Mechanics

IDLE PROBLEMS

1990 Honda Accord • 200,000 miles

I have a 90 honda accord lx model that my father and I recently have got running again. The previous owner knew little about cars and started taking the engine apart due to a loss in power. The actual problem turned out to be the catalytic converter, which has been removed. This car became my project for my first car but turned into a much longer endeavor. We just recently got the engine completely back together and started up. The idle immediately shot to 5000 rpms and kept climbing. After doing some research, I replaced the IAC valve and adjusted the HIC valve hoping to fix the problem. The problem persisted so we unplugged the IAC to see if that did anything. Starting the car with the IAC unplugged made the idle start fluctuating between 1000 and 2000 rpms like a typical vacuum leak would cause. We checked for vacuum leaks as best we could but did not have any success. I then started the car with the IAC plugged back in and the rpms were straight back up to 5000 and climbing. There is coolant going to both valves so it is not a coolant control problem. We are pretty much out of ideas so any suggestion you might have would be very appreciated. Thanks in advance.
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Iamopi
June 24, 2013.



Do you have access to a scanner that displays live data? If so, you can use that to view what the Engine Computer is doing to the idle speed valve. If it is requesting that high idle, that is in response to something. Normally you will find the computer is trying to reduce idle speed but not having success. That is the result of the vacuum leak you're looking for.

You may be able to find a smoke machine at an auto parts store that borrows or rents tools. That will allow you to inject white, non-toxic smoke at up to 2 psi into the intake manifold through a vacuum port, then you watch for where it sneaks out.

Caradiodoc
Jun 24, 2013.
Unfortunately I do not have access to a live data capable scan tool at home.

I will try to find a smoke machine tomorrow and update on how it goes.

Would the excessive idle when the IAC is connected be from the computer trying to counter the vacuum leak?

Tiny
Iamopi
Jun 24, 2013.
I don't know how that valve works on your car but if it has only two wires, it is similar to what Ford uses. If it has four wires it will be similar to a GM or Chrysler system.

With the Ford system, 12 volts is applied to one wire, then the Engine Computer varies the voltage on the other one. The lower it brings that voltage to 0 volts, the harder the spring-loaded solenoid pulls the valve open to allow more air in. If that control wire gets grounded it will make the valve open fully. You'll get the high idle but without the corresponding fuel delivery and power.

As a point of interest, I watched a Chrysler trainer demonstrate how much those valves affect idle speed. He was able to disable six cylinders on a V-8 Jeep engine by unplugging the injectors, and while it obviously ran very poorly, it was able to maintain the desired idle speed on just those two working cylinders. That is a lot of control. On Chrysler engines we can read the "steps" the computer has placed the valve at. There are 256 steps and you'll typically find it at around step 32 for a good-running engine. When you find it at step 0 and the idle speed is too high is when you know the computer has lost control of idle speed but it is trying to lower it as much as possible. That's when you start looking for vacuum leaks. Even with six cylinders disabled it still hadn't reached step 200, so you can imagine how fast the engine would run if that valve was wide open and all eight cylinders were working.

You can't measure any voltages on the Chrysler automatic idle speed motor. The wires are pulsed with varying voltages and polarities to slowly turn the motor which turns the valve on a threaded shaft. You CAN measure the voltage on the Ford unit, and if yours has two wires I would do that too. If you find full battery voltage between the two wires, ... Let me rethink that. You should find 12 volts on one wire, or possibly 5.0 volts. It's the other one you're interested in. If that has 0 volts it has to be because either the circuit in the computer is shorted or that wire is grounded on the engine. By far the most common cause would be the wire got pinched when something was reassembled.

If you do find 0 volts on that control wire, turn the ignition switch off, then measure the resistance of that wire to the engine block. If you find real low resistance, close to 0 ohms, unplug the Engine Computer. If the low resistance is still there, that wire is pinched or rubbed through and touching metal. If the wire reads open circuit, (real high resistance or infinite), that suggests the driver circuit in the computer is shorted.

Caradiodoc
Jun 24, 2013.
Unfortunately I could not get access to an automotive smoke machine the vacuum system. I did however buy a propane torch and traced the lines with the unlit propane. The idle did not change with the propane. I tested the IAC wires like you suggested and found 12 volts on one wire and only.5 volts on the other. The signal wire had around 3k ohms while the other wire had about 40k ohms. That is really low voltage but it is still not 0 so I do not know if that is within specs. I Do not think the resistance is high enough for it to be a shorted driver either though.

