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1994 Buick Regal High idle

Tiny
JINBODY
  • MEMBER
  • 1994 BUICK REGAL
Engine Performance problem
1994 Buick Regal 6 cyl Front Wheel Drive Automatic 100K+ miles

My 3.8L has been fully rebuilt. It has performed flawlessly for 25000 miles. Last trip it began to idle high intermitantly. It does it cold or hot. The PCM throws no codes even while it is hapenning. A restart sends the idle speed and IAC counts back to normal. The PCM has been replaced (again) and the IAC has been replaced (again). The interconnecting wiring and circuitry verified from both ends of the circuit. There are no vacuum leaks. Any significant load at idle( ie:shift to drive, a/c request) ratchets up the IAC count and the idle speed, but removal of the load does not drop the count or RPM. Every subsequent load application raises the indicated count and rpm (shown on the scan tool )until it gets too high to safely shift into gear. A restart of the engine brings the count and rpm down to normal. This event doesn't always happen, but is getting noticably worse.I posed this problem to your online "competition" but got only generic answers that didn't help. Any idea?
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Friday, September 25th, 2009 AT 6:19 PM

15 Replies

Tiny
MERLIN2021
  • MECHANIC
  • 17,250 POSTS
Have you checked the ECT? Here's the chart.


https://www.2carpros.com/forum/automotive_pictures/62217_ECTGM_1.jpg

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Saturday, September 26th, 2009 AT 9:32 AM
Tiny
JINBODY
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I had not directly checked the ECT sensor. A check at 2 temps shows that it tracks the table, but not exactly.
The one sensor that doesn't test with the GM book is the MAF. The book shows a DC reading over 6 V or under 4V demonstrates defective device. I understand the sensor doesn't operate on DC, but it consistantly measures 2.6VDC.
The scan tool values seem to track with the GM "normal" range. Attached is a list of scanned values.


https://www.2carpros.com/forum/automotive_pictures/413835_38L_2.jpg


Jon
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+1
Saturday, September 26th, 2009 AT 4:20 PM
Tiny
MERLIN2021
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Engin e size? 3.1 or 3.8? This works for either anyway...
Mass Airflow (MAF) Sensor A malfunction in the MAF sensor circuit will set a related diagnostic trouble code. For testing procedures, see Code P0101 in the TESTS W/CODES - 3.8L article in this section. If driveability problems exist, MAF sensor failure is suspected and no Code P0101 is present, disconnect MAF sensor connector. If driveability improves, replace MAF sensor.
Here's the flow chart I have.


https://www.2carpros.com/forum/automotive_pictures/62217_maf_1.jpg



https://www.2carpros.com/forum/automotive_pictures/62217_mafa_1.jpg



https://www.2carpros.com/forum/automotive_pictures/62217_mafb_1.jpg



https://www.2carpros.com/forum/automotive_pictures/62217_mafc_1.jpg


Dont forget to check for a bad MAP, unplug it and see if conditions improve.


