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Discussion Starter · #1 ·
Well I don't smell anyything like rottewn eggs but my mpg is down to 16 from 23-24 and o2 replacement did absolutely nothing to help and in fact it continues to drop (18-->16 now) even though car runs smoother with new o2. Power does feel a bit laggy so I bought a straight pipe and plan to do a cat elimination after I compression test motor to rule that out. Anyway has anyone seen that big of a MPG loss due to a bad Cat even though the power loss wasn't COMPLETELY obvious? The other thought is maybe a clogged fuel filter and I'm also curious to know if that could cause such a huge MPG drop? This is on my 1g 2.0 N/T A/T, thanks.
 

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Perhaps check to see if your fuel pressure regulator is leaking fuel through the diaphragm and through the vacuum line into the motor. This will give you TONS of unwanted fuel loss with few symptoms, other than long crank times, terrible MPG, and way rich fuel trims. Depending on the size of the leak, it may foul plugs, o2 sensors, and plug converters. Not to say that it's definitley the problem, but it would take seconds to check. Simply pull off the vacuum line at the regulator, turn the key on, and see if any fuel squirts out. sometimes it won't immediatley, but if you (lightly) tap the regulator with something, and the regulator diaphragm is at all perforated, it should leak. Just something to consider.
 

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OOPS!! sorry bout the boost leak test suggestion! looked at your sig and noticed it said TSI, didn't notice the end of your post. Also a normal mpg average for a n/t only 24/25? I get that out of my tsi! with the old NT I used to get 28-30ish...
 

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Plugged cat is more of a symptom then the cause. Leaky injectors, bad fpr, plugged or damaged RETURN lines, bad ect or maf are more likely the primary cause. Plugged fuel filter would probably cause a lean condition not a rich one.
 

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I said:
Plugged cat is more of a symptom then the cause. Leaky injectors, bad fpr, plugged or damaged RETURN lines, bad ect or maf are more likely the primary cause. Plugged fuel filter would probably cause a lean condition not a rich one.
+1 clogged filter means less fuel thru the lines. I would also say that cat's can go bad after many miles. Just because the cat went, doesn't mean something else HAS to be wrong. A previous problem could have caused the clogged cat as well. If you've already got a strait pipe on the way, throw it on just because!
 

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Discussion Starter · #8 ·
I did pull 1 plug today and it looked ok, if the others AREN'T fouled what could I rule out that was already mentioned as possible causes?
 

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91-Bluebird said:
What's ECT and will hooking up a scanner tell you if there's a leaky injector and indicate which one?
ECT is engine coolant temp, you should be able to check it with a logger. If it is a general rich condition (such as bad fpr) all plugs would be fouled. If it is just one it will be a problem with that specific cyl like bad plug wire or leaky injector. If there is a problem on two companion cyl like 1 and 4 or 2 and 3 then I would put money on it being a coil. And if it is on all then it will be a genral rich condition, or spark issue (ignition module/coil pack/ maybe cam angle sensor).

Cats have a fairly long life span and general don't die, they are killed by overly rich/lean conditions, coolant or excessive oil burn. They are made to work within certain parameters and if all is well they normaly have a long life, not to say they can't fail even within their operating parameters but is far less likely.
 

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Discussion Starter · #11 ·
I said:
ECT is engine coolant temp, you should be able to check it with a logger. If it is a general rich condition (such as bad fpr) all plugs would be fouled. If it is just one it will be a problem with that specific cyl like bad plug wire or leaky injector. If there is a problem on two companion cyl like 1 and 4 or 2 and 3 then I would put money on it being a coil. And if it is on all then it will be a genral rich condition, or spark issue (ignition module/coil pack/ maybe cam angle sensor).

Cats have a fairly long life span and general don't die, they are killed by overly rich/lean conditions, coolant or excessive oil burn. They are made to work within certain parameters and if all is well they normaly have a long life, not to say they can't fail even within their operating parameters but is far less likely.

Thanks that was some really helpful info :)


As far as engine coolant temp sensor my dash gauge would indicate whether that's good or bad right?

The cat may be original on my 166,000 mi car and I guess there would be a mitsu/diamond logo on it if it is right?
 

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I said:
Cats have a fairly long life span and general don't die, they are killed by overly rich/lean conditions, coolant or excessive oil burn. They are made to work within certain parameters and if all is well they normaly have a long life, not to say they can't fail even within their operating parameters but is far less likely.
I do agree with you, but was just saying the cat going doesn't mean that there is a current problem. Either way, if you have a strait pipe, run it. You'll get better gas mileage just from that. Even it that wasn't your problem, it will help ease the blow until you find the root of the problem.

