My engine idles very high upon starting, and I've checked or replaced pretty much everything I can think of, the engine WILL drop down after several minutes( I haven't actually timed it, but I'd say it's at least five minutes), and once it does, I can rev it, and it returns to normal. If I shut the engine down, and immediately restart it, it goes back to a high idle for several minutes before returning to normal idle. I've replaced the idle air controller, the throttle position sensor, the vacuum control solenoid(broke it trying to get the vacuum lines loose, cleaned the throttle body thoroughly, cleaned the MAF sensor, re-torqued the lower intake manifold, replaced the upper intake gasket, replaced the EGR gasket, cleaned the EGR, and tested it, checked all the vacuum lines and connections, and checked the amount of vacuum(my guage is pretty old, it's showing right on the lower line of normal, so I think it's ok0, I've tried isolating parts of the vacuum system, by unplugging and capping off the lines, no changes noted. It has shown trouble codes P0171 and P0174, which is system adaptive fuel too lean on both banks. I recently replaced my heads, and rebuilt the injectors at the same time, everything was running just fine afterward, but I'm wondering if maybe running some seafoam thru the fuel would be a good idea. Any thoughts/information would be GREATLY appreciated. Sorry for writing a novel here, but I wanted to try giving a decent history. Thanks for reading.

 

I would check and clean or replace the IAC..Idle Air Control valve...its what regulates the idle speed. Do a google search for cleaning or replacing instructions.


JC

 

The one item you did not mention as replacing is the engine temperature sensor for the ECU. You have two; one for the guage cluster, and one for the ECU. The idle gets set by the ECU, basically controlling the amount of air allowed by the idle air controller. It adjusts this, using the reading of the engine temp sensor. If this sensor sees "cold", the idle is high.

The temp sensore used by the ECU is a two wire unit, the one used for the guage cluster is a single wire unit. They both may have two pins, but the harness that connects to them indicates what each sensor is used for.

It sucks tossing parts at a problem, not to mention the hurt it places on your wallet.

 

Has fuel pressure been checked? You either have too much air OR not enough fuel... Any other DTCs other than P0171/P0174?

 

Temp sensor..broken wire at the sensor...air pocket in the motor not getting coolant to the sensor till after the thermastat opens.. my exp with lean codes led to a bad /clogged filter limiting gas.. is the comp making up for it by running rich to give it the fuel it wants to run...

 

Now that I read this again, I'd be looking at your oxygen sensors themselves. Sounds like it idles high until the engine goes from open- to closed-loop fuel control. Have access to and kind of scanner or datalogger to be able to monitor the O2 sensor inputs into the PCM? Once again, are the P0171 and P0174 the only DTCs that are being set?