'97 Astro cam/timing data

[Alex_V]

A customer with a '97 Astro (4.3, AWD) is trying to rule out a stretched timing chain as the cause of some problems: mainly, when placed under load it intermittently stumbles and then will run awful (missing and carrying on) until it's shut off and restarted. Codes: P0337 "CKP sensor A signal low frequency" P0339 "CKP sensor A signal intermittent" P0341 "CMP sensor circuit out of range" Watching the live data as it idled in my shop, ignition timing was running right around 20 while "cam retard" was showing -6 or -7. How exactly do these two numbers correlate? Is the cam retard number comparing crank sensor to cam sensor, saying that it sees the cam (and therefore, distributor shaft) running 6 degrees behind the crank? I had no time to do any physical checks, and the weather was going downhill fast so I didn't take it for a drive at the time - we agreed I'd do some homework on the codes and follow up later. He has changed the cam and crank sensors but says he's not moved or removed the distributor. Are the 4.3 dizzy's prone to excess wear on the driven gear like the Vortec V8's are?

 

[clinebarger]

I would ignore the P0341 for the time being. Check the CKP wiring, Especially right at the CKP Connector as oil leakage/seepage can rot away the wiring insulation. Cam Retard Offset readings are not accurate at idle, You have to rev the engine to 1200-1500 RPM.

 

[Alex_V]

I did, up to maybe 3,000 RPM. I didn't make great note of any of the readings but if memory serves, cam retard stayed about the same with ignition timing climbing up to about 30 degrees.

 

[clinebarger]

Cam Retard in the single digits will not cause performance issues, And you don't have a P1345?

 

[oldhp]

We always would put the timing mark on TDC. Now turn the crankshaft backward while watching the distributor. If you can go more than 10° before the rotor starts to turn, change the chain.

 

[Alex_V]

Originally Posted By: clinebarger

Cam Retard in the single digits will not cause performance issues, And you don't have a P1345?

Nope, no 1345 - only those three. The first thing I'm doing if it comes back in is inspect the wiring to the sensors.

 

[Alex_V]

Originally Posted By: oldhp

We always would put the timing mark on TDC. Now turn the crankshaft backward while watching the distributor. If you can go more than 10° before the rotor starts to turn, change the chain.

Good idea.

 

[14Accent]

Sounds like a failing crank sensor. Can't recall seeing a timing chain stretch on any 4.3, not that it's impossible. But I would doubt it.

 

[Linctex]

Originally Posted By: oldhp

We always would put the timing mark on TDC. Now turn the crankshaft backward while watching the distributor. If you can go more than 10° before the rotor starts to turn, change the chain.

REALLY? I would think that's about normal. I wonder what a new one does.

 

[Alex_V]

Originally Posted By: 14Accent

Sounds like a failing crank sensor. Can't recall seeing a timing chain stretch on any 4.3, not that it's impossible. But I would doubt it.

I'm also skeptical it's the chain, even with 230K on it. "Bad timing chain" gets thrown around a lot in recent years because of all the OHC engines, but a lot of people don't understand how rare it is for these short OHV chains to ever become a problem.

 

[oldhp]

Originally Posted By: Linctex

Originally Posted By: oldhp

We always would put the timing mark on TDC. Now turn the crankshaft backward while watching the distributor. If you can go more than 10° before the rotor starts to turn, change the chain.

REALLY? I would think that's about normal. I wonder what a new one does.

2°-4°....