No Fast Idle
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From: Alpharetta, Ga.
No Fast Idle
91 4.0L no fast idle. Also, the temperature gauge never goes above the"C." I was wondering if anybody knows what controls the fast idle? I've read some things on here about the IAC valve or idle air bypass valve or something like that. Does that have anything to do with it? I also have read about the ect sensor (engine coolant temperature) but that it rarely goes bad. What is the normal operating point on the temperature gauge? Any suggestions would be appreciated. Thanks!
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From: Mitchell, SD
That sounds like the ECT sensor. How good (or bad) is your gas millage? The computer might think that your engine is at operating temperature when it is not, stuck open thermostat, or a bad gauge/sender for the gauge.
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From: Alpharetta, Ga.
That's what I was thinking. The gauge not working and not having fast idle would both seem to be related to the ECT sensor. I believe I'm getting about 20MPG in mixed driving. I just got the thing operational a couple of weeks ago (see my gloating post about transmission rebuilding.) Still working the bugs out. But everytime I open the hood and look into that black hole, I just stand there with this blank stare, and mumble something like "Maybe NEXT week I'll..." I'm going to go stare at it some more right now.
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If there is no fast idle on cold start up, then it is not the ECT sensor since it is not "in the loop" until after the engine warms up. Sounds more like the IAC (idle air control) valve to me. You can try removing and cleaning it with carb cleaner but around 50% of the respondents here wind up replacing it. While you're at it, you might want to pull the MAF (mass air flow) sensor and clean that as well with electronics cleaner - not carb cleaner. Each item takes only a few minutes of maintenance time to clean.
My '91 4.0 has always registered a "cold" reading on the dash gauge, usually hanging around between the "N" and the first mark on the gauge. I have plenty of heat (150 degrees) at the center dash register so I know the engine is at proper operating temperature. I've replaced the sending unit - to no avail, so I can only surmise the problem is the gauge itself. Chances are, this is the same problem with yours.
My '91 4.0 has always registered a "cold" reading on the dash gauge, usually hanging around between the "N" and the first mark on the gauge. I have plenty of heat (150 degrees) at the center dash register so I know the engine is at proper operating temperature. I've replaced the sending unit - to no avail, so I can only surmise the problem is the gauge itself. Chances are, this is the same problem with yours.
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From: Mitchell, SD
Originally Posted by aerocolorado
.....then it is not the ECT sensor since it is not "in the loop" until after the engine warms up. ....
Aero, If the ECT is not "in the loop," then how does the PCM know that the engine IS at operating temperature and stop the rich warm up mode?
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The ECT comes on line at some point during warm up but on the intial cold start it is not involved. This is why I suggested doing the idle test on a cold start up to as to eliminate this component. As the ECT warms up it sends a varying resistance signal to the EEC module until such point the EEC switches to different timing/injector profiles to better fine tune the engine/powertrain operation. (This is not to imply the ECT is the only sensor involved, this is just a basic explanation of EEC operation.)
Last edited by aerocolorado; Feb 23, 2005 at 10:20 AM.
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Freshman User
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From: Alpharetta, Ga.
I found the IAC valve and took it off and cleaned it. Didn't seem to do much, but it was really dirty. Might be idling a little smoother, or may be my imagination. Anyway, I was planning on replacing all the upper engine gaskets, to stop this darn oil leak, so I'm going to change the plugs, wires, vacuum hoses while I'm in there. Clean the intake manifold. Maybe that'll help. Does anyone have a reccommendation for plug wires and plugs? I usually use Autolite or Motorcraft stuff.
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Joined: Jan 2004
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From: North Alabama
Go with the Motorcraft wires and plugs. You won't regret it. I tried the super-duper lifetime warranty big name wires, and had nothing but issues. I finally replaced them with OEM and haven't had another problem. (on that van, anyway. The other van is another story.)
Originally Posted by htbird
Does anyone have a reccommendation for plug wires and plugs? I usually use Autolite or Motorcraft stuff.
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