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Base timing 1995 5.7 TBI

This is a discussion on Base timing 1995 5.7 TBI within the Technical / Maintenance forums, part of the General Discussion category; Among other things I've been trying to figure out, I was wondering about base timing on a '95 5.7 TBI. ...

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    Base timing 1995 5.7 TBI


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    Among other things I've been trying to figure out, I was wondering about base timing on a '95 5.7 TBI. I was checking the base timing on my truck tonight and while shining the timing light on the H.B., I cracked the throttle. The timing advanced from 0 to about 8 to 12 degrees. I thought the these systems didn't advance timing when the wire is disconnected. Are there other issues I should be concerned with?

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    Re: Base timing 1995 5.7 TBI

    Are you having engine problems to begin with?
    08 HHR LS - 2.2 | 5 speed | 30 MPG

    98 K1500 Z71 - 204k | L31 | 60E | 3.73's | 3" BL | 33x12.5x15 Dick Cepek F-C II's | Dynomax Ultra Flo X DI/DO | 14 MPG

    84 S15 Jimmy 2dr 4x4 - engine/tranny all rebuilt

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    Re: Base timing 1995 5.7 TBI

    Yup. It runs OK, but I've been chasing a stumble just above idle under a load. I've posted other threads on the driveability issues in detail.

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    pfk
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    Re: Base timing 1995 5.7 TBI

    Direct from the service manual:


    Refer to the Vehicle Emissions Control Information label under the hood. Always follow the Vehicle Emissions Control Information label procedures first before using the following procedure.
    Put the IC system in the bypass mode by disconnecting the "set timing" connector. This is a single wire sealed connector that has a tan with black stripe lead. This connector breaks out of the wiring harness below the heater case in the passenger compartment.
    With the ignition switch "OFF," connect the pickup lead of the timing light to the number one spark plug. Use a jumper lead between the wire and plug or an inductive type pickup. DO NOT pierce the wire or attempt to insert a wire between the boot and the wire. Connect the timing light power leads according to manufacturer's instructions.
    Start the engine, and aim the timing light at the timing mark. The line on the balancer or pulley will line up at the timing mark. The timing should be OTDC . If a change is necessary, loosen the distributor hold-down clamp bolt at the base of the distributor. While observing the mark with the timing light, slightly rotate the distributor until the line indicates the correct timing. Tighten the hold-down bolt, and recheck the timing.
    Turn "OFF" the engine and remove the timing light. Reconnect the number one spark plug wire, if removed.
    Reconnect the "Set Timing" connector.
    1995 GMC K1500 Suburban, 385cc, .040" over stroker,Patriot heads ,1.6:1 roller rockers,224-236 113 lsa cam , Edelbrock Intake (performer RPM) Holley TBI Adapter, BBC TBI with vAFPR,Transgo Shift Kit, Dynomax cat back with Edelbrock ceramic TES Headers. Paxton SN93 s/c, electric fans 4.10 posi

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    Re: Base timing 1995 5.7 TBI

    I'm familiar with the procedure, but I'm wondering why it would still advance when the IC is in bypass mode. It doesn't make sense.

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    pfk
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    Re: Base timing 1995 5.7 TBI

    I'll do something I usually don't like to do and that's guess at an answer :) (somebody that does a lot of tuning or datalogging may be able to confirm or refute)

    More importantly than timing advance, spark will not be retarded when it is disconnected.The timing disconnect wire is part of the EST circuit which its main input is the knock sensor, used for retarding the timing.

    Either way, if you know that the connector is disconnected, set the timing to OTDC at idle.
    1995 GMC K1500 Suburban, 385cc, .040" over stroker,Patriot heads ,1.6:1 roller rockers,224-236 113 lsa cam , Edelbrock Intake (performer RPM) Holley TBI Adapter, BBC TBI with vAFPR,Transgo Shift Kit, Dynomax cat back with Edelbrock ceramic TES Headers. Paxton SN93 s/c, electric fans 4.10 posi

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    Re: Base timing 1995 5.7 TBI

    Here's a shot in the dark about your stumble just above idle. I was actually having the same problem with my 91 and was messing with the timing and got no where. I unplugged the egr and plugged the vacuum hose and that took care of it, but we get into a whole new can of worms about why my egr valve is being commanded to open while taking off. Also I can't verify this with my chevy since I just jumped into chevy world, but with my honda, it would advance when i gave it throttle when i was timing it. That's how you can check for a problem hold it steady at a certain rpm and make sure the timing isn't jumping all over the place.

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    Re: Base timing 1995 5.7 TBI

    I'm not sure if the timing advancing while in bypass mode is anything to worry about. I finally found my problem and it was also the EGR valve. I did the unplug trick and voila, it lost the sputter. I put a new GM one on tonight and it's running good. I think it'll take days for the block learn to get readjusted, but it should remedy itself.

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    Re: Base timing 1995 5.7 TBI

    Quote Originally Posted by Matt K
    Among other things I've been trying to figure out, I was wondering about base timing on a '95 5.7 TBI. I was checking the base timing on my truck tonight and while shining the timing light on the H.B., I cracked the throttle. The timing advanced from 0 to about 8 to 12 degrees. I thought the these systems didn't advance timing when the wire is disconnected. Are there other issues I should be concerned with?
    Matt,

    You didn't indicate mileage on your vehicle. You could have issues with a worn distributor gear or timing chain/gears.

    With moderate mileage, you could expect to see a wiggle in the timing with rpm change, but not as much as you describled.

