Help! Won't idle. Barely runs.
Thread Starter
Instructor
Joined: Nov 2023
Posts: 169
Likes: 32
From: East Tennessee
Help! Won't idle. Barely runs.
Bought a 98 6 speed with known issues. I am new to the C5 generation and LS platform.
Here is what I know: The only modifications are to the valve train. It has a high lift/duration cam with components to support it. This includes a tune. There is also an AIR delete. The engine is otherwise unmodified. The previous owner states that it was running well then all the sudden began running poorly.
When starting the engine, I must hold the throttle open or else it won't do much of anything. It will begin to spit and sputter then barely come to life. If I let go of the pedal, it will die. Here is a video of how it runs:
The clattering is the door/hood and everything around the engine shaking from how poorly its running.
It hasn't thrown a code, but it will have pending codes for MAF and random/multiple misfire. If I unplug the MAF and force it to run in limp mode, it still runs the same.
Engine has even compression. Upon feeling the exhaust, I can tell only a few cylinders are actually firing or getting hot.
Any ideas at where to start?
Here is what I know: The only modifications are to the valve train. It has a high lift/duration cam with components to support it. This includes a tune. There is also an AIR delete. The engine is otherwise unmodified. The previous owner states that it was running well then all the sudden began running poorly.
When starting the engine, I must hold the throttle open or else it won't do much of anything. It will begin to spit and sputter then barely come to life. If I let go of the pedal, it will die. Here is a video of how it runs:
The clattering is the door/hood and everything around the engine shaking from how poorly its running.
It hasn't thrown a code, but it will have pending codes for MAF and random/multiple misfire. If I unplug the MAF and force it to run in limp mode, it still runs the same.
Engine has even compression. Upon feeling the exhaust, I can tell only a few cylinders are actually firing or getting hot.
Any ideas at where to start?
Last edited by OneRedVette; Jun 20, 2026 at 01:32 PM.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,707
Likes: 4,103
From: Cape Coral, Florida
For any driveability concern I ALWAYS start with the battery...where to start is for you to find a diagnostic shop in your area and go there and pay for an hour or two of diagnostic time...without a factory level scan tool and a labscope to be able to observe secondary ignition waveforms and to current ramp your coil packs and injectors ...without some extensive knowledge you will only be guessing and that gets VERY expensive...that diagnostic shop will "follow the data" and quickly diagnose it !!...I would check the MAP sensor KOEO and if you are at sea level you should read about 101kPa, 29.9 inches/mercury or 14.7 psi if you have a scan tool...BTW with the mods you have and with a tune I wouldn't touch it !!...most "tuners" don't know WTF they're doing !!
Last edited by C5 Diag; Jun 20, 2026 at 12:23 PM.
Le Mans Master
Joined: Jul 2010
Posts: 6,039
Likes: 732
Bought a 98 6 speed with known issues. I am new to the C5 generation and LS platform.
Here is what I know: The only modifications are to the valve train. It has a high lift/duration cam with components to support it. This includes a tune. The engine is otherwise unmodified. The previous owner states that it was running well then all the sudden began running poorly.
When starting the engine, I must hold the throttle open or else it won't do much of anything. It will begin to spit and sputter then barely come to life. If I let go of the pedal, it will die. Here is a video of how it runs: https://youtube.com/shorts/hPVZiuFCR...bhFCDmlO0BW_FZ
The clattering is the door/hood and everything around the engine shaking from how poorly its running.
It hasn't thrown a code, but it will have pending codes for MAF and random/multiple misfire. If I unplug the MAF and force it to run in limp mode, it still runs the same.
Engine has even compression. Upon feeling the exhaust, I can tell only a few cylinders are actually firing or getting hot.
Any ideas at where to start?
Here is what I know: The only modifications are to the valve train. It has a high lift/duration cam with components to support it. This includes a tune. The engine is otherwise unmodified. The previous owner states that it was running well then all the sudden began running poorly.
When starting the engine, I must hold the throttle open or else it won't do much of anything. It will begin to spit and sputter then barely come to life. If I let go of the pedal, it will die. Here is a video of how it runs: https://youtube.com/shorts/hPVZiuFCR...bhFCDmlO0BW_FZ
The clattering is the door/hood and everything around the engine shaking from how poorly its running.
It hasn't thrown a code, but it will have pending codes for MAF and random/multiple misfire. If I unplug the MAF and force it to run in limp mode, it still runs the same.
