Problem: Over 4000 RPM = mixture too rich

[Tokyo_Drifter]

Hi, weird problem ahoy!

I have a 1989 Civic DX HB that idles nicely & passed emmissions etc, but when it revs to 4000, the fuel mixture gets way too rich. This was worked out while testing the O2 sensor at a mechanics when getting the car ready to pass emissions. Since I wanted to get the car on the road, I left it at that. Only other thing happening is that rarely the engine check light comes on, but no code is flashed. I reset it & it doesn't come back for a couple of months.

Engine is the 'mighty' DX 1.5L with an SI MPFI intake & ECU (#'s verified) swapped on. I've done a tune up, new leads, plugs, IACV & O2 sensors swapped (although admittedly from junkyard parts) etc. Gets pretty good economy despite it having some 240,000 km (160,000 odd miles), so I think everything else is ok.

I don't drive the car that hard (name used relates to an old movie, not drifting), but every now & then it'd be fun to be able to open it up. Right now when I rev past 4, I can feel the car actually accellerating when I ease off the throttle! I've searched DX to SI conversions, but haven't seen this problem. Call me paranoid, but I don't like things not working & want to fix this. Thanks for any help!

[sblkcamaro70]

Do you mean the car accelerates when you let off the gas pedal completly or it accelerates when you ease off the pedal. And how much does it accelerate. If it accelerates for a split second that is just the centrigual force that the engine is exerting but if it accelerates for more than a second you may have a problem

[Tokyo_Drifter]

No, it doesn't accelerate with no gas pedal-age at all (which yeah, would be a major problem).

It accellerates fine up to 4 & then bogs down. Lift my foot & the car goes well. I haven't been able to hold it at the exact point for it to accellerate smoothly, but tapping the gas pedal & the car feels like it wants to rev, backs off, revs etc. Could be the centrifugal force at that, but still doesn't quite explain why this is happening.

I was just thinking that instead of looking at too much fuel, maybe I should look at not enough air, since air + fuel =
I don't think this is it, as it was a fresh K&N filter when the problem started, but I'll remove the filter completely & take it for a test run to check.

[Hybrid1990crx]

check your o2 sensor, cold enrichment start up. what is the code it throws?

[Tokyo_Drifter]

O2 sensor checked out ok just before emissions about a year ago & I haven't noticed any changes in economy or perfomance since, which leads me to think that's it's still ok.

"cold enrichment start up"? Like if the engine was still doing warm up procedure when it warmed up? I had that problem at one stage where the air intake temperature sensor was faulty & the ECU thought it was arctic conditions & kept throwing fuel in. Fixed. After that it passed emissions, so again I don't... think... that is it.

Unfortunately the code is the dreaded '0' (no flashes, meaninging ECU). If it was happening often, I'd fix it, but it only comes up every few months. Anybody else had that happen? Maybe the ECU is faulty, but the car works fine in all respects until the 4k rpm mark. Anyone know if a mechanic can run tests on an ECU to see if it is ok?

A test drive on the highway shifting down to 3rd gear saw it accelerate smoothly up to around 4. Then it's like it hits a wall & you can feel it stuttering. Ease off the gas pedal to about half throttle & it accelerates, like at W.O.T it dumps too much fuel in for the engine to handle. Maybe that sounds like the ECU again, but wanted to check advice here before anything else.

[Figori]

Maybe it's the wrong ecu? Almost sounds like you're hitting V-tec and it's flooding the motor.... Just a thought.

[Tokyo_Drifter]

Originally Posted by Figori

Maybe it's the wrong ecu? Almost sounds like you're hitting V-tec and it's flooding the motor.... Just a thought.

Good idea & I hadn't thought of that, but nope, checked the serial number & it's the right ECU.

Also figured it could have been the Si ECU was programmed to work with an Si cam, larger engine etc & because it was hooked to a 1.5 DX block this was causing it, but I haven't read of any MPFI swaps where this happened.

I'm still scratchin' my head...