Slight surging while decelerating / coasting

[stuntmanmike]

Hey guys, noticed that my 90 tends to "surge" slightly when coasting and decelerating in gear, happens in any gear around 2400 rpm and again around 2200 rpm. It's usually accompanied by some light popping of unburnt fuel in the exhaust. (I have a borla cat back and radio doesn't work so I'm listening closely!) Above 3000 rpm car has a smooth, linear decrease in engine speed and seems that the computer or some sensor may be ever so slightly increasing RPM or changing timing or something below 3000 rpm?

Runs very smoothly at idle and no issues at all when on the gas. The car has replaced injectors and I put a Haibeck chip in last weekend...but pretty positive the car was doing this before chip went in. (it is nice to not have the CAGS and notice a quicker return to a lower idle!)

I'm new to ZR1s, is this normal or do you think something is up?

[S.hafsmo]

I've got a similar issue on mine while decelerating. Seems it cannot always deside whether to engine brake (cut fuel) or just coast/idle, which gives a slight jerking sensation. Also with Marc-chip, but I also believe this was present before.

[appelen]

My 90 does it as well, not worrying to much about it.


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[XfireZ51]

Is this w closed throttle? In gear? What about free wheeling decel?

[S.hafsmo]

Closed throttle, in gear. At lest in my case. Also in the transition between closed and slightly applied throttle.

[stuntmanmike]

Originally Posted by S.hafsmo

Closed throttle, in gear. At lest in my case. Also in the transition between closed and slightly applied throttle.

Exactly, closed throttle in gear. OK cool, seems like nothing is wrong with the car if others experience this! Thanks

[Young1]

My 91 yet has it slightly. Marc's chip lessened the surge for me. The stock chip was annoying in the mountains.

[tpr60]

I to have this surge. It all started after changing the stock exhaust to a flow master. I had the luck to sit next to Dave McClellan at a Corvette awards dinner decades ago and asked him about the surge. Seams the after market low back pressure allows the surge to be more pronounced when the computer tries to control idle on a coast down. Nothing to worry about, or go back to a stock exhaust system to hide the surge.

[appelen]

I installed a Corsa system and I think it might have increased at bit after that, but it is a bit louder maybe that is that I hear it more now. The better sound is worth it to me


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[Paul Workman]

My 90 started the rhythmic surging immediately after installing the SW headers and the necessary tune (Marc's chip). (Best description would be as tho the rpm was following a sinusoidal or smooth wave pattern, peaking ~ every 2± seconds, and @ ~200-300 rpm.) However, it is minor and only when coasting either in neutral or with the clutch depressed. No surging if engine braking, however. And, once (practicality) stopped, it quits.

I don't know if it is related, but after first starting the (cold soaked) engine when the outside temp is below ~ 60ºF, the engine will surge at the same cadence as (above described coast down). The duration and the depth* of the cycling varies; both increases the farther below ~60ºF the engine was at start up.

*The depth of the cycling when recovering from cold soak is typically as much as 1000 rpm, ranging from ≤ 500 to as much as ~1500 rpm. On rare occasion, the motor rpm will dip to the point of cut-off and have to be restarted.

Just another tid-bit, for ppl like Dom who might be reading, a very light - barely touching - throttle pressure and the rhythmic surging stops. When I asked Pete about it, he suggested I turn the TB idle air screw clockwise about 1/4 turn and reset the TB voltage to 0.4. My idle now is about 850-900 rpm, but the depth of the cycling is diminished significantly. (Now it is only a minor annoyance, but doesn't really cause any other issue besides annoying me a little.)

BTW, Ami's 91 with cams does NOT surge/hunt. But, then her idle is set around 950-1000 due to the cam overlap. Maybe that is masking the surging it may otherwise have?

However, far as my 90 surging goes, it's innocuous enough to fall below the radar (for now). I've "other fish to fry" right now...like Pete's 391 upgrade perhaps??