97 2.3 rough idle issue

[84tony]

I have a 97 2.3L 5 spd. ext. cab ranger that is having an rough idle issue for the past month. When the engine is cold and I start it up it runs fine when the high idle is on. Once the rpms drop down to about 800 after it comes off high idle, it idles rough. After the engine gets above about half way to normal operating temp. it will idle fine. So far I have cleaned the throttle body and the IAC, I put new plugs and wires on about 3 months ago. I did have a code for a misfire on cylinder 1 when it first started doing this. I cleared the code and it has not came back on since. Would the power steering pressure switch make it idle rough if it is faulty?

 

[tomw]

No, the PSPS is just used to 'bump' the idle when the power steering pump load is going to be added at idle. It closes when pressure raises, and the computer responds to that signal and uses the IAC to prevent stalling due to the added load of the pump.
If the flaky idle is when cold, check that the air temp sensor and coolant temp sensors are working. The computer uses them to richen up the idle mix when cold as the cold air and coolant make the fuel mixing with air not work as well.
tom

 

[84tony]

I changed coolant temp sensor 2 days ago, it made a little improvement in the idle when the engine is still cool. Still rough but not as bad as before. I am going to change the air temp sensor tonight, hopefully this will fix the problem.

 

[tomw]

If you have a thermo vacuum switch in the airbox housing the air filter, it may be operating a vacuum motor to move a flap forcing the intake air to come past the exhaust manifold and get a bit of pre-heat. My 85 has a vacuum bleed that opens when warmed, and allows the flap to spring back and let in cool air. When closed, it will pull the flap, and make the air come from around the exhaust. I have a weak cold idle, and just accept it, even with EFI instead of a carb. The idle speed is good, but it will tend to stumble if I try to accelerate. It is OK because I want the engine warmed a bit before I start 'using' it. Going down the drive, if I keep my foot off the gas, it will go fine and fast enough I can even shift into 2nd gear.
The ACT and ECT are used by EFI systems. If you don't have EFI, I don't think you have them as used by a computer. The coolant sensor on the side of the block near the oil filter is for the gauge or 'HOT' light, and has no effect on carb'd engines.
A carb engine will stall if the choke comes off too soon.
SORRY.
I am confused. Is this a 97 or an 84? You listed both... The above applies partially to both....
Never. Mind.
tom

 

[84tony]

It is a 97 2.3L. I changed the intake air sensor and it made a little improvement, but the engine still idles rough when the it is cold. Ordered a IAC and I will change it out next week. Other than this annoying rough idle the truck runs great for having 175K miles on it, would like to put another 100K it before I give it up for good.

 

[tomw]

I am not sure if your truck has one, but a MAP sensor can fail, and it will cause the idle to be all over the place. It senses load on the engine based on manifold vacuum, and informs the computer, which then can adjust the fuel flow to provide enough power to 'run' the engine smoothly, but not more than it needs. If the signal is goofy, the fuel flow will be goofy too, and cause wandering idle.
If you have a 'hard' miss, just about every time a cylinder fires, then you are more likely to have mechanical problems, such as valves, cam lobes, rings, and head gasket problems. These failures are more notable at slow rpms, and get 'hidden' at higher rpms.
A good compression test or leakdown test will inform as to the mechanicals, except cam, for the most part. An individual bad cam lobe is unlikely, as far as I know. If your compression test is good, then you start looking at ignition and fuel systems.
tom

 

[84tony]

No MAP sensor that I know of for this truck, there is an EGR valve pressure sensor. It only idles rough when the engine has not gotten warm up, once it reaches about half way to normal operating temp it idles smooth. It also idles smooth with the ICM in "choke" mode, but after it turns choke mode off and the engine is below half way to normal operating temp. it idles rough. I would think mechanical issues would be present all the time.

 

[tomw]

Read what I said about 'mechanical' issues. Higher rpm will mask the problem, in more ways than one, so it is not as noticeable. The problem does not go away, but you as a human will not be able to detect the problem. A bad valve will cause a definite miss at slow idle speeds, but the cycle time on the highway will make YOU unable to feel it, or hear it. (me too) Bad rings can cause low compression, leading to misfire at idle, moreso than at highway speeds, as the compressed gas does not have 'time' to escape past the weak wobbly rings, and the engine 'fires' on that cylinder, with 'higher effective compression' than you would get from a compression test. That is why a 'leakdown' test is used for a more definitive check of valves, cylinders, pistons, rings and head.

 

[84tony]

Put on new IAC valve and so far it is idling smooth at all temperatures.