My wife has a Mitsubishi 91 Eclipse with an with a 1.8 liter engine and automatic that periodically bucks and surges, especially noticeable under load like going uphill but can occur during straight line highway driving. After doing a complete tuneup, changing the throttle position sensor, O2 sensor, having both the ecu and tcu rebuilt, check all vacuum lines, replaced ignition wires, replaced fuel pressure regulator and checking fuel pressure, replaced injectors,checking all electrical sensors and connections, etc. etc. I, nor professional mechanics, cannot determine what may be causing this condition. At this point I’m thinking either the fuel tank needs removal and cleaning or its the fuel pump, coil, ignition switch, or torque converter. Here’s a clue… I’m finding an oily substance on the pavement that appears to be coming from the transmission area. It doesn’t appear to be atf nor oil. I’m suspecting the torque converter is leaking, any ideas?
Have you ever checked the fuel pressure while the car is under load? Such as rolling down the highway or trying to pull a hill? Fuel pressure in your driveway and fuel pressure out on the highway pulling a hill are two different things.
Presumably there have been error codes? Report?
Disconnect the vacuum line from the EGR valve. Cap the line and the port. This should turn on a check engine light, but drive it for a while that way and see what happens. Listen for pinging and avoid it and/or use higher octane gas while “testing”.
That is a crazy bunch of work to throw at the thing. I’d tell you to stop but you may be in so deep that you don’t want to turn back.
If no fluids are low, whatever is leaking is probably not related at all.
Not to be deliberately unhelpful but I would have gotten rid of a car that old before investing all that time and money in a 20-plus year-old car.
I like the gas tank idea. What if there is a small amount of water in the tank. the car might only pick it up under high demand operation.
Oil leak detection requires a hands-on inspection.
Torque converters operate with the transmission fluid. Thinking that it might be the torque converter because the puddle does not appear to be ATF is illogical. Dip a clean white rag on the fluid and you’ll be able to see its color, and perhaps see what it is.
Also, check all your fluid levels and their condition. Color, contamination, that sort of thing. Post the results.
Based on this statement
"I, nor professional mechanics, cannot determine what may be causing this"
I’m going to assume that someone has checked the ECU for stored codes.
I confess that your statement “having [had] both the ecu and tcu rebuilt” leaves me suspecting that there is a misunderstanding of something that was done. These units are not rebuildable. Can you clarify?
Right now there are still some questions in my mind, but I’m going to posit a wild guess that you have a vacuum leak in the induction system. Perhaps a manifold gasket or such. If the Mitsu runs cooling channels to the head(s) through their manifold gaskets, such a leak might also be a source of the leakage.
By the way, a little man in the back of my brain is suggesting a cylinder leakdown test. If you find a cylinder that won’t hold pressure, of you confirm a headgasket breech, everything will suddenly become clear. Post the results of the test.
On the tail end of mountainbike’s wondering about an induction leak, I would also include a close inspection of the intake snorkel. Especially if it is soft hose, those can hide razor-like splits that only leak under certain conditions - such as under load - and can be impossible to find unless you really work the hose to see it.
Thank you for all the responses, most appreciated. Yes, the car has been the scourge of my existence. Seems every month there’s an issue of some sort. The weirdest recently was the right hideaway headlamp going up and down constantly after turning off the headlights. Very funny to watch, like it was blinking. The problem corrected itself and hasn’t returned, bizarre to say the least. According to Consumer Reports, electrical was a trouble mark on this particular year. My wife likes the car and is reluctant to part with it so that’s why its still in the picture.
The TCU and ECU were sent to a firm where it was gone through and had parts replaced as needed, such as capacitors and any other components. This was a long shot but another attempt in rectifying the problem. Another reason for having the TCU worked on was there was some coolant dripping on top of it from the leaking heater core and making its way inside the unit. I placed a plastic shield as a deflector on top of the TCU for the time being until the heater core can be addressed (a big job involving dash removal).
The fuel pressure was tested by a mechanic who claims drove it hard on the back roads of Petaluma for half an hour with a fuel pressure gauge connected. He stated the fuel pressure was good throughout the drive.
