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Rough idle! Think its dumping too much fuel I need help

10K views 36 replies 10 participants last post by  97ebaygst 
#1 ·
I just recently purchased a 97 gst with 125,000 miles on it. When I bought it the guy that sold it to me said it ran real rough and that u could smell unburnt gas coming from the motor. When I got it home and pulled out the plugs they where black as could be. So to see what it would do I bought a new set of plugs and those blacked out after letting it warm up a little.
When I start the motor it has trouble starting. It'll turn, chug and repeat once or twice before it'll turn on. Once it's on it'll idle real low, like it's barely above 0 on the tach. When i try and rev it it'll jump like 500 rpm up then act like its gonna cut off before i take my foot off the gas. If I do this to or three times then press the gas slowly it'll crawl to between 4 and 6 grand before it sounds like it's backfiring and then bog down again.
I have done a compression test and it read between 170-180 across the board. I hope I did it right cause It was my first time doing it. Things I checked are the coil pack and the tps. The coil primary resistence read .3 on both leads. Where I read it said the norm is .75 anything else to replace, but it still runs, so I was wondering if the sence of extra fuel could be from the coil not firing hard enough to burn it all.
Thats all for now but if anybody could help me out i would be very grateful. This is my first turbo car so im a little lost on working on it compared to a n/a motor
 
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#3 ·
Sounds like you have a fuel injector that is stuck open, issues with your FPR, or problems with your Mass Airflow Sensor. If your car is in fact running too rich, it would probably come down to one of those three problems. I can not post in classifieds, but if you in fact find out you have bad injectors, PM me I have an extra set of stock 450cc I'm never going to use off of a 99 GST (I don't want to get banned from the forums, I'm just trying to help out).

It def sounds like you're running rich though. Have those things tested.
 
#6 ·
I'll make sure to check that out. Also i was just snooping around under the hood checking wires and stuff and i noticed that the coolant temp sensor pig tail is broken could that have anything to do with it even though its throwing fuel even when its cold?
It's pulling air intake temp sensor circuit malfunction and coolant temp sensor circuit malfunction.
 
#13 ·
I had the same issue wit my car could be the temp sensor....but for the blackening spark plugs I am clueless. It can also be the cam angle as stated befor. Maybe even your ecu.... I myself had that happen when my ecu was on its way out. Was the car in running order when you bought it?
 
#14 ·
Ok guys I found a pig tail for the temp sensor and fixed that. I also changed out the cam sensor but it stills runs rough. It's better where it doesn't back fire or chug anymore but still low rough idle. I haven't taking it out on the road yet but now the gauge isn't showing any boost, I hear the bov opening but it's just not reading. I've heard that it doesn't build boost while sitting still but when i got it the gauge still read some boost. Did I disconnect something on accident?

After looking more into it I notice that he disconnected the wastegate actuator and the flap moves freely. I had my wife sit in the car and look at the gauge and rev the engine while i held the flap shut thinking that could part of the problem, but still no sign of boost on the gauge but bov still pops off.

So looks like i just added to my problems. I think i fixed part of the problem but still something there that i cant fix. Could the IAT code that pops up have anything to do with it? Well I'm gonna take it out for a test drive any see if it builds boost.
any help would be appreciated. thanks
 
#16 ·
i worked on it all day today and I'm more confused then ever!!! Everytime I start it it acts different. I got the coolant temp sensor pigtail on like I said earlier and it ran a little better, like i could rev straight to redline without a hiccup but it reved slow. Now when I crank it (no stereo) it makes a clicking nosie from the stereo area and lights start to dim while the car dies then comes back on then dies again. It's really hard to type what its doing. It's more a u got see to know kinda problem.
 
#18 ·
How is your charging system and battery. The car is running better which is an indication that you have fixed a couple of things. I know from experience if the charging system is going out it will cause the car to idle poorly. This could also explain why the stereo is shutting off and the lights dimming. Check the battery and the alternator.
 
#17 ·
(I am new to DSMs BTW) but I think it could make boost when revving if it were running rich so it not making boost now may not be a problem but a sign that you did fix it, somewhat.

but i am also having a similar sounding problem in my 93 talon, where it does not start right away i have to crank it a bit and wait and crank again... till it fires some, than fires enough to run. but it revs up and down for a little while than finally calms down and runs well. it will also start up fine as long as it is still warmed up.

