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techspert, former shop owner
Category: Chevy
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Experience:  Former ASE cert tech,shop owner of a diagnostic shop
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I have a 1995 chevy 350 TBI. Its gas mileage dropped from ...

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I have a 1995 chevy 350 TBI. Its gas mileage dropped from 18 to 12 mpg, and it dropped suddenly. I replaced plugs, wires, oxy sensor, etc. Took it to a chevy garage, they said it was running too cold for the computer to go into closed loop. Replaced thermostat and BOTH temp sensors, still bad gas mileage. Runs fine, no error codes. cleaned injectors. Changed gas pressure regulator. Local auto zone says thay the too cold reason was BS, because it is an OBD1 computer.
Submitted: 6 years ago.
Category: Chevy
Expert:  techspert replied 6 years ago.
Hi! Sounds like you have a good mechanical aptitude for vehicles. Gas mileage is a very touchy things these days. So many factors go into it. Driving habits, weight of payload, what brand and type of gas etc. 1994 was the transition year for OBD 2. IF you don't have it, your production date is probably late 1994. As far as the engine not warming up and staying in open loop, that is absouletely true!!! Autozone is incorrect. The ECM looks for the temp signal as its warming. The colder the engine, the more fuel you need. If you never get to closed loop, you will use more fuel. That's also why manufacturers started using heated O2 sensors in an attempt to speed up the computers ability to control fuel and also emissions. Now with all that said, since you are having no adverse drivability issues, no SES light, try a different brand of gas. Preferably name brand. Also if you are just adding injector clear to the tank-STOP! It's a waste of money, and name brand gasoline already has the additives in them. You also have a G30 van, they are not very fuel efficient, you probably are also fully loaded. I don't think you have a problem per say, but try a different brand of fuel and make sure you are testing your MPG tank full to tank full. Some people think they can get a true MPG 1/2 tank to 1/2 tank-NOT SO!
Customer: replied 6 years ago.
I WAS getting 18mpg for several months, so it wasnt just a fluke. I use several types (brands) of gas, and still use the same brands, so thats not it. Is there any way to check if the ecm is going into closed loop? I am an electrical engineer, so not afriad of using a voltmeter. BTW, also replaced the egr valve.
Expert:  techspert replied 6 years ago.

The best way to check if your going into closed loop is to have a scanner. You can then monitor your coolant temp and your O2 voltage. However if you don't have one, use your DVOM on MILLIVOLTS and tap into the O2 sensor signal line. It's located in the exhaust manifold or Y-pipe. There maybe 2, either way each one would be UPSTREAM the cat converter. The signal wire should produce 0-1.0 v. Select your MV scale and monitor this with the engine running. Should flucuate above and below 450 mv about 1 1/2 times per second. You may need to come off idle slightly as the O2 may cool and not give you an accurate reading. Also make sure your grounds are good, tight, and clean. Again using your volt meter disconnect the negative battery cable and use your DVOM in parellel. You should have LESS than .50V, with everything off. One more thing, there maybe a flash programming update for the ECM. Call your local Chevy dealer and give them the VIN. They will be able to tell you if there are any reprogramming updates.

Customer: replied 6 years ago.
Sorry, it has been raining here, so could not make tests till just now.
With the engine cold, I got 375mv, no fluctuation while revving engine.
I let the engine warm 30 minutes, and got a SLIGHTLY higher reading..   408mv. Did not fluctuate at all, even by revving engine.
Shut engine off, disconnected negative batt terminal, and had almost 0 volts (well less than .5 volt).
There is only 1 sensor.
It looks to me, an untrained mechanic, that there is no change, so probably NOT going into closed loop?   If you agree, any ideas why it doesnt (and assuming the chevy garage was correct, why the ecm doesnt think the engine is hot enough)?
Expert:  techspert replied 6 years ago.

OK, things are starting to make sense. The fact there is no fluctuation COLD doesn't concern me as this is a normal thing untill engine temp is at normal operating temp. However after 30 minutes, there should be a big fluctuation above and below 450 mv. You are correct in saying it's NOT going into closed loop. Now lets see if it's a fuel control issue or a temp issue or something else. With your DVOM connected to the O2 sensor and engine running, pull off a vacuum line. There should be a change in voltage almost instantly. Voltage should rise to aprox 650 or higher. Then reconnect it, it should drop back down. Since it's TBI, spray some carb cleaner down the TBI unit while running and the DVOM should change once again. If these things are happening, the O2 is ABLE to work and respond to changes and therefore should be able to control air fuel ratio. If this doesn't happen, make sure the mating surface where the O2 threads into the exhaust is clean. This is a ground. How many wires is the O2? You can also connect the DVOM on VOLT scale and read your voltage at the coolant temp sensor. Voltage will be high on cold engine-aprox 4-5 volts and lower as engine warms. Let me know

 

Customer: replied 6 years ago.
Well, that was interesting. First, my can of carb cleaner was empty, I will pick some up today and re-run test with that.
Your info on the coolant sensor (which has been replaced) was interesting. You said it should start COLD at 5 volts and lower as it warms up. I got just the opposite. It started cold at 2.5 volts (read 2.7 kohm) and as engine heated up, climbed to 5 volts (260 ohms).
O2 sesor gave the same readings as before....cold 375 mv hot 410 mv.   This climbed to 420mv when vacuum line was pulled and returned to 410 mv when re-connected.
O2 sensor has 1 wire.
Are you sure about the coolant temp sensor reading? Maybe an Autozone sensor has the wrong temp coefficient?
Customer: replied 6 years ago.
Checked with carb cleaner.....same results.

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Former ASE cert tech,shop owner of a diagnostic shop