1990 Mighty Max 2.4L- covert from fuel injection to weber carburetor??? Possible???

[dbunch]

I bought the truck for my son as his first truck...thinking we could get it going as a father-son project. It's been a nightmare with this fuel injection. The truck originally would start- run rough and begin to smoke. The guy I bought it from and I both thought it was probably the valve seals. The compression is great at about 158-162 across all 4 cylinders. My son and I pulled the head and rebuilt it anyway, cleaned the injectors, replaced the timing belts (taking care to use the notch and not the head-block seal as the reference for timing the camshaft), and water pump. Compression still about the same. Replaced the O2 sensor. Completed a full tune up. Set the timing by grounding the wire by the firewall. It doesn't smoke much now, but still runs very rough as before most of the time, however a few times, it has started and run very well. Eventually, it returns to poor idle and power.

After examining the freshly fouled plugs, I am fairly confident that the truck was just running rich rather than burning oil (rebuilt the head unnecessarily) and something was wrong with the fuel injection.

So, we the checked the ecu which was a rebuilt unit and replaced one capacitor- didn't help, then I bought a new rebuilt ecu from rock auto- no change. Bought a new distributor without improvement because I thought the crankshaft sensor might be bad-nope. Replaced the fuel pump and filter- nothing (checked the fuel pressure and it's in spec).

Finally, I unplugged the MAF sensor and the truck runs much better but still not smooth and has low power. So, I bought a reman MAF sensor- nope... runs way worse with it.

I'm guessing there must be a ground issue somewhere or I've got a bad injector (but they all tested great). I've wasted a can of starter fluid looking for a vacuum leak. I checked the basic trouble codes with a needle multimeter- says it fine with everything plugged in.

I'm at my wit's end.

Here's my question:
From what I've read, a 2.0l manifold should bolt up to this 2.4L. If I get one of those and throw a weber on it, will it work? Will the existing computer advance the spark, etc.? I am really sick of this fuel injection system.

Any help/wisdom would be great appreciated.

Thanks

[Spectre]

I've read all the above... converting to carburetor isn't really a solution and will make your truck worse.

I noticed that unless you failed to mention it, you didn't check the single most common cause of rich running, the coolant temperature sensor. Not the one for the dash gauge, but the one for the EFI system - this is where Bosch-type EFI systems get their baseline fueling from. If the computer sees that the sensor is reading as if it were cold outside, it will enrich the mixture accordingly. This is given priority or more importance than the intake air temperature sensing in the MAF. Check that, check the connections between it and the computer and you will probably find your problem.

[xboxrox]

dbunch, Spectre has good advice ~ I have the G54B 2.6L engine with the stock Mikuni carb 'n all the associated vacuum lines ECU sensors, etc... Things that helped the most 1) new cylinder head 2) Mikuni DIY rebuild 3) new O2 sensor 4) removing plugged catalytic converter; not to mention new fuel pump, new coolant sensors both gauge & engine coolant temp for ECU... etc, etc, etc...

However, this dang truck still has an intermittent varying type engine miss

Some days it runs fine..!

There is one other sensor not tested or replaced yet ~ it has two or more vacuum lines & I suspect it senses coolant temp..?

All I can say is don't give up ~ found this yesterday, reading it might help you sleep
https://www.cars-directory.net/probl...icle-1689.html

My truck tends to run rich based on tailpipe blackness & every spark plug change had black fouled plugs ~ sometimes the truck gets 20+ MPG but last gas up it only got 13 MPG

If the Japanese can make it then we can fix it

[Spectre]

Unfortunately, I don't have enough experience with the carbed G54 to offer you any diagnostic help. My experience with the 4G64s comes mostly from the DSM world (Mitsu Eclipse/Plymouth Laser/Eagle Talon/etc.) but it's basically the same thing for the purpose of the topic of this thread. That said, that was the era of computer controlled carburetors and they did have coolant temperature sensors. Earlier Japanese trucks with carbs had mechanical temperature compensation based on thermally operated vacuum switching valves; it seems like yours might have something like that.

dbunch, a couple more points: A failing, failed, "sticking" or "drifting" coolant temperature sensor or cylinder head temperature sensor basically never sets a code on an EFI system because almost no EFI system has a means of crosschecking it. xboxrox also has a good point about the catalytic converter possibly being plugged - something else that won't set a code in this era of truck. Check that with an infrared laser thermometer.

[xboxrox]

rich running can also be caused by vacuum leaks ~ the computer will add extra fuel to compensate for the extra air caused from the vacuum leak sucking in extra air...

[Spectre]

rich running can also be caused by vacuum leaks ~ the computer will add extra fuel to compensate for the extra air caused from the vacuum leak sucking in extra air...

