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...

93gsxturbo said:

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.  

 

well, if that's actually accurate then it wouldn't be the coolant sensor since it will start and run just fine. MAF replaced as well.

however, I have been noticing a slight coolant leak around the thermostat, and upon further investigation, the thermostat housing has a crack in it and is leaking. That doesn't really seem as if it could affect much of anything (other than losing coolant), but now I have to replace that before I do anything else anyhow :P

Point taken on the diagnostic tools, but at this point there's not much more I can throw parts at anyhow (at least nothing that costs more than a few bucks) short of this being something internal, so not sure that's worth the cash at the moment. Hindsight is 20-20. But will definitely look into them.

Recon1342 said:

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...

No, only the 4cylinder models

irish44j (Forum Supporter) said:

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.

WEllll, you say that, but what if it's a temperature/oil related thing where you have more crank wobble when it's cold until you build enough oil pressure to bring it near the crank sensor..  Some of those old hall effects sensors only have a +-.002 window or so when they get old or the reluctor/pickup wheel gets gunky. 

You did say you get good spark immediately on first crank, right?  That should rule it out...

Do these even have a crank angle sensor? I don't  think Mitsubishi used those until the mid 90's. There might be a cam angle sensor? But this version of the 6G72 has a distributor, right? 
 

Now I don't have much experience with 6G72's, but lots with 4G63's of the era. The only thing I've ever seen cause the symptoms that you are describing were temp sensor, or the wiring to it, related. Huge vacuum leaks, dead injectors, wrong sized injectors, bad coils and plug wires, you name it, the cars still started as long as there was fuel pressure. Going by memory, when there was a problem with the temp sensor circuit, it would read some in between temp that was too high for a cold start. Now this is a foggy 20+ year old memory, so it may be wrong. But I remember  long cranking when cold when I had a bad wire. After repeated cranking with a no start, it would then flood. I'd unplug the fuel pump, crank the engine, it would start, then I'd run back and plug the fuel pump back in before it died. Then it would be fine. Must have been funny to watch. 

Boost_Crazy said:

Do these even have a crank angle sensor? I don't  think Mitsubishi used those until the mid 90's. There might be a cam angle sensor? But this version of the 6G72 has a distributor, right? 
 

Now I don't have much experience with 6G72's, but lots with 4G63's of the era. The only thing I've ever seen cause the symptoms that you are describing were temp sensor, or the wiring to it, related. Huge vacuum leaks, dead injectors, wrong sized injectors, bad coils and plug wires, you name it, the cars still started as long as there was fuel pressure. Going by memory, when there was a problem with the temp sensor circuit, it would read some in between temp that was too high for a cold start. Now this is a foggy 20+ year old memory, so it may be wrong. But I remember  long cranking when cold when I had a bad wire. After repeated cranking with a no start, it would then flood. I'd unplug the fuel pump, crank the engine, it would start, then I'd run back and plug the fuel pump back in before it died. Then it would be fine. Must have been funny to watch. 

 

No, of course, duh..... Right after posting that I looked for the crank position sensor and of course it's not there because yeah...it has a distributor (I was thinking of my e30......). So duh, that's not the issue :P

This weekend I run new wiring from the sensor all the way to the ECU so that can be definitively ruled out (or hopefully not). The wires tested out good for connectivity, but maybe just not *enough* connectivity. So time to suck it up and just do the wires smh. The worst part is that at the ECU end the wiring is REALLY hard to get to down in the footwell, so that's sure to be annoying....

Ah, the joys of old cars (and trucks).

With my Power Wagon, it took about two days of troubleshooting to determine the ignition resistor was wired backwards.

irish44j (Forum Supporter) said:

93gsxturbo said:

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.  

