I had my motor rebuilt from a very well known engine builder in my area. I got the engine put back in at replaced everything back but with all brand new parts either ac Delco or Dorman parts. My issue I'm having is it runs like crap spitting, sputtering, and backfiring but once circuits close runs great just a slight miss can be felt. Spent 2 days adjusting the distributor was watching the spark advance on the scan tool was reading -25 to -30 no matter what we did wouldn't change. Found out that after each adjustment you have to shut the engine off then restart won't read live and started seeing results but could only get it down to -13. I was told by a mechanic that the spark advance on the scan tool is the timing and not sure if that's true because first it new to me second when we finally did get it down to 0 to -1.5 truck runs even worse. I have researched and can't find any info if it's timing or where the piston fires because if it is firing position means they are now firing pretty much top dead center and I know that's not good for the motor will bust the pistons. I guess my main concern is the spark advance being set at 0 now where should it be set at and I'm putting it in the shop Friday to get it straightened out after 6 months and lots of money I'm exhausted but just worried about driving it to the shop now. Do I need to adjust the distributor back or forward more. My scan tool doesn't read cam shaft position ordered one but hasn't arrived yet. Any help would be great appreciated going insane.
Having a bad issue with my 5.7 Vortec
- Thread starter Bigdawg69
- Start date
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Greetings Bigdawg69,
Welcome to the GMT400 forum! We have lots of talent in the audience with
years of hands-on experience...but alas we have no crystal ball capability. :0)
Seriously, help us help you. The GMT400 family has both the TBI and Vortec
versions of EFI, but I have no idea what you are working on. Please share
the year, engine, drivetrain, pure stock or modded, etc.
Reading between the lines I'm thinking that you are experiencing confusion over
the 'timing' of your Vortec engine, but we need some specifics from you in order
to proceed. And you will discover that sharing photos are a big help as well.
No doubt we'll eventually get this sorted out...
Cheers --
Welcome to the GMT400 forum! We have lots of talent in the audience with
years of hands-on experience...but alas we have no crystal ball capability. :0)
Seriously, help us help you. The GMT400 family has both the TBI and Vortec
versions of EFI, but I have no idea what you are working on. Please share
the year, engine, drivetrain, pure stock or modded, etc.
Reading between the lines I'm thinking that you are experiencing confusion over
the 'timing' of your Vortec engine, but we need some specifics from you in order
to proceed. And you will discover that sharing photos are a big help as well.
No doubt we'll eventually get this sorted out...
Cheers --
Don't confuse ignition advance with cam offset. Both tend to be listed in degrees; and both involve the ignition system (in different ways.)
My scan tool doesn't have a cam offset on it have one ordered that does but what should my spark advance be set at at idle right now it's set a 0 to -1.5 doesn't feel right or sound right to me. I'm putting it in the shop tomorrow but I have to drive it there just wanted to make sure I'm not gonna tear something up before I get it there. Thanks
SableSlayer
Newbie
You need to adjust the cam retard on the scan tool while holding RPMs at $1,000 and it needs to be within 2° negative or positive of zero. I was having the same issue reading it off of ignition timing and not Cam retard. You can read Cam retard with a Bluetooth scanner and the dash command app as long as you buy the extra PIDs.
On a Vortec engine you don't set the ignition timing / spark advance. You simply can't. Regardless of distributor position, the ignition timing is not affected... unless, of course, the distributor it SO far mis-aligned that the rotor is positioned nearer to the wrong spark tower in the cap(!) and the spark is routed to the wrong cylinder
Moving the distributor around only changes what's often called the "cam offset". Other members have commented about cam offset, above.
If the cam offset is wrong, the engine will still run more or less fine. The downside to incorrect / misadjusted cam offset is, IIRC, (a) the fuel injectors will fire in batch mode instead of sequentially and (b) the ECU will not be able to determine which cylinder(s) are misfiring, in the event that a misfire occurs, and (c) perhaps other downsides.
This all assumes your Vortec engine is being controlled by an appropriate ECU. You didn't describe your vehicle in your initial post but if, for instance, you happen to have a Vortec engine in a retrofit application where you're using a different ECU, then everything said here is wrong. I for one have an L31 5.7L "Vortec" crate engine in a 1995 S10, controlled by an OBDI 16197427 ECU, and that's a different can-o-worms.
