Strange Idle

[HER2702]

OK, I talked about my "new" boneyard engine earlier but I do have 1 aggravating issue that I need a little help with. When I pull up to a stop light and kick the clutch in the engine stays at roughly 1500 rpm until my speed falls below 10 mph. When it reaches 10 mph the rpms drop to normal, around 700. I've checked and cleaned the MAF and IAC but it still persists. Yes I've checked all possible sources of vacuum leaks. All vacuum lines were replaced during the install and the intake gaskets are new FelPro installed while the block was on the engine stand. It's not really hurting anything but it isn't right. Besides, it's a little embarrassing to pull up to the light and have your engine sound like it's about to come apart for a couple of seconds.

Any ideas? Suggestions?

 

[rusty ol ranger]

Throttles not stickin a bit is it? Maybe a weak return spring? Or some gunk tiein it up from sittin?

 

[rangerenthiusiast]

I know you said you cleaned the IAC, so how did it look inside? I ask because I recently had a similar problem (except that it would never kick down to 700). I took it off and cleaned it well, but the metal shaft inside was all pitted looking. I could actuate it back and forth just fine, but when I hooked it back up, I still had the same problem. Disconnected it altogether and it idles nice now, even at stops. I just ordered a new one. Maybe try disconnecting it for a day and see if your problem persists?

 

[2trux]

I will be interested to see if you solve this as my 2.9 does the exact same thing. It's intermittent, sometimes running at much as 2K RPM as I coast up to a light. It always drops back to normal idle when the speed gets below 10 MPH. I believe I replaced the IAC when I rebuilt the engine 2 yrs ago, but I will have to look to see. Other than that the IAC works perfectly, Increasing the idle when cold and dropping it back down when warm.

 

[RonD]

Idling engines are big polluters, they have to run Rich or engine will start to overheat.

With computer control and electronic speed sensors, car makers could hold idle higher while coasting or above 5MPH, 10MPH sounds high, as speed dropped below that then warm engine idle would be set, 650-700 manual, 750-800 auto

1995 and up with the newer EEC-V computers would have this.
And RPMs should also hold high when shifting, for 5 seconds or so, also helps to lower emissions, but not sure when this was added.

It would be hit and miss on 1994 and earlier computers having this software.

1,500rpm does sound too high, 1,100-1,200 would be expected, so could be throttle cable or throttle plate, even cruise cable is holding throttle open a bit.
Throttle plate needs a good strong spring to hold it closed all the way against 20+ inches of vacuum as engine RPMs drop


Just to take it off the table test for a vacuum leak
Warm up engine fully, and let it idle
Unplug IAC Valve's 2 wires, it will close all the way and RPMs should drop to 500 or so, or engine may even stumble and stall.
Either is GOOD, it means no vacuum leaks

If RPMs stay at 700 or so then there is a leak somewhere.
OR.......someone has messed with the anti-diesel screw on the throttle linkage, it looks like an idle screw but idle screw can't be used with fuel injection, no idler Jets.
With IAC Valve still unplugged and high idle, turn this screw counter clockwise, if idle doesn't change then turn it back to where it was, this also adjusts TPS(throttle position sensor) voltage.
If RPMs start to drop as you turn the screw then keep turning until RPMs are at 500 or so, below 600 is fine.

Purpose of IAC Valve is to set idle RPMs if throttle plate is mechanically set, screw, to 700RPM then IAC can't do its job

 

[HER2702]

Throttles not stickin a bit is it? Maybe a weak return spring? Or some gunk tiein it up from sittin?

No, it's the "original" throttle body from the 94 and I did clean it just because since this was a "new" build.
I had to use the upper and lower intakes and fuel rail from the 94 (already had new injectors). The 95 was a God-forsaken EGR motor originally. Needless to say all that garbage is where it belongs.

But thanks for the thought.

 

[HER2702]

I know you said you cleaned the IAC, so how did it look inside? I ask because I recently had a similar problem (except that it would never kick down to 700). I took it off and cleaned it well, but the metal shaft inside was all pitted looking. I could actuate it back and forth just fine, but when I hooked it back up, I still had the same problem. Disconnected it altogether and it idles nice now, even at stops. I just ordered a new one. Maybe try disconnecting it for a day and see if your problem persists?

I am suspicious of the IAC. It was cruddy inside. I cleaned it well and put a drop of 3in1 oil on the shaft. It seemed to be OK but I'mm not 100% sure about it. I tried the IAC from the 95 but it revs to 2K with it. Of course I have no way of knowing if it's perfectly good or not.
I had it disconnected a couple of days ago, not on purpose tho. The connector didn't latch completely and it slid apart. She idled down to 650 - 700 but it stumbled really bad until it was up to full temp. That's also pointing me toward the IAC being bad.

I guess, unless there's a better way to test it, I'll just have to break down and buy one. It couldn't hurt, I've had the truck for nearly 20 years and it was on it when I bought it.

Thanks for the tip.

 

[HER2702]

Idling engines are big polluters, they have to run Rich or engine will start to overheat.

With computer control and electronic speed sensors, car makers could hold idle higher while coasting or above 5MPH, 10MPH sounds high, as speed dropped below that then warm engine idle would be set, 650-700 manual, 750-800 auto

1995 and up with the newer EEC-V computers would have this.
And RPMs should also hold high when shifting, for 5 seconds or so, also helps to lower emissions, but not sure when this was added.

It would be hit and miss on 1994 and earlier computers having this software.

1,500rpm does sound too high, 1,100-1,200 would be expected, so could be throttle cable or throttle plate, even cruise cable is holding throttle open a bit.
Throttle plate needs a good strong spring to hold it closed all the way against 20+ inches of vacuum as engine RPMs drop


Just to take it off the table test for a vacuum leak
Warm up engine fully, and let it idle
Unplug IAC Valve's 2 wires, it will close all the way and RPMs should drop to 500 or so, or engine may even stumble and stall.
Either is GOOD, it means no vacuum leaks

If RPMs stay at 700 or so then there is a leak somewhere.
OR.......someone has messed with the anti-diesel screw on the throttle linkage, it looks like an idle screw but idle screw can't be used with fuel injection, no idler Jets.
With IAC Valve still unplugged and high idle, turn this screw counter clockwise, if idle doesn't change then turn it back to where it was, this also adjusts TPS(throttle position sensor) voltage.
If RPMs start to drop as you turn the screw then keep turning until RPMs are at 500 or so, below 600 is fine.

Purpose of IAC Valve is to set idle RPMs if throttle plate is mechanically set, screw, to 700RPM then IAC can't do its job

Ron I have noticed the RPMs staying up between gears tho I haven't held the clutch to see for how long. I'll do that this afternoon. And, since I have this nasty habit of owning up to my own screw-ups, I will admit to tweaking the anti-dieseling screw in the past. Not trying to set idle speed tho. Trying to adjust TPS voltage. If I'm not mistaken the ADS will change the signal return voltage from the TPS. 1.0 volt is the target with the throttle closed I think. It sounds like you are much more familiar with these than me so please correct me if I'm wrong. Maybe I need to check and/or reset that before I go any further. I'm almost certain I need a new IAC I just don't want to pull the trigger on a $60 part right now unless I have to.

Thanks for the info. I'll let you all know what we find.