Enginge Misfire Hesitation in 4th and 5th gear
 

I have a 96 Civic HX 5spd 129k miles. Runs great and smooth 1st thru 3rd gear and 4th and 5th as long as there is load on engine and I am accelerating. When in 4th and 5th, running between 55-80 mph and just cruising flat or slight down hill, lightly on or just off of the accelerator, engine will start to misfire/stutter until I go heavier on the acceleration. Once I back off again, misfire comes back. Mechanic cannot seem to diagnose this. Original code was for 02 sensor which mechanic replaced. Still had problem. I thought it might be fuel filter so I replaced it but still same misfire at higher speeds under light load. I pulled the MAP sensor as an experiment and misfire seemed to magically go away but car didn't run as smoothly and idle ran higher. My assumption is that the MAP tells the engine to run leaner when low load on engine so it ran richer when disconnected, hence no misfire. So, given this scenario, what could be causing this problem?
Thanks in advance,
Jeff

Re: Enginge Misfire Hesitation in 4th and 5th gear
 

So did you try switching the map? Go to a junk yard and troubleshoot. What about ditributor cap, rotor, coils? What kind of plugs are you running? Air filter clogged?

Re: Enginge Misfire Hesitation in 4th and 5th gear
 

I inspected air filter, dist. cap and rotor, and plugs, all seem to be in reasonable condition, not sure how to check coil. I like your junkyard MAP switch idea. I'll try this weekend. Thanks. BTW nice job on DIY timing belt. I rebuilt my VW engine in high school just for fun but have lost the allure for major tinkering in my old age :)

Re: Enginge Misfire Hesitation in 4th and 5th gear
 

Visiual is one thing, knowing when they were last replaced is another. I would replace the whole setup if they haven't been changed for years.

What make of plugs? If they are Bosch, replace them.

Re: Enginge Misfire Hesitation in 4th and 5th gear
 

The MAP sensor sounds like one possible culprit. When you remove the MAP sensor the ECU is resorting to a very simple fuel injection calculation based upon RPM, throttle position, etc. which is probably a bit rich which is why the idle is higher and the car is running smooth at the higher RPM. A problem with the MAP could also manifest itself as a incorrect o2 sensor reading - do you happen to know which DTC was set?

I would also check the base ignition timing. Pulling the MAP sensor might indirectly "fix" your high RPM issue as the ECU will resort to a more conservative ignition advance. If the base timing were a bit too advanced you could get misfires from the ECU ignition advance at high RPM. Under high load conditions, the advance will not be as great which is why you may notice it go away when stepping on the throttle a bit (increasing the manifold pressure and load).

Re: Enginge Misfire Hesitation in 4th and 5th gear
 

Additional searching on forums is leading me to believe that this could be a problem with EGR system. Once I find a good step by step with pics perhaps, I will delve into cleaning the EGR valve and the ports. If this does not help I'll start replacing parts as suggested above.

Re: Enginge Misfire Hesitation in 4th and 5th gear
 

Did you figure out what the problem was? I'm having this exact issue with a 2000 Civic. Any pointers would be greatly appreciated!

Re: Enginge Misfire Hesitation in 4th and 5th gear
 

first things to check are plugs, wires, and cap/rotor. get new plugs first, they are cheap. if you have a MAF (mass air flow) sensor in the intake, take that out and clean it.