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L31MaxExpress
I'm Awesome
On a Vortec engine you don't set the ignition timing / spark advance. You simply can't. Regardless of distributor position, the ignition timing is not affected... unless, of course, the distributor it SO far mis-aligned that the rotor is positioned nearer to the wrong spark tower in the cap(!) and the spark is routed to the wrong cylinder
Moving the distributor around only changes what's often called the "cam offset". Other members have commented about cam offset, above.
If the cam offset is wrong, the engine will still run more or less fine. The downside to incorrect / misadjusted cam offset is, IIRC, (a) the fuel injectors will fire in batch mode instead of sequentially and (b) the ECU will not be able to determine which cylinder(s) are misfiring, in the event that a misfire occurs, and (c) perhaps other downsides.
This all assumes your Vortec engine is being controlled by an appropriate ECU. You didn't describe your vehicle in your initial post but if, for instance, you happen to have a Vortec engine in a retrofit application where you're using a different ECU, then everything said here is wrong. I for one have an L31 5.7L "Vortec" crate engine in a 1995 S10, controlled by an OBDI 16197427 ECU, and that's a different can-o-worms.
I drove my Express van for nearly a year with a bad cam sensor pigtail. Only repaired it to get it through the emissions test. I noticed zero difference in the way it ran or MPG. After that I have run a LT1 with an 0411 with only a 4x OBD2 reluctior and no cam sensor.
@Bigdawg69, I earlier told you that the ignition timing isn't determined by the distributor position, but what I didn't tell you is how the timing is determined.
This is my understanding of the 1996 and later Vortecs, off the top of my head, subject to other's comments.
The ignition timing is determined by the crank position, i.e., via a crank position sensor near the snout of the crank. From it, the ECU can infer TDC, say of cylinder #1, but it can't determine whether TDC is occurring on an intake or exhaust stroke because the crank position doesn't provide enough information to do so, by itself.
Normally this lack of information (above) doesn't matter, because the distributor is by-design aligned such that the rotor tip is nearer the spark tower (in the cap) of desired cylinder-to-be-fired, so... when the ECU-determined-spark (with ECU-determined-timing) is generated, it finds its way to the desired cylinder-to-be-fired (say #1) and not the cylinder that's on the exhaust stroke (#6 in this example).
As noted in prior posts, the cam position sensor in the distributor is used by the ECU to determine which cylinder is being fired. This information facilitates the ECU's control of sequential fuel injection and the subsequent measurement of the AFR by the O2 sensor during exhaust blowdown, e.g., for cylinder by cylinder misfire determination and perhaps other engine performance measures.
So that's the rest of the story.
I hope I'm right
This is my understanding of the 1996 and later Vortecs, off the top of my head, subject to other's comments.
The ignition timing is determined by the crank position, i.e., via a crank position sensor near the snout of the crank. From it, the ECU can infer TDC, say of cylinder #1, but it can't determine whether TDC is occurring on an intake or exhaust stroke because the crank position doesn't provide enough information to do so, by itself.
Normally this lack of information (above) doesn't matter, because the distributor is by-design aligned such that the rotor tip is nearer the spark tower (in the cap) of desired cylinder-to-be-fired, so... when the ECU-determined-spark (with ECU-determined-timing) is generated, it finds its way to the desired cylinder-to-be-fired (say #1) and not the cylinder that's on the exhaust stroke (#6 in this example).
As noted in prior posts, the cam position sensor in the distributor is used by the ECU to determine which cylinder is being fired. This information facilitates the ECU's control of sequential fuel injection and the subsequent measurement of the AFR by the O2 sensor during exhaust blowdown, e.g., for cylinder by cylinder misfire determination and perhaps other engine performance measures.
So that's the rest of the story.
I hope I'm right
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nightowl3130
Newbie
I would check the crank-cam coalation carefully. I had this prob after repl 5.7 with longblock. I also foung an intermittant connection (1 wire only) at ckp sensor.