    With the EST wire disconnected, the ecm can't command any timing changes. So I would be looking for a mechanical fault.

    Good luck,
    Buddy
    73 Chevy PU, 350
    78 Corvette, orig owner, 48K miles
    87 Chevy PU, TBI, Lunati 256/262/108 Deg, Hyd flat, Custom chip
    98 Lumina, V6, commute
    03 Tahoe, wife's ride
    and the only Ford ever owned, 1965, 3000 w/6' finish mower.

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    Re: Base timing 1995 5.7 TBI

    I removed the distributor last week when I did the general tune up. I thought I better inspect the dist gear for slop or wear. It looked good. I also had thoughts about timing chain slop, but I agree, it wouldn't advance the timing 8-12 degrees from that alone. I had another thought. Could the bypass mode only set the initial timing to ZERO? The timing procedure explicitly states to time while at an idle. My thoughts are the the bypass mode only sets the initial timing to zero, but will still allow for advance, to a point, when the RPM's are increased. Would anyone be willing to try this on their truck as a little experiment? My truck is a 95 5.7L. It didn't take much RPM increase to get the timing to advance...maybe around 1200 RPM it would pop up from zero to 8 degrees. It was instant too, not gradual. After that, if I increased the RPM more the advance became more linear with the RPM's...

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    Re: Base timing 1995 5.7 TBI

    Matt,
    I was under the impression that no timing changes could be made with the est wire disconnected, but I'm not certain. I'm doing some tuning on my spark tables now and will give it a try to see what changes, if any, I get. Will let you know.
    Buddy
    73 Chevy PU, 350
    78 Corvette, orig owner, 48K miles
    87 Chevy PU, TBI, Lunati 256/262/108 Deg, Hyd flat, Custom chip
    98 Lumina, V6, commute
    03 Tahoe, wife's ride
    and the only Ford ever owned, 1965, 3000 w/6' finish mower.

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    Re: Base timing 1995 5.7 TBI

    Let me know what you discover. The advance from zero in bypass mode sounds reasonable. Perhaps it's a failsafe mode where if there's a sensor failure that affects timing, the ECM can still advance to some degree...is this maybe what people refer to as "limp mode"? Again, I don't know for sure, so anyone with an early 90's 5.7 TBI rig could try this at home. Put the ECM in bypass mode, fire up your timing light, then crack the throttle up to 2000 RPM. Note if the timing is advancing up to around 8 degrees.

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    Re: Base timing 1995 5.7 TBI

    I wonder if your timing chain is worn and getting stretched out slightly and causing this? Looser chains can cause erratic timing issues, but it sounds like your timing is fine at idle, not jumping around..just spit balling here..
    08 HHR LS - 2.2 | 5 speed | 30 MPG

    98 K1500 Z71 - 204k | L31 | 60E | 3.73's | 3" BL | 33x12.5x15 Dick Cepek F-C II's | Dynomax Ultra Flo X DI/DO | 14 MPG

    84 S15 Jimmy 2dr 4x4 - engine/tranny all rebuilt

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    Re: Base timing 1995 5.7 TBI

    Yes, the timing is rock steady at idle. I would think it would jump around a degree or two intermittantly. Anyways, for a stock setup, it seems to run better than ever, even when I hammer it WOT.

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    Re: Base timing 1995 5.7 TBI

    Quote Originally Posted by Matt K
    Let me know what you discover. The advance from zero in bypass mode sounds reasonable. Perhaps it's a failsafe mode where if there's a sensor failure that affects timing, the ECM can still advance to some degree...is this maybe what people refer to as "limp mode"? Again, I don't know for sure, so anyone with an early 90's 5.7 TBI rig could try this at home. Put the ECM in bypass mode, fire up your timing light, then crack the throttle up to 2000 RPM. Note if the timing is advancing up to around 8 degrees.
    Matt,

    Getting back to you on the above, checked my timing with the est wire disconnected and there was no movement in the rpm range up to about 2000 prm. My engine was up to operating temp when I made the check. Looking at the various tables for my 7747 asdu bin, there is a coolant compensation vs spark advance table that adds about 10 deg advance below 15c/60f.

    Again, I was under the impression that no spark advance could be commanded with the bypass wire disconnected but I could be wrong. You may want to make sure you're making your check at operating temp.

    Buddy
    73 Chevy PU, 350
    78 Corvette, orig owner, 48K miles
    87 Chevy PU, TBI, Lunati 256/262/108 Deg, Hyd flat, Custom chip
    98 Lumina, V6, commute
    03 Tahoe, wife's ride
    and the only Ford ever owned, 1965, 3000 w/6' finish mower.

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    Re: Base timing 1995 5.7 TBI

    Matt,
    Here is another take on possible timing problems. Know you said you examined your dist, but if you don't find the problem, may be worth another look.
    http://www.fullsizechevy.com/forums/...d.php?t=269577
    Re: timing ques.
    Posted by 1ugly
    i thought i'd share. i finally figured out why i had to keep retiming my truck. turns out the roll pin holding the distributor gear to the shaft was broken off on one side so the gear could wander around on the shaft. i have never seen this in my life but a new roll pin fixed my problems.

    Good luck,
    Buddy
    73 Chevy PU, 350
    78 Corvette, orig owner, 48K miles
    87 Chevy PU, TBI, Lunati 256/262/108 Deg, Hyd flat, Custom chip
    98 Lumina, V6, commute
    03 Tahoe, wife's ride
    and the only Ford ever owned, 1965, 3000 w/6' finish mower.

 

 

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