I have a 95 Diamante. Its running, and i have changed the fuel filter and i am in the process of checking the o2 sensors one by one to see if they are the problem. In the process of the work i found this forum so i thought i would see if some one on here might have some ideas.

What happens is it idles fine, if you accelerate slowly normally it runs fine. I can bring it up to over 4000 rpm and it runs smoothly. If i quickly press the gas, it will go but then it sounds rough and seems to lose power. If i then back off the gas it again starts running smoothly and will keep going. A friend of mine told me to detach each o2 sensor and see if any of them being out makes an improvement in the performance.

Anyone have any other ideas as to what it could be? I am going to run some gas treatment and fuel system cleaner next time i get gas.

how old are the sparkplugs ?
when was the last service?

Plugs are new according to the guy i got the car from. He maintained it for his son who drove it daily. I got it and and after driving it for a while i had problems on night where in wouldn't start. The problem showed up that afternoon before it would start. It has sat for a while and now i need to have it running again. I would say the plugs have less than 5000 miles on them. At idle it runs smooth as glass, and i can ease it up to high rpm not really slow but not stomp on the gas. When i stomp on it is when the problem shows up. The guy said he rebuilt the engine shortly before i got it, as well as the timing belt and a new alternator.

sounds like injector problem, ISC stepper motor
or the engine management computer.
reset the computer by undoing the battery terminals briefly
see if it improves

also have a compression test done on the engine

i have taken the battery out a few times like when i replaced the fuel filter. There was no change after that. I am not familiar with What the ISC stepper motor is so i will have to look that one up. I will run some good injector cleaner through it this weekend and see if it improves. anything else i should try.

Is the malfunction indicator lamp illuminated? These models have HO2S , heated oxygen sensors and can cause the engine to run excessively rich.. This and the MAF. But it is less likely to be the MAF that is doing this. One best thing to do is check the HO2 sensors. If the MIL is illuminated there is a way to manually retrieve the diagnostic codes frum the Mitsubishi diagnostic port with a multimeter.

if you ask me it sounds like you are running rich cuz its fine when its calm but when you dump gas (stomp on it) it gets too much gas and when its at high rpms its getting enough air to even it out. first check for a vacuum leak and make sure that your plug wires are good cuz if the plugs are new and they aren't getting enough power they still won't create enough spark to burn all your fuel. do you have any aftermarket stuff on there?

Ohh okies i just reexamined wut yu wrote. Seems like yu may have a fuel pressure issue. There is not enough pressure there on tap to give it the punch it needs when it needs a strong burst of fuel spray frum the injectors. This is wut it seems to meeh.

As stated yu can check for vacuum leaks but sumthin this apparent would be heard at idle and cause idle surge and a higher than normal idle. Also yu can tho check that the intake tubing leaving teh throttle bosy and going to teh MAF is not cracked or broken in any section. If when yu open teh throttle and large amounts of air is escaping thru the intake ducting before it gets to the MAF this can cause a similar issue.

I have not lights on. There are no aftermarket products on the car. Hopefully it will stop raining this weekend long enough the test some of the other things. How would i test these other theories that you guys have purposed? the fuel pressure and the running rich?

well if its running rich you will be smoking more than normal and the fuel pressure is basically the same prob. if the fuel pressure is off your either running rich or lean. to me it sounds rich.