes the Fuel Pump Control Relay. This closes relay contact B and forces current to flow through the resistor, causing the pump to run at low speed." I can only conclude that a rapid on-and-off of this cycle caused the surging. Since the default to relay is going through the resistor I am assuming that the ECU rapidly and repeatedly tried to close contact A but the relay could not consistently maintain that mode and then that the default rapidly and repeatedly activated. I'm not sure how to test that assumption but one thing that is clear was that the rig ran stronger for the few seconds it was a little leaner.
Now the next thing on my mind is if its actually possible that with the fuel pump pumping in "slow mode" the rig can really still maintain an AFR of 11.2 !?! That would be absolutely astounding to me. What I think is much more likely is that as soon as contact A closed again the fuel pump fully spun up and rapidly recovered the leaning AFR. I'm not really willing to test this theory but it is something to think through and it is easy enough to test for someone much more daring than me! If there really is enough headroom that the stock fuel pump can provide AFR's of 11.2 on a Forced Induction rig when still in slow mode I'd be absolutely astounded. Even if not, the fact that WOT richens to 10.0 with a stock MAF and 10.8 with the LT MAF and the fact that 10.8 runs stronger than 10.0 and 11.6 runs stronger than 10.8 shows me that these rigs have huge headroom from a fueling perspective, that they like things leaner to a proper point, and that I now need to add an Exhaust Gas Temperature sensor to see if the little leaner operation is increasing exhaust temps too much and then finally to add more air via more boost and continue to test the theory of optimal AFR's for performance and for safety. Stay tuned (pun intended).
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