Recently built L98 with MAP. New electric parts, splugs, sensors,etc Engine idles poorly, like misfiring but tach is steady does not fluctuate. I check vacuum withvacuum gage and it looks ok. However, I can hear a hissing coming from somewhere,but again vacuum gage on green, no leaks. Timing on 6 BTDC. Could it be vacuumleak in plenum? runners? Or defective IAC? (new by the way) TB seems to beworking fine. Maybe O2 sensor? I ruled out any electrical issues since the tachneedle is steady and not jumping all over the place. Would appreciate anyassistance. Thanks.

 

did u change the temp sensor?... i would also change the rear fuel filter under the trunk..maybe put on a new set of wires and maybe a new coil.

 

Thanks for responding. Temp sensor, fuel filter, coil, and wires are new.

 

First thing I would do is spray either starter fluid or carb cleaner around the general area of where the hissing is coming from. What your looking for is a change in idle. Many of the components on the car have the same symptoms when they are bad so you have to test each one separately. For instance, my TPI wouldn't idle, I found the ECM and Distributor to be bad so I replaced them only to have a very poor idling engine, at that point it ended up being the EGR vacuum relay. It runs smooth now. My brother's '84 Corvette with the crossfire injection that has only 40K miles slowly started idling poorly and then began stalling. Everything on that engine tested good but I could have saved two days of time by checking the fuel pressure first. The two inch piece of fuel hose that connects the fuel pump to the fuel pump line was cracked and split not allowing the minimum fuel pressure to run properly.

 

Thanks for the reply. I believe I fixed the problem. The IAC was not properly set. Initially I had done the procedure but I left the comp on diag mode, never removed the paper clip from the ALDL during IAC setting. I did it properly this time and the hissing is gone. Still not properly idling but at leat the hissing is gone. I'll check the fuel pressure, fuel lines/hoses/filter. The EGR was deleted. Also, I'll do a balance/compression/leak test. I'll update later.

 

Most engines will idle well between 12-17* of timing. no reason to be using 5*, that is poor idle quality (part of it)