TechTips
TechTips is a monthly feature in AutoInc. Each month, specialists who field calls for autoline Telediagnosis, a technical hot line service, share some of the most common vehicle problems and their solutions.
It Had A Mind Of Its Own
I was recently working with one of our subscribers and his customer's complaint was that his 1992 Dodge Stealth with a 3.0 liter dual overhead cam V6 engine had a mind of its own. It seems that the engine would idle very erratically; the rpm would fluctuate between 900 to 2000 rpm. He tested everything to no avail. The Throttle Position Sensor (TPS) was rock steady at 0.5v. The Air Flow Sensor was reading what the engine was doing and all of the other sensors were correct on their readings.After extensive testing, he discovered the fuel injectors were being turned on and off, causing the erratic idle situation. Since the sensors were reading what they were supposed to and there was no input causing the problem, he felt it had to be a problem with the Powertrain Control Module (PCM).
Replacing the PCM did not solve the problem. That's when I became involved. After confirming all of the tests that had already been performed, I had the technician block off the air bypass for the Automatic Idle Speed Motor. Lo and behold the surging went away. Now the fuel injector pulse was steady and the engine was running smooth, although it was idling too slow even for base idle. I had the tech remove and clean the throttle body and go through the base idle procedure.
Now the base idle was restored, but the surging would reappear if the air bypass was opened. I instructed the technician to ohm-check the Automatic Idle Speed Motor and the motor was bad. Replacing the motor cured the problem and now the car was ready to be delivered to the customer.
But the question still existed why was the computer shutting off the injectors? I did some research and came up with nothing in print, but after discussing the problem with some of our other specialists (another capability at a hot line), I discovered that the PCM was shutting off the injectors in an effort to slow down the idle. Did this car have a mind of its own? Or did our Asian friends leave a little out of the service manual? By Tom Vadnais, autoline Chrysler, Mitsubishi and Hyundai specialist (20 years experience)
So Where's The Cool Air?
This is a mystery that can be described as involving big oil and too much hot air. The problem: After replacing a bad air conditioning compressor for making noise, you didn't get cool air from the car's air conditioning (A/C) ducts. You evacuated the A/C system, blew out the A/C lines, changed the orifice tube and receiver/dryer, added an in-line filter and installed the correct amount of refrigerant. Still no cold air. Your pressures look pretty good high side at 150 psi to 200 psi; low side at about 35 psi to 40 psi.What in the world is going on? Must be a defective new compressor, right? So you put on another new compressor, evacuate and recharge again. Now it's no better maybe even a little warmer. Could it be another bad compressor? Not likely. Pressures are a little high, so you take out some refrigerant. Still no improvement.
The cause: Right at the very beginning, you didn't check the oil amount in the new compressor compared to the old one. New compressors come shipped with enough oil for the whole system. So when you put it on, you probably had three to six ounces of excess oil. When replacing the second compressor, it adds that much more oil. Too much oil in the system will not allow it to cool properly.
The correction: You need to remove as much oil as possible, which includes blowing out the lines and removing the receiver/dryer and dumping the oil from it. It may even be necessary to remove the condenser and dump the oil from it as well. The main thing to remember is to measure the oil coming out of the old compressor. Dump the oil from the new compressor and then add back only as much as you removed from the old compressor. An extra ounce usually isn't a bad idea to allow for loss of oil over time. By Rocky Schubel, autoline GM specialist (19 years experience)
©1996, Automotive Information Systems.
Autoline Telediagnosis with its 29 phone specialists handles over 12,000 phone calls per month from shops all over North America. As an ASA member, you're already signed up to use the hotline service at a discounted rate. Call (800) 288-6210, Monday - Friday, 7 a.m. to 7 p.m., Central Standard Time.
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AutoInc. Magazine ®, Vol. XLIV No. 3, March 1996