Chrysler Repair: high idle- 318, intake air flow, fuel injection systems

Question
Hi Roland- '91 Dodge Dakota, 5.2l, 2wd, no air cond:  suddenly, I have a very high idle speed. Used to be around 800, now it's 1800 and have to keep both feet on brakes to keep it stopped.  The idle is constant, the pcv line is sucking, but I don't hear the idle speed control activater kick in when I turn the key to start (but not run).  I took a chance and bought a TPS, but it didn't make a difference.    This is a TBI carb.  ANY help would be greatly appreciated, Roland. Thanks, Paul (Toronto)

Answer
Hi Paul,
I suspect the cause of the high idle to be a cracked vacuum line or disconnected line at a fitting. In contrast to conventional carbs or fuel injection systems that monitor the intake air flow to set the mixture (and if there is some tramp air getting into the manifold it will over-dilute the mixture), the injection systems on many of the Chrysler cars have a feedback correction via the 02 sensor which will respond to a leak by lengthening the fuel pulse width and so it is as if you had opened the throttle a bit. So I would suggest that you trace each of the vacuum lines shown in the underhood labed to try and find an air leak.
If you can't find one you might take spray can of starter fluid and use a very small squirt at each of the vacuum line connections while the engine is idling to see if any site produces a momentary modification of the idle.
The other diagnostic test might be to readout the fault codes on your engine controller. If you turn ignition switch: 'off-on-off-on-off-on' and leave it on, all within an elapsed time of 5 seconds or less, then watch the check engine light to begin to flash, pause, flash. pause, flash... Count the number of flashes before each pause and make a mental note of the number or write them dowm. The last two groups of pulses will have 5 in each. Thus when you take all the numbers you have written down and pair them in order, the last number you have is 55, which is the code for 'completion of readout'. All the other two digit numbers that you will get are a code for some fault observed by the engine controller. Either write back with the numbers or go to www.allpar.com/fix/codes.html for a translation of the code numbers. You can readout the codes reapeatedly to make sure that you are getting them correctly.
Roland