| '92 LT1 idle issues (long post) |
Subject: '92 LT1 idle issues (long post) by screamin_conure on 2010/1/28 20:32:29 Hi all, Newb 'Vette owner / Guru member from Northwest Indiana. I posted this over at CAC a few weeks ago, but cabin fever and the need to do something "Corvette" has me wanting to post it here to get a little more input, so here it goes. I have a '92 LT1/A4 coupe with 62k miles. I picked up the car early last fall and to date, I've had the serpentine belt changed, new front seal, new intake gaskets, new IAC valve and TB cleaned, engine smoked for vacuum leaks (none found), new TPS, new air filter, and new PCV valve. I'm now trying to troubleshoot a poor quality idle situation. The car starts every time with the tap of the key (knock wood), but settles into a "bouncy" idle with lots of rich smelling exhaust. My own personal thought/diagnosis is a lean miss at idle due to a faulty or failing injector that's causing the 02 to read lean, thus telling the ECM to pour on the fuel. Being batch fired, there's just too much fuel now at idle and I'm getting the bad smelling exhaust, and an idle that feels "lower" than normal and "bouncy" (I love that term! ). I have an AutoXray EZ-Scan 3000, a fuel pressure gauge, a vacuum gauge and a digital volt/ohm meter. I recently spent an afternoon running a battery of diagnostics on the car and the results are below: Fuel pressure check: I hooked up a FP gauge to the schrader valve and upon turning the key (no engine start), the needle briefly went to 50 while the pump ran for a few seconds and then settled back down to 45. Shut the key off and pressure stayed right at 45 psi. I left it there while I did the cold injector readings, and pressure dropped to around 40 by the time I was done. Started the engine and pressure stayed right at about 42-43 psi. I think I'm good there. Vacuum gauge: I teed a vacuum gauge to the port that feeds the EGR solenoid/ASR line, underneath the driver's side fuel rail cover. Started the engine and vacuum jumped to 15" Hg. Revved the engine a couple of times and vacuum dropped to 0, shot up to about 20, and then settled back down to about 17" Hg. The needle was rock solid stable at idle. I think I'm good there too. Injector resistance check: COLD Cyl. # Ohms 2 12.8 4 13.1 6 13.0 8 12.6 1 12.7 3 12.7 5 12.6 7 12.7 HOT - after a 20+ minute drive where coolant temp hit a maximum of about 225 degrees. Cyl. # Ohms 2 13.3 4 13.1 6 13.1 8 13.3 1 12.9 3 12.9 5 13.2 7 13.3 Road test: Just to be sure it wasn't the MAP sensor, I pulled the connector for it and started the car. It threw a code and SES light for the MAP sensor voltage, but it had no affect on the idle quality/miss. I plugged the MAP sensor back in, cleared the code and started the engine again with the scanner running. Both left and right BLM and left and right integrators were locked on 128 while in open loop. As soon as it went closed loop, the right BLM shot up to 160 and stayed there while the left stayed right around 128, fluctuating only a bit. The right integrator shot up to 180 and was flipping around like mad while the left was right in the normal range. As soon as I drove off, the right BLM dropped from 160 to under 128, and then started to slowly level back off to a more normal range. When I would come to a stop, the right BLM would just climb right back up to 160 and the low/bouncy idle would return. The whole cycle would repeat with every movement in traffic and every stop. Right 02 sensor readings were all over the place (from 200 mV up to 800+ mV), while the left sensor, while varying, seemed to be ranging more in the 450 mV range. At no point that I was able to see, did the EGR duty cycle move off of 0%. I thought that was kinda' strange. TPS voltage was bang on at .62 volts at zero throttle and ranged normally with throttle input. When standing near the idling engine on the passenger side of the car with the clamshell open, I can also hear a clicking of sorts. It's not really the metallic-sounding tap of a bad lifter, but more of just a rythmic mechanical clicking. Driver's side sounds fine. As I've mentioned, even though the resistance readings are normal and the fuel pressure holds well, I'm still about 85% convinced that I have a bad injector somewhere on the passenger-side bank. The other 15% of me thinks it could possibly be the right 02 sensor. Thoughts? Thanks in advance! Ron |