I don't think a scanner will tell you that you've got a leaky injector. It will tell you that a cyl is running rich, but I think thats about all...
 

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91-Bluebird said:
As far as engine coolant temp sensor my dash gauge would indicate whether that's good or bad right?
On these cars, I believe 2 different sensors are used. One is for the IPC guage, and the other is for the ECU. Someone feel free to correct me if I'm mistaken on this.
 

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I have the same problem. So far I have swapped plugs, coil, O2 sensor, wires and Im using a afpr now as well. 16ish mpg, 200 miles per tank with almost no boosting........

Weird question how should exceleration feel at 14psi on a stock t25 with a FMIC.(auto) As my car hits around 3k she opens up like magic. Should I feel a surge of speed at 3k. Or should a stock turbo have a more even take off?

Sometimes on the freeway I hit the gas and watch the revs sag 2-3 hundred rpms, The car is not shifting just a loss of Rpms?

I was in my buddys GSX and my other buddys turbo spyder. Mine feels "different". Only thing I have left to swap is the TPS and the cat.
 

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What led you to replace the O2 sensor? Hook the car up to a logger and compare values. If the rest of your plugs are not fouled then it might be the age of the Catalytic Converter. An idea that just popped in my head. Converters melt due to mifires and reaching extreme temperatures under load. Of course a melted Converter would result in a restriction in your exhaust system causing you to develop backpressure after a while. Especially at higher speeds. With backpressure, exhaust gasses go back into the cylinder reducing the space for mixture in the combustion chamber. Resulting in a loss of power and poor fuel economy. My suggestion would be to see if you hear some kind of whistle out of your tail pipe to see if its clogged or lightly tap on it with a rubber mallet to see if you hear some type of rattling. Rattling usually indicates that the honeycomb inside is bad.

I base this only on the fact the plugs are good and dont show wear. There are no leaking injectors or current misfires. Definetly something to look for. It could be time to replace that Converter.

Also ask yourself this. Are you the original owner of this car? If not think of the driving habits the previous owner had. Maybe they ran the car when it was misfiring for a period of time. Hurting the Converter but not enough to notice any power loss or fuel economy?:dunno:

Let us know what happens. Good luck
 

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Discussion Starter · #16 ·
JuanG said:
What led you to replace the O2 sensor?
Because my mileage dropped all of a suuden by 5-6 mpg and I've read many threads over the years with people seeing similar drops due to bad o2's. The one I took out had about half the ?holes? on it corroded with carbon and although mpg did not improve with new one engine does run noticeably better so it wasn't a complete waste imo. Also I don't have a logger and tried testing old o2 via chilton's instructions but didn't get too far with that lol.

Either way I'm removing the cat because the car does feel a bit restrictive especially at highway speeds and I'm betting it's the original with 166,000 mi. on it.

As soon as I get my new used starter in (separate problem lol) I'll be able to compression test motor and determine condition of plugs while I'm at it. The one plug did look black from a rich condition and I'll see what the others look like.
 

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I am also having an issue like this where my MPG is poor and lack of power on the highway.Also my friends are telling me to clean there cars from the black im shooting out my exhaust. I am overly rich. quite possibly the fpr???
 

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Discussion Starter · #18 ·
I said:
ECT is engine coolant temp, you should be able to check it with a logger. If it is a general rich condition (such as bad fpr) all plugs would be fouled. If it is just one it will be a problem with that specific cyl like bad plug wire or leaky injector. If there is a problem on two companion cyl like 1 and 4 or 2 and 3 then I would put money on it being a coil. And if it is on all then it will be a genral rich condition, or spark issue (ignition module/coil pack/ maybe cam angle sensor).

Just got around to pulling the plugs and this is how they look:

#1 pretty normal

#2 a slight dusting of white carbon on very top of outer electrode

#3 normal, possibly a little rich

#4 looks to be running a bit rich

Anybody want to guess from that what might be causing the horrible gas mileage and remember I already changed the o2 and removed cat which made virtually no difference.
 

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Discussion Starter · #20 ·
Joescool2008 said:
Maybe your plug wires are becoming shot. Have you noticed any misfiring?

I do have a good bit of misfire/stuttering and changed plug wires didn't help so I took them right off and returned them to store. Motor ran smooth as silk with exception of a very small slight mis UNTIL the day my ecu caps leaked which allowed FP to run non-stop and filled all cylinders to the top with fuel so I got a used ecu but engine has never been close to running the same as before the hydrolock so either that fouled something up or the used ecu is not operating at 100% even though it's been in there for over 1 yr now. Is a 'semi-bad' ecu a possibility?
 
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