Where is #1 on cap???

[hubidaddy]

I rebuilt the top end of my 87 350 tbi with the help of a friend. It seemed when the distributor was put back in it was not where I had drawn the rotor to be pointing. After the timing was properly set and it still didn't run right, we tried to correct for this by shifting the wires on the cap. Since then I have tried 3 different positions of wires, 6 positions of distributor to cam gear placement and now just learned the cam sensor is in the distibutor. The only way to get the distributor in the only acceptable tooth position is to be sure the rotor is pointing at #1 at tdc. None of my documentation specifies this, and I failed to document it as well. Cars were alot simpler before computers!!! Any help??

[GMMerlin]

Put the #1 piston on TDC.
Install the distributor with the rotor tip pointing at #1 cylinder.
Firing order 18436572 clockwise.

87 was electronic ignition..no cam sensor in the distibutor

[dirty dan]

With the timing mark lined up to "0", you'll either be at #1 TDC or #6 TDC. Normally when you set a distributor in a GM engine, you'll want the #1 wire position on the cap aimed toward the #1 cylinder.

In the future when you re-set a distributor and the crank hasn't moved, you'll have to move the oil pump slotted shaft back about a 1/8th turn so everything can line up as before.

[Ketch]

Originally Posted by dirty dan

With the timing mark lined up to "0", you'll either be at #1 TDC or #6 TDC. Normally when you set a distributor in a GM engine, you'll want the #1 wire position on the cap aimed toward the #1 cylinder.

In the future when you re-set a distributor and the crank hasn't moved, you'll have to move the oil pump slotted shaft back about a 1/8th turn so everything can line up as before.

Have a 99 5.7L R that I just replaced intake manifold gasket. Thought I'd marked position of rotor but seems to have a mind of its own. As I drop it back down it will not go into same position but will go into different positions for each different attempt, if that makes sense. As if something down there is moving around on me although I have not moved the crank. I was wondering if the advance is built into the rotor shaft and has something to do with the slot and gear teeth not lining up.

If I get close to what it was but the engine runs rough after, what is the easiest way to tweek the timing? Do I need a scan tool or something or can I just play with rotating it until it smooths out? Merlin, dirty dan, hope your round somewhere...

[GMMerlin]

Originally Posted by Ketch

Have a 99 5.7L R that I just replaced intake manifold gasket. Thought I'd marked position of rotor but seems to have a mind of its own. As I drop it back down it will not go into same position but will go into different positions for each different attempt, if that makes sense. As if something down there is moving around on me although I have not moved the crank. I was wondering if the advance is built into the rotor shaft and has something to do with the slot and gear teeth not lining up.

If I get close to what it was but the engine runs rough after, what is the easiest way to tweek the timing? Do I need a scan tool or something or can I just play with rotating it until it smooths out? Merlin, dirty dan, hope your round somewhere...

Sometimes you will need to turn the oil pump drive shaft a bit to get the dist. to drop in the right spot...the biggest problem most people have on the 96 and up trucks is getting the dist. set in the correct position..
If the dist is installed incorrectly (but close), the engine will run rough and the SES light will come on..
If that happens, you will need to set the camshaft position offset...you will need a scan tool to do this....turn the dist. until the camshaft offset reads 0 degrees at 2000 rpm...

[Ketch]

Originally Posted by GMMerlin

you will need a scan tool to do this.

Thanks Merlin.

Would you recommend a scan tool brand/model that would do the job (without costing me a half grand.) I see some scan tools that cost $750 and beyond. What about Actron?

[GMMerlin]

Most of your good bi-directional scan tools are in the $2500.00 range.
Personally I use the Tech 2 from GM tools...

[Ketch]

Well I really need a basic scan tool for DYIer's but would like it to be upgradable for future cars I may own. Don't need backward compatibility for OBD I.

Anyway, on the timing issue I got it put back according to the marks I made and it seems to run fairly smooth. I just wonder how many degrees (rotation of base relative to rotor) of error does it tolerate? I mean, I can't possibly have it exact to a millimeter back to where it was. Yet it "seems" to run fine - so far.... Does anyone understand this well enough to know if the computer compensates for minor timing offsets at all?

[dirty dan]

What type of timing advance on your distributor?

[Ketch]

Originally Posted by dirty dan

What type of timing advance on your distributor?

there appears to be a swivel mechanism on the dist. shaft just above the gear that I ASSume has something to do with the advance. But since putting it back together it seems to run fine so I must have got it dead on.

At the risk of getting way too technical, I would speculate that perhaps the onboard computer has somehow compensated for any fractional variation I may have induced. I mean, when I pushed the dist shaft back into its gear slot and then rotated the housing back to my markings, I could have been off by half the width of the Sharpie pen marks I made on the rotor and housing - which would be appx. 1/16" wide. Just for argument, let's say if the distributor housing has a radius of say 1.5 inches (i don't have one in front of me right now to check but stay with me here) - Formula for circumference is 2 pi R so 2 pi times 1.5 gives radius of 9.42 inches. Divide by 360 degrees to get appx .025 inches per degree. 1/16 inche equals .0625 inches so you can see I could easily be off by some fraction of a degree! Plus, if you look at the shaft just above the gear you'll see the swivelling mechanism that could also induce variation (still don't know what that thing is)

So the question is, how many degrees +/- tolerance of rotation is there and how much does the computer compensate. When I first drove her out about three miles and stopped to head back, the engine at one point started to rough-idle but only for about a minute (she was fully warmed too) then smoothed herself out. So, without the luxury of a $2500 scan tool - but armed with my trusty Sharpie marker pen - I just have to guess the computer somehow is compensating for any errors in timing I may have created. So, who really understands GM onboard computer firmware well enough to confirm or deny this? Any GM software engineers hanging out here?

[dirty dan]

Timing check does not require a scan tool, just a timing light. Base timing specification is indicated on your emmisions label on the radiator bulkhead cover. Timing mark is on the harmonic balance and the scale should be located either at 12 o'clock, or around 2 o'clock as you face the harmonic balance.
On the older 5.7 you do not have a crank or cam shaft position sensor. The base timing set is important more for the ECM generated curve of the advance than for the amount of advance itself. If you're within four degrees your in good shape.