Idle Stumbling HELP
Thread Starter
Cruising
Joined: Mar 2023
Posts: 12
Likes: 4
Idle Stumbling HELP
Hey everybody, '88 L98 here (automatic) 51k Miles
I'm fighting a very rough idle that stumbles to a near stall 500 rpm then ramps up to 1200 rpm in a cycle 5 seconds high 5 seconds low.
Things I have done: New alt and serp belt (14.5V running)
Fuel pressure 39 psi at idle, ramps to 44 psi during the bogging phase of idle.
New IAC
TPS set to .538 volts (closest I could get to .54 lol)
Plugs are new
Cleaned MAF
Wires also new
Cap and rotor new
New EGR valve (OEM acdelco)
New battery
Cleaned all sensors with sensor cleaner.
all fuel injectors measure 15 ohms +/- 0.5 ohms
Repaired 2 vac hoses and inspected all others. (not to say there couldnt be something i missed but this idle is very "off" for a small vac leak
NO CODES. (EGR fixed my previous 32 code)
Thanks all for any input before i throw in the call and call a shop
I'm fighting a very rough idle that stumbles to a near stall 500 rpm then ramps up to 1200 rpm in a cycle 5 seconds high 5 seconds low.
Things I have done: New alt and serp belt (14.5V running)
Fuel pressure 39 psi at idle, ramps to 44 psi during the bogging phase of idle.
New IAC
TPS set to .538 volts (closest I could get to .54 lol)
Plugs are new
Cleaned MAF
Wires also new
Cap and rotor new
New EGR valve (OEM acdelco)
New battery
Cleaned all sensors with sensor cleaner.
all fuel injectors measure 15 ohms +/- 0.5 ohms
Repaired 2 vac hoses and inspected all others. (not to say there couldnt be something i missed but this idle is very "off" for a small vac leak
NO CODES. (EGR fixed my previous 32 code)
Thanks all for any input before i throw in the call and call a shop
Last edited by SpinsTires; Mar 22, 2023 at 04:57 PM. Reason: forgot to add mileage
Burning Brakes
Joined: Jun 2021
Posts: 1,199
Likes: 449
From: NJ
O2 sensors? You’ve replaced many parts that could have contributed to it. Also check further for vacuum leaks. I know your car only has 51k on it, but the rubber vacuum hoses are 34 years old. I’m not super familiar with the procedure for checking them and common problem locations, but I’m sure some others can help you out with where and how to check.
Melting Slicks
Joined: Apr 2001
Posts: 2,105
Likes: 220
From: Ca.
The best way to figure it out is with live data off a scan tool. You can see if the O2 sensor is toggling etc. As for vacuum lines-the ones most missed for cracks run from the driver side under the headlight area to the engine.
Racer
Joined: Jan 2012
Posts: 456
Likes: 113
From: Hill Country TX
Subscribing. I have same issue as OP. Mine is when the car is in gear (drive) at idle, it fluctuates between 500 (almost dies) and 700-750 rpm. Have done the exact same things as OP, no joy. I suspect vacuum leak somewhere? I think my next move will be to take to a shop and see if they can do a smoke test to hopefully find the leak. But monitoring this thread for other thoughts.
Thread Starter
Cruising
Joined: Mar 2023
Posts: 12
Likes: 4
Thread Starter
Cruising
Joined: Mar 2023
Posts: 12
Likes: 4
Update, Installed a ACdelco PCV valve. the old one was a knock off and i noticed I could blow into either side and it wouldn't fully close off. test drive this morning on cold start was smoother ran fine 25 miles of city/highway.
Will update further to see if that was a fix or i just had 1 good startup
Will update further to see if that was a fix or i just had 1 good startup
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Thread Starter
Cruising
Joined: Mar 2023
Posts: 12
Likes: 4
Update: Did a cold start today (it was 70 degrees outside) car started like usual. bogged way down to 400 RPM right after startup and threatened to die. I didn't touch the gas and it went back to 800-1000 for the warmup normal phase before settling at 700 RPM.
SO: I'm still seeing an issue with the initial startup and the car choking then running relatively normal.
I have heard a faulty CTS in the block can be an issue with cold starts. I have a new one but tested the resistance on the old one and it checked out so I didn't want to drain coolant to change it
EDIT: wanted to add that warm starts or hot starts i DO NOT have this issue. it is only with COLD starts (engine temps)
SO: I'm still seeing an issue with the initial startup and the car choking then running relatively normal.
I have heard a faulty CTS in the block can be an issue with cold starts. I have a new one but tested the resistance on the old one and it checked out so I didn't want to drain coolant to change it
EDIT: wanted to add that warm starts or hot starts i DO NOT have this issue. it is only with COLD starts (engine temps)
Burning Brakes
Joined: Jun 2021
Posts: 1,199
Likes: 449
From: NJ
Update: Did a cold start today (it was 70 degrees outside) car started like usual. bogged way down to 400 RPM right after startup and threatened to die. I didn't touch the gas and it went back to 800-1000 for the warmup normal phase before settling at 700 RPM.
SO: I'm still seeing an issue with the initial startup and the car choking then running relatively normal.
I have heard a faulty CTS in the block can be an issue with cold starts. I have a new one but tested the resistance on the old one and it checked out so I didn't want to drain coolant to change it
EDIT: wanted to add that warm starts or hot starts i DO NOT have this issue. it is only with COLD starts (engine temps)
SO: I'm still seeing an issue with the initial startup and the car choking then running relatively normal.
I have heard a faulty CTS in the block can be an issue with cold starts. I have a new one but tested the resistance on the old one and it checked out so I didn't want to drain coolant to change it
EDIT: wanted to add that warm starts or hot starts i DO NOT have this issue. it is only with COLD starts (engine temps)
Last edited by C4 Steck; Mar 25, 2023 at 09:16 PM.
Thread Starter
Cruising
Joined: Mar 2023
Posts: 12
Likes: 4
Another update,
I replaced the coil, rotor and distributor cap (all were 1988 era) AS well as a new CTS, (the old one tested fine but wanted to rule out a sporatic one.) Tested all units prior to installing to verify working at various temps.
Also did a full smog delete. (no vaccum lines on a 1988 smog system) all plumbing removed/capped.
STILL when starting for first time car will start, idle for 2 seconds then stumble hard with a low idle that hunts up and down for about 5-7 minutes before smoothing out (closed loop im assuming by then)
Unplugged MAF when running and car dies.
I replaced the coil, rotor and distributor cap (all were 1988 era) AS well as a new CTS, (the old one tested fine but wanted to rule out a sporatic one.) Tested all units prior to installing to verify working at various temps.
Also did a full smog delete. (no vaccum lines on a 1988 smog system) all plumbing removed/capped.
STILL when starting for first time car will start, idle for 2 seconds then stumble hard with a low idle that hunts up and down for about 5-7 minutes before smoothing out (closed loop im assuming by then)
Unplugged MAF when running and car dies.