Mitsubishi Repair: 90 Eclipse GS 2.0 Non-turbo (Surge at idle), sec intervals, mileage engine

Question



Installed low-mileage engine from engine importer. Swapped original intake w/all related attachments/sensors etc., exhaust manifold, and remaining original sensors from previous engine. Starts and runs fine(1800-initial, drops to 1500rpm within 3 minutes) until closed-loop operation at which time engine begins to surge between 1200-1400 at approx 8 min from start-up. At approx 15 minutes (normal op temp) the engine will go into a die/recover cycle, also with a 200rpm bandwidth (<1 sec intervals). When the engine dies and recovers, I can feel it torque then relax (a fairly good work-out for the mounts). Bring the throttle just off idle and it returns to a nice rolling 100rpm surge @ 3 second intervals. Above 2000:50rpm surge w/same 3 sec frequency. Turn the A/C on at idle and idle drops to 900 w/50rpm surge @ 3 sec interval (barely noticeable). In drive: idles @ 800 w/30rpm variation (not cyclical). If I just turn on all electrical accessories; Headlights, Rear defroster, blower fan etc.,and with the cooling fan on, it will idle steady. As I add the loads one by one, the recover time to 1400rpm lengthens until sufficient load has been achieved to maintain steady idle. I watched the timing marks during this whole process.  They toggle between 8 degrees and about 20,(switching like a transistor)in exact time with the surge.

I installed a new timing belt, checked and rechecked the marks three times after two complete cycle rotations.

The cam sensor can be installed in two positions; Correct & 180 off. I was reasonably careful to watch for correct positioning. I'm not sure if it would even run if it were 180 off.

I checked & adjusted the timing, but I couldn't get any power out of it until rpm gets above 2000. I tried several different positions and left it where it performed best.

The O2 sensor may be a little slow (switches from 200mv to 800 in a second or so), but it doesn't seem like it could cause this symptom.

I tested some of the other sensors, but couldn't find anything out of specs.

Symptom Summary:

Idle @ normal op temp:
    
    No loads = Cyclical 200rpm surge
    (die/recover; like fuel shut off)
     @ <1 second intervals.
     

    w/12v loads =  Die/Recover
    200rpm cycle w/increasing recovery
    time to 1400rpm until most or all
    loads on which produce smoothe
    idle.

    w/ only A/C on = 900rpm w/50rpm
                     rolling surge @ 3 sec
                     intervals

    Drive = 800rpm w/30rpm non-
             cyclical variation

  OFF IDLE in Neutral :

            1500rpm : rolling 100rpm
                      surge @ 3 sec

            >2000rpm : rolling 50 rpm
                       surge @ 3sec

  Have you ran across this type of problem and if so, could you suggest anything I might try to locate the cause?

Answer
Kelly: When I ran across this problem it was due to the computer going bad. Your car is the right year for the bad computer problem. But you said your checked the sensors, I guess I would have the car coded just to rule out anything problems with that. Sensors or computer. You know don't you that Mitsubishi farmed out the building of their computers to another company that did not make very good ones. The problem often shows up just as your car is doing. There is a piece in these computers that starts to leak onto other parts in the computer, so with time killing off the computer. AutoZone will code your car for free and that would rule out any problem with the computer. I shall check around with friends that work on cars and the interent to see what I can find. If it is not the computer or a bad sensors. Keep me posted. Hope this helps and good luck!