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Doug, ASE Certified Technician
Category: Mitsubishi
Satisfied Customers: 8594
Experience:  Mitsubishi employed and Factory trained ASE certified technician
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2001 Mitsubishi Mirage ES: cold start..sedan..smooth..hesitation

Customer Question

Service engine soon light came on after a cold start and has not shut off. (2001 Mitsubishi Mirage ES sedan) I've taken it to Advance to see if it can pull a code with their OBD2 scanner but it couldn't pull a specific code. Engine runs as well as it ever has with a glass-smooth idle and revving without hesitation to redline and hasn't lost any power. It has 155k miles on it and the timing belt was replaced and tune-up done 40k miles ago. Some of my friends have told me to just disconnect my negative terminal, but I'm loath to do that because I'm anal about this car (I've had it since new and it's still on the original clutch, brakes, and suspension) and hate to just ignore a potential problem on a car that's done so well for me. Any ideas? Ran Seafoam in the tank and my mileage has improved from 36mpg highway to 38, but no changes aside from that.
Submitted: 5 years ago.
Category: Mitsubishi
Expert:  Doug replied 5 years ago.

Your best bet here is to take the vehicle elsewhere to get the code read. We do have compatibility issues with many of the generic ("cheap") code readers on this era Mitsubishi, and it is not uncommon for these generic tools to return no fault codes or garbage (P0000 or other non-existent codes).

If you have any other auto parts chains you can try one of them (Autozone, Oreilly, etc), as most chain stores will read your codes for free. There is a good chance that one of the other stores will have more luck if they have a different tool. Otherwise you will need to take it to a shop or someone with a proper scan tool to determine the fault.

All that being said, with you having no drivability side effects, you can rule out your fuel filter most likely, and certainly your plugs and wires or air filter as causes. Not to say that it wouldn't be wise to replace them all if they are due, however I think you should focus on determining the cause of the light first, correcting that, and then performing your regular maintenance.
The other thing to consider is that if you were to just guess at it, some of the trouble codes can take weeks to go out on their own... meaning you could be out a lot of money in parts trying to fix it without ever knowing if you had already corrected the issue or not.

It would be in your best interest to take it to some other stores to try to read the codes, and if that is unsuccessful as well take it to a shop with a proper scan tool or best a dealership with the Mitsubishi scan tool to see what is going on. Even if it costs you $50 to determine what is turning the light on, that will more than be saved over the cost of unnecessary repairs if you just start replacing things and hoping it fixes it. Just remember as long as that light is on, a code is set... just because one universal/generic scan tool can not read it does not mean no one can read it.. these systems are very picky about certain off-brand code readers.
Customer: replied 5 years ago.
Thanks for the fast reply. Short question...could a bad valve cover gasket cause the car to throw a code? I have enough mechanical ability that I can handle anything short of a timing belt replacement myself and I have noticed more of an oil odor since I got the oil changed by Walmart (possibly a mistake).
Expert:  Doug replied 5 years ago.
No problem at all.

In short, there isn't any likely reasons for the valve cover gasket to cause a code to be set. In some special circumstances you can have problems arise when oil leaking makes it's way to electrical connectors, shorting them out. On this particular model, due to the lay out of the engine electronics, this is highly unlikely.

With the 'new' oil smell since the oil change, even though you know you have an existing oil leak at the valve cover, you -really- want to go underneath there and make sure the drain plug and oil filter are both tight. Beyond that, there may have been a small spill when filling it back up, or it could just be that the valve cover leak has gotten bad enough to finally start smelling and it could be just coincidence with the timing.
Before leaving anything to chance however, double check that drain plug and oil filter; it only takes a minute and could save you a bundle if something was left loose.

With regard to taking care of the valve cover gasket, you sound pretty comfortable around the car and this should not be any major issue. There are a few things to be prepared for though.

1) the valve cover gasket can be a royal pain to remove from the cover. With age and heat the rubber turns more to plastic and melts into the cover. A right angle pick and lots of patience will help you out if yours is this way.
2) replace the plug tube seals. When you remove the valve cover each of the spark plug tubes will have a round seal at the top (may stay stuck to the valve cover on removal). These are very important to replace, as they typically fail sooner than the valve cover outer gasket. When they fail they allow oil to go into the spark plug tube eventually, which can cause premature plug wire failure.
3) be prepared for plug wires. If the tube seals were leaking, it is common for the plug wires to either come apart on removal or the rubber can swell up once removed and no longer fit properly in the plug tube opening.

If you have any other questions, just let me know; I would be happy to assist you further.
Customer: replied 5 years ago.
Finally got a chance to take it to O'Reilly's and use a scan tool that actually worked. Pulled a P1400 code (manifold differential pressure sensor circuit malfunction). Is this something I should get replaced ASAP, or can I wait a while and not have it hurt my engine?
Expert:  Doug replied 5 years ago.

For optimum fuel mileage and minimal side effects, it would be wise to take care of this as soon as you can. I would not want to put this off long term. Additionally, I would strongly recommend getting this part from a dealership even though it will be a bit more money. Engine control electronics on these cars are notorious for not getting along well with aftermarket components. It isn't a 100% failure rate on these or anything, just on occasion I have seen these come in to my dealership with brand new MDP's from Autozone, etc, and the shop that installed them insisting there is another problem when a genuine part is all that was needed.

If you do not do it right away, the main area of concern is that your fuel control is going to be skewed some. The MDP is used to help judge the volume of air in the intake and the computer uses this in part to determine the amount of fuel to use. An incorrect reading can result in over or under fueling, which can cause drivability problems in certain circumstances. Long term damage is not likely except in the case of severe over fueling, which can cause puddling in your catalytic converter, causing premature failure.
BotXXXXX XXXXXne though, if the vehicle is driving normally, I wouldn't be too concerned; just get it fixed as soon as you are comfortably able. If the drivability conditions change, move the repair up sooner.

In most cases this will be nothing more than the sensor itself; I would go around the engine compartment carefully looking for any broken/damaged vacuum hoses first though. If you have abnormal intake vacuum it can cause this code to come on erroneously (not real common, but worth mentioning).

If you have any questions just let me know.
Doug and 3 other Mitsubishi Specialists are ready to help you
Customer: replied 5 years ago.
Probably just going to go ahead and replace vacuum hoses while I'm at it along with the sensor. One final question...where exactly is the sensor? Last question, I swear. :D
Expert:  Doug replied 5 years ago.
It's no problem at all :)

The MDP sensor is located directly on top of the intake manifold, right near the throttle body.

It is pictured here: