1990 Honda Accord Idle problems

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.

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.

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.

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:

https://www.2carpros.com/articles/honda-car-1990-1995-obd1-code-retrieval-procedure-and-definition-table

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.

The valves and connecting rods did not get bent or warped. Valves can get bent on some engines if the timing belt breaks, but then the engine wouldn't run at all. The connecting rods are really tough and will withstand anything you can do to the engine as long as nothing flies apart while it's running.

I can offer a possible alternative to selling the car. Look for a nearby community college with an Automotive program and ask if they would like to try to solve the problems. We were always looking for live work to give our kids real-world experience. We also had a dozen people from the community who would sit on a car problem until it fit what we were teaching because they understood the value of working on real cars with real problems, not silly trainer boards. The students are well-supervised and conscientious. Every school is different, but we charged ten bucks per hour for what the procedures were supposed to take, (same as any other shop does), not for the time we actually spent on it, and we bought parts at a real good discount, then marked them up only ten percent to form a "breakage fund" in case we damaged something. That way taxpayers weren't footing the bill.

The downside is each of our eight subject areas was only taught twice per year for eight weeks, and sometimes just once per year, and it could take weeks to get your car back. The kids spent some time in the class room and they had other classes to run off to so they usually only got to work in the shop about 15 hours per week. Only work related to the class was allowed because otherwise we were taking work away from the shops that might hire our graduates. That meant engine performance problems were only worked on in Engine Performance class, not in Brakes class or Automatic Transmission class.