Tiny
Iamopi
Jun 25, 2013.
The half a volt says the wire is not shorted to ground or pinched but it also is the reason for the high idle. The computer can't really draw that voltage down any further. Normally I would suggest the computer is suspect but I don't know how engine repairs would affect that.

Caradiodoc
Jun 25, 2013.
Maybe something could have been damaged when the previous owner took everything apart. He managed to mess up things my father or I never thought were possible. I really didn't want it to be a computer problem but it has always been an option when everything else is checking out. Would that be a PROM or EPROM problem that could be fixed or would a whole new unit be a better solution?

Tiny
Iamopi
Jun 25, 2013.
PROMs are mainly a GM thing. They only are the memory that sets the personality for the specific model. On newer cars we install different software for different models and we update that software to address running problems. We had to replace PROMs years ago when an update was needed.

The problem you're having wouldn't be addressed by replacing a PROM. Assuming it is the computer, either the power transistor that drives the idle valve is shorted or the circuitry that runs that transistor has a problem and is turning that transistor on fully. In a tv or vcr, with a shorted transistor it would be like a piece of wire so you'd have a dead short to ground and 0.0 volts on that control wire. If the circuitry had a defect that turned that power transistor on fully, 0.5 volts is about as low as it would be able to draw the voltage down to. It can't hit 0.0 volts when it's not shorted.

Unfortunately no manufacturer gives out a schematic diagram of their computers like we can get with tvs so there's no way to know how the circuits work or what the likely failures might be. The good news is those computers don't fail real often. What you can do first is to check for diagnostic fault codes. Most manufacturers use similar systems with some form of crankshaft position sensor and / or camshaft position sensor. The Engine Computers use those signals to know when to fire an injector or ignition coil, and based on those signals it knows engine speed. On most cars the engine will not start or run if one of those signals is missing. On some newer models the engine will continue running if one sensor fails but it will not restart once it has been turned off. It uses the one good signal as a backup strategy and calculates approximately when to fire an injector.

This page will explain how to read the codes and what they mean:

http://www.2carpros.com/articles/honda-car-1990-1995-obd1-code-retrieval-procedure-and-definition-table
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Caradiodoc
Jun 25, 2013.
That is sound advice, but the trouble is it is not throwing any codes. It has a bad battery so we have to jump it every time to start it up and when the cables are disconnected, all power is lost. This probably clears any codes since every time we shut it off we are in essence disconnecting the battery until the next jump. It has not actually been moved in years so do I actually need to drive 25 miles like the link says in order for it to have codes present?

Tiny
Iamopi
Jun 25, 2013.
That 25 miles comment surprised me too. Every car I've worked on will turn on the Check Engine light as soon as you disconnect a sensor while the ignition switch is on. On '96 and newer models the computer will run self-tests on various systems while you're driving, and some of those require very specific conditions to be met before they will run. For example, you may need to hold a steady speed above 50 mph for at least 15 minutes, you may need to hit wide-open-throttle for a certain number of seconds, or you may need to let the engine idle in neutral for two minutes, and things like that. I never heard of any car model needing to run those tests on a '95 or older. On those older models, if there is no heater inside the oxygen sensor, they may not reach 600 degrees during prolonged idle or low speeds, so perhaps they want you to drive on the highway for up to 25 miles to get them warmed up and working so the computer can see if they're operating properly. I find it hard to believe the computer could detect a problem and not set a code right away.

Caradiodoc
Jun 26, 2013.
Yea I thought it should be okay heating up too for the oxygen sensors so we got it up to temperature and tried to pull some codes. We have a Hynes manual which said to create a short between the pins of the connector near the computer behind the kick panel on the passenger floor board. I pulled up the carpet to look for it and noticed that one of the bolts holding down the computer cover was missing, which means that someone previously messed with the computer in some way. My dad also happened to stumble across another computer that was hidden in the trunk with the spare. We plugged in the other computer just to see if it would make a difference in the performance but nothing changed between the two modules. I found the aforementioned connector and connected the two pins with a paper clip, which according to the manual, should start the DTC process and make the check engine light start flashing with the key on. Still nothing happened so we were thinking maybe by some weird chance it needed the 25 miles to register codes. I started backing it up and did not even make it out of the driveway before it sputtered out and died. We now think, due to the excessive noise when I started moving, that the prolonged exposure to the elements before we got the car caused rust to the valves and possible warped the valves and rods when we got them broke loose. Given how much continues to turn up wrong we finally called it quits today and are going to cut our losses. Thank you still for all your help, it was very appreciated and good insight to an unfortunately not fixable problem with our limited time and budget.

Tiny
Iamopi
Jun 26, 2013.
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