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1G 1991 TSi wont run right when warm

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MyTalonTSi1991

Proven Member
48
28
Jul 12, 2019
Bloomington, Illinois
I have been chasing this issue with my Talon where it has been sputtering and stalling out. Ive replaced the PTU, tried a salvage yard MAF, replaced the plugs and wires, replaced the fuel pump and fuel filter and even put in a reman ECU and I still am left with this issue.

The car runs just fine when its cold, drives just fine. Its when it warms up to operating temperature where I have this issue. If I give it any more than about 25% throttle it sputters and stutters and is very unhappy. I gave up chasing the issue and took it to storage in the fall. While driving it there I had to 2 foot it at stop lights to stay running and my friend that was following me noticed when I would accelerate it would puff black smoke. When I got to my barn it seemed to be running VERY lean, smelled hot. Went back and after it cooled down the spark plugs look like the engine was running very lean.

I'm not sure whats left here. Nobody in town really knows much about these cars and the Mitsubishi Dealer is hesitant to start digging into it when I take it out of storage in the spring.

Somehow the air/fuel seems to be off leading me to the MAF. I have a laptop with data logger software, not DSMLink, I cant recall which software it is but the ambient and coolant temperatures seem to read accurate. I tested the injectors by disabling them one at a time in the program and all the injectors seem to be functioning as intended. I'm kind of at a loss here. My Local O'Reilly can get a reman MAF for $114 plus core, should I go that route? I could also throw in a coolant temperature sensor and even a coil. It acts the same with my old ECU which when I pulled it was also a reman and the board/caps looked perfect.

Id love to get this back on the road so I can enjoy it, but right now it really cant be driven anywhere reliably. Thanks for any assistance!

The last shop I took it to had it for 2 weeks, they did the fuel pump, filter and injector cleaning, that was in August.
 
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Funny that nobody in town knows anything about a car build down the street.

I suspect a problem with the Engine Coolant Temp Sensor but who knows. Figure out your logging software. Start a log with the engine cold and watch it heat up.

Lean cars don't usually puff black smoke, rich cars do. Not idling points at looking into the IPS, ISC, TB and ECU to make sure the IPS is telling the ECU the throttle is closed, that the ECU is able to move the ISC to respond and that the BISS is correctly set. Again this will show up in a log.

How old is the O2 Sensor? As soon as the ECT goes over 86F (assuming a cold engine) you should see the O2 voltage cycling as the ECU starts closed loop.

Just a few thoughts.
 
O2 sensor on the turbo was done in April 2021. The ISC I replaced with one from Rock Auto. I grounded out the points and set the BISS, seems to idle fine with no surging.

I'm not familiar with the IPS, what is that? I guess most of the people in town who worked on these have retired or something. The dealer was willing to do the timing belt a few years ago but most places say no thanks when I bring it there.

The software is TMOLogger that I have ion an old laptop and I have a serial to the communication connector on the fuse panel by the brake pedal
 
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My initial thought is the coolant temp sensor or maybe throttle position sensor or their wires. You can check those with your TMO logger. Also log your O2 sensor and make sure it's cycling. I ran with a faulty idle switch (ips) for a few weeks with very minor symptoms, so probably isn't that alone.

Do you have any modifications? What about a check engine light? Have you done a boost leak check?

Something is causing a rich condition if you're seeing black smoke. Then going lean, so probably an electrical component on the verge of failure.

Pretty much what Steve said.
 
The car is stock, no CEL. I have tested the CEL by unplugging the MAF....it will immediately trigger the light so that system is functioning properly. I haven't done a boost leak test...that was on my list to consider, just not sure how to do it!
 
Idle Position Sensor (IPS) is on the firewall side of the throttle body and does two things, Tells the ECU that the throttle is closed by grounding the green wire connected to it and acts as the butterfly stop. The throttle body have a strap from the top of it to one of the bolts holding it to the intake manifold that makes sure the TB is grounded so the IPS works. (Somewhere I have pictures of it but I know they are on the site here)

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The TMO logger will work fine. There used to be a site that explained how to use everything. The Old TMO site is still online but it doesn't go into detail.
 
If you are able to log your O2 sensor voltage you can tell quite a bit about rich or lean from that.
You probably have a pretty good feel for when the engine should be running richer than stoic and when it should be leaner than stoic and when it should be just right around stoic. So look at the O2 voltage log, vs throttle opening basically, and see if it is doing the right thing.
When the O2 voltage is cycling it's stoic and it should run ok at light throttle and idle.
When the O2 voltage is staying high (not cycling), like around 0.7 to 0.9 volts, it is richer than stoic and you want that at heavier throttle. But of course there can be too much rich and that could be the black smoke.
If the O2 voltage is just laying there at 0 volts, it's lean, and normally you should only see that during decel, or in the first minute or two after a cold start.
 
The first thing I'd suggest is stop throwing parts at it. Of course that might work but the more new parts (and especially the more other people who install them) the more question marks you have. New defective parts are not rare, mistakes in buying and installing are possible ... When you get down to "It has to be one of these two things" and both are pretty cheap and you can change them yourself, sure. But you are not there yet.

A good first question with most problems is: "Did it ever run right?" Or has your car been like this from the day you drove it home? If it used to be okay, then did the problem come on gradually or overnight? Was it solid from day one or sometimes there, sometimes not? Did something change right before the problem showed up?

"Runs fine when cold, lousy when warmed up" might mean the problem happens in closed loop operation. (Because 02 sensor is heated only by exhaust this takes a while on 1G.) The other possibility is that something is okay when cold but fails when hot: problems with the ignition system are probably the most common of those.

One quick test that would be useful now is to warm it up, confirm that the failure is still there, and then disconnect the 02 sensor. That forces open loop operation and if it's a closed loop problem, it will go away. If it doesn't, then you have a 'bad when hot' issue.

If it is a closed loop problem then collect all the numbers your data logger gives you and post them. With a problem this bad, something will be very wrong with those numbers. If it's 'bad when hot' then since you've replaced plugs and wires the best bet is either a coil or transistor (no distributor on that car, right?) and there are things to do to either gain confidence in that diagnosis or point in another direction.
 
OP, Work to learn more about how to do a better job troubleshooting. Most “professionals “ can’t do this well. Make a diagnosis plan and trust your measurements/data. Don’t trust your gut feeling and throw parts at it. Waltah’s advise above seems like a solid plan to start with.

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"Runs fine when cold, lousy when warmed up" might mean the problem happens in closed loop operation. (Because 02 sensor is heated only by exhaust this takes a while on 1G.)

I see O2 sensor heaters in the ECU wiring diagrams....
 
I see O2 sensor heaters in the ECU wiring diagrams....
I guessed wrong -- the 90's were a period when the O2 sensor situation switched from just one to two and from heated by the exhaust to having electrical heaters built in. Different Mitsu models switched in different years and California cars are different from 'federal' (all non-California) in some years.

Unplugging the two heated sensors will give you a CEL and codes but if I'm understanding correctly you'll still be able to drive in open loop and thus find out if closed loop is where the problem lies.

OP, Work to learn more about how to do a better job troubleshooting. Most “professionals “ can’t do this well. Make a diagnosis plan and trust your measurements/data. Don’t trust your gut feeling and throw parts at it.
This is good advice. My gut feelings are about 75% wrong. But -- this forum is a fantastic resource. Bring data, you will get answers.
 
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