I have a bone stock 93 touring, aside from a 3” downpipe, Magnaflow cat, 3” cat back, and a modded airbox. I’ve had an issue with low idle since I bought the car three years ago. I installed a wideband recently and it shows 11:1 at idle. Cruising is stoic, and it holds steady at 10:1 under WOT. All emissions equipment is still intact, and the sequential system seems to be working just fine. Plugs are new and compression checks out ok. I have a Walbro 255 pump with the resistor bypassed, and I deleted the FPD. Where else should I be looking? Poor O2 sensor readings? Vacuum leak? Any help is greatly appreciated.

 

Check the idle bleed screw under the throttle body.

 

I raised the idle to ~750 RPM with this screw, but the rich condition hasn’t changed.

 

I assume no check engine light?

Could be weak/dead O2 sensor or non-functioning air pump. Make sure the air pump's magnetic pulley is engaged and it's spinning at idle. Should disengage when you rev up then re-engage at idle.

Also, hate to say it, have you done a compression test?

Dale

 

No CEL, compression is ~100 on all six faces, and O2 sensor is brand new (I have my AEM wideband outputting the narrowband nerst signal to the ecu). I’ll check my air pump.

 

Regarding the rich idle, with the resistor bypassed, idle fuel pressure will be higher than normal and higher than what the ECU is expecting. I have read that some users reported 1-2 points lower AFR at idle afterwards and before any re tuning. So what you are experiencing could be normal considering that the fuel pump is outputting substantially more pressure than it otherwise would be doing and the ECU (I am assuming you are running the stock ECU) is calibrated on the lower fuel pressure.

 

11AFR is perfectly normal for the stock ecu to idle at if your air pump is doing nothing. With the simplified sequential set up on my car, it would do the exact same thing your car is doing with the stock ecu. There's nothing to worry about.

 

The air pump is NOT spinning at idle.

 

If the air pump isn't running, that's the first place to look. It's common to blow the fuse for the air pump since the wiring to the pump gets pretty crispy over time. Start with that.

Also, with the air pump running, the AFR won't be 100% correct. The extra air being injected into the exhaust manifold skews the numbers. The stock ECU knows this and compensates accordingly, though.

Dale

 

The wires were broken at the air pump connector, and the plug exploded when I touched it. I soldered on a new pigtail and everything functions normally again. Still rich at idle, but nowhere near as rich as it was.

 

As a data point, my FD with stock fuel pump control and a good working airpump has something like 17:1 AFR at idle IIRC.