How do I lower my idle speed???

BigTirez · Jan 2, 2008

I am failing my emissions testing because my idle speed is too high, it has to be between 400 and 1100 rpm and I can't get this thing to idle below 1300 rpm.

84 4x4
2.8L
5-speed TK
A/C -- I think it may be aftermarket, not sure
Ignition timing verified @ 10°BTC with engine at operating temp
Verified all vacuum connections, don't seem to be any leaks

The manual says that the adjustment for curb idle is different depending on whether or not it has A/C.....I can't find the "knob" that the manual shows for a engine with A/C and the screw that it says to adjust for an engine without A/C has no effect on curb idle speed when I adjust it either way.

I have backed off the only screw that seems to effect idle speed until it no longer touches the linkage and it still idles high.....( This is called the "throttle sop adjustment screw" in the manual)

Any help would be greatly appreciated....I need to pass this last year of emissions

skippy · Jan 2, 2008

BigTirez said:
I am failing my emissions testing because my idle speed is too high, it has to be between 400 and 1100 rpm and I can't get this thing to idle below 1300 rpm.

84 4x4
2.8L
5-speed TK
A/C -- I think it may be aftermarket, not sure
Ignition timing verified @ 10°BTC with engine at operating temp
Verified all vacuum connections, don't seem to be any leaks

The manual says that the adjustment for curb idle is different depending on whether or not it has A/C.....I can't find the "knob" that the manual shows for a engine with A/C and the screw that it says to adjust for an engine without A/C has no effect on curb idle speed when I adjust it either way.

I have backed off the only screw that seems to effect idle speed until it no longer touches the linkage and it still idles high.....( This is called the "throttle sop adjustment screw" in the manual)

Any help would be greatly appreciated....I need to pass this last year of emissions

back off the idle speed control.it's on drivers side,mounted in front of throttle linkage.one screw holds it down between isc and carb,and one screw adjusts between linkage and isc.

BigTirez · Jan 2, 2008

So, that will only help if the ISC plunger is not already going all the way out right? As it sits now, I am pretty sure that when the throttle is released the plunger doesn't touch the linkage thingy. When you increase throttle, the linkage pushes in the plunger on the ISC and then when you release the throttle, the plunger slowly pushes back out....is that what actually controls your curb idle?

I will try what you suggested when I get off work....hopefully it helps. It is frustrating because the truck runs great but I have one more year of emission testing to get thru. The first time I went through emissions, it failed HC (hydrocarbon) high at the 25mph test, the next time I went through it failed HC high at the idle and also failed for idle speed too high.

I am going to advance the timing a little to help with the idle HC reading...I looked at the base timing after writing my first message and it looks like it may actually be 8°btc rather than 10°btc. Don't know if this will help or not but it never hurts to have everything dialed in.

Once again, thanks for the help with the distributor shaft...

skippy · Jan 2, 2008

if you have a fax i can send you the page out of the tech manual.shoot me a pm if you want to do that.

BigTirez · Jan 3, 2008

I adjusted it as instructed...my neighbor had a printout from when he had a ranger and it said to disconnect the connector to the "TSP?" and then start the motor and adjust the ISC untill the tach reads 2000, then tighten the mounting screw and connect the plug and it should drop to the proper idle.....I did this and it still idles at around 1300 - 1400 bouncing.

Can I just disconnect the plug, start the motor and adjust the position of the ISC untill idle is around 950 and then plug in the connector or will this cause problems at other thottle positions?

I also replaced the Air Charge Temp (ACT) sensor, put in a new PCV valve and re-verified no leaky vacuum lines (at least I don't think there are any leaks) by spraying carb cleaner around all of the vacuum connections with no change in engine speed.

one of my co-workers asked me if I checked the voltage output of the "Throttle position sensor".....don't really know how to do that.

This is getting very very frustrating.....

enginepaul · Jan 3, 2008

First: Don't replace anything unless it tests bad. There are test procedures for everything; unfortunately you will need the proper manuals to see what needs to be done. Pull codes and repair as needed because some of the things may influence your idle speed. I had to adjust mine a little low, about 1900 rpm to get the idle right, but it did pass California smog.

As for the speed of the idle, look at the base of the carb on the firewall, driver's side. When the throttle lever is released and it doesn't hit the idle motor (the ISC plunger that goes in and out) then the bottom part of the lever rests on a screw that holds a minimum idle speed as indicated on the decal located on the radiator support. That screw is hard to see at first and I think - not sure from memory - it might have a lock screw.

BigTirez · Jan 4, 2008

Yeah, that is the screw that keeps the throttle plates from sticking....I backed that out right away (now throttle plates stick on close)....

I moved the Idle Speed Controller (ISC) all the way back away from the carb to see what my curb idle would be and it is still around 1200rpm.

Seeings as I get a code 23 (TPS error) I think that will be the next test I will run...as I understand all I do to test this is measure the voltage at the middle wire and slowly yet smoothly open throttle to WOT and make sure there are no jumps or drops in voltage and it should go from ~0.95v to ~4.5v......is this the correct procedure?

SO, if the TPS is faulty, how the heck do I replace it....it looks like I have to take the whole choke assembly off to get to the TSP off.

skippy · Jan 4, 2008

bigtirez,unfortunately the choke has to come off to change tps,and the test procedure sounds about right.my idle hangs up on me every now and then,can't wait to get rid of the feedback carb.i may tune on it some this week end.good luck to you.

Hotrodlincoln · Jan 9, 2008

The TPS doesn't have anything to do with the idle speed. Im not entirely sure why its there, or the O2 sensor as neither one can make any sort of changes to the fuel mixture like they do on FI cars.

You may have to adjust the idle speed fuel mixture screws on the carb to get it right.

edit: FAIRTAX FTW!!!!!

skippy · Jan 9, 2008

Hotrodlincoln said:
The TPS doesn't have anything to do with the idle speed. Im not entirely sure why its there, or the O2 sensor as neither one can make any sort of changes to the fuel mixture like they do on FI cars.

You may have to adjust the idle speed fuel mixture screws on the carb to get it right.

edit: FAIRTAX FTW!!!!!

what?two words,"shop manual"

84Ranger4X4 · Jan 11, 2008

My choke on mine always caused my idle to stick at high rpms(sometimes at like 2,000). I backed off the adjustment on the choke to where it doesn't work anymore. It idles at 950 now and hasn't stuck once in 8 years now. It also passes California smog with flying colors.

How do I lower my idle speed???

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