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Mitsubishi Idle And Heater Problems

Q. I am an original owner, and the car was maintained relatively well - all fluids were changed regularly, spark plugs and wires, rotor, distributor cap, air filter, fuel filter It's been running extremely well with very smooth and quiet engine in spite of quite high mileage. I'd like to ask you two part question if I could.

Mitsubishi Idle And Heater Problems

#1: About four months ago I went to the dealer to have a fuel filter replaced. I did this myself before a few times. This time I was busy and decided to have it done, after all it's quite innocent and inexpensive procedure. And ever since I regret that I did not do that myself.

He replaced the filter all right which I asked, but he also was messing around with adjustments of the throttle body, which I absolutely did not ask. He said the engine was too quiet. I went to the manager and ended up not paying for this so called adjustment. Basically, the guy turned two screws: one is the idling screw, two (near by), I don't know how you call it, it's just above throttle sensor, I suspect regulating high idle cam. Both of them cushioned by springs.

From the moment I started the car I already realized the problems. It did not start as before. Normally when cold starting it goes to about 1,500 rpm and than gradually to about 900 to 1,000 when the temperature gauge just about at first left mark. Then within five minutes or so driving temperature gauge goes just to the left of middle mark and when you stop, idling for the first couple moments rpm was usually 900 to 1,000 and then goes to 750 to 800. That's how it was. But, when I started the car that day, the rpm was only 600 to 650, the car was shaking and vibrating.

I stopped and tried again; the same thing. Only this time kept gas pedal slightly depressed until the engine became warm. When temperature gauge was between very left and middle mark I released the gas pedal and rpm was at about 750 and still shaking and vibrating, but not as much though. Then I started to drive. There were a few jerks during first five minutes. Then it seemed to run fine.

After driving on a highway for about 15 minutes, exiting, stopping on red light, idling - funny, erratic, alternates from 900 to 1,200 back and forth. So, the next day and pretty much all these four months when I start the car, I keep gas pedal depressed for about three to four minutes at 1,500 rpm then slowly release it as engine warms up, but even then while shifting in reverse to back up I still keep it slightly depressed.

Then after driving for about 10 minutes I can stop and it idles at about 900 to 950. I did went back with this problem not just once but four times. And every time he was playing with this two screws and test driving. Seemed to me more like trial and error method. The last time I raised a hell in that dealership and they offered me $300.00 for aggravation. That particular service line was eliminated in that dealership, there were to many complains.

Since then I did not want to go anywhere else to make it worse. I've been trying to correct it myself. First, by inspecting that throttle screw (I referred to it as #2) I noticed a trace of slightly greenish color, about 1/8" from the end of screw. I suppose that's how much it was projected below bottom support. But after it was turned clockwise it's projected more than twice of that distance. I turned it counterclockwise to return it back to that trace position.

Things improved somewhat. Occasionally the car tries to start normally, it does occasionally go to about 1,500 but stays there only few seconds and goes down to 600 very fast. And then, the same thing. I have to keep pedal depressed for a few minutes. I tried to turn neighboring idling screw #1. It improves starting, but then at normal operating temperature idling becomes too fast 1,000 to 1,100.

During these four months I realized that these two screws affecting so many things. The question: how much that screw #2 should project? Does it control throttle, amount of air directly or indirectly? I understand once this screw positioned correctly, than, when the car idling at normal operating temperature simply adjust screw #1 to idling 750+-100 rpm just like label under the hood says, right?? Also I noticed that any electrical load decreases idling in much more dramatic way than before. For example, during evening ride with just lights and fan on (not A/C) idling goes down by as much as 300 to 350 rpm

#2: The heater does not blow not just hot air, but not even slightly warm air with temperature control on max hot. It started to happen during last 3-4 days from barely warm air to outside air temperature.

Upper hose gets warm and hot in synchrony with engine temperature. No thermostat problem here, I assume. But I did change it two months ago anyway along with flushing the whole cooling system. Heater hoses, they do seem to be just barely warm. Are they supposed to be just as hot as upper radiator hose with the heater on or regardless of heater settings?

They are warm only at the thermostat housing, and the other one close to connection to lower hose. But if I touch both hoses near firewall they are barely warm, probably just from other near by components' temperature.

