91 Ranger High Idle

[RedNeckRanger]

My '91 Ranger 3.0L has developed a high idle that I can't seem to figure out. The truck starts fine, but idles at about 1800 rpm. With the IAC unplugged, the idle does not change. I backed out the throttle stop screw until it was not touching, but no change. The throttle plate is closed, and if I block off the IAC where it bolts to the intake, there is still no change! I have checked for vacuum leaks at the manifold, IAC etc and found none. I tried cleaning the IAC anyway in case it was stuck open, but no change. If I scan for codes, there are none present. If I run a KOEO test, it passes. If I run a KOER test, I get a code 13, unable to control idle at low rpm idle test. I am thinking maybe there is something going on with the timing and this is affecting the idle speed? I am going to disconnect the SPOUT and check the timing. Anyone have any other suggestions?

 

[RedNeckRanger]

Update

I checked today for any vacuun leaks again, didn't find any. Disassembled IAC and cleaned it again. Also made sure the valve was moving freely. Removed SPOUT and checked timing, was right where it should be. Ran engine at idle and did wiggle test of all connectors and hoses, no change. Unplugged IAC while idling, slight change in RPM. Unplugged MAF while idling, slight change in RPM, engine seemed to run a bit rougher. I had scanned for codes yesterday using a Snap-On scanner and there were none on the KOEO test. Ran test again today using my Canadian Tire OBD1 scanner, had code 29 - VSS Vehicle Speed Sensor problem and code 66 - MAF signal low. Both were continuous codes according to the scanner. I think that the two codes are obviously unrelated to each other, and the MAF code is what is causing my problems. I have another known-good MAF sensor that I am going to swap into the truck and see what happens.

 

[Rearanger]

The throttle plate is closed, and if I block off the IAC where it bolts to the intake, there is still no change!

If you're also blocking the throttle plate by-pass (IAC port) then the engine should stall for lack of air. Nothing will compensate for that. Has to be a vac leak. How about the PCV hose? Does the idle change when engine hot?

 

[RedNeckRanger]

Update

No, the idle does not change once the engine was hot. I checked the pcv hoses and they seemed fine, but since they were 21 years old I changed them anyway. I know that the engine should have stalled out when the IAC was blocked off, but it kept running the same. I checked for vacuum leaks at the upper and lower intake manifolds, but found none. I found an exhaust leak at the right exhaust manifold where it bolts to the head. I was wondering if that could be drawing air in and giving the O2 sensor a false lean reading? I'm grasping at straws now.

 

[Rearanger]

I was wondering if that could be drawing air in and giving the O2 sensor a false lean reading?

So what if it was, how would that cause high idle? False lean would tend to make combustion rich. If the ECU was detecting lean at the O2 then it would attempt to add more fuel - but without air from the intake you wouldn't get high(er) idle.

I'll still go with vac leak - somewhere, considering how you choked the air supply and still got high idle. Can't run combustion on no air.

 

[RedNeckRanger]

While checking for vacuum leaks, I ended up removing the air intake tube from the air cleaner at the throttle body. The engine ran rough and stalled instantly. It would not keep running with the tube disconnected unless I kept my foot on the gas. Once I reconnected the air intake tube to the throttle body, it kept running on its own but idled high again. Any suggestions where the air could be coming from that is making the engine idle high but stall when the intake tube is disconnected? The only things attached to it are the air cleaner, MAF and the hose to the oil filler neck. This may sound stupid, but the oil cap on this engine has no rubber seal of any kind to seal it to the filler neck, it is just a plastic to plastic seal. Could air be entering here?

 

[Tractor Dan]

Not sure if this could cause an issue but where the intake duct goes onto the manifold, are you getting a good tight seal and seals at all your connections for that? Mine wasn't sealing tight, previous owner had buggered it up and i kept it working sicne 08 but it finally gave me problems in June or so. I ended up replacing it with 3inch chrome duct that is modular so you can add pieces and elbows. That and i put a 6 inch cone filter on it. So maybe make sure all your connections are tight. Not the most high tech answer but as i have learned sometimes the oddest thing or thing you wouldn't think of to be an issue at all is the root of the problem.

 

[locovaca]

While checking for vacuum leaks, I ended up removing the air intake tube from the air cleaner at the throttle body. The engine ran rough and stalled instantly. It would not keep running with the tube disconnected unless I kept my foot on the gas. Once I reconnected the air intake tube to the throttle body, it kept running on its own but idled high again.

It stalled because you essentially disabled the MAF. The ECU had no clue how much air was going into the engine and thought that none was at all. You didn't really learn anything from this test other than what a MAF does