Hello, I'm new to this forum but have been lurking for some time now. I am hoping to have input shined on my problems. I recently purchased a Mitsubishi Montero gen 3, with the 3.8 SOHC engine. I have read on these, on how good these vehicles are for an expedition platform and commuting abilities. I decided to pull the trigger and get one. The Montero I found was in good condition, cosmetically, and mechanically... Or I thought. The previous owner had stated how it misfired "sometimes when starting the vehicle in cold weather". And showed me that he "fixed" it with taking it in and getting coils and injectors checked, both good. Tested for vacuum leaks - passed, cleaned throttle body, cleaned mass airflow sensor, and fuel injector cleaning. He provided me with the shop receipt, so I bit on the proof. As well as a good test drive, good shifting, and no misfires. The next day after driving it from school, a code pops up, so I took it to get scanned, and it shows a dreaded P0300 code (random assortment of misfires). So I messaged the guy, and to my surprise, he felt horrible. He actually paid for it to be taken to a shop, and the shop thought it was a bad intake manifold gasket due to their being a crummy silicone gasket. After replacing the gasket, and putting new iridium spark plugs in, it was still misfiring! The guy spent $450 to attempt to fix the problem, and it was not fixed. So I get the Montero back with a note from the mechanic saying they were thinking the coils where bad, so I checked them and to no avail that was not the problem. I have no clue what to check next, maybe the spark plug wires??? It does sound like it may be skipping a spark on cylinder 5, or is it the butterfly valve problem? I can't pinpoint it... it does somewhat sound like something is raddling in the engine near where the manifold is, which makes me scared to even start the thing. So as of now, my goal is to remove the manifold and reach the intake plenum, thus removing the butterfly valves and bolts as a whole. While I have the manifold out of the way I'm just gonna go ahead and replace the spark plug wires. Sorry for the whole run down, I would appreciate if anybody knows of anything else that could make these vehicles, in particular, have this code pop up. Can a corrupt butterfly valve cause this code and slight misfiring? The vehicle is not running super rough, more of a slight unrecognizable rattling/chugging sound. The montero does have good compression. Any help or input would be awesome, thank you for your time!
2003 Mitsubishi Montero XLS P0300 code
- Thread starter Ashton_59
- Start date
Michael Brown
You followed me, so now we're both lost
You can also check fuel pressure/injectors and do a compression test. Maybe a dirty MAF or throttle body, but the random misfire without another cylinder means it is in the overall system and not just one bad part. Also keep checking for vacuum leaks.
The rattling noise can be a loose heat shield on the exhaust or something else simple. If you clear the code, how long does it take to return?
The rattling noise can be a loose heat shield on the exhaust or something else simple. If you clear the code, how long does it take to return?
nwoods
Expedition Leader
The little screws inside the intake at the butterfly valve are known to fall out, get sucked into the engine cylinders and potentially do Bad Things. Do a search here in the Montero Channel on this, it has been discussed within the past year or so.
It’s a bit odd to get the generic 0300 code, rather than a specific cylinder. Another common issue with these engines is that the seals the valves set up against when closed go bad. This causes low compression and leakage and thus, misfires. A compression test is both easy and a PIA to do because you have to remove (or at least unbolt and move out of the way) the intake manifold to access the spark plugs to do the compression test.
Also, just to rule it out, do you see any bubbles or exhaust gases looking down into the radiator? If so, that could be a head gasket. Not unheard of, but rare compared to bad valve seals.
It’s a bit odd to get the generic 0300 code, rather than a specific cylinder. Another common issue with these engines is that the seals the valves set up against when closed go bad. This causes low compression and leakage and thus, misfires. A compression test is both easy and a PIA to do because you have to remove (or at least unbolt and move out of the way) the intake manifold to access the spark plugs to do the compression test.
Also, just to rule it out, do you see any bubbles or exhaust gases looking down into the radiator? If so, that could be a head gasket. Not unheard of, but rare compared to bad valve seals.
Thanks for the reply! The Mechanic the Montero got taken to did a compression test, and it came back good. The previous owner got both the fuel injectors and MAF cleaned, less than 15,000 miles ago. As for the fuel pressure, I have no clue where to find the pressure release valve. I looked all over the engine bay for it and could not find it! The code usually returns withing about 35 miles. As of now, my next step is to remove the manifold, remove the butterfly valves, and change the plug wires. The wires look newer, but I have no clue when they were changed last. It does kind of sound like cylinder 5 is getting a bad spark but I'm not sure.
This code does suck! I'm gonna have to look a little more into the valves, because the previous owner did get them replaced, and had a receipt from about 15,000 miles ago. The mechanic did a compression and it came back good as well. When I get home today I will need to check out the radiator. I did do it when I bought the Montero and did not smell any strong fumes or anything out of the ordinary. But I will check it out once more. Thanks for the helpful tips!
tj90
Member
I had rough idle, low idle and stalling issues that resolved with 50 dollar ebay MAF. It got worst after cleaning MAF - probably damaged it mot using the correct cleaner. I was shocked how much it helped.
