Weird problem
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Joined: Sep 2001
Posts: 720
From: Indianapolis
Weird problem
A friend of mine just sent me this on her LT1 SS. So far none of the shops she has taken it to can figure it out.
When you are in drive, and stoped at a light, you can feel the rpm's drop, like it is missing, but you can almost time it. And if it is at night, you can see the lights dim at the same time the miss happens. But the engine is not coding, so people are having a hard time figuring out what is wrong. We had the injectors cleaned, a tune up done, to no avail. I think I am going to have to get the computer flashed. This also affects the car coming off the line. Yesturday I was stoped, and the miss was so bad, I thought the car was going to die.
When you are in drive, and stoped at a light, you can feel the rpm's drop, like it is missing, but you can almost time it. And if it is at night, you can see the lights dim at the same time the miss happens. But the engine is not coding, so people are having a hard time figuring out what is wrong. We had the injectors cleaned, a tune up done, to no avail. I think I am going to have to get the computer flashed. This also affects the car coming off the line. Yesturday I was stoped, and the miss was so bad, I thought the car was going to die.
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Joined: Apr 2004
Posts: 459
From: East Greenbush, NY
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Joined: May 2003
Posts: 2,275
Re: Weird problem
When ever you have a condition where the idle is low and the system voltage goes low, it's hard to determine if the idle is causing the voltage to go low or is it the voltage going low and causing the engine to idle erratic. Usually if the voltage going low caused a poor idle, it would start setting codes. So my first inclination is the idle is going low first, then the voltage drops from low rpms. If the vehicle has any aftermarket electrical items such as a big amp, they should be suspect first. Try removing them from the cars electrical system before doing anything else.
IMO you should check 3 things. The EGR, the IAC and the alternator output for bad diodes. You can check the EGR simply by disconnecting the vac line to it and plugging it. Take it for a ride. The IAC will have to be removed and cleaned or at least checked to be sure the pintle is not contaminated. Both of these 2 items can cause erratic idle behavior.
The last item, the alternator can be checked without even getting your hands dirty and is easy. Start the car and check the voltage across the battery with a meter set to DC voltage. It should read about 14 volts DC. Now change the meter to AC voltage and put one lead on the negative terminal of the battery and the other lead on the output post of the alternator. It should not read above 0.03VAC. If it does read above 0.03VAC, the diodes in the rectifier are defective and allowing AC on the DC supply through out the vehicle.
IMO you should check 3 things. The EGR, the IAC and the alternator output for bad diodes. You can check the EGR simply by disconnecting the vac line to it and plugging it. Take it for a ride. The IAC will have to be removed and cleaned or at least checked to be sure the pintle is not contaminated. Both of these 2 items can cause erratic idle behavior.
The last item, the alternator can be checked without even getting your hands dirty and is easy. Start the car and check the voltage across the battery with a meter set to DC voltage. It should read about 14 volts DC. Now change the meter to AC voltage and put one lead on the negative terminal of the battery and the other lead on the output post of the alternator. It should not read above 0.03VAC. If it does read above 0.03VAC, the diodes in the rectifier are defective and allowing AC on the DC supply through out the vehicle.
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