Advice for tracking down a stumble and misfires

oefvet0708 said:

Looking for advice here. At first I was getting a 1345 Ckp to Cmp correlation code. I was able to re-set the distributor forward a tooth and got the timing set at +1*.

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(edit) See later post https://www.gmt400.com/threads/advice-for-tracking-down-a-stumble-and-misfires.69324/post-1444932)

Umm... just moving the distributor a tooth isn't necessarily the answer.

The distributor has to have a specific orientation as well, which is discussed in the FSM, otherwise this can happen:

The distributor can be SO far mis-aligned that the rotor is positioned nearer to the wrong spark tower in the cap and under certain conditions the spark mis-routed to the wrong cylinder.

See the page from the 1998 FSM, attached.

Well, vortecs are bad about moisture building up, inside of the distributor cap. This , in time, corrodes the cap, and rotor. Pull the cap, off, ( you may want to presoak the screws that hold the cap, to the distributor, with some penetrating oil first), and look inside to see what things look like, in there. If that checks out, good, then you can move on to other things. If you still have the OE, plastic distributor in you truck, be very careful. It is very easy to snap off the plastic ears that hold the distributor cap, in place. I swapped to a bullet distributor years ago.

1998_K1500_Sub said:

Umm... just moving the distributor a tooth isn't necessarily the answer.

The distributor has to have a specific orientation as well, which is discussed in the FSM, otherwise this can happen:

The distributor can be SO far mis-aligned that the rotor is positioned nearer to the wrong spark tower in the cap and under certain conditions the spark mis-routed to the wrong cylinder.

See the page from the 1998 FSM, attached.

Click to expand...

That’s what I thought too but no matter what I tried I could only bring the total timing to -8* until I did reset the distributor. Again, I bought the truck in this condition so I cannot tell you what brand or how old the distributor is. But in my research I did see where ppl had issues with the aftermarket distributor gears being off. All I know is that this is what fixed my p1345 code and it runs a lot better then it did and I no longer have a light on. There have been other ppl that have had this issue and this was the solution.

kennythewelder said:

Well, vortecs are bad about moisture building up, inside of the distributor cap. This , in time, corrodes the cap, and rotor. Pull the cap, off, ( you may want to presoak the screws that hold the cap, to the distributor, with some penetrating oil first), and look inside to see what things look like, in there. If that checks out, good, then you can move on to other things. If you still have the OE, plastic distributor in you truck, be very careful. It is very easy to snap off the plastic ears that hold the distributor cap, in place. I swapped to a bullet distributor years ago.

Click to expand...

The cap appears to be new. The distributor is aluminum. Unknown age or brand.

So as I mentioned yesterday, I did a data log this morning on my way to work which about 7 mi from home to my lot driving through my town. I was pretty surprised how well it ran compared to when I bought it a couple weeks ago and before I replaced the fuel pump this past weekend. I was able to get at least 75% throttle without any noticeable stumbling or hesitation. But on the scan tool it is showing a cyl 6 misfire upwards of 200 within a few seconds at cruising speed and rpm’s which I think is also transferring over to cyl 5 as well. Cyl 5 shows maybe 20 every few seconds but I think it’s a phantom misfire in the firing order.

I think at this point a good tune up is in order just to rule out the possibility of an injector issue. The previous owner did tell me that he “re-gaped” the plugs to .60 thou but when he checked them they were reading .80 thou. While I am in there I will do a compression check to check out the health of the motor since it’s got 240k on the clock

oefvet0708 said:

I did see where ppl had issues with the aftermarket distributor gears being off. All I know is that this is what fixed my p1345 code and it runs a lot better then it did and I no longer have a light on. There have been other ppl that have had this issue and this was the solution.

Click to expand...

I wonder what would happen if you removed the distributor gear, rotated it 180 deg, then re-installed it on the shaft. That gear has 13 teeth, an odd number; so, doing this would change the phase of the distributor shaft w.r.t. the cam by half-a-tooth, about 360 / 13 / 2 = ~14deg.

FWIW.

As @kennythewelder mentioned, the distributor caps on these can be a problem, and a lot of guys on GMT400 have posted about spark-jumping problems within / across the cap's electrodes.

Best to luck!

1998_K1500_Sub said:

I wonder what would happen if you removed the distributor gear, rotated it 180 deg, then re-installed it on the shaft. As I remember, that gear has an odd number of teeth; if so, doing this would change the phase of the shaft w.r.t. the cam.

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Ya that was going to be step 2 if just advancing the gear by a tooth and bringing it backwards wouldn’t have worked. Either advancing it by a tooth was going it advance it too much where I wouldn’t have been able to retard the timing enough which would’ve been the exact opposite of being retarded too much where I couldn’t advance it enough. So then rotating the gear 180*and finding that middle ground with the odd number of teeth would be the next step. But this way worked.

oefvet0708 said:

Ya that was going to be step 2 if just advancing the gear by a tooth and bringing it backwards wouldn’t have worked. Either advancing it by a tooth was going it advance it too much where I wouldn’t have been able to retard the timing enough which would’ve been the exact opposite of being retarded too much where I couldn’t advance it enough. So then rotating the gear 180*and finding that middle ground with the odd number of teeth would be the next step. But this way worked.

Click to expand...

(edit) See this later post https://www.gmt400.com/threads/advice-for-tracking-down-a-stumble-and-misfires.69324/post-1444932

It “worked”, but your rotor may not be in the optimal orientation with respect to the distributor cap. So you can say it worked…. but with the possible loss of optimal phase between the rotor and the cap, a spark jump to the wrong spark tower may occur under certain conditions of ignition timing and cause what would appear to be a misfire condition.

Check your installation against that detailed in the FSM, proactively. It should be pretty easy to do.

Anyway, learn as you go. I never had to solve your exact problem, so I’m speculating. When I stuck my distributor back in I didn’t have any phase issue to figure out, so call me lucky.

oefvet0708 said:

The cap appears to be new. The distributor is aluminum. Unknown age or brand.

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Think is, the cap can appear new on the outside, but without pulling it off, and looking inside, at the contacts, there is no real way to know. I have put a new cap and rotor on, in the past, that was fouled out in about 2 months. Yes, your fuel pressure, needs to be at or just over 60 PSI, pre starting the engine, when you cycle the key on, and the F/P comes on for a few seconds, and low F/P will effect performance, but don't underestimate the cap and rotor. Also the spark plugs play a part in all of this, as well. Personally, I run AC Delco, cooper core plugs. They are cheap, and what GM put in these trucks originally.

kennythewelder said:

Personally, I run AC Delco, cooper core plugs. They are cheap, and what GM put in these trucks originally.

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I thought they were platinum or over the years they updated the manuals to say platinum. I have had good luck using ngk tr5 platinums in pretty much everything gm