sancarlos1

I have a 1993 LS400 with 125k miles that was running great. I changed the air filter yesterday and drove it 15-20 miles and it ran perfect. Today after driving a few miles the idle climbed to 1100 while driving in gear and began surging between 1100 and 1600 rpm in park. I shut it off and it ran fine after a few minutes wait. Then it began the high idle in gear and the surging while in park again. No codes. Any suggestions? Coincidence or did I screw something up while changing the air filter. Other than the high idle it runs like it should, no missing, good throttle response, plenty of power, no smoke.

dicer

Explain what all you did in changing the air filter. What all did you remove etc.

sancarlos1

just removed the front large plastic cover and the front portion of the air filter housing, replaced the filter and put the plastic pieces back into place, nothing else was bothered

dicer

Maybe the air meter wires were disturbed or something could have been moved enough to cause a small hose to break causing a vacuum leak. Things to check out.

python

iTrader: (4)

have the alt/battery checked for low voltage

sancarlos1

Elec system is fine. I removed the air filter unplugged the air flow sensor and removed it checked the large hose, no leaks or holes, carefully replaced everything. Starts up fine, idles fine while cold, as soon as it warms up idle in gear goes up to 1100 rpm, with or without a/c, in park it surges from 1100 to 1600 back to 1100 continuously.

sancarlos1

Elec system is fine. I removed the air filter unplugged the air flow sensor and removed it checked the large hose, no leaks or holes, carefully replaced everything. Starts up fine, idles fine while cold, as soon as it warms up idle in gear goes up to 1100 rpm, with or without a/c, in park it surges from 1100 to 1600 back to 1100 continuously. I seems that the mixture is cycling from rich to richer.

dicer

That is a pretty common issue with these cars, with no good answers yet. I have one that developed that same deal. I lean towards the bearings in the idle air control motor. I have not checked it out yet in mine the temps just too cold and other things to do as well.

dicer

Also suspect the ecu, but others with this problem have changed the ecu and still had the problem afterwards.

deanshark

When the AFM went bad on my 93 it would sometimes idle and surge just like you explained. When I unplugged it then it would idle perfect. Mine didn't throw any codes when it was bad. If you have an Ohm meter you can check the resistance on the AFM. I forget the procedure but if you search I'm sure it's here somewhere. Hopefully you didn't try to clean the AFM.

oldskewel

iTrader: (1)

To help with debugging, it may be helpful to know that the engine _should_ idle nicely with all that stuff removed completely. Yes, everything upstream of the throttle body can be physically removed. Alternator belt removed, and it should still idle nicely. You should even then be able to cover the throttle body intake with your hand, and it should still idle nicely. At that point, it will be regulating intake air through the idle air control valve, so if you cover the intake hose for the IACV it will die right away.

This is general information for debugging things, not specifically for just your problem here.

Knowing this, you can start taking stuff off the car (maybe starting with deanshark's simple recommendation to just unplug the AFM connector) and see what causes a change. All the intake stuff is pretty easy to remove, as you've found with the air filter change so far. This approach may be more effort, since it is more methodical than guessing, but it should eventually get you to the root cause. If you can guess well, that might be a good approach here.

Since it was running perfectly beforehand, and then you did some things and this problem appeared soon afterwards, yes it is a pretty good bet that the problem is related to something you did. But it may not be easy to find. Maybe you touched a brittle vacuum hose that then cracked??? The hoses to and from the power steering pump idle up valve are notoriously brittle. If you did break something, don't feel bad about it - the only way to avoid that is to not touch it at all, and you can't maintain the car without touching it.

sancarlos1

If I unplug the electrical plug to the MAF while idling the car dies. The problem is now intermittent, I drove it about five miles and it ran as it should, A/C on still ran like it should, parked it for about thirty minutes. When I started it the idle went high 1100 rpm, drove home and idle was high all the way. Put it in park and idle began to surge from 1100 to 1600 rpm. I sprayed starting fluid near all the hose connections could not detect any leaks that way. I went to the local self service wrecking yard and picked up a couple of IACVs, both off of wrecked cars so I assume they were running at the time of the collisions. Paid $9 each. The ones I bought cannot be disassembled. Any way to clean these. They are easy to remove but I imagine difficult to install because of the hose to the water outlet. I cut that hose to remove them. I am thinking the problem must be the IACV perhaps something came loose when I changed the air filter and entered the IACV. Can I try to clean the IACV on the car by squirting cleaner into the large hose that provides air to the IACV?

deanshark

It shouldn't die if you unplug the AFM. I never heard of that happening. Next time it happens unplug it with car off and then start again, see what happens. When I had mine unplugged (for over 2 years ) it ran perfect, better then ever. But ran very very rich.
I would check the vacuum lines again. Check out pic #8 in this: http://www.lexls.com/tutorials/ignition/sparkplugs.html Those are the hoses you were closest to. The big plastic piece that comes out the air intake pipe is known to crack. I've never cleaned my IAC but I did read somewhere on here, a few years ago, how to clean it.

sancarlos1

I disconnected the AFM connection and started the car it idled fast 1100 RPM cold w/a/c on, check engine light came on. Reconnected the AFM and the idle was high but steady 1200 rpm in park w/a/c off. I found an IACV that can be taken apart looked clean inside. What is the initial setting when putting it back together? the plunger that seals off the air is threaded into the rotor in the stepper motor. turning the rotor moves the plunger in and out. what should the plunger position be on reassembly? fully closed or does the computer set it to the right position?

sancarlos1

I think it was just a coincidence that the problem happened after changing the air filter. I picked up an ECU, Air flow meter and idle air control valve from the local self service wrecking yard. Problem was complicated by the fact it was intermittent. Changed the easiest first the AF meter didn't solve the problem. Changed the ECU didn't solve the problem, then changed the IACV, pain in the A but it seems to have solved the problem. Idle has returned to normal. I don't understand how a faulty IACV can cause the idle to surge from 1100 to 1500 RPM and back. If the IACV sticks too far open because of bad bearings, that would cause a constant high idle. A surging idle would mean the valve is moving back and forth. It is a stepper motor with 125 steps perhaps some of the steps don't work. Maybe changing the air filter caused the ECU to direct the IACV to some different position thats not available in the defective IACV. Cant we go back to the time when an engine wiring harness had about 6 wires.