I just inspected the distributor. The coil pack was checked with an ohm meter and met specs. I did detect some rust under the rotor cover plate and some corrosion where the coil packs electrical connections were. It appears moisture was getting kicked up by the drive belts and went past the distributor cap, which probably wasn’t sealing to the distributor body. I cleaned off all the rust and corrosion, treated electrical contacts with dielectric grease, checked the cap for cracks then made sure it was sealing snugly and properly. The engine started up immediately and ran smoothly and during a brief jaunt across town but have yet to take it for a more extensive drive.
As far as ECU or TCU codes are concerned, none were present. They have to be checked the old fashioned way with a voltmeter and service manual codes then counting how many times the needle swings to determine the malady.
I’ll double check the vacuum hoses again and attempt your suggestions on the EGR and leak down test eventually but I get burned out working on this thing and have to pace myself.
Just did a more extended run involving stop and go driving. Didn’t notice any peculiarities with engine performance, my wife feels the problem is solved but I’m more skeptical knowing the history of this machine. I’m taking a wait and see attitude. If all the problem was corroded electrical contacts in the distributor, fantastic! Time will tell.
Apologies on the ECU/TCU comments. I overlooked the early year of the car.
By the way, I know you said the vacuum lines were checked, but vacuum line is dirt-cheap by the foot, and if it were mine I’d simply buy a roll and replace the lines one at a time. Vacuum leaks can be tough to find, but replacing lines is easy. You can even keep the old lines to inspect close up on the kitchen table. You might find cracks that you never would have found under the hood.
You might have gotten lucky with the distributor. It happens, albeit always after doing much good work, which you’ve done. If so, sincere congratulations.
Thank you for the comments. My wife took the car for a lengthy drive that normally would have showed the problem. No surging or hesitation, the engine and trans responded normally. Can the situation been resolved? We’ll wait and see. My method has been doing one procedure at a time then seeing if it had any positive effect.
Forgot to mention I did a standstill transmission procedure I saw on YouTube. With the engine running and placing my foot firmly on the brake pedal, I accelerated to around 2500 rpm in park, neutral, drive, reverse and low, done for only a few seconds to avoid overheating the trans. I understand this might free up any stuck valves in the transmission, so was worth the try.
Changing all the vacuum hoses seems like a good idea, it’ll be next on the agenda. It’s just a pain working on this car, hard on the back stooping over the engine compartment and difficult getting my hands in to access various components as everything is packed in there so tightly.
An update on the Mitsubishi. Rechecking all things, I found the fluid was transmission fluid and that its level in the trans was way too high that involved some extraction. Based on two manuals, the Haynes and the official workshop manual from Mitsubishi, there were conflicting ways to test accurately for correct fluid level. The Haynes manual stated checking with the engine hot, the selector in PARK after slowly going through the the gear range and after driving 30 mins. The workshop manual states checking it the same way but in Neutral. Before I went by the workshop manual, this time by the Haynes manual. Now the level is correct and maintains it with no evident leakage on the ground.
Since doing this the trans was behaving itself…until very recently when the symptoms of bucking reappeared to our dismay. I’ve cleaned and checked the sensors and their connections in the engine compartment, all seem fine, now I’ve cleaned and checked sensors and their connections in the passenger compartment related to sending info to the computer including a sensor that was quite dirty located by the gas pedal. Time will tell if what I did rectified the problem of bucking and hesitation. If not, the next chore is to pull the gas tank and fuel pump and check for contamination or blockage. Stay tuned!
Finally found the cause. After being scanned by an experienced mechanic, the problem turns out to be a bad crank position sensor. With this model it’s located within the distributor and not sold separately, necessitating replacing the whole distributor. Now the engine runs normally…so far! I’m very pessimistic with this car due to its troubled past but we’ll see how it goes!
Thank you for posting back with the cause of the problem. That can be beneficial to others who may be experiencing a similar problem with a similar car.
My daughter’s first car was a 1991 Mitsubishi Eclipse with an automatic and the 1.8. It was a great little car. It drove well at speed, had ice cold A/C, and would tick off 35 MPG on the road while driving the snot out of it.
Thanks for the update
It’s always nice to hear the the final outcome
Here it is Aug 4 and the Eclipse is still running as it should, no bucking or hesitation. I’d say at this point the diagnosis and the repair was successful. Replacement distributors can be found at most auto parts sites for varying cost. The dealer was the worst at $700, the new one (not a rebuild) I found was on Ebay for just under $110…big difference! Hope the quality of this one holds up.