2000 Tacoma (Very High Idle & Negative Fuel Trims)

Working on a 2000 Toyota Tacoma with a idle at warm up around 1100-1500 RPM's. When I first start the truck the idle seems to be a little low in open loop at around 900 RPM's, but when it goes to closed loop it idles very high from 1100-1500 RPM's. Also, the short term and long term fuel trims are negative at around 14-20 percent. Any help is great appreciated. See attached picture.

Brian

Let's check out some items that can cause a rich condition. High fuel pressure, leaking pressure regulator if there is one, ECT sensor, leaking injector(s), O2 or A/F sensor, MAF or MAP sensor, saturated evap canister.

Are you able to force the truck lean by pulling a vacuum line and rich by introducing propane or some other fuel to the intake?

Any DTCs?




Nissan Technician

If I pull a vacuum line the truck will idle down considerably. I have verified that the ECT sensor is good. The O2 sensor B1S1 is reading correctly as well. Originally the truck had no power and when I did a WOT test, the O2 B1S1 was reading very lean and this was because of the dirty MAF in which I cleaned & eventually replaced. Either way though the high idle was before and after the MAF replacement. The power has been restored to the truck, but there is still a very high idle and negative fuel trims. I also checked the fuel pressure before the regulator which was 44 PSI. I have not checked it after the regulator as it is hard to get access to. Originally, it had a P0171/P0174 lean codes, but I believe that had to do with the dirty MAF as no codes have come back since then.

what is your PID data on the following

RPM
Desired RPM
IAC position

RPM - 1393
Idle Air Control Duty Ratio - 22.6%
There is no desired RPM PID available.

Is there a vacuum line on the regulator that you reach? Pull it off and see if the regulator is leaking. If it is, that vacuum line will suck fuel right into the intake.

Nissan Technician

I have checked and pulled the vacuum line on the fuel pressure regulator and and it is not leaking. I appreciate the responses.

IAC position % is showing that the IAC valve is almost closed. Check the throttle bore where the throttle plate comes in contact with the bore and look for black carbon build up. Clean with a tooth brush and carb cleaner. Too much air getting past the throttle plate, IAC compensates by closing.

At what percentage is the idle plate considered closed? I cleaned the throttle body as well, but it still has a very high idle. Thanks for the response.

20% closed... 30-40% at idle... 50-60% full open.

unplug the MAF does idle correct?

Nice info on the IAC. Thank you. I unplugged the MAF and it dies. If the IAC is at 22.6%, it must be getting air from somewhere else or like you said maybe a worn throttle body perhaps? I am not sure why the fuel trims are so negative though. Although they are only negative at idle for that RPM and load range.

have you done any checking for vacuum leak/s... use a squirt bottle with water and check the brake booster line as well (pinch it off or disconnect from booster).

I checked & pinched off any vacuum lines and found no vacuum leaks. I honestly believe now that its a worn throttle body perhaps.

did read about a guy who found that the throttle shaft was slightly worn and mis aligned causing the throttle plate to hit against the bore and not allow it to fully close.

It seems to be the worn throttle body as I can manually move it down and it does idle lower. Thank you.