Posted: 8/4/2008 4:21:34 AM EDT
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I have a 1998 F150 with the 4.6 V8 in it. It usually idles around 850RPM, but lately it has started idling between 1100 and 2100. This is adversely affecting my gas milage, and since it is not what I would consider normal, what should I check about this? The accelerator pedal is not sticking in the cab, but I dont know about the cable beyond there. Thanks |
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The cable will go up to the Throttle body. The TB will on on the intake manifold. I would pull the air cleaner duct work off the TB and physically look to make sure its not bound up, and make sure it closes all the way. I would maybe get some good intake spray cleaner to spray on the "butterfly" (the part of the TB that opens/closes). If you spray it while truck is running, be ready to be opening the throttle while spraying cuz when you spray it, itll kill the motor (not in a bad way) Things that could be wrong: 1) TPS (throttle positioning sensor) I know on the old 5.0s they were known to go bad or get out of wack. 2) Vacuum line cracked , pulled off or broken 3) Ignition timing, hit it with a timing light. 4) Electronic choke stuck (not sure how this is in an FI vehicle, but there has to be something to turn the choke on and off) |
| From the passenger's side valve cover, follow the tube coming off the PCV valve to the throttle body/intake manifold. More than likely, somewhere you've got a rubber hose or elbow rotted through on that tube, allowing unmetered air into the engine causing a fast/high idle. |
This would be true if it was a MAP system this is a MAF system and therfore this information is not correct. The Idle Air Conrol Motor controls idle speed. This needs to be checked. Possible to to accelerator cable but unlikely ensure that you cannont close throttle anymore and ensure that butterfly(s) are closed. Also there are no chokes on fuel injecton. Chokes are neccicary on carburated cars to compensate for condensation of fuel on intake manifold and there is minimal condensation due to minimal lenght from injector to end of intake runner, This condensation is compensated by Engine coolant temp sensor. For more help PM me SS |
This. The IAC can adjust for a certain amount, but the PCM is looking at fuel trims more than IAC duty cycle, it'll notice a lean condition before it notices an idle discrepancy that needs to be corrected with the IAC. The IAC is still a viable suspect though in regards to the OP's problem, but those PCV hoses are notorious for rotting through and causing running problems. As with anything on Arfcom, he should |
I see where your going with that and it makes sense but i have dealt with many cars with maf systems and vaccum leaks and this doesnt show up as a usual symptom SS |
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This would be a correct statement by Quintin also, You could easily monitor the Fuel trims and the IAC pids to determine unmetered airflow as well as comparing readings to the Baro which will be off due to the leak |