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1988 4g64 misfires and bad idle

Posted by big_mama, Sep 1, 2020

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  1. big_mama

    big_mama Probationary Member

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    Hi dsm tuners, a Mitsubishi companion here that needs your help. I thought I would write here on this forum because I think here are the most people with knowledge of these older Mitsu engines, and because I am totally lost here. First of, I don't have an Eclipse, but a Mitsubishi Sapporo (Europe) aka Galant Sigma (States) E16a from 1988. Technology wise the engines should be very similar to the 1G Eclipse. It's a 4G64 SOHC however.

    So, my problem is the engine has terrible misfires, especially noticeable in the 2000-3000rpm range. It also fails to start 1-2 times when cold or stalls after a few seconds. The idle is also very rough, especially cold. After narrowing down the possible reasons, I guess the engine is simply running much too rich (--> black sparks).

    The things that I have done so far:
    * I checked all the sensors (as far a I could) that the ECU needs to calculate the mixture, including air flow, air temp, air pressure sensors, engine coolant temp sensor, throttle body position sensor and mixture regulator resistor. I also checked all the lines from the sensors to the ECU, swapped the ECU and swapped the MAF unit. So on the electrical side I don't know if there is anything more I can do, seems to be all fine.

    * Compression wise I did a simple dry compression test with the following results (168, 170, 175, 170 psi). It should be 170, so 3 is a little high (hmm :idontknow:). Vacuum is constant at around -10psi at idle.

    * Fuel pressure regulator is new. I also tried new injectors (cheap Chinese ones though).

    * I checked the EGR valve and cleaned it, and I changed the PCV with a new one.

    * Coil, distributor cap, sparks (tried BPR7ES-U11 and BPR6ES) and cables are all new.

    * Double checked timing belt alignment, and set idle ignition timing.

    * Cleaned the throttle body and checked for intake leaks.

    * The car doesn't have a cat, so no O2-sensor either. It also has no knocking-sensor that could go mad.

    And now I don't know what I can do anymore. Unfortunately I am far from an expert when it comes to engines, so maybe it's just a totally stupid thing that I missed, but I don't know what to look for, or how to approach this. So please, any help is very appreciated. What could cause it to run so rich?
     
    Last edited: Sep 1, 2020
  2. ThunderChild

    ThunderChild Supporting VIP

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    To me it sounds like you have a vacuum leak somewhere. Might try smoke testing the inlet system and see if you have any bad gaskets/seals.

    When did this problem start occurring?
     

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    1987 Toyota Pickup/Hilux
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  3. big_mama

    big_mama Probationary Member

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    But should a vacuum leak not lead to a leaner mixture? I did a smoke test already, only found a small leak at the throttle body shaft which I sealed up. Nothing however that could have caused the issues in my opinion.

    When exactly it started is hard to say because it's a restoration project and I haven't driven the car for some years. But it definitively had random misfires and a tendency to stall before.
     
  4. big_mama

    big_mama Probationary Member

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    DAA11038-4BCA-4985-8344-BB27E9ED55EB.jpeg C46DDF54-FD27-4A19-8E1E-129CE9CBF2E6.jpeg
     
  5. bravos91tsiawd

    bravos91tsiawd Probationary Member

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    Probably not your issue but you can double check your spark plug cables are routed correctly. I had a similar issue when I routed mine improperly causing misfires.
     

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  6. ThunderChild

    ThunderChild Supporting VIP

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    It's possible it could run lean, but if it's EFI, I'd suspect the engine to read that lean mixture from the leak at the o2 sensor, and be trying to compensate by dumping more fuel in.

    Double check for a leaks, and triple check all ignition components. Might be a stuck injector?
     

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  7. tstkl

    tstkl Proven Member

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    Does the throttle body have an idle speed controller? Did you check the resistance on the coils for that? If it is a vacuum leak causing it to run rich I would suspect it is dumping out metered air which is unlikely without forced induction. Can you run any form of mmcd to monitor the engine while it's running? An exhaust leak before the o2 sensor may be causing this too, or just a bad o2 sensor in general.
     
  8. big_mama

    big_mama Probationary Member

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    The thing is, the car has no o2 sensor because it has no cat, so I don't see how the ECU could try to (over)compensate a lean mixture and then run rich. The only place I couldn't really see during the smoke test was the underside of the intake manifold to cylinder head seal, so I changed it preventively. So, I really don't think it's an vacuum leak.

    Maybe it's a fluke in the ignition system somewhere before the distributor. I mean, the clogged up sparks indicate unburnt fuel, so it's either too rich or the ignition gets interrupted at random, so that carbon builds up on all sparks.

