my car is a 90 civic si. it has 20,000 on the engine/tranny, and 140,000 on the body. here's the problem. at highway speeds 40-70 mph, the car will hesitate and the tachometer shows that the revs are falling. if i keep driving the car will shake more violently and then stop shaking once the check engine light comes on. after the lights on it runs louder than before but i don't notice the hesitation anymore.

here's another piece. when it is hesitating, and before the code comes on, i can take it out of gear and rev it just fine. i can hold 5000 rpms in neutral no problem, but as soon as i put it in gear the car will start shaking and losing power.

and yet another piece. at times when i engine brake the light comes on. is this related or a seperate problem, i don't know.

i thought the fuel filter, replaced that. the o2 sensor is new, just gave it some injector cleaner too. now i'm thinking a vacuum leak. i checked for obvious vacuum leaks.... nothing. it might be some sensor i don't know about. please someone help my baby! i hate seeing her choking like this!

 

i'm thinking it is a clogged cat. anybody know the diameter and length for a test pipe? the part store doesn't have a clue.

 

I was gonna guess o2 sensor but you said it was new. maybe replace it anyway as they are cheap? or unbolt your exhaust at the header and plug your ears to see if it drives better that way(?) My car had a similar hesitation but only after the car warmed up. It only tripped the CEL once but it went off and for me all it ended up being was the o2 sensor.

 

i just looked at the o2 sensor two weeks ago when i took the header off. it was clean, some carbon build-up but it wasn't white or burned or anything. i'm taking the cat off to see if anything is melted in there. ******* exhaust bolts are crazy tough!

 

i'm looking into the front of the cat. it isn't melted at all, just a white mesh screen with no defects. it looks fine to me, but i guess i don't know much at all about cats. anyone know what a clogged cat looks like? i won't know for sure until i put on a test pipe.

quick question: can i test drive the car without the exhaust? i know it will be loud but i should be able to tell if the problem is gone. however will driving without the exhaust give me more problems? i don't want to hire a lion to get rid of my mouse problem. that's a metaphor.... a shitty one too....

 

am i talking to myself here? c'mon help me out. i'd appreciate even a "good luck" post.

i know now it isn't the cat. i can see through to the other side of it. i'm convinced it is a vacuum problem. the car had the same exact problem when i bought and all it was fine after i replaced a collapsed hose. i just checked every vacuum hose, nothing.

could this be my tranny slipping? perhaps some sensor on the tranny?

 

in case in body else has a similar problem. it was the dizzy cap and/or rotor.

 

Hi, I know that we have two totally different vehicles however, I just lucked up and seen where you were posting a similar issue that i am having with my 1990 Mitsubishi Mighty Max truck. I just purchased it from someone that mentioned that it had been sitting up for a year or 2 however he has taken care of it. He seemed to think that this hesitation issue under highway speeds is because of a slipping gear in the tranny, however, I really don't believe its the transmission or it would slip and show its behind all the time not just once in a while or just at driving speeds like when it goes into over drive or reaches cruising speeds of 60+ mph. How did you find it was your rotor button if i may ask? Thanks!
Syrs

 

Well I don't think I would have picked that one as the cause of the problem. I change my cap, rotor, plugs and wires yearly just to rule them out as any issue.

I have had all kinds of hesitation issues including the O2 sensors being swapped, the TPS incorrectly wired, vacuum leaks, O2 sensors not getting to temp, fuel pump / filter issues, timing, cam timing, and intake and coolant temp sensors. I have also seen where the gas tank will hold too much vacuum and popping the gas cap to relive the pressure will solve the problem.

My current one only happens after extended wide open throttle runs it will start to break up on the top end and during quick deceleration the car will nearly die regardless if I put the clutch in or not. I can try to peddle it and it will respond but if I take my foot off the throttle it will die. Fuel pressure looks slightly low at 36.5 PSI. Although everytime I have checked it at home it is at 39 PSI. If I turn the car off and try to start it again it will take a minute or two before it will fire back up and then exhibit no issues. I have never had any cold start issues.

Pretty sure I am having vapor lock issues as this has only become a problem with the ambient temps over 100 degrees and only seems to happen after my 3rd or 4th run. Probably just need to put some vents into the hood to let the heat out. I am going to check the tune next week and look for up stream exhaust leaks to verify I don't have any issues.