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Old 03-22-2004, 09:35 AM
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computer controlled timing

I recently bought a palm pilot powered scanner made by otx. As I was driving to work I was watching the live data and noticed that the timing was dropping considerably under acceleration. It would drop as low as 9 deg under accel and then return to 25 when cruising (it says 25 deg at idle) Does the computer tell me the total timing, is there mechanical or is it all computer?. Seems to me it should stay up in the 20's when the tach is over 2000, not the case with mine. Could the knock sensor be bad? The truck is a 98 4x with the vortec 454.

Any one have any Ideas I am used to the non computer stuff.

Thanks

John

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Old 03-22-2004, 12:03 PM
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rev. the engine in neutral.. what does your timing read. why did you get the palm scanner in the first place? what is the truck doing to make you go out and purchase one.. ?
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Old 03-22-2004, 01:07 PM
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When I rev it up in neutral the timing goes up to about 30 32 @3500 rpms. Now that I think about it I have been buying cheap gas. You think its knocking? This engine does have a little piston noise that gm told me was normal in these engines, it goes away when it warms up.

I bought the scanner to check error codes on my company trucks. The factory computers dont like propane.
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Old 03-22-2004, 01:27 PM
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Your PP is reading total timing. The computer must have the Idle timing set at a prescribed timing with out any computer input and this is called base timing. There is no vacuum or mechanical advance in your distributer so the computer does all the advancing and retarding based upon the various sensors such as oxygen, RPM, inlet air temp, coolant temp, and throttle position and vacuum. With all these inputs, the computer adjust the timing and the injector impulse to provide the appropriate ignition and fuel/air mixture. Wide open throttle under load is requires a retarded ignition timing to prevent "pinging". Cruise under light engine load is advanced. Sounds like your big block has a base timing around 6 degrees. My SBC 383 has a base timing of 10 degrees and my cruise timing is near 45 degrees and under acceleration is 34 degrees max. Every time I add throttle at cruise speed, the timing retards and when I lift the accelerator, it may advance but then will slowly retard and when I reach idle (warm engine), it will read 28 degrees total timing.. You should find out what your base timing is supposed to be and go from there.

Trees
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Old 03-24-2004, 07:51 AM
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Thanks trees. It seemed to me that the computer was pulling out a lot of timing. In my older truck with the mechanical at 2500 I have all my timing, around 26deg. The computer controlled at cruise is around 21, the only time it is higher is when I am coasting. I was thinking that the computer would give similar timing curves to the older mechanical style. This led me to believe that the knock sensor might be giving false signals thus causing the timing to be lower. Like I said before I haven't had much experience with the computer stuff.

John
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Old 03-24-2004, 02:03 PM
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Popaniac,

I think you might be right about the knock sensor. I have heard the piston noise can cause this. Your engine should have more than 9 degrees advance on accleration. It will not be as much as at cruise, but should not be that slow unless you are at like 1000 RPM. For a test, you might locate the knock sensor and just unplug it temporarily.

It is also possible that the distributor has been moved and that timing is too far advanced so it really is having some preignition. You need to check what a timing light reads and what the computer reads at idle. If they are different, adjust the distributor untill they match.

All the computor is doing is delaying an ignition event based on the known number of crankshaft degrees from the time it sees an ignition event on the sensor(probably hall) and the time that the rotor lines up with the cap electrode. If the distributor gets moved the ECU will not time properly.

Chris
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Old 06-09-2004, 09:25 AM
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Turbo( sorry for the delay) I went ahead and disconnected the knock sensor( after it was running for awhile) and drove it. The timing still drops considerably under acceleration, down to 6 degrees. The interesting thing is when its running in open loop the timing stays up and the truck runs like its supposed to.

So what other sensors would be causing the computer to pull out timing under acceleration. Or where could I find info on what sensors should read what and when.

Thanks guys

John
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Old 06-10-2004, 07:31 AM
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I am not sure what else could cause timing to decrese to 6 degrees at cruise. I dont know what that particular system is suppose to run, but something sounds very wrong with the way it is operating. You should be seeing around 40 degrees advance when cruising down the freeway. From there, the timing should retard back to 32-36 when you start to accelerate. Those numbers may vary a little, but should be close.

I dont know what else to suggest since I have never worked on one of these systems. You really need to talk to someone who has tuned them before to find out how it is suppose to act. It almost sounds to me like someone tryed to tune the timing table and made a big mess of it. Something is definately not right. If you can find someone to restore factory settings it might just fix it.

Chris
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