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1G 1991 Mitsubishi Eclipse - rough idle

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romeroom

Probationary Member
5
0
May 28, 2024
Michigan
This is my first time here. My sincere apologies if I am posting where I'm not supposed to. Please direct me otherwise.
Please note I have made the same post on a different forum.

Bought the 1991 Eclipse 2.0 recently. It idles very rough after warming up to the point it will often quit if I don't give it some gas when I'm at a stop light. It runs great when traveling down the road.

Here are some details:
  1. Maintenance/Repairs: Recent oil change, new plugs, new alternator, replaced exhaust flex pipe, new air filter, sprayed a significant amount of throttle body cleaner into the throttle body.
  2. Checked vacuum leaks by spraying throttle cleaner around vacuum hoses I could see on top and around valve cover. No noticeable change in RPM
  3. Regarding the Mass Airflow Sensor
    1. Unplugged and plugged back in the wiring harness while the car was running. No noticeable change in RPM.
    2. Opened the air filter chamber to access and inspect the MAF sensor. Noticed two of the
      "squares" on the laminar flow screen were plugged.
    3. I have not checked for voltage (12 & 5) on the harness plug yet.
    4. I noticed the two sections of the air filter housing where the MAF sensor resides does not have a tight seal when secured together with the three clamps. Would that make a difference in the functioning of the MAF sensor?
I would appreciate some suggestions on what else I should be checking. Thank you.
 
Turbo car? Do you have anyway to datalog it?
Filling out a vehicle profile will help a lot. Be sure to include your mod list, if any.

Explain "rough." Like really low all the time (below 750), hunting (from ~800 - ~2000 RPM over and over), or like it's missing? Feel free to post a video.

Have you tried adjusting the BISS? The Factory Service Manual (FSM) has a special procedure for setting it.

Do you have a check engine light (CEL)? You should normally see it for a few seconds on startup but if it stays on or comes on randomly on you should pull the code.

Perform a boost leak test, if turbo. This is very important since intake leaks wreak havoc on the idle system. You can't always find them via spraying carb cleaner or visual inspection.

Remove your ECU from under the dash and inspect or post a picture of the circuit board. You're inspecting the three capacitors for leakage and corrosion on the board. This can cause the idle circuit to be inop. Also you're looking for any blown driver chips.
ISC/IAC could also be a culprit. There's a testing procedure for this in the FSM but a datalogger can help. A failed one has the potential to blow a driver chip in the ECU.

The idle switch is another thing to inspect. Does it have a ground and is the single wire still attached (top/back side of the TB)? The FSM also has a procedure for setting the height/stop adjustment.

FIAV could also be another culprit if you're get high or hunting idle and everything else checks out. Blocking it is the most common fix.

I think the coolant temp sensor plays a roll too, but it should trigger a CEL.

I'm not following what you mean with the MAF. Post some pictures if you can. Granted, unplugging it will usually affect idle but also trip the CEL.
 
This is my first time here. My sincere apologies if I am posting where I'm not supposed to. Please direct me otherwise.
Please note I have made the same post on a different forum.

Bought the 1991 Eclipse 2.0 recently. It idles very rough after warming up to the point it will often quit if I don't give it some gas when I'm at a stop light. It runs great when traveling down the road.

Here are some details:
  1. Maintenance/Repairs: Recent oil change, new plugs, new alternator, replaced exhaust flex pipe, new air filter, sprayed a significant amount of throttle body cleaner into the throttle body.
  2. Checked vacuum leaks by spraying throttle cleaner around vacuum hoses I could see on top and around valve cover. No noticeable change in RPM
  3. Regarding the Mass Airflow Sensor
    1. Unplugged and plugged back in the wiring harness while the car was running. No noticeable change in RPM.
    2. Opened the air filter chamber to access and inspect the MAF sensor. Noticed two of the
      "squares" on the laminar flow screen were plugged.
    3. I have not checked for voltage (12 & 5) on the harness plug yet.
    4. I noticed the two sections of the air filter housing where the MAF sensor resides does not have a tight seal when secured together with the three clamps. Would that make a difference in the functioning of the MAF sensor?
I would appreciate some suggestions on what else I should be checking. Thank you.
Play with the BISS screw.
 
Turbo car? Do you have anyway to data log it? Filling out a vehicle profile will help a lot. Be sure to include your mod list, if any.
Explain "rough." Like really low all the time (below 750), hunting (from ~800 - ~2000 RPM over and over), or like it's missing?

DWB: OMG, thank you so much. What great questions, useful not only for diagnostics but teaching this novice who has been fixing his own cars for years, yet is clueless about many things.

In the next week or so I will go through your questions in detail and respond best way I can.

Play with the BISS screw.
Thank you. Since I'm brand new to this car, some questions:

1. Where would I locate the BISS screw? (I suspect its near the throttle body.)

2. In a post I found, it says the following about the BISS: "You have to ground out the ECU then adjust the BISS to desired idle. Disconnect the ground you just made and restart the vehicle. You should now be set at your desired idle. Having a scan tool helps you put the idle on the right numbers." How do I ground the ECU? I thought all ECU's were grounded.

3. Is there a service manual online for a 1991 Eclipse 2.0 non-turbo?

I think the coolant temp sensor plays a roll too, but it should trigger a CEL.

I'm not following what you mean with the MAF. Post some pictures if you can. Granted, unplugging it will usually affect idle but also trip the CEL.
A couple of quick responses before I fully tackle your reply. (By the way, it is non-turbo) Also, the car is at my son's garage and I won't get to it for about 3 weeks.

1. CEL: I don't recall seeing a CEL light triggered. Relatedly, since this is a gen 1, OBD1 I believe, how would I find out how to scan or check for trouble codes?

2. MAF: My apologies for not being clear. It was my understanding that unplugging the electrical harness to it would impact idle speed. Plugging and unplugging it yields no noticeable change in RPM. To be sure, I do need to check the harness for power.

Thank you again. I appreciate your willingness to help and patience with this noob.

Play with the BISS screw.

Please, anyone, where would I find the BISS screw. A picture would be great!
 
Last edited by a moderator:
If you are keeping this car, you're 100% going to want to get ECMlink.

This sounds to me like a faulty FIAV. I was having this issue even after replacing it so I deleted my FIAV entirely. One less thing to go wrong and possibly damage the ECU. Any air leaks after the airflow sensor will cause idle issues too, get a cheap smoke machine and check to make sure you don't have any air leaks.
 
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