klak1212

First post! Alright, well, I just bought this 97 civic ex sedan from a private seller, with 229k on it. I am getting some bad mileage on it (in my opinion), have gone 169k (105 miles) on half tank. I haven't completely drained the tank yet so it could just be an inaccurate fuel gauge sensor in the tank, so I am going to just keep driving until the fuel light comes on. However, one of the bigger problems I am having is a sporadic shift in RPM's while driving.

I will be driving, doesn't matter what speed, and the RPM's will drop ~1000, it will almost feel like the car is going to stall, (sometimes I have to release the gas pedal and press it again) and then the engine will rev up to 2000 again after 1-2 seconds to continue driving . This only happens 80% of the time while driving the car. I talked to the previous owner and they said that the distributor cap, rotor, plugs, and wires were replaced 3 years ago, and the air filter 2 years ago. I checked all of these things and they still look fairly new. I asked them about the fuel filter and they can't remember replacing it (I am thinking this could be the cause as the bolt on top holds the filter has a lot of rust). I checked the transmission fluid and it looks to be fairly low (although I am not sure if this could cause my issue, going to stop at the dealership to get it refilled). Could it be the fuel filter?

Any help is appreciated!

klak1212

Bump.

Brian Parr

had this same problem and it never threw a check engine light, ended up being one of the o2 sensors cant remember which but ever since i changed them it was never an issue.. i had to monitor them on live datastream with a scanner to find this out

klak1212

Makes sense for the decreased mileage and weird drop in RPM's, but I had to get the car safety and etested (being in Ontario), and it passed the emission test. Could it be an o2 sensor still even if it passed emissions?

klak1212

Brought the car to the dealership. They checked the tranny fluid and found it to be 3/4 to full (suppose I am to check it when the engine has been running ). Anyway, RPM's still jumping around. Got new tires today and changed all the spark plugs. Checked the fuel filter as well. I could blow through it using my lungs and gas sprayed out, but it wasn't constant, I guess more like spitting fast. Gas looked kind've yellow/brown when it came out.

Former User

Dealership couldn't figure out rpm problem?

klak1212

No, I told them I researched a few components that could be making it do that. The mechanic told me if it was the ECU there would be a CEL. He told me it *could* be the Fuel Filter (but I am starting to doubt it...). I didn't ask him about the IACV, TPS or O2 Sensors.

/e Drove on a 100km/h highway to see what happens. I had to put the pedal to the floor for me to get enough power to get to 100km/h, then merged. Speed would only increase if I floored the pedal, so I would get to 110km/h, then release a little so as to not go too fast, and the speed would decrease. Started going 90km/h, and there were times if I put the pedal to the floor but it would not increase speed, so I had to release and try pressing the gas again. Car was really jerky going up and down in RPM's and shifting. I couldn't go a steady speed.

On the way back I tried using cruise control, and the car went a steady 100km/h all the way back. Nothing bad.

klak1212

Put a new fuel filter on, problem still happens.

DeLSoLRo

So you will be driving, holding the gas pedal at the same angle, then the car will just drop down to under 1k rpms, while still holding the pedal at same angle? My guess is prob. a dirty fuel system/injectors try some seafoam and make sure to follow instructions and research on here first before doing it yourself.

Some free/cheap fixes: Try doing an ECU reset and see if that helps by removing the ground from the battery for about 5 mins or 15 if you want to be safe, turn the car back on and make sure you have the a/c off, dont touch anything and let the car idle for 15 minutes and turn it back off. You can also remove the intake from the throttle body and see if theres black stuff on the walls of the throttle body and if the gold hinge can move easily and freely. If not, take it off the manifold and spray it down with carb cleaner. If nothing works atleast youve done some cheap/free maintenance to the car

DeLSoLRo

Another thing you can try is swapping ECU's with a buddy for a day and see if problem still occurs, or if it happens to him. Have you tried pressing the gas while the car is "parked" and see if it will do it then?

klak1212

The throttle body looks 'okay'. The gold flap can move freely but there is some black gunk inside on the walls (assuming carbon buildup). The problem doesn't happen when I am in park. I also found out today that the problem doesn't happen in 2nd gear (Not D4/D3, but 2). Could this be a transmission issue?

Also, wouldn't the ECU throw a check engine light or not always?

klak1212

Guess I was just lucky for it not happening in 2. It does now!

I am stumped, I have no idea what to do now.