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Idle cycles up and down and engine shuts off

Tiny
SSSUR32
  • MEMBER
  • 1989 GMC SUBURBAN
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 130,000 MILES
Engine starts fine. Idle cycles up and down in approximately 3 second interval. Sometimes the engine shuts off because the idle goes too low.

I have changed the idle air control valve, the throttle position sensor and the mass air sensor, and still have the problem. Also, if I press on the gas pedal the engine hesitates and sometimes shuts off.

I have tried the ECM reset procedure by disconnecting the battery for 5 minutes.

If I disconnect the connector to the mass air sensor, the engine idles fine, and I can press the gas pedal and the engine accelerates and acts normal. It seems as the problem completely vanishes.

I haven't driven the truck yet (with the MAS disconnected), as I'm concerned it might stop working and I'll be stuck out on the road.

Any help/ideas would be greatly appreciated. Thank you.
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Saturday, December 5th, 2020 AT 9:49 AM

8 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,700 POSTS
Hi,

When you disconnect the MAF sensor, it forces the computer into limp mode. That is a preset calibration to keep the engine running in the event of sensor failure.

Now, you already replaced the sensor, so we need to check for other possible causes. First, have you confirmed there are no engine vacuum leaks? Anything that would allow unmetered air to enter the combustion chamber will cause issues for the MAF.

Next, if you can, check to see if there are any diagnostic trouble codes. This vehicle has an OBD1 system which can be read without a scan tool. All you will need is a short jumper wire or even a paper clip. Do this and see if there are stored codes that may help.

Here is a link that shows how it is done:

https://www.2carpros.com/articles/buick-cadillac-chevy-gmc-oldsmobile-pontiac-gm-1983-1995-obd1-code-definitions-and-retrieval-method

Again, check to make sure there are no engine vacuum leaks. Here is a link that shows how that is done. Also, these engines are known for intake manifold leaks, so make sure to check there.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Let me know what you find or if you have other questions. Also, if it is possible for you to record how it is running and upload it for me to hear, it may help.

Take care,
Joe
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Saturday, December 5th, 2020 AT 6:50 PM
Tiny
SSSUR32
  • MEMBER
  • 5 POSTS
Thank you for your response.

I mistakenly called the third part that was changed, a mass air flow sensor. I looked it up again, and it was a Manifold Absolute Pressure Sensor. It has an electrical connector and what looks like a small diameter tube that comes from the engine. This is the sensor that when I disconnect it, the engine goes into limp mode. Sorry about the error.

Tried shorting the A and B terminals on the error code connector, then turning the key to the "on" position, but don't see any dash light blinking. Maybe the check engine light is burnt out.

Inspected all of the vacuum hoses that I could see. All seem okay, but will spend more time checking them.

Attached is an mp4 file so the idle problem can be listened to.
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Sunday, December 6th, 2020 AT 11:23 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,700 POSTS
Hi,

For some reason, the recording didn't attach. Could you try that again?

Thanks,
Joe
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Sunday, December 6th, 2020 AT 10:02 PM
Tiny
SSSUR32
  • MEMBER
  • 5 POSTS
Tried uploading the mp4 file. The file uploads fine, and I can see a thumb nail image.
When I click 'Reply', the browser goes to:
https://www.2carpros.com/content/questions/view/process.php
and I see a blank page except for the word:
needlog

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Tuesday, December 8th, 2020 AT 4:03 AM
Tiny
SSSUR32
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  • 5 POSTS
Put the video on youtube.com here:
https://www.youtube.com/watch?v=KhiM7YLJM6o
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Tuesday, December 8th, 2020 AT 5:08 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,700 POSTS
Hi,

I heard it. Okay, because a few different things can be causing it, we need to try to determine why the check engine light isn't coming on.

Does the light turn on when you first turn the key on? There is a fuse for instrument lights that could be blown. See the attached pic. Check that fuse and if it is good, confirm power to it and from it.

Next, when you replaced the IAC, did you check inside where it mounts for possible carbon or something that would prevent the IAC from working properly?

Let me know.

Joe
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Tuesday, December 8th, 2020 AT 1:13 PM
Tiny
SSSUR32
  • MEMBER
  • 5 POSTS
Pulled the fuse and tested it for continuity, okay.
With the fuse out, none of the instrument lights come on.
The 4WD light works. Right next to the engine check light.
Pulled off the instrument panel cover and some of the instruments.
(Broke the speedometer needle off) :(
The check engine bulb is under the speedometer.

Tried to remove the engine check light bulb and it started blinking after twisting it.
Codes are: 12, 12, 12, 34, 34, 34.
Looked up the code (34) which points to MAP sensor.

I'm confused as to what the part I replaced is called.
The sensor I replaced has a single tube connected to it.
This is the one that has been replaced.

There is another sensor that has two tubes connected to it.

Thanks for hanging in there while I muddle through this!
I'll attach a picture showing the parts that I've replaced (in red), and the other
sensor.
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Wednesday, December 9th, 2020 AT 10:29 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,700 POSTS
Hi,

It certainly related to the MAP sensor. Here are the directions for diagnosing this code. Take a look through it and let me know if it helps.

___________________________

1989 Chevy Truck Suburban 1/2 Ton 4WD V8-350 5.7L
Powertrain Controls - ECM/PCM
Vehicle ALL Diagnostic Trouble Codes ( DTC ) Testing and Inspection Manufacturer Code Charts DTC 34 Powertrain Controls - ECM/PCM
POWERTRAIN CONTROLS - ECM/PCM
Code 34 - MAP Sensor Circuit Signal Voltage Low (High Vacuum)

see pic 1

Wiring Diagram For Code 34 - MAP Sensor Circuit Signal Voltage Low (High Vacuum)

see pic 2

CODE 34 - MANIFOLD ABSOLUTE PRESSURE SENSOR CIRCUIT (MAP) SIGNAL VOLTAGE LOW - HIGH VACUUM

Circuit Description:

The manifold absolute pressure (MAP) sensor responds to changes in manifold pressure (vacuum). The ECM receives this information as a signal voltage that will vary from about 1 to 1.5 volts at idle to 4-4.5 volts at wide open throttle.

If the MAP sensor fails the ECM will substitute a fixed value and use the throttle position sensor (TPS) to control fuel delivery.

Test Description: Numbers below refer to circled numbers on the diagnostic chart.

1. Code 34 will set when:

When engine is less than 600 rpm.
Manifold pressure reading less than 13 kPa, conditions met for 1 second or
Engine is greater than 600 rpm.
Throttle angle over 20%.
Manifold pressure less than 13 kpa conditions met for 1 second.

2. This tests to see if the sensor is at fault for the low voltage or if there is a ECM or wiring problem.

3. This simulates a high signal voltage to check of an open in CKT 432. If the test light is bright during this test, CKT 432 is probable shorted to ground. If "Scan" reads over 4 volts at this lest CKT 416 can be checked by measuring the voltage at terminal "C". (Should be 5 volts)

Diagnostic Aids:

An intermittent open in CKTs 416 will result in a Code 34.

See Diagnosis By Symptoms - No Trouble Codes Stored.

The "Altitude to Voltage" scale at the right may be used to test the MAP sensor at a specific altitude level to evaluate the possibility of a "slewed" (mis-scaled) sensor. A "slewed" sensor could result in poor driveability complaints.
_____________________

Pic 3 shows what the map looks like and pic 4 shows the location.

Let me know if this helps.

Joe
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Wednesday, December 9th, 2020 AT 7:59 PM

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