troubleshooting help - at my wit's end (Mitsubishi 6G72)

Boost_Crazy said:

In reply to irish44j (Forum Supporter) :

It's neither flapper or hot wire. That honeycomb is a vortex generator, it uses it to count the incoming airflow. I don't recall them being problematic. There is an air temp sensor and I believe a barometer in there too. If you think the sensor is bad, unplug it when it won't start. It should bypass the counts and start off a limp home table. 
 

I'm leaning heavily towards coolant temp sensor related. I know you checked the wiring and sensor, but it sounds just like what used to be a very common starting issue with turbo DSM's when the wires got brittle. 
 

Is the fuel pump running when it won't start? I know fuel pump relays were an issue sometimes. I know most Mitsubishis had test connector under the hood on the fire wall where you could run the fuel pump with a jumper off the battery. 

yep, I have a momentary switch hooked to the test connector so I can run the fuel pump manually. I've actually been priming it in advance so it should be on full pressure on all these tries. I considered running standalone wires from the coolant temp sensor directly to the DME but the wiring tested out fine so it doesn't *seem* like that should be an issue.

it is notable that on the MAF the honeycomb is slightly damaged (think similar to a radiator, just mushed in one spot). It's been like that since I got the vehicle though so unsure why it would suddenly be an issue. In any case, a new MAF is on the way so will be able to rule that out as well hopefully (then of course there's MAF  wiring, etc.....)

I just started the truck. Took about 3 ties in 45 degrees after sitting overnight, so problem is still there. I'll test it with the MAF unplugged tomorrow once it's cold again. 

So you guys have been more than helpful, and I feel bad continuing this but might as well do so until (if) I solve the issue, which of course will happen when I've replaced every single part of the engine lol.

1. unplugged the MAF as suggested: no start at all. Plugged it back in and it eventually stumbled to life.

2. new MAF came in. Installed: no change in the situation, not the MAF apparently. 

3. on a hunch, I unplugged the coolant temp sensor entirely: started quickly (though Ihad been cranking testing the MAF, so meaningless diagonstic) but would just stumble to life and then die (twice).

4. plueed coolant temp sensor back in and started immediately, no stumble. But again, it was probably "almost there" already so doesn't mean much.

So I have to wait for it to cool overnight again and try tomorrow again.

- at this point, it keeps coming back to the coolant temp sensor (which would have to mean the wiring itself). The next test may be to run a separate pair of wires directly from the sensor to the ECU pinouts and see if that helps. Otherwise, one of I suppose two other things that I haven't replaced/messed with yet:

1. the igniter:: not something known to go bad on these, and IIRC I put a new one in just a couple years ago pre-emptively so that would be doubly surprising. Plus, also unclear how temperature would affect it variably depending cold vs. warm. 

2. maybe the crank position sensor? If that were bad maybe the fuel pump isn't getting an immediate signal, idk? But if it were bad I would think that would make the car not run at all. Plus unsure why the engine cold vs. warm would have any effect on that. 

3. some other wire, someplace else, that does something, anywhere in the engine harness of a 36-year-old vehicle ......

On the upside, the truck always DOES start eventually and then runs fine So if I can't solve it, just means i need to make sure when it's cold that my battery has a good charge for extended cranking, and deal with this minor annoyance. 

Not entirely related to your post, but I love how no matter how much experience and knowledge you can have with cars, they will still find the weirdest ways to make you pull your hair out. My favorite 2 are:

Had a 91 ranger that wouldnt start, no starter engagement. Replaced everything related to the starter and nothing worked. Ended up being a cracked firewall that I fixed with a 10 cent washer 

00 explorer had a vacuum leak that would appear after hours of driving. Ended up being a spark plug that was just loose enough that when the engine was cold enough it wouldn't leak, but when it warmed up, massive vacuum leak. That took awhile to find .

Best of luck and at this point I'd color outside the lines for ideas. Old security system? Interlock somewhere? Small automotive gremlins bent on destruction?

Okeydoke - lotta stuff going on here.

As noted, the MAF on these machines is a Karman Vortex, not a hotwire.  The screens are important to straighten the airflow and make the MAF read correctly - most notable at idle/lower airflow scenarios.

The intake air temperature sensor is integrated to the MAF, so when you replaced the MAF the intake air temperature was replaced.

OBDI Mitsubishis are very sensitive to the coolant temp sensor and straight up will not start if they are getting bad data, they are not like the old GM stuff that will just YOLO it and half-ass run with bad sensors.

You really need a scan tool, otherwise you are just parts-cannoning and not going to fix it.

The OBDI Mitsus use ALDL (Assembly Line Diagnostic Link) protocol.  

You need EVOScan

https://evoscan.com/product/evoscan-obdi-aldl-12pin-1989-1994-mitsubishi-datalogger-cable/

or a Scanmaster

https://www.extremepsi.com/store/product.php?productid=17998

or MMCd

https://mmcdlogger.sourceforge.net/

Alternate options include an old Palm Pilot with Pocketlogger (good luck), a TMO Datalogger and appropriate cables (good luck), or a Snap-On MT2500 (ebay) with the Mitsu-1 Cable (ebay) and correct cartridge (ebay) for the year of your vehicle.  

Once you have your scanner working you can fix this easy peasy including the trouble light for the TPS.  Note the TPS on these OBD1 vehicles should never report fully closed if it is adjusted correctly, ususally reports something like 5% open.  

In reply to irish44j (Forum Supporter) :

Does this year/configuration of engine have Jet valves? If so, could one be sticking when cold and causing compression issues?

Just a wild thought...