Timing randomly drops 4-8 degrees

[95 Tahoe]

What will cause the timing to randomly drop 4-6 degrees at idle? The distributor is new and I disconnected the knock sensor for a moment to see if it was related. It still did it. I plugged in a vac gauge and was watching it. I was only seeing .25-.5 inhg fluctuation. Not much. It's a 95 5.7L (TBI Engine)

 

[Erik the Awful]

What year and what engine?

 

[95 Tahoe]

What year and what engine?

sorry, it's a 95 5.7L (TBI Engine)

 

[Erik the Awful]

There's a plug you're supposed to undo when you check timing, but I forget where it is. Also, check your timing at 2000 rpm, not at idle. Inspect your distributor's guts to ensure they're clean and not crusted up with rust or metal filings. Inspect the magnet under the pickup to make sure it's not cracked. Look critically at the cap to ensure it's not cracked, doesn't have any carbon traces, and the terminals aren't crusty. If you have a known-good ignition module, swap it out and see if the problem goes away.

 

[someotherguy]

On a '95 the disconnect on the tan/black wire for timing bypass is usually in the loom of wires underneath the glovebox -

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Small block base timing spec is 0°, checked at idle, with a fully warmed up engine. Not at 2K - if you do so there's an advance curve built into the ignition module that will advance the timing. Erik, you must be thinking of checking cam retard on a Vortec, that reading is done while holding rpm's to at least 1K to get a good read.

With the tan/black wire disconnected the timing should not move around. Once it's hooked back up, the ignition module will take over and you will see an advance of around 8-12° and constantly fluctuating.

Richard

 

[Frank Enstein]

Perhaps a worn timing chain.

 

[Scooterwrench]

Idle speed is controlled both by ISC and ignition timing because the timing will react faster than the ISC such as when the A/C kicks in or when you drop it in gear. Small variations in idle speed are controlled more by timing than ISC hence the reason you see the timing changing at idle.

 

[tayto]

Idle speed is controlled both by ISC and ignition timing because the timing will react faster than the ISC such as when the A/C kicks in or when you drop it in gear. Small variations in idle speed are controlled more by timing than ISC hence the reason you see the timing changing at idle.

you know ISC and IAC are not the same things right?

only TBI i've seen with an ISC is on a 7.4L.

 

[Scooterwrench]

you know ISC and IAC are not the same things right?

only TBI i've seen with an ISC is on a 7.4L.

Really?
Too many acronyms to keep 'em all straight. IAC,ISC,throttle kicker. Pick one,they all do the same thing. Still the fact is ESC also plays a part in idle speed.

 

[PlayingWithTBI]

you know ISC and IAC are not the same things right?

Yeah TBI's idle speed is controlled by the ECM/PCM adding/subtracting IAC steps. SA may be changed if feedback makes the SA table move to a different MAP/RPM cell (more acronyms, LOL).