An interesting high idle issue (1 Viewer)

[Spook50]

Wow this gets more and more interesting.

Today I ran through the complete O2 sensor test (yeah there went a quarter tank of gas). The test, per the FSM, is actually done twice to verify results. The first time I ran the test, I got a fail (voltage fluctuating too slowly once the O2 sensor was hot). Second time, I got a pass. So I ran it a third time and still got a pass. I had actually gotten excited that I narrowed something down when the CEL flashed a code 51 as soon as I throttled up the engine with TE1 and E1 connected, but when it stopped on returning to idle, I realized (and verified in the FSM) that it was only coming on due to the "off idle" condition that the TPS was indicating to the ECU. I guess at least now I know for sure that the TPS is in fact set correctly.

Anyhoo, I got curious after this and pulled the TE1/E1 jumper wire and then checked voltage between VF1/VF2 and E1 with the engine warm and running at 2,500 RPM. Instead of the 0 VDC I should've gotten (keep in mind this part of the test as related to the O2 sensors is only actually done if the earlier test is a confirmed fail, which tells me this is not pointing to either of the O2 sensors), both VF1 and VF2 showed a steady 3.54 VDC on my multimeter!

So now it's time to do some more FSM homework and see what direction this should be pointing me....

Edit: Looking through the FSM more, one possible cause of high idle is a faulty fuel pressure regulator. I did replace my FPR when I had everything apart with a new one that was posted about several years ago in a thread discussing viable alternatives. I'll have to check the box again if I still have it but I believe it's a Standard Motor Products part. I do know it's not an OEM Denso part. I do have the old unit but rather than just swap it out to see, I'm going to get a fuel pressure gauge (like what I used to track down my non-running fuel pump when my 62 stopped on Christmas Eve in 2021. Yes, that SUCKED) from O'Reilly and check what actual pressure I'm sitting at. To be fair, I'm doubtful that this is the case, but want to confirm one way or the other rather than just brush it off.

 

[Spook50]

Fuel pressure test seems to have been a bust. Gauge showed 42 PSI with the fuel pump connected via the diagnostic port, and actually low at idle/high vacuum with rapid fluctuation between 30 and 34 PSI (minimum per the FSM is 37 PSI). Disconnecting vacuum from the pressure regulator bumped it right up to stable 42 PSI.

Was really hoping the new pressure regulator I put in was just faulty on the high side and I could just swap the old one back in. It wasn't causing any problems when I replaced it. Just the factory original that I replaced for the sake of preventative maintenance. Honestly I'm tempted to swap it back in anyway so I can compare fuel pressure readings on that too. The kids are having fun hanging out in the driveway with me and watching me work and explain what I'm doing, so I'm going to go for it

Stay tuned....

And after realizing that the schrader valve to connect the gauge was itself faulty and correcting that, I've found that the new pressure regulator is most definitely faulty, unfortunately I don't think the high idle is resulting from that. The old regulator sits rock solid at 42 PSI at idle, and when I remove vacuum to the (old) regulator, it jumps to 52 PSI. Given the fluctuation with the new regulator, I decided to just leave the old one in place for now. But it seems the regulator, while faulty, is not contributing to the high idle.

Back to the drawing board.....

 

[Spook50]

Well after driving normally for over two months, going over EVERYTHING multiple times and verifying it's all good and set right, my warm idle is locked in at 850 RPM, no matter how I have the throttle body screw set. It's as if the ECU is for some reason wanting to hold the idle at 850 RPM instead of the 650 it held before I did the injectors, gaskets and throttle body.

Certainly weird, but I'm now averaging 15-16 MPG overall regardless, so there's that