 

well, if that's actually accurate then it wouldn't be the coolant sensor since it will start and run just fine. MAF replaced as well.

however, I have been noticing a slight coolant leak around the thermostat, and upon further investigation, the thermostat housing has a crack in it and is leaking. That doesn't really seem as if it could affect much of anything (other than losing coolant), but now I have to replace that before I do anything else anyhow :P

Point taken on the diagnostic tools, but at this point there's not much more I can throw parts at anyhow (at least nothing that costs more than a few bucks) short of this being something internal, so not sure that's worth the cash at the moment. Hindsight is 20-20. But will definitely look into them.

OK go ahead and ignore good advice and keep parts cannoning, guessing, and not fixing it.  Glad I posted.  JFC.  Did you even look at MMCd?  Its completely free, can be run in a Palm OS emulator, and the cable is DIY and costs like 10 bucks to make.  Or get a used Snap On brick from Ebay, that will do what you need all day every day.

Does it have at least some compression cold - like 125 psi on 2 cylinders?  If so, its not a mechanical issue preventing it from starting cold.

Does it start immediately on ether when its cold?  If so, you can rule out all mechanical and spark related components.

Just for you - and because I am home today since a transformer blew up at work - I fired up my good ol Snap On MTG-2500, typed in a few VINs of Dodge Raiders, and it would work just fine with the HYUN-2 adapter plugged into the under dash ALDL diagnostic port.

My recommendation, if you dont want to mess with the homebrew scan tool solutions mentioned above, is hop on Marketplace or Fleabay, get an MT2500 with the Asian cartridge through the year you need (protocols are not available under Dodge, so Domestic cart won't work) 

If you are feeling frisky, even the oldest Snap On Solus would work too, they are basically just the MT(G)2500 in a prettier case and with a bigger screen.  An out of date Solus is worth basically free dollars.  

https://www.ebay.com/itm/267098246277?_skw=MT2500+asian&epid=1411827082&itmmeta=01JJYQWR28462046FVRNQTF5AJ&hash=item3e304bcc85:g:FlAAAOSwCWVnX2RD&itmprp=enc%3AAQAJAAAA8HoV3kP08IDx%2BKZ9MfhVJKndENwNL76Yms2farTGsxyzzGbtd3khfy6LJqRS2F5EchNOz4%2Fwi6U2D2Mqwf9CXxZTSVdy9nznbyZzdcNeZMCeuyManQjWEdJQsORbA0BNQW2%2B5kAn5wFadS7m4tQ5qmJMfxnylBKuvK8CfN3bozIct8YWvOK6QjdwZPpmVmeoeZ5gel9ASdTay0HqZBbTXJZRXM%2Fe7CnOtPmHbOzSR9ZUBTuSzL8nkQUuHY8htwdNxqNRoYQHlS5UC4F2NYg6LziFB%2BMHtAGquXJ4MVIlAePdrghoHHNnWq7t2jfyjufj8w%3D%3D%7Ctkp%3ABk9SR6iB89eXZQ

This would work just fine, may be able to use the Mitsu adapter otherwise the correct adapter is $10.

https://www.ebay.com/itm/226515597112?_skw=hyundai-2+adapter&epid=1522248017&itmmeta=01JJYQZE5FCYT2PQEJHTPX70RY&hash=item34bd61b738:g:SPAAAOSw92lnbk4y&itmprp=enc%3AAQAJAAAA8HoV3kP08IDx%2BKZ9MfhVJKlqkKIoR2H8bC9vQisdedl%2FM1XpO6CNr592GOqhgwtWsq0pndrX5%2FTN%2FTHwupdGxBQLbyKTm3wYTB6EUJdJL6z9LDcUKz9S%2Bif%2B%2FrmTqfjq%2F7YBGLbxkeClNjc6WQ5ml8EUHDcn7%2Fd2%2B7DLvCh0gQS2O83ezR9g2%2FCuUoiza5A6%2BGOpz2pNeyWmazJzJRhzzpydS%2BfxK862Rc%2FBRVnikuystviA40O5hCfxzXMLkhlfYq%2BstF0U1a36vc32Ur9%2B%2B6idn%2FTpE0QEpZNdDWLZhwo1FUCkrXCqsdxc%2BN5FuPGBjw%3D%3D%7Ctkp%3ABk9SR_ji_deXZQ

93gsxturbo said:

irish44j (Forum Supporter) said:

93gsxturbo said:

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.  