I removed instrument panel and tried to see if the temperature control cable not broken (it was the case in the past with directional air control) and whether it moving or not the temperature cam. The cable in place and it does move the cam. Although, I did not go further to black box (heater unit).

There is a heater relay (silver cylindrical object) just above fuse box on the passenger side. If that relay goes, could this be the result of this?? Are there any other electrical components related to the heater which could do this?? I think, I smelled something burning, but, then again, it could be coincidence.

Generally speaking, the car still runs quite well on highway, 30 to 31 mpg, but once you get in stop and go mode getting sluggish. But even in this mode 24 to 26 mpg. It does like oil, at about 220,000 or 230,000 miles it started to burn about one quart every 800 miles or so. Should not be that bad considering all these miles. This is the end of my S H O R T story. Vincent, thank you very much for your time and help.

  • 1990 Mitsubishi Galant
  • 262,000 miles
  • 2.0 liter SOHC
  • Manual transmission
  • A/C

Best regards,

A. WOW! This is going to turn into a novel very quickly! I can't believe this guy said he made the adjustment because the engine was too quiet. What was he thinking? I guess he doesn't believe in "If it ain't broke, don't fix it".

The Fast idle speed is controlled by the Powertrain Control Module (PCM) through the Idle Speed Control Servo (ISCS). Under normal conditions those two screws never need adjustment. In fact they are painted to alert technicians that it is not a normal adjustment.

Since both of these screws have been tampered with, they will have to be adjusted properly. Here is the procedure for adjusting both screws properly:

Base Idle Speed:

  1. Confirm initial inspection conditions:

    1. Engine coolant temperature is 185 to 205°F.
    2. Lights, cooling fan and all accessories are OFF.
    3. Transaxle is in neutral or park.
    4. Steering wheel is in straight-ahead position, (on vehicles with power steering).
  2. Check the ignition timing and adjust, if necessary. (Refer to procedure)
  3. Connect a tachometer to the engine speed detection connector, Fig. 54 . Do not unplug the connector.
  4. Start the engine and idle for two minutes.
  5. Check the engine idle speed. If it is NOT within specifications, check the idle speed control system. Curb Idle Speed: 750 ±100 RPM
  6. Stop engine and disconnect tachometer. (Leave tachometer connected, if further adjustments are required).


  1. Loosen the accelerator cable, so a slight amount of slack is present.
  2. Connect a tachometer to the engine speed detection connector.
  3. Turn the ignition switch ON, but DO NOT start the engine.
  4. Wait at least 15 seconds and check the idle speed control plunger position. The plunger should extend to a fast idle position, then retract after 15 seconds. This is the initial position.
  5. Turn the ignition switch OFF and disconnect the electrical connector for the idle speed control servo. This will lock the idle speed control plunger at the initial position. Fig. 55
  6. Loosen the fixed speed adjusting screw (SAS) fully.
  7. Start the engine and check that the idle speed is within specifications. Basic Idle Speed: 750 ±50 RPM
  8. If idle speed is not as specified, adjust the idle speed control adjusting screw with a hexagon wrench. Fig. 56
  9. Turn the fixed SAS in until the engine speed rises slightly.
  10. Back-off the SAS to the "touch point", where the engine speed ceases to decrease.
  11. Back-off the SAS an additional half turn from the "touch point".
  12. Stop the engine. Readjust the accelerator cable.
  13. Reconnect the electrical connector for the idle speed control servo and remove the voltmeter from the engine.
  14. With the ignition switch OFF, disconnect the negative battery terminal for at least 15 seconds. This will clear any diagnostic codes that may have occurred during testing.
  15. Reconnect the battery, start the engine and check that the engine is operating in the normal idle range.


As for your heating problem, If both heater hoses are cool, relatively speaking, there is no flow going to and from the heater core. In your vehicle there is no heater control valve. Temperature is determined by how much or how little heat is vented into the cabin by the blend door.

But since there is no flow through the heater core, we can eliminate anything inside the vehicle as the problem. SInce the only thing that would prevent the flow of coolant through the heater core is a clogged heater core, that's a pretty good bet that is the case. Under normal conditions, with the heater on full hot, the inlet heater hose should be at normal engine operating temperature and the outlet heater hose should be significantly cooler.

Additional Information provided courtesy of ALLDATA

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© 2003 Vincent T. Ciulla

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