High idle in park and neutral

BENOMI84

MEMBER
1988 GMC C1500
5.0L
V8
2WD
AUTOMATIC
235,000 MILES

Just purchased the truck listed above with a 305, engine idles high in park and neutral. Normal idle when in drive and reverse. I have not found any vacuum leaks, PCV valve was making a rattling sound, so I replaced that, no change. New cap, rotor, and plug wires because the old ones were old and brittle, no change. I know there are many reasons why an engine can idle high, but I am hoping that you can help me to narrow it down. I have limited diagnostic equipment, there was a DTC 44 that popped up but then shut off. I am thinking I have a vacuum leak, but why would it idle high in park and idle normal when in gear?

Thursday, December 9th, 2021 AT 4:52 AM

3 Replies

JACOBANDNICKOLAS

MECHANIC
110,119 POSTS

Hi,

Since the vehicle is in gear, it may be lowering the idle just due to load. How high does it idle in park?

As far as possibilities, you were on the right track when checking for vacuum leaks. Since it is running lean, a leak can cause it. I'm not sure how it was done, but here is a link you may find helpful when testing:

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

Have you checked fuel pressure? Low pressure can also cause a lean fuel mixture.

An oxygen sensor can be the cause as well. Since you don't have a scan tool (OBD1), you would have to physically check voltage at the sensor with a multimeter.

Let me know what all has been done. Also, let me know if the technique listed in the vacuum link was how things were checked and if the vehicle has throttle body injection or if it is carbureted.

Take care,

Joe

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Thursday, December 9th, 2021 AT 5:05 PM

BENOMI84

MEMBER
2 POSTS

I was unable to find any additional vacuum leaks but when I tested the O2 sensor I kept getting wild readings fluctuating from 0.01 volts to 5 volts. I pulled the sensor, and it was all broken apart inside. It looked like something had hit it from underneath. New sensor, truck idles great. Thanks for the help!

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Sunday, December 19th, 2021 AT 11:26 AM

JACOBANDNICKOLAS

MECHANIC
110,119 POSTS

Hi,

Thanks for the update. I'm glad to hear you got it fixed. Please feel free to come back anytime in the future if you have questions.

Take care and I hope you have a great Christmas.

Joe

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Sunday, December 19th, 2021 AT 7:29 PM