vehicle forgeting idle speed?

Factual Questions
1

I’ve had this happen before, but it was due to disconnecting the battery for an extended period of time. This occurred a couple of years ago when I replaced the ignition switch located on the steering column. I called a buddy of mine who told me that the computer had basically forgotten where it needed to idle. Therefore I needed to reprogram the computer by either driving around or sitting there with my foot on the gas until it figured it out.

Recently my truck, a 97 Dodge 1500 with a 318 motor, has forgotten the idle speed at least five times within the past two weeks. What’s going on. Do I need a new ignitions switch? New battery? What? :confused:

2

“forgotten the idle speed”? Can you be more specific as to what’s actually happening? Is it stalling or idling too fast?

3

“vehicle forgeting idle speed”? - huh? That’s a new one. Be sure to tell your buddy to fill up on blinker fluid before winter hits. :wink:

If you could be more specific with the symptoms the vehicle is having. (Rough idle, engine dies? If so, does it occur with the engine hot or cold? Warning lights on the dash?)

4

Ok forgetting idle speed may not have been the best way of explaining it. It was late and I was tired. The truck won’t idle at all unless I have my foot on the gas. After a short drive everything seems to be back to normal. Then in a few days it’s the same thing all over again. It has only died on me once while driving as I was pulling into a parking space. The buddy I was talking about is a mechanic who my father and I have taken all our vehicle issues that we could not tackle. It doesn’t matter if the engine is cold or hot. It has happened in both circumstances. The only way I know it is doing this is when I go to start the motor and it will not idle after it starts. It just dies. So I start it up with my foot pressing the gas.

5

FYI, I had a similar problem with my Dodge 1500. The battery died and took the settings in the computer with it (perhaps it takes a smidge of juice from the battery to keep it going when the ignition is off). I jumped my truck but had to keep my foot on the pedal to keep the truck running.

According to the guy at the shop, it’s difficult to jump start a Dodge Ram truck because of this problem – you have to keep your foot on the gas at stop lights, etc. to get its brains back together. An excellent way to kill someone.

The solution, the mechanic said, was to get a fresh battery into the truck, then wait a bit for the computer to reset itself. I did and in the amount of time it took me to pay for the new battery, the computer came back online. No idle problem.

I’m wondering if your battery is near death (or some other electrical problem preventing current from getting to your ignition computer).

6

IANAMechanic, for what it’s worth…

I might suggest the EGR valve (exhaust recirculation valve), but since it happens with both a hot and cold motor, I don’t think this is likely. If you have an EGR valve with an exposed diaphram, a quick check for functionality is to depress it with a cold start and the motor should die when at idle. But since you can’t idle to well, you may need someone to help keep the RPM’s up a notch and then test it. A haynes manual will tell you precisely how to test this valve and/or the EGR control solinoid (if equipped) with an ohm and volt meter.

Next I would look into the TPS sensor (throttle positioning sensor). I mention this because you shouldn’t have to press the gas to start a fuel injection system. If out of alignment or faulty will cause idle problems. Also the IAC valve (Idle air control sensor). It can be removed and cleaned, but don’t spray rough chemicals directly on the sensor, just clean the plunger and the orifice it mates to on the throttle body and replace the o-ring. FYI, both sensors are found on the throttle body. And can also be tested via a haynes manual with a ohm and volt meter.

I was thinking about the crank sensor, if you have one. But a failure with this one is usually a no start condition right away.

The MAP sensor (Manifold absolute pressure sensor). This one can mess with the fuel calcuations in the PCM tables. Maybe what your friend was referring to about “forgetting idle speed”. Check the vaccum hose for cracks and replace if necessary. Can also be tested via a haynes manual with an ohm and volt meter.

The MAF sensor (Mass air flow sensor). I hate this one because it’s usually expensive. If you have one, it’ll be in the ducting before the throttle body. Make sure the ducting has no air leaks before and after this sensor. Can also be tested via a haynes manual with an ohm and volt meter. The little wires on the inside can be cleaned with electrical parts cleaner, do not use brake clean or WD-40. Also don’t rub the wires with anything, just spray them with the cleaner to get the crud off.

Also check the basics - Spark plug wires, ignition coil(s), PCV valve, and vaccum hoses. A leak in the vaccum system will cause all kinds of weird problems. Check them all for cracks and replace as necessary, especially the EGR valve, EVAP solinoid, and MAP sensor.

About the computer “losing its brains”. Like I said, I’m not a mechanic, and maybe some mechanic guys experience this, but I really don’t get this. Unless the computer is truly faulty, this sounds bogus. If it’s not, maybe some can come along to dispell it and show me the light. The computer, when functional, should never “lose its brains”. If it loses power due to a battery issue or such, the OPEN loop functions (cold engine program) should always be there hard coded to use when the power is good again. The CLOSED loop functions (warm engine program) are variable based on O2 readings, timing, and fuel trims that have been learned over time. And I suppose this can be lost or erased, but if this table needs to be “re-learned” again for what ever reason, it won’t run like crap to get to get optimum trims again.

7

It is possible for the computer to lose its idle memory?
Well yes, but in my experience it is not common.
In general as long as battery power is supplied to the computer, it should maintain its memory. If battery power is lost the computer’s memory will be wiped. It may lose its idle memory, and it will lose it mixture memory, and this can for sure effect the idle. How long the battery has to be disconnected varies from system to system. I should also note that a loose connection or corroded connection that does not supply enough voltage could also cause this type of problem.
Things I would check:
Is the check engine light on? If so fault trace that first.
What is the battery condition? Have the battery tested. Replace as necessary. Are the battery connections clean and tight? Remedy as necessary.
Are there any holes in the air intakes that allow any unmetered (what is called pirate air in the industry) You may have to feel around to find the hole, or remove the intake trunk to examine it for holes. Along the same lines, a leaky intake manifold gasket(s) can also give the same issues.
Throttle plate: As the throttle plate gets dirty (they do get dirty) the computer has to adjust the idle to compensate. if the throttle gets too dirty, the computer may not be able to compensate. Clean the throttle.
Plugged injectors can give uneven idle and perhaps a failure to idle. Injector cleaning can remedy this. IMHO injector cleanings are oversold by many shops, but if in fact you have a plugged injector, this will clear it up.

ParentalAdvisory mentioned many items that can cause issue with idle. (TPS, MAF, MAP, EGR, plugs, wires) In most of these cases I would expect a fault code to accompany these failures, but it is possible that Chrysler’s system might have a failure mode that the computer does not detect. I doubt that the crank sensor is at fault, the description of the fault does not support a bad crank sensor.

If and only if when all other possibilities have been ruled out, then the computer itself may be suspect.