https://www.2carpros.com/forum/automotive_pictures/62217_map_1.jpg


CODE P0101, MAF SENSOR CIRCUIT The Mass Air Flow (MAF) sensor measures the flow of air which passes through it in a given time. PCM uses this information to monitor the operating condition of the engine for fuel delivery calculations. A large quantity of air movement indicates acceleration, while a small quantity indicates deceleration or idle. MAF sensor produces a frequency signal, which cannot be easily measured. Diagnose sensor using procedures on this chart. Code P0101 will set when the following conditions are present. Ã Â Â Engine is running. Ã Â Â No MAF signal for over 4 seconds. With Code P0101 set, PCM will use a default value for airflow based on throttle position, IAC motor position and engine speed. PCM will illuminate Malfunction Indicator Light (MIL). 1. This step checks to see if PCM recognizes a problem. 2. Voltage reading at sensor harness terminal "A" of less than 4 volts or greater than 6 volts indicates a fault in circuit No. 492 (Yellow wire), or poor connection. 3. Verifies that both ignition feed voltage and a good ground circuit are available. Diagnostic Aids An intermittent may be caused by a poor connection, mis-routed harness, rubbed through wire insulation or a wire broken inside the insulation. Check for: Ã Â Â Poor Connection At PCM pin: Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection. Ã Â Â Mis-Routed Harness: Inspect MAF sensor harness to ensure it is not too close to high voltage wires, such as spark plug wires. Ã Â Â Damaged Harness: Inspect harness for damage. If harness appears okay, observe Tech-1 while moving related connectors and wiring harness. A change in display would indicate intermittent fault location. Ã Â Â Plugged Air Intake Filter: A wide-open throttle acceleration from a stop should cause MAF reading on Tech-1 to range from approximately 4-7 grams per second at idle to 100 or greater at time of 1-2 shift. If not, check for restriction. NOTE: Test numbers refer to numbers on diagnostic chart.
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Sunday, September 27th, 2009 AT 7:32 AM
Tiny
JINBODY
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We are reading from the same manual. Following the troubleshooting flow chart is why I did a second replacement of the PCM. Had exactly the same reading with the new one. The harness checks fine, the routing is factory. Unplugging the sensor puts the idle at approx 2500 in N. IAC count is way high and the code for sensor low sets. Been trying not to just shotgun parts. Stores won't credit back electronic parts. Direction?
Jon
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Sunday, September 27th, 2009 AT 12:19 PM
Tiny
MERLIN2021
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MAP sensor been checked? Is there vac at the line? Unplug it and see if that changes anything. Electrical! Check MAP for vac leaks too.
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Tuesday, September 29th, 2009 AT 12:59 PM
Tiny
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No MAP sensor in '94 3.8L. Vacuum sits at 20-22 inches and drops with added throttle. Looks normal
Jon
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Tuesday, September 29th, 2009 AT 3:29 PM
Tiny
MERLIN2021
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Only other thing I can think of is a shaky TPS, these dont always throw a code either, if you need testing info let me know, like you said, we are looking at the same manual!
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Tuesday, September 29th, 2009 AT 3:38 PM
Tiny
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The TPS reads steady and responds like the book says. Only thing I can't noodle out is why the TPS reading(on the scantool) does come up off minimum(usually 0 to.39) when the idle goes up during this failure. If the throttle plate is hard against the stop, where does that increased reading come from? I think I'll unhook the cruise hard cable to make sure it isn't causing a problem.
Jon
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Tuesday, September 29th, 2009 AT 3:45 PM
Tiny
MERLIN2021
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Throttle Position (TP) Sensor Install jumper wires to enable connection of a DVOM in parallel between TP sensor harness connectors. Connect DVOM positive lead to Dark Blue TP sensor signal wire terminal. Connect negative lead to Black, Black/Orange or Purple sensor ground wire terminal. See Fig. 3. Turn ignition on, with engine off. Signal voltage should gradually change from less than one volt at closed throttle to about 5.0 volts at wide open throttle position. If reading is not as specified, adjust or replace TP sensor. See ADJUSTMENTS article in this section. A malfunction in the TP sensor circuit should set a related diagnostic trouble code. For further information, refer to the TESTS W/CODES - 3.8L article in this section. You can also take readings in OHM, watch meter, if it spikes and jerks, replace the TPS.
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Tuesday, September 29th, 2009 AT 3:53 PM
Tiny
JINBODY
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I had previously made those measurements and they followed the book values, with no spikes or jumps. I just noticed something when I disconnected the cruise cable. With the idle "ratcheted" up to about 2500 and the TPS reading near 2.0, a very hard pull on the throttle arm brings the idle back to normal. Is it possible that there is pivot wear or something causing? But if it's mechanical, why would a restart of the engine also bring the idle back to normal? There is no electol/mechanical throttle plate device here, is there?
Jon
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Tuesday, September 29th, 2009 AT 4:14 PM
Tiny
MERLIN2021
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The shaft on the throttle plate may be worn, or bent. Does happen, may need new throttle body.
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Tuesday, September 29th, 2009 AT 4:18 PM
Tiny
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I guess it's time to take apart the throttle body. How would a restart change a bound plate or shaft? Air pressure maybe?
Jon
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Tuesday, September 29th, 2009 AT 4:24 PM
Tiny
MERLIN2021
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As it turns off, vacuum is a bit stronger. ANY play in the throttle plate means the shaft is worn, so idle cant be controlled. Or maybe the air horn part of it has a groove worn into it?
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Wednesday, September 30th, 2009 AT 1:02 PM
Tiny
JINBODY
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Disasembled throttle body. No significant slop or stickiness found,however.. turns out you had the problem pegged a couple posts back. While I had the thing apart, I checked the TPS sensor using 5 volts and a scope. At the very bottom of the travel of the arm, the pot had a tiny dead spot. Must have been enough to confuse the PCM but not stable enough to throw a code. I still can't see it with the Fluke meter on volts or ohms. I replaced the TPS and the problem is gone! Nice shootin' Merlin.
Thanks!


https://www.2carpros.com/forum/automotive_pictures/413835_164_6415_1.jpg


That 3.8 Buick drivetrain is in a salveged french built motorhome.
Jon
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Wednesday, September 30th, 2009 AT 3:14 PM
Tiny
MERLIN2021
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Nice rig! I might get one when I retire in five years!
Good work on the TPS, some would have given up on it by now!
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Saturday, October 3rd, 2009 AT 7:32 AM

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