The other thing that I was thinking it could have been was blocked cats. My buddy at work cleaned up poor highway runs with new cats - not a cheap endeavour on a Monty.
Sorry for the anecdotal info but something to think about.
Tj
Sent from my SM-G960U using Tapatalk
The other thing that I was thinking it could have been was blocked cats. My buddy at work cleaned up poor highway runs with new cats - not a cheap endeavour on a Monty.
Sorry for the anecdotal info but something to think about.
Tj
Sent from my SM-G960U using Tapatalk
Michael Brown
You followed me, so now we're both lost
I would not assume that the MAF cleaning was done correctly. If the wrong cleaning chemical is used, it can damage the sensor. There are more details in the FSM, but I would start your investigation assuming that nothing else has been done, or that it may have been done incorrectly. Only eliminate it when you have personal information that it is not the source.
Make sure you have a FSM copy from my link. It will help with troubleshooting.
Make sure you have a FSM copy from my link. It will help with troubleshooting.
SONICMASD
Adventurer
1. The MAF cleaning was likely done incorrectly since Monteros have a different kind of MAF than the vast majority of vehicles and if you use store bought “MAF cleaner” you’re running a risk. The MAFs on these rigs dont really ever need to be cleaned given the way they operate (karman vortex) but ive heard “cnc lectra motive” is safe in them. Getting a replaement maf in a junkyard ir ebay could be a cheap (<$50) gamble.
2. When the previous owner replaced the butterfly valves and you have a receipt for it, what exactly did he replace? The only real part you can buy to fix that issue is a whole new surge tank from Mitsu for around $700 and if he did that, that is likely not the issue as they are good for at least 100k.
3. More likely source of vac leaks would be the vac lines all around the intake plenum or the exhaust manifolds. A $60 amazon smoke machine is the easiest way to test both the entire intake and exhaust systems for air leaks.
4. I am not 100% sure but i dont think a loose or even completely detached butterfly valve could cause a code (until it makes it down into the piston of course). You can run the engine with no butterflies at all in there and no codes are triggered (as i and several others do). And if a butterfly was totally off the rod and clanking around the intake you’d hear it and know it. I think the problem being butterfly related is slim, same with plug wires if they look newish.
5. Fuel pump - how many total miles on your rig? The fact that the previous owner did a fuel injector cleaning makes me guess it has high miles and he was noticing a fuel pressure issue. I only know of one other montero that has had a proper fuel injector cleaning but know lots of guys who have had to replace fuel pumps. If you’re around 200k, this is more likely. Theres a youtube vid on how to do the fuel pump.
6. Check to make sure your air filter box and hose to the throttle body are all good. The tabs on the box lid break and then there isnt a good seal there. And make sure the throttle body is clean to rule out that it is sticking.
I hope that helps.
2. When the previous owner replaced the butterfly valves and you have a receipt for it, what exactly did he replace? The only real part you can buy to fix that issue is a whole new surge tank from Mitsu for around $700 and if he did that, that is likely not the issue as they are good for at least 100k.
3. More likely source of vac leaks would be the vac lines all around the intake plenum or the exhaust manifolds. A $60 amazon smoke machine is the easiest way to test both the entire intake and exhaust systems for air leaks.
4. I am not 100% sure but i dont think a loose or even completely detached butterfly valve could cause a code (until it makes it down into the piston of course). You can run the engine with no butterflies at all in there and no codes are triggered (as i and several others do). And if a butterfly was totally off the rod and clanking around the intake you’d hear it and know it. I think the problem being butterfly related is slim, same with plug wires if they look newish.
5. Fuel pump - how many total miles on your rig? The fact that the previous owner did a fuel injector cleaning makes me guess it has high miles and he was noticing a fuel pressure issue. I only know of one other montero that has had a proper fuel injector cleaning but know lots of guys who have had to replace fuel pumps. If you’re around 200k, this is more likely. Theres a youtube vid on how to do the fuel pump.
6. Check to make sure your air filter box and hose to the throttle body are all good. The tabs on the box lid break and then there isnt a good seal there. And make sure the throttle body is clean to rule out that it is sticking.
I hope that helps.
Thank you for the amazing advice and help! It's awesome how helpful these forums are. I had the Montero idling and I disconnected the MAF sensor to see if it changed idle. It did change idle, more like a deeper lower idle, so I know the rpm's dropped from that. But I am curious if the MAF could be fine during idling, but become corrupt under load? Also, I'm not sure if it is the butterfly valves, but I do hear some distinct rattling from the manifold area. I've already made plans to remove those this weekend, due to the anxiety I get for the possibility of those falling into the pistons. And the Montero has 172k miles. I do not really hear any misfire and or idle issues, only when the engine is under load. Or after I've been driving for some time, I can somewhat hear it at lower speeds. My main concern is that I don't really want to take it to a shop, I already have seen what happened with that, the previous owner had receipts and it seems like the shop would fix some random ******* they they would assume was the problem and charge a fortune for it.