My dad has a 1992 GMC Vandura 1/2 ton work van with a throttle body fuel injected 4.3 liter V6. Recently he just replaced the engine. Before he replaced the engine, when the engine was cold or when it was sitting for awhile, when you start it, the rpms would pulsate, from like 500 -1000 rpms, up and down for like 5 mins unless you stomped on it or drove it. Since replacing the engine, it still does this. We took the throttle body apart, cleaned it, put a new TPS on it, Idle Air Control Valve on it, and it still has the same problem. The van also has a new oxygen sensor on it, but it has no catalytic converter. I have some thoughts of what the problem might be, but would like any input on what you guys think it might be. Thanks
Yep, every imagineable was replaced except the coolant temp sensor and the oil pressure sensor. Van still does it, even with this "brand new engine". You think it not having a cat would cause it to do this? Like I said before, its just when its cold, or when its been sitting anywhere 30 mins all the way up to past an hour.
Did your Vandura have an oxygen sensor located at the outlet of the removed catalytic converter? This appears to be a common situation.
Before reading the following possibly-useless information, keep in mind that I have already been told the following by an extremely-knowledgeable contributor to this newsgroup:
"Hardly possible... I've removed dozens of cats from OBD-1 vehicles without issue."
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"The oxygen sensor reports live information about the engine's air/fuel mixture to the powertrain control module. This information is used primarily to help calculate fuel delivery to the engine, which changes continuously while it is running. If the engine is running lean, the powertrain control module will sense this from the oxygen sensor's signal and increase the air/fuel mixture to the engine. Conversely, just the opposite occurs when the engine begins to run rich. On OBDII-equipped vehicles, the sensors are also used to help determine the efficiency of the catalytic converter. The powertrain control module does this by comparing the signal of the sensor located at the inlet of the catalytic converter with the signal of the sensor located at the outlet of the converter."
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This test can be performed on the sensor installed at the outlet of the Catalytic converter to verify its condition as well. These are looking at Oxygen levels on the outlet side of the Cat, and should remain flatlined if the Cat is working efficiently.
The O2 sensor was replaced before the engine was replaced. So it's in place and also only a couple months old. I don't drive the van, but just listening to it start and run is annoying. My dad doesn't seem to find it entertaining either.
Did your Vandura have an oxygen sensor located at the outlet of the removed catalytic converter? This appears to be a common situation.
Before reading the following possibly-useless information, keep in mind that I have already been told the following by an extremely-knowledgeable contributor to this newsgroup:
"Hardly possible... I've removed dozens of cats from OBD-1 vehicles without issue."
*** ******
"The oxygen sensor reports live information about the engine's air/fuel mixture to the powertrain control module. This information is used primarily to help calculate fuel delivery to the engine, which changes continuously while it is running. If the engine is running lean, the powertrain control module will sense this from the oxygen sensor's signal and increase the air/fuel mixture to the engine. Conversely, just the opposite occurs when the engine begins to run rich. On OBDII-equipped vehicles, the sensors are also used to help determine the efficiency of the catalytic converter. The powertrain control module does this by comparing the signal of the sensor located at the inlet of the catalytic converter with the signal of the sensor located at the outlet of the converter."
formatting link
This test can be performed on the sensor installed at the outlet of the Catalytic converter to verify its condition as well. These are looking at Oxygen levels on the outlet side of the Cat, and should remain flatlined if the Cat is working efficiently.
I was replying to One Shot Scott... as far as his suggestion that the van might have 2 Oxygen sensors. That didn't start untill OBD-II came along in mid-year 1995.
As far as the problems with the idle, the idle air control was my first thought, but someone already suggested that. Only other thing I can think of would be maybe the fuel pump is getting weak? I had a '91 S-10 that used to want to die right after you first cranked it up, unless you gave it some gas it would die out. Once it ran for 20-30 seconds, it would kind of try to die out, but would catch itself right before it died and rev back up. It would then repeat the process untill it was warmed up. Replacing the fuel pump and filter solved that one for me.
Make sure that you are getting correct voltage to the injectors. I had a '95 pickup that had one of the injector wires shorting against the motor and causing an erratic idle.
They are grounded by the ECM, so it's battery voltage
My fathers would surge at idle too. Turned out to be the EGR. The computer can't compensate for some things fast enough. EGR is one of them. Remember when the engine is cold, it's running in open loop, so O2 sensor and other emission stuff isn't having an effect. Leaking EGR, however, does. I'd also look for a vacuum leak somewhere. Squirt carb cleaner around the intake manifold gasket and listen for a difference.
Without the motor running, you should be seeing battery voltage across both terminals of the injector, as the computer end will be open circuit. When the motor is running, the ground side of the terminal will have a square like wave on it, with the off time changing width with demand from the computer. You can't see it without a scope, so this test is pretty much useless unless you know what you are looking for. IOW, start looking for the problem somewhere else. The computer is slow at making corrections to the mixture setting.
What is happening is after the engine starts, the computer looks at TPS, MAP sensor, RPM, Temp, and adjusts the IAC accordingly. But the combination of these things isn't in the range the computer is expecting to see. The engine starts out idling high, because of the initial IAC setting programmed in, and then starts backing down. The engine will start to stumble, which the computer sees through the MAP sensor and RPM, and tries to adjust the IAC back up. Then the engine revs past where it was trying to go, and the cycle repeats. Disconnect the IAC when you start it to prove it to yourself. The mixture is way off, and the computer can't compensate for it.
So, something is out of spec. Look for a vacuum leak first. EGR is a vacuum leak source, as far as this example is concerned. Also look at the intake gasket, TBI base gasket, brake booster hose, vacuum hose to Heater/AC stuff, PCV hose, etc.
Next, look to a leaky or faulty injector. Check the spray pattern, and look for a steady dribble coming out the middle of one or both of them. If you see it, chances are that may be the culprit. The injector should be throwing a nice inverted cone pattern out. A trouble lamp is enough to usually see the pattern well, but sometimes a timing light is useful for this if you have one handy.
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