Erratic timing problem
Thread Starter
Joined: Nov 2005
Posts: 1,946
Likes: 0
From: Sacramento
Car: 91 RS
Engine: 350 TBI
Transmission: WC T5
Axle/Gears: 3.42 Posi
Erratic timing problem
I'm working in a 91 L98. With the EST PLUGGED IN, the flash of the timing light is very erratic. If I rev the engine sometimes it stops flashing for almost a full second as the RPM come back down, then starts up flashing again. I disconnect the EST and this problem goes away (other problem though see below). New cap, rotor, wires, and the plugs look fine. [correction: it happens on all plug wires]
Also, with the EST disconnected, I noticed the timing jumping around about 2 degrees. As I rev the engine it will start to creep up 2 or 3 degrees then snap back as I let off. Timing chain? The erratic flashing described above does not exist with the EST disconnected.
Also, with the EST disconnected, I noticed the timing jumping around about 2 degrees. As I rev the engine it will start to creep up 2 or 3 degrees then snap back as I let off. Timing chain? The erratic flashing described above does not exist with the EST disconnected.
Last edited by Darkshot; Dec 29, 2008 at 07:33 PM.
Junior Member
Joined: Sep 2008
Posts: 18
Likes: 0
From: San Diego
Car: 1991 Pontiac Firebird
Engine: 3.1L V6
Transmission: 700r4
Re: Erratic timing problem
I'm working in a 91 L98. With the EST PLUGGED IN, the flash of the timing light is very erratic. If I rev the engine sometimes it stops flashing for almost a full second as the RPM come back down, then starts up flashing again. I disconnect the EST and this problem goes away (other problem though see below). New cap, rotor, wires, and the plugs look fine. [correction: it happens on all plug wires]
Also, with the EST disconnected, I noticed the timing jumping around about 2 degrees. As I rev the engine it will start to creep up 2 or 3 degrees then snap back as I let off. Timing chain? The erratic flashing described above does not exist with the EST disconnected.
Also, with the EST disconnected, I noticed the timing jumping around about 2 degrees. As I rev the engine it will start to creep up 2 or 3 degrees then snap back as I let off. Timing chain? The erratic flashing described above does not exist with the EST disconnected.
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Joined: Jan 2000
Posts: 20,981
Likes: 11
From: Mercedes Norte, Heredia, Costa Rica
Car: 1984 Z28 Hardtop
Engine: 383 Carb
Transmission: 4L60
Axle/Gears: 3.54 Dana 44
Re: Erratic timing problem
I'm working in a 91 L98. With the EST PLUGGED IN, the flash of the timing light is very erratic. If I rev the engine sometimes it stops flashing for almost a full second as the RPM come back down, then starts up flashing again. I disconnect the EST and this problem goes away (other problem though see below). New cap, rotor, wires, and the plugs look fine. [correction: it happens on all plug wires]
There's some slack in the distributor drivetrain which can give you a few degrees of variability. You can shim it out if it's a problem. The small amount of advance you see is built into the ignition module, and is normal.
Thread Starter
Joined: Nov 2005
Posts: 1,946
Likes: 0
From: Sacramento
Car: 91 RS
Engine: 350 TBI
Transmission: WC T5
Axle/Gears: 3.42 Posi
Re: Erratic timing problem
It's not the timing light. It's a brand new Craftsman (Christmas gift) and it works perfect on my other car and it works perfect with EST disconnected.
The car has a slightly rough idle (TPI should run smooth as silk I presume?) and I'm thinking it's related.
The car has a slightly rough idle (TPI should run smooth as silk I presume?) and I'm thinking it's related.
Thread Starter
Joined: Nov 2005
Posts: 1,946
Likes: 0
From: Sacramento
Car: 91 RS
Engine: 350 TBI
Transmission: WC T5
Axle/Gears: 3.42 Posi
Re: Erratic timing problem
Anything else that could cause the spark to cut out with EST plugged in? Or at least, the timing light to not flash? I tried the coil from my other car and no difference.
Junior Member
Joined: Sep 2008
Posts: 18
Likes: 0
From: San Diego
Car: 1991 Pontiac Firebird
Engine: 3.1L V6
Transmission: 700r4
Re: Erratic timing problem
The spark voltage traveling through the spark plug wire is too weak for the timing light to pick up. You could possibly have a bad alternator. What you need to do is ramp up your idle speed over 1000rpm to set the timing, then you can take it back down once you're dialed in. That way the alternator will be charging at full capacity and increasing the amperage of the ignition sparks.
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