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Doug, ASE Certified Technician
Category: Mitsubishi
Satisfied Customers: 8579
Experience:  Mitsubishi employed and Factory trained ASE certified technician
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Mitsubishi Diamonte: My 1995 Mitsubishi Diamonte has a miss

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My 1995 Mitsubishi Diamonte has a miss while running.Seems worse in hot weather.No idiot light has ever come on.Have changed all filters and had several mechanics look at it but they can't find anything-they say that until a light comes on on the dash,they don't know what it is.Car runs great otherwise.

Is this miss constantly occurring?

If it is, can you isolate the cylinder that is missing by unplugging one plug wire or injector at a time to see if any one cylinder does not affect the miss (I know the back injectors are not easy to do this)?
Customer: replied 3 years ago.

I would have to take it to someone.Could you tell me all the possible possibilities and I will print this out and take it to a mechanic. Thanks

I understand.

To have a consistent miss is going to be a pretty wide range of problems however they are all relatively easy to check out (compared to some other problems anyway).

To have a rough operation or miss in the running you are going to be looking primarily at an ignition related issue, followed by compression based issue, and lastly fuel based, by order of likelihood.

The first thing you are going to want to do is see if you can isolate the problem to an individual cylinder. This is called a cylinder balance test. The theory here is that if you have a single cylinder (or two etc) that are consistently missing and the others are not, when you cut power to the "Bad" cylinder, the engine will not really change condition any, whereas if you cut power to a "Good" cylinder it will cause the engine to stumble since it is losing power when the good cylinder is turned off.
On newer cars this can be done by scan tool, however on a 95 this is a hands-on vehicle, as you've experienced already... these cars are what separate the veterans from the new kids, as less experienced techs have a hard time diagnosing without a computer that tells you everything for you... these older cars you have to get your hands dirty on them.

The easiest way to do this balance test is to unplug the injector for each cylinder one at a time. If the engine does not change on any one cylinder being unplugged at the injector, you know that cylinder is not firing properly. The rear injectors are very hard to get to due to the intake manifold, and it is easier to unplug the spark plug wires from the distributor instead for them... the same effect is had.

If you isolate it to an individual cylinder, you will then want to inspect the spark plug and especially the wire to that cylinder. Note that generic spark plugs do not work well on these cars.... No Bosch, No Autolite, No Champion.... NGK or Denso only, and only original equipment replacement (no gimmick plugs like four electrode, glow in the dark, whatever they are pushing). The plug wires are OK to use any decent quality wires that are "fitted" (correct length for the specific vehicle). Plug wires are extremely difficult to verify condition accurately, especially when partially concealed. If it is unknown how old they are, it is best to replace them. Note that Mitsubishi plug wires have manufacturing date printed on them which can give you an idea of the age.

If nothing is found with the ignition system, then looking into the compression would be next.
This also happens to be what you would do if you did not have a specific cylinder issue. A compression test would be in order to verify the internal health of the engine. This can tell you two things.... first if there is any internal issue (leaking piston rings, leaking valves not seating properly), and second if the timing belt jumped a tooth which would cause the same miss but would be identified by low compression values. You should see about 200 psi per cylinder ideally, 185 if you have the rare DOHC model... if you see any major differences between cylinders for example 180 180 190 120 180 185 or something like that, you found your miss... and unfortunately it is time to go internal for repair. If you see all of them low (140 140 150 140 130 155 etc), then we would first suspect a slipped timing belt, followed by a severely worn out engine... the former corrected with a new timing belt set properly, the latter with internal repairs.

If the compression is good (say 170s or higher and all relatively even), then we would want to check out your fuel supply.... checking operating pressure (with the engine running, not just pump pressure) to see if there is a pump or regulator issue. A fuel sample would be in order to make sure you don't just have a bunch of muck in there. And finally a spray pattern check by lifting the injector rail off the engine and cranking the engine to make sure the injectors are all working properly and spraying nicely (no kitchen faucet looking sprays, just fine atomizing).

It will be very rare to see anything outside that range.... while it is possible to have wiring damage (rodents, accident damage etc) cause some odd behavior, it isn't the norm. In typical situations you will always be able to narrow the issue down to one of those three areas if you have no malfunction lights coming on, as those areas are rare to set warning lights (on a 95 model that is.... 98 and up is a different story) until they get very bad.
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