YAIT - Yet another idle thread... (1 Viewer)

[CharlieS]

I'm still chasing idle issues, and realistically I probably just need to bite the bullet and buy a new ($$$$) or good used ($$$) IAC valve.

I've documented it in other threads, but I've done a lot of work on the rig, plugs, cap, rotor, ECT sensor, FPR, vacuum hoses, PCV, air filter, intake tube, cleaned intake and upper plenum, cleaned and lubed IAC, etc. Timing is at 3 degrees BTDC at idle with diagnostic jumper in. I believe I have addressed all vacuum leaks.

It runs great driving down the road.

But, it idles at lower RPMs, and erratically WHEN IN FORWARD OR REVERSE GEAR, AC off.

I know it can idle smoothly - it idles smoothly and at a higher rpm rate if I put it in N or P.

AC on raises the idle (as expected) and helps mask the issue.

Why would it idle roughly and lower in D or R, and idle smoothly in P or N?

Guessing the ECU has an sensor input to tell it the vehicle is in a drive gear, and adjusts idle via the IAC accordingly.

If this speculation is reasonable, would adjusting the static depth of the IAC plunger to open it up (reduce the static extended length) be a reasonable thing to do, letting more air bypass the throttle plate...? In my mind this is the same effect as opening the throttle a teeny tiny bit.

 

[mudgudgeon]

There was a technical page on the old LCOOL website ( now defunct) that described inserting orings to limit the travel on the IAC to reduce cold start high idle speed.

Some 1fz-fe will idle up to 1500+RPM on cold start.

I know it's the opposite of what you're trying to do.

If you limit its travel so it stays open does the ecu adjust fueling to match the air flow? Or will the ecu adjust fueling to try and manage idle speed?

 

[ZackR]

It's my understanding that the IAC can be reinstalled at any position (not static) and the ECU will send a command to open or close it as needed. When I removed mine I just put it back as it was after running it fully open and closed on the bench.

When you had the IAC on the bench did it pass all of the FSM tests (resistance, stepping all the way open and all the way closed)?

If so I'd check the adjustment of the throttle position sensor vis the FSM procedures. I recall it taking a bit of trial and error with the multimeter connected to get it just right.. It's my understanding that the ECU needs to know that the throttle position is at idle so it can properly adjust the IAC to allow the correct airflow to maintain idle.

Once these are adjusted the ECU will need to be reset (unplug the battery) and it will take several drive cycles before things really smooth out. I had a rough idle for YEARS and only when I finally dug into these during my wiring harness replacement was I able to solve it.

Since you have a 94' this is one of those times the TOYOOBD tool comes in handy (it shows the values from the IAC, and TPS live). I didn't have mine when I was doing these adjustments (multimeter was fine).

 

[CharlieS]

Based on the FSM, I understand that the ECU adjusts the IAC plunger depth based on inputs from the ECT, AC (on/off) - and it must get input from the TPS too?:

3. Idle Air Control (IAC)The ECM is programmed with target idling speed values to respond to different engine conditions(engine coolant temperature (ECT), air conditioning (A/C) ON/OFF, etc.). Sensors transmit signals to the ECM which controls the flow of air through the bypass of the throttle valve and adjust idle speed to the target value.​

It must also take a reading from the transmission based on this off behavior of idling fine in P/N and not idling well in R/D-2-1.

Yes, after disassembly and cleaning the IAC passed all of the multimeter resistance and powered bench tests.

I reassembled the plunger depth to the same depth as one measured by another 'mudder.

I'm thinking that the low idle position is with the plunger at the most extended position, limiting airflow.

I've learned that I can adjust the plunger depth in or out with a torx. That was how I set the starting depth when I reassembled after cleaning.

I checked the throttle position sensor and it looked good, but I will test it again.

I've been chasing a TOYOOBD tool, but they seem to be no longer available. I'm strictly multimeter and FSM for now.

 

[gummycarbs]

I believe I have addressed all vacuum leaks.

I don't. Post your peak RPM at cold startup, then RPM once warmed up, both in gear and in neutral. And specify whether AC is on or off in each case.

 

[CharlieS]

I don't. Post your peak RPM at cold startup, then RPM once warmed up, both in gear and in neutral. And specify whether AC is on or off in each case.

Will do and report back.

 

[ZackR]

When you check the TPS also check that the throttle plate closes fully and that there is some slack in the throttle cables (FSM will say how much). If the throttle plate is open slightly (or the throttle body is dirty not allowing it to close) then some air is getting past and the IAC isn't able to fully control the idle.

A scrap piece of vacuum hose and some vice grips are handy to pull the throttle opener back (apply vacuum to hold open and clamp with vice grips). I removed the throttle body during adjustment so I could be sure the throttle plate was all the way closed and everything was easier to access.

When adjusting keep a close eye on the multimeter, it will go from an open circuit to within the expected range as you fine tune it. You really want to be sure its within the specified range when you let go of the throttle plate and the spring closes it.

 

[CharlieS]

Some data points:

Test 1: Cold start, AC off, in Neutral, no accelerator input.
Elapsed Time RPM Quality of idle (s=steady/l=lumpy)
0 2200 s
10 1200 s
45 800 l
55 200-400 l
125 1200 s
140 800 s
150 200-400 l
160 800 s
180 200-400 l
185 1200 s
190 200-400 l
210 1200 s
215 200-400 l
225 1200 s
230 200-400 l
245 stalled

Test 2: engine warm, foot on brake, AC on
Gear RPM Quality of idle (s=steady/l=lumpy)
Park 800+ s
Reverse 400-600 l
Neutral 800+ s
Drive 400-600 l (same in D 2 and 1)

Test 3: engine warm, foot on brake, AC off
Gear RPM Quality of idle (s=steady/l=lumpy)
Park 300-500 l
Reverse 300-500 l
Neutral 300-500 l
Drive 300-500 l (same in D 2 and 1)


Test 4: Warm start, AC on, in Neutral, no accelerator input.
Elapsed Time RPM Quality of idle (s=steady/l=lumpy)
0 2200 s
5 900 s
20 650 s
25 400 l
45 600 l
65 900 s
75 600 l
89 900 s
105 800 s
stayed at 800 and steady

turn off AC immediate drip to 400 and lumpy
turn on AC and back up to 800 and steady

 

[CharlieS]

I adjusted the TPS spec per the fsm (what a pain getting feeler guages in there!), no difference.

Then removed and adjusted the IAC plunger. No change.

I hunted all over the engine bay with brake cleaner and then propane, and no changes in RPM.

I'm prerunning trails tomorrow for an upcoming event and hoping this isn't going to get in the way.

This one is realy eluding me.

 

[CharlieS]

Oh, forgot to say, the throttle plate closes fully against the stop, there is slack in the cable, and I thoroughly cleaned the entire intake, throttle plate and upper plenum with BG Chemclean and brake cleaner very recently. It was clean as a whistle when I put it back together last month.

 

[ZackR]

When you did the TPS checks did you check and see if the resistance changes when its at idle vs just slightly open (step 2 below)? I didn't find any issues when I was checking with the feeler gauges but did find it was out of spec when checking resistance.

The really low idle down to 200-400rpm (with an occasional stall) is exactly what mine was doing as well. When cold or fully warmed up it wouldn't do it quite as much but the idle was still uneven. Even after replacing all my vacuum lines (including all of the lines under the manifold) it still had the issue, the only thing that really fixed it was when I pulled the throttle body for a deep clean including the IAC valve, and TPS adjustments.

 

[ZackR]

Almost forgot - also try running with the 02 sensors disconnected and see how it does. Mine were not throwing codes but it ran a lot better (albeit rich) when they were disconnected. Might not be the issue but worth checking as a possible contributor to the problem.

I replaced them around the same time all of the other adjustments were being made but didn't take notes if I did them before or after.

 

[CharlieS]

When you did the TPS checks did you check and see if the resistance changes when its at idle vs just slightly open (step 2 below)? I didn't find any issues when I was checking with the feeler gauges but did find it was out of spec when checking resistance.

The really low idle down to 200-400rpm (with an occasional stall) is exactly what mine was doing as well. When cold or fully warmed up it wouldn't do it quite as much but the idle was still uneven. Even after replacing all my vacuum lines (including all of the lines under the manifold) it still had the issue, the only thing that really fixed it was when I pulled the throttle body for a deep clean including the IAC valve, and TPS adjustments.

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Yes, I did. I adjusted it so I had resistance (0.490 K ohm which was within spec of 0.2-5.7 K ohms) at 0.020" and infinite/open at 0.030".

Throttle body and plate had a deep clean a few weeks ago.

 

[CharlieS]

I'll give that a try in the morning. They are brand new O2 sensors. But I guess disconnecting them would throw the ECU into open loop mode?

 

[ppc]

It must also take a reading from the transmission based on this off behavior of idling fine in P/N and not idling well in R/D-2-1.

This behavior is normally caused by by a failing intake tube that when the engine is shifted into gear the motor rotates via torque on it's mounts causing a crack in the intake tube to further open up allowing unmetered air into the engine. You stated the tube had been replaced but I would double check that.

 

[CharlieS]

Will do, but as you noted, it is brand new, and I've removed, reinstalled and rechecked it multiple times. I tried the propane leak test on that area last night too.

Thank you for the ideas, please keep them coming.

 

[mudgudgeon]

I personalty don't think using brake clean, carby clean, ether to test for vacuum leaks is a rentals way to test. Unless you have a major leak.

There's lots of possibilities for smalll vacuum leaks.
Oil cap seal
Dipstick - tube seal
Spark plug tube seals
Valve cover gasket.
PCV grommet
All the small hoses. Cracked hoses, or hardened hose ends with a loose fit
Intake tube
Manifold & throttle body gaskets

Probably others

 

[fjbj40]

I had a erratic idle condition similar to yours.

It was moisture in the afm plug!

 

[gummycarbs]

0 2200 s

Way too high. Especially with the AC off and during summer months! You have vacuum leaks.

Don't give me some B.S. about spraying whatever chemical around the engine bay, either. Replace every single vacuum hose, and don't forget the brake booster hose.

 

[CharlieS]

Okay.

I've replaced every vacuum hose, tube and grommet in the engine bay, except the brake booster line. I'll do that one too now.

replaced cap - Oil cap seal
replaced - Spark plug tube seals
replaced - Valve cover gasket.
replaced - PCV grommet
replaced - All the small hoses. Cracked hoses, or hardened hose ends with a loose fit
replaced - Intake tube
replaced - Manifold & throttle body gaskets

I'll look into the dipstick tube seal. I didn't know there was a seal there.

Happy to check for vacuum another way, I just don't know how.

I'm sitting in it now. Warmed up. Parked. Shifter in P. AC on. It is idling smoothly and steady at 800 rpm. for 15 mins solid. How is this even possible?