Engine idle surge - Pontiac G8 Forum: G8 Forums - G8Board.com
 
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post #1 of 10 (permalink) Old 04-05-2016, 08:38 PM Thread Starter
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Engine idle surge

I am looking for help in trying to diagnose a idle surge. The chains were replaced because of a P0008. When I got the car back, I replaced the belt because it was slipping. The car has been running great for 3 weeks. Now I have a idle surge of about 200 rpms. When I start the car, it runs fine. I can do short hops with no issues. If I do a long drive such as going to work which is about 45 minutes. The rpms will be start to be erratic. If I rev the engine up to 1000rpms, it stays steady. What is also interesting is that if the engine is running normal at idle. The rpms will surge if I turn on the air conditioning. When I drive the car, the car drives with no problems. It is so far just an idle issue. Any suggestions will be appreciated. I attached a video of the gauge in engineering mode.

[YOUTUBE]
[/YOUTUBE]

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post #2 of 10 (permalink) Old 04-06-2016, 06:36 AM
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I had a similar issue recently. I pulled the throttle body and it had a nice layer of carbon build up on it. I cleaned it up nice and pulled the negative cable on the battery for about 20 mins and it fixed the problem. Keep in mind that for the first 500 miles or so your ecm will still be relearning everything...

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post #3 of 10 (permalink) Old 04-06-2016, 07:33 PM Thread Starter
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So remove the throttle body and give it a good cleaning. Disconnect negative terminal to reset ecm. I'll give it a try. Someone else mentioned cleaning the tb. Thanks for info.

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post #4 of 10 (permalink) Old 04-06-2016, 08:03 PM
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Make sure it's completely dry before putting it back on. Just in case you didn't know
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post #5 of 10 (permalink) Old 04-07-2016, 11:22 AM
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Did that work, CCG8 brethren?

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post #6 of 10 (permalink) Old 04-07-2016, 11:51 AM
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The ECM has two primary methods of adjusting for idle speed - airflow (throttle) and timing. However, it has limitations for how much it can adjust, and, if one of them is just too far out, it can oscillate like that. The first two things that I would look for are a vacuum leak or a dirty MAF sensor. Seeing as you have a (presumably oiled) rotofab intake, it wouldn't hurt to give your MAF sensor a good clean. The sensors aren't very accurate at low airflows (hence the combination of MAF and speed density lookups for fueling at lower RPM), and a buildup of filter oil or anything else can have a significant effect on idle, while it gets better while under more throttle. I cleaned mine last night and it made a huge difference in fueling and has actually turned me away from oiled filters. You should also check to see if you have a vacuum leak that might not be big enough to trip a check engine light, but could affect idle. However, the most impactful thing that you could do is scan it with an OBD2 scanner and check to see what your fuel trims and O2 switching look like. Large positive fuel trims mean that it's having to put in a lot of extra fuel, either due to unmetered air (vacuum leak) or a contaminated MAF sensor that is reading less airflow than is actually flowing through. If the fuel trims differ greatly between the two banks, then the issue is something further downstream, possibly an O2 sensor. There are many further steps to track down the issue (failing MAP sensor, clogged catalytic converter, misfires that aren't being picked up by the misfire monitor, etc.), but the MAF sensor and vacuum leaks are what I'd personally look into first to eliminate them as the culprits.

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post #7 of 10 (permalink) Old 04-07-2016, 08:45 PM Thread Starter
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Quote:
Originally Posted by jaydubb View Post
Did that work, CCG8 brethren?
Its been raining here, so hope to get to it tomorrow.

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post #8 of 10 (permalink) Old 05-07-2016, 01:07 PM Thread Starter
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The idle issue unfortunately is tied to a bigger problem. I have a bad reluctor wheel on the crankshaft. So I was given the option of a new crank or replace the engine. I'm getting another engine because the engine will have a good warranty on the whole engine compared to a 12 month warranty on the crank and labor.

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Appearance: 20% Tint, 6K HIDs 35W Lows, 50W LED Fogs,Debadged,GXP Diffuser, Blacked Out Grilles and Trim, VG Sharkfin, Rear Roof Spoiler
Performance:
Roto-Fab CAI, GT Brakes,

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post #9 of 10 (permalink) Old 05-07-2016, 01:17 PM
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Just out of curiosity, what engine are you getting and how much?

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post #10 of 10 (permalink) Old 05-08-2016, 08:24 PM Thread Starter
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Its a LY7 engine coming out of GM. I have a buddy that is getting me employee discounts on the engine and labor. Total is going to be 6K.

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Performance:
Roto-Fab CAI, GT Brakes,

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