Lately I've been having a lot of detonation and some power loss on my 2001 Astro. Full tune-up with AC cap,rotor, coil, plugs and wires. Tried various octanes with no difference. To me it feels like the timing isn't changing under load? Would a bad anti-knock sensor cause this, and if so would it set a dtc code, because I have none as of yet?

Thanks

Does the Service Engine Soon, (SES) light come on when the ignition is turned to the "ON" position and then go off after the engine is running? Does P0325, P0327 or P1345 show up either in the Freeze Frame or history file? Are there ANY Diagnostic Trouble Codes, (DTC's) in freeze frame or history files?

When the Knock Sensor, (KS) detects detonation, the VCM retards ignition timing until the knock is gone. If there is a problem with the KS, or its circuits, it will set a DTC in memory and the SES light will come on.

Excess moisture in the fuel might cause a knock, and a can or two of fuel line anti-freeze should cure that. Also, have you checked fuel pressure and leakdown rate?

Old Master....

Yes, the SES light comes on for it's "check" when the key is turned on. As of yet, no codes have been set.

My fuel pressure is 60 with key on, leaks down to about 55 after 5minutes, then stays there for hours.

I put in a new distributor along with the tune up (my top bearing went bad). I aligned everything when reinstalled, but new distributor can rotate a few degrees either direction in the clamp. I'm guessing the computer doesn't care much about a degree or two, but I could be wrong? I just lined up the rotor tip with the #6 pointer and left it at that.

Thanks

You have the distributor aligned correctly. If not aligned correctly, it would have set P1345 and turned the SES light on.

Your leakdown results are great, however, fuel pressure is at the lowest allowable limit. Factory spec is 60psi to 66psi with the ignition on, engine off, and fuel pump running. Must remain above 55psi for 3 to 5 minutes after the pump shuts off. If it drops below the 60psi you have now, you'll notice other driveability issues.

It's possible that the spring pressure in the fuel pressure regulator is too high. There is no adjustment, replacement is the cure. The only way to know for sure is by checking fuel pump maximum output pressure.

It's also possible the injector nozzles have a buildup of varnish. Varnish buildup is more common with the CSFI system as opposed to the CMFI system due to the injectors fire less often on the CSFI system. There is a GM Technical Service Bulletin that addresses this which can include changing over from CSFI to CMFI. Most of the time adding a good quality injector cleaner every so often keeps the varnish in check. Have you added fuel injector cleaner to the fuel?

Old master,

Thanks for getting back to me. I checked that pressure again....it's actually around 64psi. The pump is about a year old.

Yes, I have run a few different brands of injector cleaner through (Gumout and Techron) over the past month. I often wonder though how much good it does since it's so diluted by the gas? I did a seafoam treatment through the pcv, and it seemed to help a litttle. I was kicking around the idea of spending $90 for an injector cleaner tool from Harbor Freight and mixing a 50/50 gas & seafoam. I figure that has to do better cleaning since it's directly hooked into the fuel port? Any thoughts?

Techron is an excellent choice. If you use as directed on the bottle, it should do the job. A second treatment might be necessary depending on how gummed up they are. The stand-alone injector cleaner would certainly do the job faster, and theoretically better.