distributor - '98 K2500 5.7L Vortec

[dilerin]

!998 4WD K2500 with towing package, 5.7L Vortec, 190K miles. I started out overhauling inop heat and A/C systems which led to a host of other deferred maintenance fixes including upgrading of the FI system. Plugs, wires, cap, rotor & distributor all replaced. Original dist, cap, & rotor were still in reasonably serviceable condition in spite of the miles, and the thing still ran like a top.
Now to my problem. I set the base timing on the new distributor and am positive that it's correctly done. Everything looks right, the engine starts instantly, idles and runs smoothly and well from 700 - 2500 rpm (driveway no load), and took me nicely around the block which included several stop sign stops and starts. My code reader (Actron CP9580) indicates 25.5 advanced @ 1100 rpm. Next, rotating distributor CW as far as it will go, until #1 spark plug wire is against the upper manifold, reduces advance to 13.5 and that's it. At that setting increasing rpm from 1100 to 2000+ produces no change (It may be that computer has compensated all that it can?). Engine still starts instantly and, as before, runs well under light load. Don't know how well it runs under load as I haven't put my foot down yet or stretched out on the freeway.
To summarize; with base timing properly set, indicated advance is 25.5. With dist then turned max CW, 13.5 is the min achievable advance. At that dist position increasing rpm does not change advance in either direction. In both dist positions truck starts instantly, runs well under light load and low speeds. Performance under load or hi speed so far unknown.
I don't suspect the cam position sensor as it's new. Crank sensor is original but was working before starting this project, and I've read that engine performance suffers quite noticeably if either are bad. I don't yet suspect my code reader as other Live Data points are as they should be, but I may try another if I can find one. I am the orig owner so know that no mechanicals or electronics have been jacked with.
Have any of you experienced this circumstance, or anything similar? Any input from you forum folks will be appreciated. I'm hoping I'm missing something obvious and easily fixed. Thanks very much.

 

[454cid]

I must be hungry..... I read "inop" as iHop!

You don't set timing on a Vortec V8, the computer does it. You're adjusting the cam/crank offset.

 

[dilerin]

I must be hungry..... I read "inop" as iHop!

You don't set timing on a Vortec V8, the computer does it. You're adjusting the cam/crank offset.

I know, but I'm an old points/dwell/timing guy and it's hard to shake that terminology (I'm working on it). However - in spite of what one calls it, rotating the distributor should induce change in cam/crank offset but in my case it does not. Either way, thanks for your help.

 

[Jeff Janssen]

are you reading cam/crank offset or timing? They are not the same thing. The fact that you list 25 degrees and not + (value) or - (value) makes me thing you are reading and reporting on ignition timing. Ignition timing is completely set by the computer. You can only adjust the cam/crank offset by turning the distributor.

 

[Awest623]

I gotta agree with Jeff on this one. Make sure you're looking at the cam/crank alignment, not the timing advance.

 

[Schurkey]

I bet he is looking at cam/crank offset, and the distributor won't turn far enough to get the signals to align.

Common problem, especially with aftermarket distributors. It can also be caused by not getting the distributor gear meshed properly with the cam gear--a tooth "off", for example.

There's two potential "fixes".
1. Lift distributor just enough to move distributor gear one tooth on the cam gear. Probably have to crank engine to get oil pump drive to align so distributor drops all the way down. Use scan tool to get to "0" offset at 1100 rpm. The direction you turn the distributor shaft depends on which way you're having interference issues.

2. Remove distributor gear roll-pin, slide the gear off, spin 180 degrees, reinstall. This isn't "right" but might work. Don't do this if #1 works for you.

I'm under the impression that aftermarket distributor are manufactured sloppily; the stamped-sheetmetal mount for the rotor (which also triggers the cam sensor) isn't pressed onto the shaft in the exact, proper location. It's "spun" a few degrees clockwise or counterclockwise from perfection.

 

[Jeff Janssen]

If this is actually about CMP offset not being able to be adjusted within range, for more info on what Shurkey says above check out this video from 12:30 onward.

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[dilerin]

All you guys are right. My scanner is looking at timing and not cam position. Dumb? Of course, but I'm an 81 year-old stone age mechanic (shade tree) with much experience in stone age automotive "technology" including points, dwell, condensers, timing lights, etc. and it's VERY hard to break that mind set.

I will leave things set at rough base as it runs well there and look for a scanner that supports CMP so I can get within the legendary (+2/-2) range. Any suggestions on a scanner under 4 million dollars that will read it? Thanks, all, for the very helpful input.

 

[Jeff Janssen]

All you guys are right. My scanner is looking at timing and not cam position. Dumb? Of course, but I'm an 81 year-old stone age mechanic (shade tree) with much experience in stone age automotive "technology" including points, dwell, condensers, timing lights, etc. and it's VERY hard to break that mind set.

I will leave things set at rough base as it runs well there and look for a scanner that supports CMP so I can get within the legendary (+2/-2) range. Any suggestions on a scanner under 4 million dollars that will read it? Thanks, all, for the very helpful input.

CMP offest can be set without the use of a scanner several ways. The most interesting to me is the person (I believe on this site) who talked about rotating the dizzy slightly and seeing how quickly it throws a code. If the code comes quicker than the previous time, (measured in seconds) then turn it the other way and reset the code. Time how long it takes for the code to come back. Eventually you'll lenthen the time it takes for the the code pops up until it doesn't throw a code anymore at all. At that point, it's within the required range. I've never tried doing it that way, but I'm curious if it works and it's free to try. The original poster said it took them 5-6 tweaks of the dizzy. You already have a scanner that can reset codes so why not give it a try?

 

[454cid]

CMP offest can be set without the use of a scanner several ways. The most interesting to me is the person (I believe on this site) who talked about rotating the dizzy slightly and seeing how quickly it throws a code. If the code comes quicker than the previous time, (measured in seconds) then turn it the other way and reset the code. Time how long it takes for the code to come back. Eventually you'll lenthen the time it takes for the the code pops up until it doesn't throw a code anymore at all. At that point, it's within the required range. I've never tried doing it that way, but I'm curious if it works and it's free to try. The original poster said it took them 5-6 tweaks of the dizzy. You already have a scanner that can reset codes so why not give it a try?

The spec is 0 +/-2 degrees. The PCM doesn't throw a code until it's over 20 degrees out, so the above does not sound like a great plan to me.