First post / 1985 surging idle
Thread Starter
4th Gear
Joined: Apr 2023
Posts: 4
Likes: 0
From: Ohio
First post / 1985 surging idle
Hello everyone, first let me say Thank you, some of you have helped me a lot over the last few months getting this 1985 barn find running. I am having a ongoing idle issue surging from 550 to 900 in park ( automatic) along with a hesitation when giving heavy throttle. Since this car was sitting for a few years I have started replacing known problem parts that I could test or see needed replacing- IAC, TPS, EGR, injectors, fuel pressure regulator, O2 sensor, fuel pump and filter, oil and filter. The car is running rough but it's a set in the right direction since it didn't run last month. The car starts up idles high 1000-1200 then drops to 550 and enters closed loop it begins to surge. I did a smoke test thinking it was a vacuum leak but found nothing. I have codes 15, 23, 44 both sensors should be here this weekend but not sure why 44 -lean exhaust is popping up. Fuel on the rail is 42psi with 24LB injectors.
Any tips on what i should do next?
Any tips on what i should do next?
Drifting
Joined: Sep 2022
Posts: 1,456
Likes: 380
From: Liberty, SC
2023 C4 of the Year Finalist - Unmodified
Wish i could help you. Ive asked an iuneplied to idle question too.
Best guess, and one im going to investigate for myself soon.... what about the EGR solenoid and the hoses downstream from it? Maybe... it is opening and introducing air through a line leak that wont show itself whem the enigne is not running.
Best guess, and one im going to investigate for myself soon.... what about the EGR solenoid and the hoses downstream from it? Maybe... it is opening and introducing air through a line leak that wont show itself whem the enigne is not running.
Thread Starter
4th Gear
Joined: Apr 2023
Posts: 4
Likes: 0
From: Ohio
Wish i could help you. Ive asked an iuneplied to idle question too.
Best guess, and one im going to investigate for myself soon.... what about the EGR solenoid and the hoses downstream from it? Maybe... it is opening and introducing air through a line leak that wont show itself whem the enigne is not running.
Best guess, and one im going to investigate for myself soon.... what about the EGR solenoid and the hoses downstream from it? Maybe... it is opening and introducing air through a line leak that wont show itself whem the enigne is not running.
I do not know if there's a way to test the EGR solenoid but seams to be a problem. Also waiting on a new coolent sensor since I have code 15 pop up randomly. I feel like I'm just throwing parts at this thing and we all know that's not the way to go.
Racer
Joined: Mar 2020
Posts: 387
Likes: 148
From: Langton, Ontario, Canada
2025 C4 of the Year Finalist - Unmodified
My 85 had a wandering idle like yours, and I finally fixed it this past weekend. It turned out to be a single vacuum hose that I had neglected to replace last year. I replaced all the soft vacuum hoses with new silicone vacuum hoses, except for the one from the cruise control unit to the brake and clutch pedals. It was mostly missed because I wasn't looking forward to doing a session of Corvette yoga, under the steering wheel. But, the wandering idle was really bugging me this year, so I yanked that thing through the firewall, with the new hose attached to it. When I put the old hose on my hand vacuum pump, and applied a bit of vacuum to it, it collapsed to about the thickness of a piece of paper. At 38 years old, those black rubber hoses are done, and all need to be replaced. The plastic ones are fine, as are the fittings. But this is the cause of the idle problems, at least for me.
Thread Starter
4th Gear
Joined: Apr 2023
Posts: 4
Likes: 0
From: Ohio
My 85 had a wandering idle like yours, and I finally fixed it this past weekend. It turned out to be a single vacuum hose that I had neglected to replace last year. I replaced all the soft vacuum hoses with new silicone vacuum hoses, except for the one from the cruise control unit to the brake and clutch pedals. It was mostly missed because I wasn't looking forward to doing a session of Corvette yoga, under the steering wheel. But, the wandering idle was really bugging me this year, so I yanked that thing through the firewall, with the new hose attached to it. When I put the old hose on my hand vacuum pump, and applied a bit of vacuum to it, it collapsed to about the thickness of a piece of paper. At 38 years old, those black rubber hoses are done, and all need to be replaced. The plastic ones are fine, as are the fittings. But this is the cause of the idle problems, at least for me.
