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1997 T100 RWD Auto
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72 Posts
Discussion Starter #1
Can anyone tell me what their RPM's are when:
  • engine is at operating temp and..
  • is in drive and...
  • stopped at a stop sign and...
  • sitting for at least 30 seconds or more.
My T100 Automatic 2wd will idle at around 700 rpms, but then drop down to around 500 after a few seconds. I am curious if this is normal.
 

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1998 T100 SR5 2WD
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13,576 Posts
My RPMs sit around 700-750 at idle, coolant around 186°F, auto transmission in drive, and sitting fer at least 30 seconds. My AC is usually on all the time so when the compressor cycles on, the RPMs will increase to about 900 RPMs. Have you accounted fer yer AC being on or if the Defrost mode is selected on yer climate control and the blower on? ;)
 

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1997 T100 RWD Auto
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72 Posts
Discussion Starter #3
My RPMs sit around 700-750 at idle, coolant around 186°F, auto transmission in drive, and sitting fer at least 30 seconds. My AC is usually on all the time so when the compressor cycles on, the RPMs will increase to about 900 RPMs. Have you accounted fer yer AC being on or if the Defrost mode is selected on yer climate control and the blower on? ;)
Thanks for the info. Yes, my idle goes up slightly if I have my A/C on (when it cycles on).

So, it sounds like 500rpm is low. I wonder if I need to clean out my IAC. I do not think I have done that yet.
 

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1997 T100 RWD Auto
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72 Posts
Discussion Starter #4
Thanks for the info. Yes, my idle goes up slightly if I have my A/C on (when it cycles on).

So, it sounds like 500rpm is low. I wonder if I need to clean out my IAC. I do not think I have done that yet.
I cleaned out TB, IAC, and checked ohms on TPS and IAC (all within range). Still getting a low 500rpm idle at stop sign after a few seconds sitting (at first it will hover around 700 ~ 750 before slowly dropping down to 500rpm)
 

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1998 T100 SR5 2WD
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13,576 Posts
I wonder if you have a slight vacuum leak. Did you ever git an OBDII scanner? Check the fuel trims at idle and see if they are trying to compensate fer a vacuum leak. ;)

You can also unplug the IAC valve connector and see if that helps or not. If it does, then the IAC valve might be gitting activated (system should be disabled when ECT hits a certain temp) some how. ;)
 

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1997 T100 RWD Auto
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72 Posts
Discussion Starter #6
I wonder if you have a slight vacuum leak. Did you ever git an OBDII scanner? Check the fuel trims at idle and see if they are trying to compensate fer a vacuum leak. ;)

You can also unplug the IAC valve connector and see if that helps or not. If it does, then the IAC valve might be gitting activated (system should be disabled when ECT hits a certain temp) some how. ;)
I have not yet purchased a bluetooth OBD2 Reader, but it is on my list for the next week (I live on a budget).


I parked in the garage (tail-end out) and let it idle until warm, put the truck in Drive with the e-brake on and waited until it dropped down to 500rpm.
Pulled the IAC plug and the idle immediately shot up to 750rpm.
Plugged it back in and it dropped down to 500rpm.

Ok. So you were saying that the IAC should not be activated. Should I look at my temp sensor?
 

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1997 T100 RWD Auto
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72 Posts
Discussion Starter #7
When I did my head gaskets, I had installed an aftermarket Temp Sensor. At the time I though I needed a new one, but my old one was actually working fine. I happened to keep the old original OEM one so I put it back on yesterday, but it made no difference to idle speeds.

I will keep looking.
 

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1998 T100 SR5 2WD
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How did you clean the IAC valve? Did you remove it from the bottom of the throttle body and make sure the shutter open and close freely without any resistance...or did you just spray some throttle body cleaner through it? ;)
 

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1997 T100 RWD Auto
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72 Posts
Discussion Starter #9
How did you clean the IAC valve? Did you remove it from the bottom of the throttle body and make sure the shutter open and close freely without any resistance...or did you just spray some throttle body cleaner through it? ;)
I removed the TB, put it in my big vice and removed the IAC (being careful to not strip the screws out).
Once removed, I cleaned it out and made sure the shutter moved easily. It was not as dirty as I thought it would be. Some debris flew out when I applied compressed air.

I did not put in a new gasket, but the old one did not look terrible. I forgot to test it for leaks, but will do that later today.
 

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1997 T100 RWD Auto
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Discussion Starter #10
I am wondering if there could possibly be a vacuum leak that only occurs when the truck is in Drive. This would explain why the idle drops to 500rpm when in drive, and also explain why I have not found it yet (since I have always had the truck in Park while searching for leaks.

Tomorrow, I think I will try putting my e-brake on with truck in Drive while I search for vacuum leaks using the Carb Cleaner method, just to see if this is a possibility.
 

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1998 T100 SR5 2WD
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I am wondering if there could possibly be a vacuum leak that only occurs when the truck is in Drive. This would explain why the idle drops to 500rpm when in drive, and also explain why I have not found it yet (since I have always had the truck in Park while searching for leaks.

Tomorrow, I think I will try putting my e-brake on with truck in Drive while I search for vacuum leaks using the Carb Cleaner method, just to see if this is a possibility.
Do you have the AC system off? ;)

It would be better to have someone step on the brake while you perform yer test. I guess you could block the wheels and tie the rear bumper off with a strap to something solid just as a safety precaution. ;)

I don't believe there should be anything that could affect a vacuum leak unless there was something mechanically wrong. There shouldn't be any vacuum hoses near the shifter linkage. However, there should be an air control valve that's hooked up to vacuum hoses that controls the power steering at low speed.

