Understanding and diagnosing surging and high idle 4A-GE 4A-GZE and other L-jetronic systems

Terms and accronyms listed at the bottom of the page.
In my opinion the BGB (Big Green Book) or FSM (Factory Service Manual) is the best tool for diagnosing and troubleshooting. This guide is designed to give a deeper level of understanding of why the systems work as they do but I recommend using the FSM as the primary tool during troubleshooting.
 
This problem is a very simple one but most people don't understand the system or why their car is surging. Most people see surging as the problem when it's really a symptom of a problem.
There are also generally two types of surging. There is high idle surging where there is too much air entering the system. The ECU sees the RPM get too high and cuts fuel till the idle comes back down to an acceptable level.
The other type is when the ISC it's self is causing the idle to go up and down.

In the TPS there is a sensor (really just an on off switch). When the car is idling and no throttle is being applied this switch is triggered telling the ECU the car is idling and to behave as such. The ECU has a limit set into it that idle speed should never go over. On the 4A-GE and 4A-GZE This is approx 1600 RPM. If the RPMs hit this limit while the idle circuit is tripped the ECU will cut fuel until the RPMs drop to an acceptable level causing a surging idle. This serves two purposes, one is when you are driving and start to decelerate letting off the gas completely the ECU will cut fuel off. This saves fuel when you don't need to be burning any. The second is if something malfunctions your revs won't shoot to the moon when the idle circuit is tripped.

So when your car starts surging to 1600 RPM then back down it's simply a symptom of your idle being too high. There are only a few common reasons why this can happen.
One of the most common is an air bubble in your coolant either by the waxstat ISC or by the temp sensor for the ECU. The Mr2 has a slightly more difficult cooling system than many cars and it needs to be bled properly or you can have problems. The NA ISC is controlled completely by coolant temp. The GZE has a coolant temp controlled primary ISC and an electrical secondary ISC.
Both thermostatic ISCs are located at a somewhat raised part of the system, particularly in the AW11 MR2 so if there are any air bubbles in the system they will be likely to collect in the ISC. Since the air doesn't transfer heat as well the ISC thinks the engine is still cold and will stay open allowing too much air into the engine which will raise the idle.

A number of people have also found their ISCV to be dirty, stuck or malfunctioning. I mention this here because I am talking about the system but I would leave this as one of the last things on this list to inspect.

Another common cause is intake leaks. These can be hard to find sometimes and can be very frustrating. I also think oftentimes people shortcut their troubleshooting of this to jump to other things and end up spending lots of time on other things only to come back and finally find a vacuum leak.
Many vacuum leak possibilities can be eliminated simply by temporarily capping the line at the manifold or whatever it's vacuum source is. The brake booster is a good example of this. It is very hard to search the whole brake booster system for vacuum leaks but if you just cap the line at the manifold you can eliminate it from the possibilities. If that fixes your problem at least you have narrowed down your problem to that one system. There are plenty of sources out there for finding vacuum leaks so I won't go into detail on it here but I will give warning. If you are using anything flammable to look for vacuum leaks be aware of the potential of things like arching spark plug wires, hot exhaust manifolds and other things that could cause your engine bay to go up in a nuclear fireball.
I prefer the boost leak test method of pressurizing the system and looking for leaks with soapy water.
There are a few seals that can be hard to find too. There have been a couple people who have had leaks in the bushings of the throttle plate shaft. It can help to take off the TPS to check that side for leaks. Make sure to properly calibrate the TPS if you ever move it.

Another place is the TVIS butterfly shaft bushings on any TVIS equipped motors. These can wear out over time and start to leak.

The TPS should be set so the idle switch goes off with the slightest throttle input but it is possible for the throttle to be open slightly and the switch still tripped. If the TPS is not calibrated properly it is possible for the throttle to be open considerably and the circuit to still be tripped. It is best to set the TPS accorgintg to the factory service manual.
Make sure the throttle plate is closing fully and make sure the cable, pedal and actuator move freely through their full range of motion.
A lot of people think that the TPS being out of spec can cause surging. This is not true but if there is any doubt it is always good to inspect it and adjust it if needed because it can cause other issues and is something that is often neglected.

Make sure your coolant temp sensor is working properly and make sure the ECU is getting the signal. On motors that are purely thermostatically controlled  this shouldn't have a significant effect on idle speed but if it thinks it's cold it will run richer. If a motor that has an electronic ISC it can raise idle if it thinks the motor is cold. Also test the electrical ISC and signals to make sure it is electrically working properly.
If none of these do the trick take the ISC apart inspect it, clean it and test it.
 

There is also the BISS (Base Idle Set Screw) This is on the TB and should have a cap on it to keep people from tampering with it.
If your motor is stock and healthy this should never need to be touched.
If your car is idling normally then starts to have an idle problem you should not need to adjust this. You have a problem somewhere else and adjusting the BISS will only be trying to compensate for that problem. If you buy a basket case or if the cap is missing and you don't know what's been done to it or if you rebuild the motor especially if it's not stock and especially if you do something big like cams you may need to adjust the BISS to get your idle to the proper place. I list this last because even though it's one of the easiest things to do it is one of the last things you should try. Of course with the exception of throwing in big cams or something where you know the reason for the idle change.
The 4AGZE is harder to diagnose since the intake is much longer and any leak between the throttlebody and the motor can cause high idle. 
The supercharger has a number of ports on it. Hypothetically a leak on any one of these ports or even at the seals in the supercharger could cause an intake leak that is extremely hard to find or diagnose. 

There are a few other things that can effect idle and should be checked while you are at it. It's unlikely these will be primary culprits but they are good things to take care of anyway.
One is making sure the ignition timing is right or at least close. It's kind of a catch 22 because you need it to be idling at the right speed as specified by the FSM  rpm to time it properly but timing can also effect idle speed. You will be able to tell if it's way off though so get it as good as you can. If that doesn't bring you down to a good idle then you need to go back and fix the other problems till your idle comes down enough to time it properly.

Beyond that just make sure everything is healthy and in good working order. Check connectors, grounds, basic tune up stuff. Most of these things are unlikely however bad grounds and electrical connections can do some weird inexplicable stuff and an engine bay in disrepair is always asking for trouble. Always fix the things you know are wrong first even if you think they are unrelated then go from there.

 
 

Terms and accronyms

TPS       

Throttle Position Sensor

AFM

Air Flow Meter

ISC

Idle Speed Controller

FSM

Factory Service Manual

 
 
Here is also a great article with some good information on L-jetronic EFI systems.
http://www.hiperformancestore.com/Ljetronic.htm
 
And another on Toyota L-jetronic.
http://www.autoshop101.com/forms/h24.pdf
 
In fact autoshop101 has a lot of great info on this and other subjects.
 

courtesy of webmatter.de