'91 V6 Fast Idle

[rkgraves]

Thank you for your help!

I have a '91 V6 5sp 4wd that idle too fast. The problem has gotten gradually worse over the last year. I've owned the vehicle since new and kept up on all the normal maintenance. To-date I haven't done anything other than normal maintenance (no sensors replace, MAF sensor, etc.)

As of today, when cold the engine idle speed will jump to around 2,000 RMP's. When warm it settles down to around 1400 rpms. The idle speed will decrease to around 1200 rpms if I put the heater fan switch into the off position. This past summer on really warm days the idle speed would drop down to around 800-900 rpms.

My plan this afternoon is to replace all the vacuum lines connecting to the throttle body. I'm also considering changing the coolant temp sensor. I'll also verify that the throttle cable is slack at idle. Lastly, I'm thinking of using a can of carb cleaner to spray shorts burst around the intake manifold/throttle assembly to look for vacuum leaks.

I'm pretty good with tools but don't understand all the interactions of what influences idle speed.

Would appreciate any suggestions of what to check - test - replace.

Thank you & Happy Thanksgiving from N. Idaho!
Randy Graves

 

[77'911]

You might consider checking your fan clutch and t-stat. If the fan clutch is failing, or failed, it will keep the engine temp too cool, ie sending it into "cold running mode"= ie high idle to warm it up. I would make sure the coolant is cycling at the proper temps with the t-stat functioning properly before attacking the idle screw or vacuum leaks(crankcase, secondary emission, or the like).
regards

 

[rkgraves]

Thanks for your reply!

The fan clutch tested OK - when the engine is cold it stayed fully engaged for several min. and then kicked-out.

if the dash temp gauge is reading correctly, and the warm air coming out of the heater seens plenty warm, while not positive I would say the thermostat is opening and closing at roughly the correct engine temp.

I changed most-all the vacuume line in and around the intake and, used carb cleaner to see if I could detect a vacuume leak. But all seems OK.

The one symptom that has me puzzled is why the idle speed will increase 300+/- rpms when I turn the heater/AC fan on. It does this just as you would expect when you turn the AC on, but without the AC. I have the factory electrical manual and have notes from working on the AC in the past, but can't pinpoint what signal increase idle speed.

Again Thanks! All Help and Comments are appreciated!
Have a great Thanksgiving,
Randy Graves
N. Idaho

 

[ongoing]

coolant temp sensor,That what I had to replace

 

[rkgraves]

Thanks for your reply! I'll change that next.

On a side note... we traveled over Thanksgiving with our 4Runner and I noticed that turning on the heater/AC fan does not always cause the idel to increase.... wish I kne why. Fan + AC always causes the idle to increase, as it should. But often just turning on the fan by it'self will have the same effect. I got out the schematics and can't see on paper why this happens.

Again Thanks! Water Temp Sensor is next on my list.

Merry Christmas,
Randy Graves

 

[rkgraves]

Thank You! Everyone who offer Help with this problem. I appreciate it!

Yesterday I changed the EFI temp sensor and that corrected 90% of the fast idle problem. Our 3VZ 6 cyl now idles down to around 900 rpms once warmed up. I think it should idle lower, but am at a loss of what to do next.

I've checked/adjusted the Throttle Position Sensor, it doesn’t read zero ohms when at idle but close (20 ohms), and does go to open continuity when the throttle is depressed. I also check the Air Flow Meter and its thermistor, both tested OK.

What do I do next to lower the engine idle speed assuming everything else is working correctly?

Thank You! I appreciate everyone time and help. I enjoy working our 4Runner, and having help makes all the difference!

I still have the problem that when I turn on the heater fan the engine increases 250-300 rpms. I have isolated this to “A/C Idle-Up VSV” relay energizing when it shouldn’t (fan is on but A/C is not) and will be troubleshooting that next.

Best Wishes for 2012,
Randy Graves
CdA, Idaho

 

[rkgraves]

Friends,

I wanted to follow-up to my own post as I did find and repair the source of the fast-idle with-fan-on problem. The problem turned out to be the A/C Amplifier. While I don't have a schematic for this little circuit board I was able to flex it and have the symptom come and go. I narrowed down the problem to what appeared to be a hair-line crack in the circuit board and was able to solder around the break. So-far-so-good and idle is back down to 750 rpm's when warm and the engine idle only increases when both A/C and fan are turned on.

Thanks Everyone For All Your Help and Input. Especially those who steered me towards the EFI Water Temp sensor as the source of my constant 1,100 - 1,500 rpm idle speed.

Best Wishes For 2012,
Randy Graves
CdA, Idaho