17630VALVE ASSY, AIR CONTROL
17630-740101$283.76

It may look similar to this and might be screwed into the back of the power steering pump.
285045


If it's bad, it may be sucking the power steering fluid through the hoses. Pulling the hoses off and inspecting it fer signs of fluid would be an easy check. It should be bone dry. If it's not, yer gonna need to replace it. BTW, have you inspected those hoses as well? ;)

Here's a used one on eBay. ;)

You might git lucky at yer local parts yard. Good luck. ;)
 

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1998 T100 SR5 2WD
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According to my FSM, the test fer the vacuum valve is to have the engine at idle, AC off. Fully turn the steering wheel. Pinch the vacuum hose to the valve. The engine RPMs should decrease. Release and the engine RPMs should increase. ;)
 

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1997 T100 RWD Auto
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72 Posts
Discussion Starter #13
I have not been running my A/C when troubleshooting this issue. When I do have the A/C on, the idle goes up a bit both in Drive and Park.

I do not think my power steering vacuum is the culprit. I changed out the two vacuum lines a couple of months ago. The power steering works fine, and my idle will go up if I turn my steering wheel (which is what it is supposed to do, as far as I understand it)

I will have a look at this all today and report back.
 

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1997 T100 RWD Auto
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Discussion Starter #14
I searched around a bit for vacuum leaks this morning (both in Drive and in Park) but was not able to find anything.

Still have idle dropping down to 500rpm when in drive.

Pulling the IAC plug while the truck is in Drive makes the idle go to 750 rpm.

Could this mean that the IAC is bad? I found a used IAC Switch (just the electrical part of the IAC Valve) on eBay for $24.00 but have not purchased it yet.
 

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1998 T100 SR5 2WD
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Have you checked fer voltage at the IAC plug after the engine has warmed up? ;)

 

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1997 T100 RWD Auto
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72 Posts
Discussion Starter #17
My FSM had some troubleshooting instructions for the IAC. Everything seemed to be within specs when I did all of the steps, so I am going to rule out the IAC as the culprit. I will post the instructions below in case anyone needs this for future.

While I was in the general area, I moved the plug on the TPS and noticed a slight change in idle speed. I did it again but nothing, then I tapped on the TPS with a wrench and got another brief idle-speed change (went up).

I did not want to continue pounding on it, so I am going to troubleshoot the TPS next.



Here are the FSM instructions for the IAC test for anyone needing this:

IDLE AIR CONTROL (IAC ) VALVE
ON-VEHICLE INSPECTION

1. INSPECT IAC VALVE OPERATION
Initial conditions:
• Engine at normal operating temperature
• Idle speed check correctly
• Transmission in neutral position
•A/C switch OFF

(a) Using SST, connect terminals TE1 and E1 of the DLC1. ( I just used a piece of wire bent into a U shape)

285158


(b) After engine speed is kept at approx. 1,000 rpm for 5 seconds, check that it returns to idle speed. If the engine speed operation is not as specified, check the IAC valve, wiring and ECM.

(c) Remove the SST from the DLC1.


2. INSPECT IAC VALVE RESISTANCE

NOTICE: ’’Cold” and ’’Hot” in the following sentences express the temperature of the coils themselves. ’’Cold” is from -10°C (14°F) to 50°C (122°F) and ’’Hot” is from 50°C (122°F) to 100°C (212°F).

(a) Disconnect the IAC valve connector.

(b) Using an ohmmeter, measure the resistance between terminal +B and other terminals (RSC, RSO).

285159


Resistance:
Cold: 17.0 - 24.5 Q.
Hot: 21.5 - 28.5 Q,
If resistance is not as specified, replace the IAC valve.

(c) Reconnect the IAC valve connector.


3. INSPECT AIR ASSIST SYSTEM

Initial conditions:
• Engine at normal operating temperature
• Idle speed check correctly
• Transmission in neutral position
•A/C switch OFF

(a) Using SST, connect terminals TE1 and E1 of the DLC1.

(b) After engine speed is kept at 900 - 1,300 rpm for 10 seconds, check that it returns to idle speed.

(c) Stop the engine.

(d) Disconnect the air assist hose from the IAC valve, and plug it.

285160



(e) Start the engine and check that the idle speed reaches 500 rpm or below (the engine may stall).

(f) If the idle does not reach 500 rpm or below, check for a leak between the air assist hoses, pipe and injectors.

(g) Remove the SST from the DLC1.

(h) Reconnect the air assist hose to the IAC valve.
 

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1998 T100 SR5 2WD
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I'm curious if the wiring harness might be compromised and allowing voltage to leak into the IAC valve wiring. The TPS is connected on the same wiring harness. It might be worth it to pull back the covering and see. ;)
 

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1997 T100 RWD Auto
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Discussion Starter #19
I looked some more at the TPS and its wiring. I beleive that either the TPS itself or the pigtail/plug is the issue,

After checking voltage, I played around with wriggling, tapping and pushing on the wires and plug to the TPS. I was able to get a noticable increase in RPM's after unplugging and plugging the TPS back in and pushing firmly on the plug.

In Park, it is idling at 1000 ~ 1100 rpm's (this seems higher than it should be)
In Drive, stopped, it is at 750 rpm's

I let the truck run for a while to be sure it was not a temporary surge, and drove around for a while.

I have ordered a new pigtail and a used TPS. I will change the TPS first, then the pigtail if that does not make a difference.
 
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