BUT.... I did find 4-5 little holes in the pipe coming off the stock intercooler i didn't think that the little holes could cause it to run sooo bad but i dont know... :(
 
#19 ·
Well i just replaced the battery and the charging seems good. I figured out that the relay that I hear is either the fuel pump relay or the mif relay. What could be making either one of these things go off and cut the car off? I've heard that the mif is called the auto shutoff so I think thats the one thats clicking but I'm not sure. I dont know if I fixed the problem because I can't keep it running long enough to tell. It's still pushing out black smoke. It did runn better for like five minutes but started acting up again so I think that the good revs was a fluke.
So I guess my question for now is what could cause either the fuel pump relay or the mif relay to click and could that clicking be why the car shutoff?
 
#21 ·
could it be the FPS that is causing to much fuel pressure? I took it off and wired it to a 12v battery and didn't feel anything coming from it, vacuum or pressure. Should I feel either one?
If it is the FPS do I have to have that if I want to run a AFPR? And if I do run a AFPR do I have to have a wide o2 sensor?
 
#22 · (Edited)
well I know I said that the alternator was fine the other day but now it took a crap on me. could this be contributing to my problem of running rich? also when I disconnected power to my fuel pump the car actually ran good at idle and I didn't smell any gas at the exhaust. Shouldn't the car idle rough when the fuel pump isn't connected. It's almost like the pump is running to hard and when i disconnected it to depressurize the fuel system it evened out the the load on the injectors. I'm totally lost on what do to. is there a way i can check to see what size injectors I have on the car?
 
#24 ·
Checked the tps and that checked out fine. biss screw? not sure what that is. The regulator is stock but i haven't pulled the pump yet to see it it's stock, I'm gonna check that next. I'll check the injectors at the same time. If it is the injectors what can i do to fix it other than put the stock ones back in?
 
#25 · (Edited)
Okay if your TPS is good and adjusted correctly then that takes care of that. If it was adjusted too far one way or the other it can cause an unstable idle. I have a 1g throttle body and the screw is on top on the front side ( near the valve cover so to speak, not next to the firewall) It is recessed in there a bit. You made need to adjust it a hair out. If your pump is a 255 then it will over run your stock fpr..not sure how it would affect idle exactly but would highly recommend an aftermarket FPR. Well stock injectors in theory should not give you idling problems. Unless they are partially clogged or something of that nature but then your normal driving conditions would reflect injector issues as well. Do you have an EMS on this? Are you running pump gas in this, What kind of plugs are you using(have you pulled them and are they black with soot?) and what is the gap on them? Tons of variables here! :eek:
 
#31 ·
I thought the same thing but I cant think of anything else. I finally got the alt back on it and it's doing the same thing as before. Turning on, rough idle, lights flicker, rev to a grand and shutting off. If I crank it enough times it'll stay on, but it's like 15 times that i have to crank it.
Could a sensor cause this or is my ecu going out.?
 
#32 · (Edited)
Id say if you got a friend with the same car just swap them out and even if it doesnt fix it, thats another thing you can rule out. It sounds to me like your turning it on getting spark and fuel then you will give it gas it will get a bit but then it will die out from lack of constant gas. Then when you try and crank it again you wont be getting enough gas for it to ignite and fire up. Its probably not helping your power issue that your cranking it over that long your battery is probably extremely weak, from enagaging the starter. If I were you I would get a mobile mechanic to your house and find out the problem then fix it yourself.
 
#33 ·
I have been looking all over this forum for how to do a fuel pressure test. Basically I want to know what tester to buy and where do I hook it up. Also are there any threads on what parts I would need to do a boost leak test and how to perform one. I have yet to do either one of these and I just want to eliminate them from the list of possible problems.
One last thing, what sensors would control the amount of gas that get dumped into the cylinder. I remind u it does it when cold too so I would assume that the o2 sensor and coolant temp sensor would not cause it because its on the closed loop circuit until the car heats up.
My list consist of....
1.tps
2.mas
3.mdp
4.cam angle
5. was wondering if the crank angle sensor could cause to much fuel.
did I miss any?
 
#34 ·
I was under the hood of my car trying to take off the tps (to no avail) when i notice a plug dis-connected, well I did some research and found out that it is my knock sensor unplugged. I can't find the knock sensor plug in. So I have two questions.
1.could this be a possible cause to the fuel dumping and the cut off problem?
2.where is the knock sensor located? I know it's on the back off the block somewhere but I was hoping for some more detail like which side would be easiest to find it, is it high on the block or low?
Oh and one more question what kinda symptom would you get from it not being hooked up.
 
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