 

well, if that's actually accurate then it wouldn't be the coolant sensor since it will start and run just fine. MAF replaced as well.

however, I have been noticing a slight coolant leak around the thermostat, and upon further investigation, the thermostat housing has a crack in it and is leaking. That doesn't really seem as if it could affect much of anything (other than losing coolant), but now I have to replace that before I do anything else anyhow :P

Point taken on the diagnostic tools, but at this point there's not much more I can throw parts at anyhow (at least nothing that costs more than a few bucks) short of this being something internal, so not sure that's worth the cash at the moment. Hindsight is 20-20. But will definitely look into them.

OK go ahead and ignore good advice and keep parts cannoning, guessing, and not fixing it.  Glad I posted.  JFC.  Did you even look at MMCd?  Its completely free, can be run in a Palm OS emulator, and the cable is DIY and costs like 10 bucks to make.  Or get a used Snap On brick from Ebay, that will do what you need all day every day.

Does it have at least some compression cold - like 125 psi on 2 cylinders?  If so, its not a mechanical issue preventing it from starting cold.

Does it start immediately on ether when its cold?  If so, you can rule out all mechanical and spark related components.

 

If I came off sounding as if I was ignoring you, my apologies. I said I'd look into them, maybe worded that sentence poorly. Right after reading that I sent messages to two semi-local friends who have older diagnotic scanners (MT2500 was mentioned by one), so we're working out a time to get together.

Incidentally, while I love the idea of the MMCd being basically free, anyone who reads my build threads knows I'm a moron when it come to even basic electrical or programming things, and everything on that page might as well been in Chinese. I aced ocean engineering, maritime propulsion systems, and naval architecture in college (and am a 25-year national expert in related fields to those), yet managed a D- in electrical engineering and berkeley up basic wiring regularly. Pun intended, my mind is not wired that way.  Call it a mental block or whatever - no amount of studying electrical has managed to make me comprehend it with any level of competence.

On my 89 Saab with Bosch Jetronic, unplugging the coolant temp sensor causes the system to run with a fixed value based on a 68 *F temp. I think you mentioned that the car started when you unplugged the sensor. So, assuming your ECU also defaults to a warm temp value when sensor is unplugged, you tricked it into thinking it was warmer. Also, if it starts, I think you can deduce that, yes, the wiring is hosed somewhere.

To answer a prior question: Yes, compression is good. Ice cold 140-155psi across the front four (didn't do the rear two as they are a pain in the ass to access)

Ran full new wiring from the coolant sensor to the ECU today, and it did not start within 3 cranks. It's not the coolant sensor, period. Unplugging it makes no difference one way or the other, it will still start with long cranking.

Right after that I checked various plugs in the engine  bay for tightness (again) , basically jiggled them. Engine started immediately after doing that. I had previously checked most of them (and cleaned/tightened the contacts) which had no effect. However, I didn't do the throtttle position sensor. This time I did, among the others. So can't tell at the moment if that has anything to do with the starting, but once I got the truck running my TPS CEL that has been on for years is now off.  So at least something else is apparently fixed lol.

A buddy in West Virginia is going to try to head out here next weekend with an MT2500, so I'll probably order up that plug for it just in case he doesn't have one (he thinks he does, but not positive). So maybe next weekend we'll see.

Leaving a link to my favorite wiring fault finding video
 

buzzboy said:

Leaving a link to my favorite wiring fault finding video
 

https://www.youtube.com/embed/UAlYcl_WFQo?rel=0

 

bookmarked. That's a good straightforward vid for an electrical dummy like me. And there's definitely a shared ground between many of these components, and that seems like a good method prior to testing with a scan tool. Thanks.