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, and how under-appreciated/ underrated these vehicles are, but how able they are for expedition 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!

 

Hey Ashton_59, welcome to the forum. Congrats on your new ride. There have been many articles and posts here on the forum about butterfly flaps on the intakes. It would definitely be worth taking off the manifold to examine it. One thing that concerns me - did the mechanic who replaced your spark plugs mention anything about the butterfly valve? The reason I'm asking this is that on these vehicles you can't get to the spark plugs without taking the intake manifold out and therefore seeing the condition of the butterfly thing. I do hope your mechanic replaced the spark plugs and not just told you he did that.

Now, as far as addressing the P0300 code. The "standard" method is to replace spark plugs, wires and coils. This is NOT my favorite way to do it, because it is often costly and unnecessary. Iridium spark plugs need to be replaced only after 50-60k miles. Wires and coils should be replaced only if they are not passing the spec measurements. One of the issues that occurs with the wires, is the end that sits over the plug may get saturated in oil collected in the spark plug well (leaks past old hardened spark plug oil seal). If that happens, the end of the wire swells and can "push" itself out of the spark plug hole and off the plug. I had that happen to me once. Again, since your mechanic claims he replaced plugs, he should have seen if any of the wires are bad and are in need of replacement.
Coils do not need to replaced unless the resistance on primary or secondary circuits is outside specifications range.

Since you are still having misfire code, pull the Freeze Frame data from it. It will tell you if the misfire is due to lack of fuel. That is a possibility.

One other thing that is overlooked but may cause misfire is leaky valve seals. Surprised? You should be... Do you burn a little bit of oil? With the 2003 year model, you probably have original factory valve seals on there. The oil is leaking along the valve stems and getting into the combustion chambers. That messes up the air fuel ratio. If the valve seals are leaky enough, they may let enough oil in that it may create rich enough mixture that it can't ignite. The fact that misfire happens more when cold, is a good indicator that this might be at least one of your causes.

I had been battling a random misfire on my 2001 Montero for several years trying to chase down if it is a coil, wire, etc. Everything would check out, but misfire persisted. I knew i needed valve seals, so one of the nice spring days, i did that job. Guess what... misfire went away. I had no misfire for over 7 month.

 

Thanks for the warm greeting! The previous owner had oil leakage from the valve seals, so he had them replaced, which he had receipts for that. And I will for sure need to pull the Freeze Frame data, thanks for the idea, I spaced that one. I was maybe thinking a fuel problem, the previous owner had the injectors cleaned and maf sensor. But I was thinking maybe something with the pump or fuel filter. I tried testing the fuel pressure through the pressure release valve, but could not find it anywhere. My next step is to remove the manifold and change the plug wires, remove the butterfly valves, and just for safe measures check the spark plugs, even though the mechanic put new ones in. As you said, if the mechanic pulled the manifold off, if the butterfly valves were bad he should be able to notice that, that's beyond me. I have a hard time trusting mechanics, so I end up doing my own wrenching. And congrats on clearing your P0300 code Such a pesky code! As for the wires, I only have previous receipts for the plugs, the guy who paid for it to be taken back to the mechanic might have
told them to hold off on the wires due to not wanting to spend that extra $$$.

 

how long did it take you to do the valve seal job? How many miles are on your montero? And do you still have your butterfly valves in it?

 

It took about half of a nice sunny day (i had to work outside). I did a lot of prep work and had experience with doing timing and manifold removal/installation on this engine. My main goal was to not hurry and do it right from the first try. My Monty right now has 247k miles on it. I did the valve job at 242k (way overdue - factory valve seals at that point). I have 2001 Montero (not Sport). It does not have butterfly valve, so I never had a misfortune having to deal with those. I think (please correct me if I'm wrong) after 2003 year they might have added them to the full size Montys.

Here is the link to the thread where several very knowledgeable people and I had a discussion about valve job:
https://mitsubishiforum.com/forum/mi...acement-52566/

If your P0300 code pops up again, do pull the FF data. That will help to figure out if your issue is fuel starvation or some other cause.
As far as fuel pressure testing, unfortunately Montero does not have fuel pressure testing port. You have to install an inline adapter to do the testing.