Engine has even compression. Upon feeling the exhaust, I can tell only a few cylinders are actually firing or getting hot.
Any ideas at where to start?
Thread Starter
Instructor
Joined: Nov 2023
Posts: 169
Likes: 32
From: East Tennessee
Yep. Got a good deal on it for that reason. I have a scanner which can read live data. I will be combing through it soon here. Hopefully something will reveal itself. I have already verified fuel pressure and compression. I dribbled some raw fuel into the intake. It did not make a difference in starting.
How are these engines timed? Is it possible for ignition timing to "jump"? I assume its a reluctor wheel somewhere.
The car does have an AIR delete and the dtc code for it has been tuned out. I hope the tuner didn't disable a bunch of other potentially helpful DTCs that would otherwise give me some hints.
How are these engines timed? Is it possible for ignition timing to "jump"? I assume its a reluctor wheel somewhere.
The car does have an AIR delete and the dtc code for it has been tuned out. I hope the tuner didn't disable a bunch of other potentially helpful DTCs that would otherwise give me some hints.
Last edited by OneRedVette; Jun 20, 2026 at 01:34 PM.
Safety Car
Joined: Jul 2010
Posts: 4,027
Likes: 388
From: Chandler AZ
How are the spark plugs and wires?
Years ago I bought inline spark tester tool at Harbor Freight, never needed to use it yet, but this would be a good time to try it.
I do wonder if say a spark plug ceramic was cracked and the spark arcs to ground, if it would show flashing on the inline tester tool.
If anything you could see each cylinder is at least getting spark through the coil/plug wire maybe buy some new spark plugs or try resistance testing the ones you already have.
Maybe pull the valve covers and make sure all the rocker arms operate the same.
You said compression is good, but still worth checking.
The fact the cam was changed makes me wonder if something is wrong with valvetrain.
Years ago I bought inline spark tester tool at Harbor Freight, never needed to use it yet, but this would be a good time to try it.
I do wonder if say a spark plug ceramic was cracked and the spark arcs to ground, if it would show flashing on the inline tester tool.
If anything you could see each cylinder is at least getting spark through the coil/plug wire maybe buy some new spark plugs or try resistance testing the ones you already have.
Maybe pull the valve covers and make sure all the rocker arms operate the same.
You said compression is good, but still worth checking.
The fact the cam was changed makes me wonder if something is wrong with valvetrain.
Running Guns & Moonshine
Joined: Aug 2001
Posts: 12,819
Likes: 7,387
From: CT
It costs nothing to pop the valve covers and make sure no lifters are dead and that the rockers are installed correctly. Also cheap to change the oil and look for glitter. As a new car also maybe send the oil out for testing which will tell you how it's doing overall as a baseline from ownership.
The rest devolves into grounds and sensors.
Did the prior guy have it running cleanly on the new cam before this change occurred?
The rest devolves into grounds and sensors.
Did the prior guy have it running cleanly on the new cam before this change occurred?
Advanced
Joined: Mar 2026
Posts: 98
Likes: 47
From: Coastal NC
It costs nothing to pop the valve covers and make sure no lifters are dead and that the rockers are installed correctly. Also cheap to change the oil and look for glitter. As a new car also maybe send the oil out for testing which will tell you how it's doing overall as a baseline from ownership.
The rest devolves into grounds and sensors.
Did the prior guy have it running cleanly on the new cam before this change occurred?
The rest devolves into grounds and sensors.
Did the prior guy have it running cleanly on the new cam before this change occurred?
When the cam was put in they may have installed trunion bearings, they wear out and will need replacement eventually, especially if the cam had high lift. I’d take off the valve covers and look at the springs and rockers themselves. I had a rocker on my 408 decided to break a retaining ring and it caused it to sound like it was coming apart.
Last edited by Smokinlmm; Jun 21, 2026 at 08:23 AM.
Thread Starter
Instructor
Joined: Nov 2023
Posts: 169
Likes: 32
From: East Tennessee
It was running prior to this. PO states it was running fine then all the sudden it didn't. He was not very technical in his explanation. He said he has 1500 miles on the camshaft.
Alrighty I've done some digging. All eight plugs look the same except the fact that 5, 6, and 7 were damp with fuel. They were OE plugs. Ceramic looks intact. Did my best to run the engine for about a minute, then sprayed water on exhaust. Confirmed that the above cylinders are not firing. Strange that it's across two banks. I will be getting a spark tester after work today.
