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Old 07-18-2012
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High idle. Engine won't slow down while coasting. No HVAC controls. P1506. Related?

I have a 98 2WD ford ranger with a 2.5L. I bought it a little over a month ago and I noticed shortly after that it was idling a little high. It seems like it's gotten a bit worse and now it idles at about 925-975 RPM. Its not a perfectly smooth idle, but it isn't too rough.

The motor also doesn't slow down while the clutch is in or while its in neutral until I come to a complete stop. If i'm at 2500 RPM when I take it out of gear, that is pretty much where it stays until the truck stops.

Also, the HVAC controls don't work. It always blows out the dashboard. Changing the temperature, speeding up, slowing down, going WOT, idling, all have no effect.

It also has a check engine light, P1506. For about two weeks it was only a pending code, but now the light is on.

Things I've checked/tried:
Clean IAC valve, throttle body, and MAF sensor
Tested the TPS. It reads .8 volts at idle and 4.75ish at WOT. Seems normal to me.
Unplugged the maf while idling. The truck dipped down to about 600 RPM for a split second, then right back up to 950. No other change
Unplugged the TPS while idling. No change.
Unplugged the IAC valve while idling. Truck drops down to about 850.
Plug it back in, back up to 950. So it seems like it may need to be replaced. I also noticed, that while it was off for cleaning, it appeared to be open slightly. I'm not sure if this is normal.
Sprayed brake cleaner on a couple vacuum hoses (I doubt I got them all, and my truck doesn't really appear to match what's on the vacuum diagram sticker) and around the throttle body and intake manifold. No change.

As far as the no HVAC controls go, I looked for the little "orb" that everyone refers to when this topic comes up, and I can not find it.

I can replace the IAC valve but I wanted to get some input first. Thoughts, tips, tricks, suggestions?


Edit: About the vacuum orb thing. The truck has larger tires, removed fender well liners, and a swapped motor (from a 99). I'm thinking they could have removed it for any one of these, and never put it back.
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Old 07-19-2012
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Upon recommendation from someone on another forum I decided to replace the idle air control valve. I reset the CEL with my Ultragauge, and then replaced the IAC valve. So far the check engine light is still off, but I only drove it about four miles after.

The idle has gone up even higher. Now it sits around 1050. Other than that, no change.
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Old 07-19-2012
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Well the check engine light came back on, so I swapped the old IAC back in. Hopefully Auto Zone will let me return the one I bought.

But, I did find something interesting...


The white one connects to a black one then appears to go up under the dash.


The black one connects to a red one then goes off with a bunch of other lines.


Then while taping these out of the way I found this...

A gray line that appears to go to the same place as the white one that I mentioned first.

I guess these are the remains of my HVAC controls...? I sprayed brake cleaner on, in and around them with no change in idle. I still haven't found any leaks. I've heard that propane works better so I'm going to look for a hose to put on my torch and try that.
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High idle. Engine won't slow down while coasting. No HVAC controls. P1506. Related?-20120719143043873.jpg   High idle. Engine won't slow down while coasting. No HVAC controls. P1506. Related?-20120719143054558.jpg   High idle. Engine won't slow down while coasting. No HVAC controls. P1506. Related?-20120719143145129.jpg   High idle. Engine won't slow down while coasting. No HVAC controls. P1506. Related?-20120719143536740.jpg  
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Old 07-20-2012
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The vacuum lines for the AC system run from the intake manifold to the vacuum reservoir (should be located in or around the right front inner fender), then from the vacuum reservoir through the firewall to the function selector switch. This line should be black.

I believe a white line runs from the function selector switch to the vacuum control motor for the recirculation door and has a tee that runs back through the firewall going to the heater control valve (I believe it’s gray). Look behind the glove box and the function selector switch to verify whats what.

Last edited by Rev; 07-21-2012 at 12:13 AM.
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Old 07-23-2012
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So, I reset the CEL light and replaced three hoses that were cracked/in bad shape. I replaced the one that goes from the valve cover to the throttle body. And also the two that are connected to this thing...

Sorry, I'm not sure what it's called.

It seemed to run a little better, but after about 2.5 days the CEL light is back on.

What is a good method for checking for vacuum leaks? I've tried both brake cleaner and a propane blowtorch with a hose on it and I haven't been able to find anything. Even putting the propane or brake cleaner directly on the cut HVAC lines doesn't do anything noticeable to the idle. I'm off to the junkyard in the morning to look for a new HVAC orb, but I would still like to be able to check for leaks elsewhere. Also, does the HVAC orb have an actual name? I'm always seen it referred to as the orb or vacuum ball...
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Old 07-26-2012
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I installed a new (old) vacuum reservoir today. The good news is that I now have full heating and cooling functionality. The bad news is that the check engine light came right back on and the motor still behaves exactly as before. In the morning I'm going to give the engine bay another once over looking for any dried out/cracked lines. If I can't find anything I guess I'll just have to swallow my pride and take it to a mechanic.
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Old 07-26-2012
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So I was searching for vacuum leaks (for what seems like the thirtieth time) and while I was standing there with the hood open I had a thought... I decided to completely back out the idle adjustment screw. I then went and drove it and it was a LOT better. Before I would put it in neutral and it would always go to about 2300 rpm and just sit there. If I was only at 1200 RPM when I took i out of gear, it would shoot up to 2300 and sit there until I came to a dead stop. But now, while coasting it goes to about 1300-1400. Muuuuch better. It also idles lower, about 870 rpm and the idle is more consistent. I feel like a tard for not trying this before. I looked at it a week ago and thought about adjusting it, but It was barely touching the throttle so I guess I just dismissed the idea. And one more thing, the Ultragauge is now reporting 0 trouble codes and 0 pending trouble codes. So, am I fixed? 870 sitting still at idle still sounds a little high, and this is my first vehicle with a standard transmission so I'm not sure what is considered normal while coasting.
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Old 07-27-2012
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One more thing I forgot to mention. Regardless of setting, the ac/heater still has a little bit of air coming out at the floor. So I think I may have a small leak somewhere, but I think it's going to be pretty tough to find.
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Old 07-27-2012
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i had the same hvac problem, and the leak was really tough to find. ended up being a puncture through the black vac line you have pictured.

we have different year motors and stuff changed but check the vacuum lines running to your evap canister. those lines dry rot, might be causing your high idle, but i don't think so.
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