99 Civic runs rough since I started hypermiling
 

This is my first post here, but I've been lurking around for a while. I've been having a hard time finding info on this kind of problem on google or alldata, so I figured someone around here may have had a similar issue.

I've been hypermiling on my Civic (D16) for a few months now and average 39ish mpg (silly Gov. Crist put ethanol in all our gas. Before that went through I was at 42). Anyway, under very light throttle and cruising around 40 mph (2000~ rpm) the engine seems to surge on and off. Letting off the throttle to engine brake doesn't seem to fix this. Whenever I drive heavy on the gas, it goes away.
I've discussed this with my old automotive teacher and he proposed anything from failing EGR valve to bad O2 sensor and everything in between. We had a hell of a time trying to diagnose it. All the sensors checked out, but we didnt have much time to check the EGR.
I thought perhaps it was spark plug heat range? But of all the manufacturers I've checked no one makes a plug hotter than stock for that model of Civic. Also, I've thought it may be a fuel delivery problem, but since it happens during engine braking makes me think that is not the issue (since fuel pretty much stops being delivered).

Besides this issuue, I have noticed a very slight miss when revving up the engine in neutral. Under 2k rpm, the needle just seems to bounce up an down a little like a misfire, but hooking up a scantool shows no signs of one.

Anyhoo, if anyone has had a similar issue with their car, let me know. This thing is driving me nuts!!


-Spoony

Also, just did a full tune-up a month ago. (Platinum Plugs, wires, cap, rotor, fuel filter, air filter, pcv). The problem existed before the tune-up as well.

So it'll actually surge during engine braking, above the DFCO?

There is a plug (blue) under the dash on the passenger side near the door, its male with two wires. Just stick a paper clip in there and see if the CEL blinks.

Do you have vtec-e? If so, maybe it has something to do with vtec engaging or disengaging?

Just to be clear, when you say surge on and off, do you mean like you are pressing the gas and letting off or like the engine cuts out (as it would if you were out of gas or at the max speed) and then kicks back in?

Hmmm ... had a 1995 Civic that had a few issues in its 132,000 miles.

How does it idle? Does it hunt at all? That might be a bubble in the cooling system ... near one of the sensors can produce a hunting idle. Might try bleeding the system down in case it got a bubble in it.

Perhaps it is loaded up with soot? What did the plugs look like? I might run a bottle of Red Line SI-1 through the gas for a quick clean-up. I think it's the best of the pour-in cleaners.

I've seen a leaky head gasket produce a surging idle ... same with a vacuum leak ... which may be the case here. Best way to diagnose this is with a UOA ... used oil analysis. Coolant shows up as potassium and sodium in the oil ... as well as accelerated iron and lead wear.

didn't mention new oil. My 01 protege starts to act exactly that way when it needs new oil (slight part load missing, less mileage, and other general roughness)

If you have indeed chaecked everything mechanical ... like sensors tune ups etc ... I would consider a quality grounding kit. My old 96 Civic always ran rough at low idle then it dawned on me that the vehicle had only 2 crappy ground cables. I built my own tuner like grounding kit and the car began to pur like a kitten from there on.

If you haven't solved your problem, then it's probably something between the gas tank and injectors. Fuel pump screen could be clogged, regulator might be bad, injectors clogged.

Might even have something to do with the MAP sensor. Even though if you checked it and it was ok, it wouldn't hurt to buy map sensor cleaner and clean it out. CRC makes some cleaning agent. Try spraying some carb cleaner into the tb under load.

There has gotta be a code stored in the ECM. Get a scan and post the results.