3.0 with rough idle

[anters]

I've got a 92 3.0 4x4 w/ 55k miles with a rough idle. Sometime it feels like a constant mild stumble and other times it's more of an intermittent miss, and it feels stronger when the revs are dropping after blipping the throttle.

Tach is rock solid at 700rpm, ignition and cam timing are confirmed correct. Ignition timing does bounce around a bit due to the stumble/miss.

For maintenance I've done plugs/wires, cap/rotor, coil, and 02 sensor, fuel filter, run a bottle of Techron fuel system cleaner and seafoam separately. As well as timing belt, all new fluids and MFI relay.

I've sprayed brake clean on every vac hose and fitting with no results, and pressing the brake pedal has no effect on idle, so I'm pretty sure it's not a vac leak. No egr from factory, but PCV, etc have been removed.

No check engine light, diagnostic port shows Normal. I do have the ABS light on, but the diagnostic port doesn't give me any reading on those pins.

Things I haven't done: pulled the injectors, tested the IAC valve (truck runs at 1100 rpm with it disconnected), pulled the ECM (which was replaced by the PO), or cleaned the air intake sensor, I've read mixed reviews about using MAF cleaner on them.

Any ideas?

[geezer101]

Have you removed the IAC and given it a clean out? It will cause idle issues if it's carboned/gummed up. It's possible that removing the PCV and letting it vent is the culprit - how have you deleted it?

[Chargerx3]

Which IAC do you have? Is it the tan and aluminum one? If so those will go bad and then kill your ECU. This is well documented on the 3000GT forums as they use the same IAC. Swap it out immediately for this black plastic one. Just make sure your part numbers match.

https://m.media-amazon.com/images/I/...AC_SL1002_.jpg

Otherwise i would lean towards the following:

-MPI relay - I've had 1 go bad before. Super easy to swap out.
-Distributor (whole unit), My fathers went bad, twice on his 3.0 4x4
-Injectors - remove and flow test or replace.
-ECU - common for the capacitors to leak fluid out and then arc across the board, killing circuits. Ive experienced it once.

[anters]

Geezer: I have not cleaned the IAC, I think it may be part of the problem. The PCV is gone and vents to atmosphere. Both valve cover ports vent to atmosphere.

Charger: Yes tan and aluminum IAC, I've read about them frying ECUs. I have a new black one sitting in the garage, I've been hesitant to pull the throttle body to install it.

MPI was replaced due to a no start issue.

Is there a source for distributors?

I've again been hesitant to pull the intake plenum to check the injectors. Is there a source for injectors?

ECU was replaced by PO, but it is on my list of things to check.....

Thanks for the help, I'll replace the IAC and see what happens

[geezer101]

I'd at least try cleaning the IAC out with carb/throttle body cleaner and give the air bypass gallery a blast too (brake cleaner works too and apparently is more effective for breaking up gunk) Injectors are a little trickier to clean out - maybe try a fuel additive first and then opt for removing them and using another method like an ultrasonic bath or spraying throttle cleaner through them and pulsing them with a 9v battery. Unless you have a particularly noisy injector or it's showing a fault I wouldn't mess with replacing them.

[geezer101]

*I've just discovered injector pulse generators really cheap online. Only problem would be waiting on one to be delivered if you're in a hurry. There are also crude injector cleaner kits that simply connect to a spray can cleaner and hook the injector to the car battery - they look like they have the potential to spectacularly erupt into flames but they also might keep the injector open long enough to blow any varnish or particles out of them.

[Chargerx3]

Geezer: I have not cleaned the IAC, I think it may be part of the problem. The PCV is gone and vents to atmosphere. Both valve cover ports vent to atmosphere.

Charger: Yes tan and aluminum IAC, I've read about them frying ECUs. I have a new black one sitting in the garage, I've been hesitant to pull the throttle body to install it.

MPI was replaced due to a no start issue.

Is there a source for distributors?

I've again been hesitant to pull the intake plenum to check the injectors. Is there a source for injectors?

ECU was replaced by PO, but it is on my list of things to check.....

Thanks for the help, I'll replace the IAC and see what happens

Chuck the tan IAC. Even if you clean it, it will randomly fail and then destroy your ECU, which you cannot buy anymore. The new black one is easy to install. 2 Philips screws and no need to remove the throttle body.

Auto parts stores might have a generic one. I had one originally in one of my v6 trucks and it caused some intermittent issues. OEM is by far more preferred, but you cannot purchase OEM new anymore.

Check the injectors last. Might be worth pulling your ecu (bit of a pain) that is located on the passenger upper footwell along the exterior. Check those capacitors to make sure damage isnt already done.

[anters]

I swapped in the new IAC to no effect, not sure why Chilton calls for removing the throttle body. This WVE by NTK (NGK) unit has the Mitsubishi logo on it, which is nice. The only difference is the new unit does not have the self check buzz when the key is turned to accessory. But at least it shouldn't grenade the ECU.

Inside the throttle body had a good layer of carbon, I shot some throttle body cleaner through it and the top vac port on the intake. There might have been a very slight improvement to idle.

The only thing that sticks out to me is the distinct miss/stumble when the RPMs drop back to 700 after blipping the throttle. It feels like a vac leak, but I checked every hose and fitting again with no luck. The miss doesn't increase with RPMs (so not a spark issue, right??), but it does get a bit less intense.

