I am stumped. I have a 1996 B2500 5.9 MPFI.
I have replaced the IAC valve, the MAP, and the intake air temp sensor
but I still have an idle problem. Oh yea, I checked the TPS voltages
and they were correct.
When it's cold it idles ok, and "sometimes" when it is warm it will
slowly raise the idle RPM and slowly fade back to normal. I can not
really call it a rumble, surge, or loping. I can call it irritating.
No codes, no vacuum leaks, no egr, and I cleaned the throttle body.
I am not sure what the water temp sensor does, but this is my next
Does anyone have any wizdom, other than take it to a dealer. I
personally would like to learn what is going on here. Of course I
could pay but what good would that do for me to learn?
The fact that it's only happening when warm is an important indicator - this
seems to suggest that it's only idling improperly when in "closed loop"
mode. There are two modes of operation on a fuel-injected vehicle:
open-loop and closed-loop. When in open-loop mode, the computer determines
the amount of fuel to inject based on a fixed table (called a "fuel map")
based on several inputs: the MAP sensor, the TPS, the temp. sensor, etc. In
closed-loop mode, the computer still uses those input as a "suggestion" of
how much fuel to inject, but relies on feedback from the oxygen sensors to
adjust the fuel mixture up or down, attempting to maintain a specific
Since you're saying the idle only gets erratic when the engine has warmed
up, I'd suspect a faulty oxygen sensor. If I had a scan tool capable of
displaying oxygen sensor outputs, I'd use that to confirm these suspicions.
I'd be somewhat leery of simply throwing an oxygen sensor (or two, depending
on your specific vehicle), because of the cost (anywhere between $50 and
$100). However, this was exactly the problem with a friend's '00 Durango
recently... erratic idling when warm, stumbling during cruise (cleared up
when he floored the throttle - that also puts the engine back into open-loop
mode), etc. Even though there were no codes set, it was the oxygen sensor
that was the culprit. After changing that out, his problems were solved.
I was reluctant to go after the temperature sensor because of its
location. Well, I bit the bullet and went after it yesterday. When I
stuck my hand down in there to pull the connection off I had the first
indication of a possible problem, the insulation was coming off the
wire. When I finally got it out it looked like the wire was all chewed
up, shorted in some spots and had that ash gray burnt look.
I cut out the bad section, soldiered in a good stretch of wire, heat
shrink wrapped the connections and taped it all up. I checked the
reference voltage and it was right on the money. Because I had a new
sensor, I put it in.
I drove it much of the afternoon yesterday with only a few indications
of an idle problem but it went right away. So I can not say for sure
that I got it worked out but I feel much more comfortable.
I am getting real good about taking the engine cover off from the
inside of the van. Because it is a conversion there is all kinds of
stuff screwed to it.
PS, the only thing really left to change now is the crank sensor and
the O2 sensor.
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