Up and down idle rich

Tiny
GOODMANJ
  • MEMBER
  • 1992 CHEVROLET SILVERADO
  • 7.4L
  • V8
  • 2WD
  • MANUAL
  • 230,000 MILES
I have been trying to figure this out but it has me stumped. When you start the truck it idles good for 2 seconds and then starts surging. 300 RPMs to 1,500. When you try to accelerate the surging is still there. It won'the get over 2,500 RPMs. If you unplug the map sensor, it idles high but smooth. When it's surging you can watch the injectors flutter when it idles down. I am embarrassed to say but this is what has been replaced:fuel pump and filter, MAP, TPS, coil, distributor checked EGR valve and solenoid and throttle body base gasket. Checked for vacuum leaks with brake cleaner, propane and finally smoke machine, nothing.
Saturday, May 8th, 2021 AT 7:25 PM

15 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,068 POSTS
Hi,

What you described sounds related to a faulty idle air control valve (IAC). All engine idle speeds are controlled by the control module through the Idle Air Control (IAC) valve mounted on the throttle body. Oftentimes, looping (up and down idle) is a symptom.

What I suggest is this: Remove the IAC and check for carbon or dirt where it enters the throttle body. Also, with it removed, have a helper cycle the key from off to on. You should see movement (slight) in the pintle.

Here are a couple of links you may find helpful:

https://www.2carpros.com/articles/idle-air-control-valve-service

https://www.2carpros.com/articles/how-to-replace-an-idle-speed-control-motor-iac

If you look at the attachments below, the first pic shows the location of the IAC. The remaining pictures are the directions for replacement specific to your vehicle.

One last thought. If you haven't checked for diagnostic trouble codes, I recommend it. The vehicle has an OBD1 system. You don't need a scan tool to retrieve codes. All that is needed is a short jumper wire or even a paper clip. Follow this link:

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

I hope something here helps. Let me know what you find or if you have other questions,

Take care,

Joe
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Saturday, May 8th, 2021 AT 10:30 PM
Tiny
GOODMANJ
  • MEMBER
  • 9 POSTS
Okay, I forgot to list that the ECT, IAC and fuel pump relay have been replaced. There aren't any codes showing up except 12. You can hear the sucking sound in the throttle body like the IAC is open.
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Sunday, May 9th, 2021 AT 5:05 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,068 POSTS
If you unplug the electric to the MAP, pull the vacuum hose off (engine running). Is there a good strong vacuum?

I have a feeling it is tied into the map circuit. I attached the diagnostics below for testing the MAP circuit. Let me know if you have already done this.

Joe
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Sunday, May 9th, 2021 AT 8:31 PM
Tiny
GOODMANJ
  • MEMBER
  • 9 POSTS
Okay, my vacuum pump handle is broken so I couldn't do that test. But here is what I have:
These are on the Map - Purple wire, key on not running -12.05v (checking ground) on the white wire 5.07v - on the green wire 1.84 v. I started the truck and checked voltage on green wire. Keep in mind the idle is surging. It was between 1.3 v and.284. I unplugged the vacuum line from the sensor and put a vacuum gauge on it. It was idling sooth at about 1400 rpm and 23hg of vacuum.
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Monday, May 10th, 2021 AT 7:34 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,068 POSTS
I have to be honest. The reason it surges is likely because of the voltage variation in the signal from the sensor. Did the 5v reference from the PCM change?

If it didn't, my first suspect is you got another bad MAP sensor. The vacuum is excellent. The only thing we aren't sure of is if the sensor holds a vacuum. Since the sensor signal goes crazy when you plug vacuum to the sensor seems to indicate the sensor.

Let me know.

Joe
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Monday, May 10th, 2021 AT 7:42 PM
Tiny
GOODMANJ
  • MEMBER
  • 9 POSTS
Okay. Just went and checked voltage on the reference wire (white) it was steady at 5.07v running at idle.
I will get another MAP sensor this afternoon and report back.
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Tuesday, May 11th, 2021 AT 5:24 AM
Tiny
GOODMANJ
  • MEMBER
  • 9 POSTS
Okay, installed the new replacement MAP sensor. Problem is still present. Little or no change. I will try to attach a video of my meter connected to the output wire on the sensor. I checked the reference wire and it was still stable at 5.07 V. This new sensor is going from "open lead" to about 2.4 v as it surges.
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Tuesday, May 11th, 2021 AT 4:48 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,068 POSTS
The OL is indicating infinite resistance. Are you sure there isn't a faulty wire or connection?

Joe
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Tuesday, May 11th, 2021 AT 7:56 PM
Tiny
GOODMANJ
  • MEMBER
  • 9 POSTS
I will check and report back. The sure seal plug is looking tired.
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Wednesday, May 12th, 2021 AT 4:23 AM
Tiny
GOODMANJ
  • MEMBER
  • 9 POSTS
Okay, I ohmed the wires out from where they go through the firewall to the computer. Back to the plug at the map. Gave the harness a good rattle, didn't see any variations. I replaced the plug as it was cracked. Started it up and no change.
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Wednesday, May 12th, 2021 AT 4:40 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,068 POSTS
Hi,

Are you certain there are no vacuum leaks? The throttle body gaskets are common on these as well. I suspect that could be causing the spikes in voltage. It's just odd that it goes to OL on the meter.

Joe
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Wednesday, May 12th, 2021 AT 6:27 PM
Tiny
GOODMANJ
  • MEMBER
  • 9 POSTS
Okay, I double checked for vacuum leak and there is none. Just some observations. The IAC is open at startup, it will close once warmed up. The vacuum line for the EGR solenoid has virtually no vacuum until you open the throttle. I can feel the EGR valve open at than point. Timing (with the EST wire unplugged is 4 degrees and the low end of the idle surge and 8-10 when it idles up.
When the vacuum line is unplugged from the map it smooths out the idle but jumps up to 1,500 RPMs and it isn't drivable like that. It will stall and die when you let off the accelerator.
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Thursday, May 13th, 2021 AT 9:30 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,068 POSTS
Hi,

Things sound normal (other than idle). I spoke with a GM tech and the owner of the site regarding this. Both told me that the TB gasket needs to be inspected because a vacuum leak may not show up.

It's a thought. For both of them to say the same thing leads me to believe there is a possible issue.

I attached a pic below of the gasket I'm referring to.

Let me know.

Joe

See pic below.
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Friday, May 14th, 2021 AT 8:24 PM
Tiny
GOODMANJ
  • MEMBER
  • 9 POSTS
I did replace the gasket on the throttle body. No change, I'm going to do the intake gaskets today. I will report back. A vacuum leak makes sense of everything we are seeing.
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Saturday, May 15th, 2021 AT 4:50 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 110,068 POSTS
Hi,

That sounds like a plan. One other thing I just thought of. Have you checked the gasket at the base of the fuel metering body? If that is leaking, it could cause a lean mixture as well.

See pic below.

Joe
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Saturday, May 15th, 2021 AT 7:47 PM

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