He did say he'd been checking for vacuum leaks with starting fluid, so I discounted that. Still, it's never a bad idea to *carefully* replace vacuum hoses when they're this old. Be extra careful on plastic or other non-metallic vacuum hose barbs as they tend to get brittle and break easily.

[SubGothius]

rich running can also be caused by vacuum leaks ~ the computer will add extra fuel to compensate for the extra air caused from the vacuum leak sucking in extra air...

It's the other way around; a vacuum leak introduces "false air" (more air than the ECU is aware of) to the intake charge, so it injects less fuel than necessary for the actual air present, causing a lean misfire that would show up as high HC with low CO in exhaust gas analysis, whereas a rich condition would show high HC with high CO -- i.e., the exhaust would be "rich" but only because of unburned fuel due to the misfire, rather than too much fuel quenching efficient combustion.

BTW, IIRC '90 had a one-year-only FI system that was replaced for '91-on, which complicates troubleshooting and parts availability.

[geezer101]

Swap the distributor and manifold from a 2.0. The distributor is analog and it'll solve your fixed timing issues with a carb. Use a 38DGES as it will keep up with the 2.4. The 32/36 DGEV will struggle with air flow when you push it hard. The distributor gears all interchange so there will be no need to modify anything.

[dbunch]

Thanks for all the advice everyone!!! I've ordered a new coolant temperature sensor. If that $10 part fixes everything, I'll let everyone know before I jump off a cliff! JK.

If it doesn't work, Weber here I come.

One question, would deleting the catalytic converter do anything to the fuel injection system? Does it need the back pressure or something?

[Spectre]

One question, would deleting the catalytic converter do anything to the fuel injection system? Does it need the back pressure or something?

Not in this era of truck. It does not need back pressure (which is actually a myth anyway) and there's no sensors post-cat unlike later OBD2 trucks. There *may* be a small pre-cat just after the exhaust manifold, then the single O2 sensor after it. Either way, nothing after the O2 sensor will make a difference in the truck's running mixture.

I presume that you are *only* removing the cat for testing purposes as removing it permanently is a Federal felony carrying pretty hefty penalties... right? Just FYI, a straight pipe has no practical power advantage over a modern universal aftermarket free flow cat - the power difference is 1hp or less.

Be advised that if simply swapping the sensor doesn't instantly fix it, you shouldn't just rip out the EFI (technically swapping an EFI vehicle that was not offered with a carb in that year out to a carb is also a Federal crime) but break out the multimeter. Check the terminals for corrosion/contamination and treat it accordingly. Get the test values for the coolant temperature sensor (usually tested through measuring voltage or resistance) and check that it's showing the same at the ECU connector pins that are supposed to be connected to it - you want to see what the computer is 'seeing'. Finally, inspect the wiring harness for damage, cuts or shorts. While Mitsubishis often have electrical problems in my experience, the core EFI isn't a major problem center. The fuel injectors themselves can be, but the core EFI electricals have at least average reliability. It shouldn't be too hard to troubleshoot, just a bit time consuming.

[geezer101]

...I wonder if the cat thieves are getting charged with a federal offense

[Spectre]

...I wonder if the cat thieves are getting charged with a federal offense

Some are, most aren't. Reasons for not charging them are... political. And I don't want to get into that in a forum like this one.

[xboxrox]

Hello dbunch 'n fellow wrenchers ~ your son is learning & helping on his truck too, right..?

dbunch you don't need the truck for vital transportation; it's a project right..?

So besides instant gratification, must the truck meet a deadline to be running..?

My truck is my only vehicle but I live in a tourist town where walking or biking works ~ however, I do need it for doctor appoinments..!

I been down the same road, the same trouble with my truck as yours (maybe worse) ever since we bought it... I FEEL YOUR FRUSTRATION.

I noticed that every sensor we replaced was toast & looked like garbage corroded burned up etc.

One sensor I have yet to remove & inspect is sensing coolant (I think) and has 2 or 3 vacuuming hoses attached & this sensor must also draw in air because it has a small air filter under a black plastic round cap.

Our truck runs but I worry about it's dependability the 13 MPG crap and weird little odd not so little sometimes MISS.

I wish you & your son the best AND hope the project stays FUN

Aloha ~ George

P.S. Be very careful what you buy online & open the packages as soon as they arrive to inspect THAT YOU GOT WHAT YOU ORDERED

[geezer101]

The biggest issue you will be dealing with is the year of your truck. It runs a different ECU and engine peripherals than other year models (including later ones...) The ECU and sensors from other year trucks aren't compatible.