    Anyway, no I don't have anything to monitor the engine. In order to do that I would basically need this ECMLink v3 cable, and a software like TunerPro, right? I see that ECMLink comes in different variations, is this because the connector is physically different, or is just the pin layout different?
     
    Last edited: Sep 2, 2020
  9. ThunderChild

    ThunderChild Supporting VIP

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    So there's no o2 sensor in n the exhaust stream? This would be your main problem. It's getting no feedback from the sensor to tell it how the fuel mixture is, so it can't adjust anything and is probably defaulting to some set parameter, hence the rich condition?

    Also, I applaud the preemptive gasket change, but generally the last things you touched are the first things to verify being correct.

    You need the upstream o2 installed in the exhaust for the car to run correctly, do that. You also need to do a smoke/boost leak test on the inlet system to verify if there are vacuum leaks. You can make a boost leak test from stuff at home depot/lowes for about $10 to test for leaks. The o2 sensor issue is in your court, I'd take it to an exhaust shop to have a bung welded in.
     

    Street Build 976  3

    1990 Plymouth Laser N/T
    fwd · manual · 1G DSM

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    1987 Toyota Pickup/Hilux
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    1997 Mitsubishi Eclipse Spyder GST
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    1992 Eagle Talon TSi AWD
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  10. big_mama

    big_mama Probationary Member

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    Hmm, I'm not sure if you or I misunderstood, but it's not like the car is missing the o2 sensor, but it's not supposed to have one. It doesn't have a catalytic converter. And I already did a smoke leak test as I said.
     
  11. Dericsh

    Dericsh Supporting Member

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    Secondary o2 sensors monitor the function of the catalyst. But what about before that? In the exhaust manifold or somewhere near there? There should be a sensor there that would give the ecu feedback about the air/fuel mixture leaving the engine.
     

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  12. big_mama

    big_mama Probationary Member

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    Well, there was a cat and a non-cat version of the car. The cat version has an o2 sensor in the exhaust manifold but the non-cat version has none. Which should also not be a problem as long as everything is okay with the engine.

    Anyway, I think I have to invest in a scope now so I can see what's going on.

    Btw, the ISC coil resistance is fine @tstkl
     
    Last edited: Sep 3, 2020
  13. delta448

    delta448 DSM Wiseman

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    If it's fuel injected it will need to have an upstream oxygen sensor to run correctly. Is it possible that you overlooked it or that the exhaust has been improperly repaired? Do you have an ECU pinout for this car? The closest I could find was one for an 89 Galant SOHC, but it clearly shows an upstream sensor connected to pin 4 of the ECU.

    Screenshot_20200903-164427_Chrome.jpg
    [​IMG]
     

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  14. big_mama

    big_mama Probationary Member

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    I have the ECU wiring diagram. It's in german so I tried to translate it as good as I could (too big to upload here. Click on image for full-screen)
    https://killkenny.spdns.org/nextcloud/index.php/s/P8m7HrEnA3aPdDK

    The O2 sensor has a double dotted line which indicates it's only for cars with a cat.

    Edit: Btw, there are also two different ECU's for cat and non-cat cars.
     
  15. delta448

    delta448 DSM Wiseman

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    So if the catalytic convertor were equipped on this car, would it be located before or after the sensor?

    Without feedback trim, you'd basically have 4 injectors working like fixed electronic carburetors. Even carbs can be adjusted. I can't imagine a manufacturer denying that functionality on purpose.

    Do you have or have access to a wideband sensor so you can tailpipe sniff it? I would be pretty surprised if the fueling is where it should be.
     

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  16. big_mama

    big_mama Probationary Member

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    After the sensor
    Tbh, I don‘t know what leads you to that conclusion? The air/fuel ratio is basically calculated with the data from the afs, engine temperature and throttle body position etc... An o2 sensor is not really needed for that. Afaik the o2 sensor is only needed to fine tune the mixture so that the exhaust gas has a more or less exact composition, so that the catalyst can work optimally.

    Oh, I‘m sure it‘s not where it shoud be, thats probably why the sparks are black. I even get backfires, so I surely have too much unburnt fuel in the exhaust. The question is why?
     
    Last edited: Sep 4, 2020
  17. delta448

    delta448 DSM Wiseman

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    I'm not sure what to suggest except for maybe an adjustable fuel pressure regulator.

    And yes, you certainly can run an engine in open loop without feedback to trim the fueling, but I am surprised that it is set up that way from the manufacturer. Filters clog, injectors get dirty, pumps wear out, airflow sensors drift, temp sensors fail... without on-the-fly adjustment how long could they expect their product to perform optimally?
     

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  18. big_mama

    big_mama Probationary Member

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    hmm, first generation injection system I guess then :idontknow:
     

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