I still need to check the ECU.

[camoit]

Check the EGR

[anters]

Check the EGR

No EGR from factory. I thought it had been deleted by a PO, but turns out the early 3.0’s were not equipped with EGR’s and instead run higher engine temps to burn up more stuff in the exhaust. At least that’s how a random forum post explained it.

Still trying to figure this out, I checked cam timing again just to completely rule out it’s not off by a tooth, but it’s dead nuts on. I’m glad I did though, when I pulled the radiator hoses they were coated in a white chalky film. Coolant is bright green, but every surface is coated. My first guess would be head gasket but that seems unlikely, and it doesn’t look like oil milkshake.

[Chargerx3]

My 89 Montero and 90 Max v6's both have EGR's. Both are first years of production for both of those engines in those platforms. Its dependent on weather or not they are California Spec or Federal Spec.

[anters]

Finally pulled the ECU, tagged as a rebuilt unit, but can't make out the company name. The 2 capacitors were clearly replaced. And beyond a fair bit of rust and oxidation on the case, the board appears to be fine.

Does anyone know of shops in Western Washington that work on these trucks? I've called a couple specialty Japanese shops, a Mitsubishi dealership, a JDM importer, and a JDM mechanic with no one wanting to touch it.

[geezer101]

Look right smack in the middle of the board and you'll see 2 balled soldered joins. Money is on those bad joins being the culprit. That capacitor replacement is horrid too - the solder protruding under it is one ugly patch job. Break out a soldering iron and see if you can get the solder to flow onto the board. Any dry or incomplete joins on the board should get a once over with a soldering iron.

[anters]

Yea I noticed solder on the newer capacitors was pretty poor. I might just replace them with new, hopefully the pads are not damaged. Fingers crossed this is the issue.

[anters]

Here's close ups of the carnage:

[geezer101]

And that was a 'repair' by a 'shop' The traces on the board look like they may have gotten roasted during the botched fix. It might not hurt to get new capacitors and DIY it. Make sure you get appropriate ones - look online or...somewhere on here for advice on which type to get. Some capacitors are known to generate unwanted background noise that affects the ECU adversely. Chargerx3 call out on the ECU was on the money, for the wrong reason. Not often we see sabotage on that level!

[Chargerx3]

Yea, cant say im surprised its a reman and that it might not be in the best of shape. Sadly they don't get to choose what condition they receive it in. Take a look at the large black and silver Diode. See all that corrosion on the side closest to the capacitor? Looks like at least one already blew and leaked down your board. I cant see the board well, but my next steps would be to hold the ecu up in the air (with the wiring harness prongs pointed down) and follow the path of gravity on your board. See if any other oxidation or corrosion exists. All of your electronical traces should be light colored. Anything that is black, or gray in color is oxidation and bad news.

Short example: The 3000GT has a similar board. The small "daughter board" you have right next to your capacitors is not well protected. That board controlled my injectors. Turns out the vapor from the caps got underneath the board and right where the races overlap each other I found some light gray spots. That is what killed my ECU. Inspect yours very closely as well.

Adding head again to your caps if their traces are compromised might net worse results. Especially if you do not already have all the proper gear and expertise. Fun tip. If i recall correctly that board has 5-7 layers. Many of which share the same hole for connections. Top and bottom might look ok, but it can be worse within.

[geezer101]

The more you look at the board the worse it gets. There are a bunch of 'dry' joins on it. Top image on the close up photo you can see 2 joins in roughly a straight line below that moon made of silver holding a capacitor on. That join closest to the west of them is suspect too. There didn't seem to be a great deal of care taken when they were being assembled but the choice of capacitors is the one issue that has come back to haunt owners...

[Chargerx3]

The more I look at this the less I like it. Im certain your larger capacitor leaked down the board and did some damage. With how much solder they used on the top and bottom it must have oxidized, or deteriorated the copper traces that it attaches to.

Also, what is that large solder glob just between the large capacitor and the large black/silver diode on the top of the board? Did they attempt to bridge where a surface resistor goes? That is highly suspect.

[geezer101]

The corrosion on that diode is something else too. I never really noticed that either...

[anters]

I ended up taking the board to an electronics repair guy I've used before, he thinks he can fix it.

I also order a Masterpro branded reman unit from O'riellys. Hopefully one of them works

[Chargerx3]

Wish you luck. Hopefully its repairable.

[anters]

Well, it's not the ECU. Both the re-repaired and new reman units had no effect. No check engine light and "Normal" signal from the diagnostic port still.

I played around with different throttle inputs, at idle RPMs fluctuate by +/-10. If the throttle is blipped drops past 700 on the way down to about 600 (with a noticeable miss/stumble) then recovers to 700. But the big one was holding at say 1000, the RPMs oscillate by 100 or so up and down rhythmically.

[Chargerx3]

Well that is some good news. Wonder if its a vacuum leak.

[anters]

That's what I keep thinking too, but I've probably sprayed a whole can of carb cleaner around the engine without any results.

You'd think a bad or dirty sensor would throw a code. Other option is fuel pressure regulator has a bad diaphragm? Clogged throttle body port?