I also read some live data and found TPS to sit anywhere from 11-18% with foot off pedal.
Pulled the valve covers and confirmed all 16 valves are moving and springs intact. No loose rockers or odd wear that I could see, but I didn't look at the rockers particularly close. It was spotless under the valve covers too. No "magic glitter" or varnish. Just oil.
When turning the engine over by hand, I noticed the harmonic balancer bolt was entirely loose and not doing anything. Could this be a clue?
Alrighty I've done some digging. All eight plugs look the same except the fact that 5, 6, and 7 were damp with fuel. They were OE plugs. Ceramic looks intact. Did my best to run the engine for about a minute, then sprayed water on exhaust. Confirmed that the above cylinders are not firing. Strange that it's across two banks. I will be getting a spark tester after work today.
I also read some live data and found TPS to sit anywhere from 11-18% with foot off pedal.
Pulled the valve covers and confirmed all 16 valves are moving and springs intact. No loose rockers or odd wear that I could see, but I didn't look at the rockers particularly close. It was spotless under the valve covers too. No "magic glitter" or varnish. Just oil.
When turning the engine over by hand, I noticed the harmonic balancer bolt was entirely loose and not doing anything. Could this be a clue?
Last edited by OneRedVette; Yesterday at 07:18 AM.
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Brett Foote
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,707
Likes: 4,103
From: Cape Coral, Florida
It was running prior to this. PO states it was running fine then all the sudden it didn't. He was not very technical in his explanation. He said he has 1500 miles on the camshaft.
Alrighty I've done some digging. All eight plugs look the same except the fact that 5, 6, and 7 were damp with fuel. They were OE plugs. Ceramic looks intact. Did my best to run the engine for about a minute, then sprayed water on exhaust. Confirmed that the above cylinders are not firing. Strange that it's across two banks. I will be getting a spark tester after work today.
I also read some live data and found TPS to sit anywhere from 11-18% with foot off pedal.
Pulled the valve covers and confirmed all 16 valves are moving and springs intact. No loose rockers or odd wear that I could see, but I didn't look at the rockers particularly close. It was spotless under the valve covers too. No "magic glitter" or varnish. Just oil.
When turning the engine over by hand, I noticed the harmonic balancer bolt was entirely loose and not doing anything. Could this be a clue?
Alrighty I've done some digging. All eight plugs look the same except the fact that 5, 6, and 7 were damp with fuel. They were OE plugs. Ceramic looks intact. Did my best to run the engine for about a minute, then sprayed water on exhaust. Confirmed that the above cylinders are not firing. Strange that it's across two banks. I will be getting a spark tester after work today.
I also read some live data and found TPS to sit anywhere from 11-18% with foot off pedal.
Pulled the valve covers and confirmed all 16 valves are moving and springs intact. No loose rockers or odd wear that I could see, but I didn't look at the rockers particularly close. It was spotless under the valve covers too. No "magic glitter" or varnish. Just oil.
When turning the engine over by hand, I noticed the harmonic balancer bolt was entirely loose and not doing anything. Could this be a clue?
While idling if your TPS is over 10% your throttle body is dirty...mine sits at about 7%.
Last edited by C5 Diag; Yesterday at 07:26 AM.
Thread Starter
Instructor
Joined: Nov 2023
Posts: 169
Likes: 32
From: East Tennessee
Bought a tester... Got spark on all 8.
Decided to plug in a vacuum gauge and run it. Major needle fluctuation. Time to look at valve train a little more closely. I smell a broken valve spring. Will update tomorrow.
Decided to plug in a vacuum gauge and run it. Major needle fluctuation. Time to look at valve train a little more closely. I smell a broken valve spring. Will update tomorrow.
Running Guns & Moonshine
Joined: Aug 2001
Posts: 12,819
Likes: 7,387
From: CT
Still curious and in for the fix.
Thread Starter
Instructor
Joined: Nov 2023
Posts: 169
Likes: 32
From: East Tennessee
Not certain on injectors. I never asked.
I had a friend spray starting fluid while it was running and it did not make it any better. O2 sensors are reading around 12 AFR for each side.
I had a friend spray starting fluid while it was running and it did not make it any better. O2 sensors are reading around 12 AFR for each side.