I've got the W vin Vortec, and ever since purchasing my truck about 9 months ago I've had a light miss under light throttle (I don't drive it every day and didn't ever focus on fixing it until I had spare time). Fair idle (no noticeable miss), problem is gone at WOT, usually misses around 2000 rpm or so, highway cruising speed.

I replaced all the usual maintenance issues: plug wires, plugs looked good, cap/rotor, TPS checked, coil/ign control module checked, no slack between crank and dist so I assumed it didn't jump time, and previous owner already had the injector spider assembly replaced (which honestly should be the last thing to look at IMO).
Trying to solve my problem, I came across some other issues (water pump was apparently installed by RETARDS at some local shop while with previous owner), but the miss never went away.

My cap was the first issue, and what I've found is it's common for these things to go out because of the horizontal plug tower design. I very recently checked the cap I installed again and noticed the spark was hitting the dist. cap posts at a weird, advanced angle. I hunted through my alldata subscription trying to figure out what sensor to replace, thinking spark advance was all electronic.

It isn't.

Inside the distributor's shaft is a mechanical advance (so I'm told), and apparently mine had worn out. If you are having that horrible P0300 code and don't want to try the spider assembly yet, take a look at the spark pattern on your distributor cap. A new distributor with lifetime warranty was about $100 with my trade-in core. :tu:


Just thought I throw out my experience, this thing had me scratching my head for a LONG time.

-Kenny

 

I will look into this. I have an SCFI distributor sitting in a box in my garage that I can take apart and snap some pictures of to show everyone else. I never noticed a mechanical advance when I had mine out of my 2000 Bravada, but I will have a look again.

The other typical thing is to get a worn gear on the bottom of the distributor that will throw off spark timing.

Thanks for sharing your experience!

 

That worn gear could be a possibility as well, I didn't take a good look at mine as I just returned it for a core, but my issue was definitely distributor related, one way or another.

 

is it true that the distributor shaft itself is a magnet? i read this, but is it true for 1995 CPI Vortec "W" engines with the plastic horizontal distributor? or not?

The magnet, or more properly the reluctor, on the distributor shaft has 3, 4, 5, 6, 8 or 12 "points" on it depending on the number of cylinders the engine has. There is also a sensor, called a pickup, which senses the magnetic field of the point as it passes triggering the spark for the spark plugs. The distance between the point and pickup is usually only 0.006". The reluctor is round and the inside is one pole and the outside is the opposite pole. One is North and the other is South. This gives the reluctor a specific magnetic field.
When there is a crack in the reluctor, one side of the crack becomes North and the other side of the crack becomes South. This changes the magnetic field and confuses the pickup because it's looking for one type of magnetic field and is getting one totally different. This change causes some of the points to be stronger and others weaker resulting in loss of signal and engine misfires.
On some distributors, the reluctor is replaceable separately. On others, you need to replace the whole distributor.

 

Hello, I'm wondering if this could be my issue. I have a 97 Blazer with 190k miles and have a problem with severe missing under light throttle from about 2 to 3k rpm, and while cruising at highway speeds. When I mash the pedal, the engine smoothes out. Idle is fine. Sometimes it takes a few seconds on the key for the engine to start, while others, it starts right up. Its giving the P0300 code, and I was about to start changing a whole bunch of things, but figured I'd poke around on this site for some help.

 

I have the same type of misfire on my 95 S10 and i replaced all the basic tune-up parts and only the idle misfire went away. but the other misfire under light throttle cruising or between 1k and 2k is still there. it only smoothes out during hard throttle like everyone else. im going to try to replace the entire distributor on my truck but only after i replace the lower intake gaskets and the injector because its leaking fuel in there anyway. then after i can rule that stuff out. but keep us posted on what you do on yours.

 

on the second design 4.3 vortec the distributor main function is to control the injection timing the crank sensor does the spark timing the distributors are junk in these and are prone to failure they will still run because the pcm has a default program to time the injectors but it will stumble and hesitate with loss of power and fuel economy possibilitys if you go cheap on this distributor it wont last long my uncle had put 4 of the stock style replacements in a 99 in 100k they wipe out the drive gear and nock out the shaft bushings due to poor design the rotor isn't balanceed causing those problems and they commonly strip were the cap fasten if you serch you can find aftermarket with alum housings and balanced rotor ect. they hold up alot better there isn't any advance in the distributor and basicly only consists of a pickup coil that sends pulse to pcm to time injectors and there is no timing adjustment by turning the distributor all you do is make sure at top dead pull distrbutor and when you reinstall new line mark with 6 on distributor housing as you put it in if it is not lined up with 6 it is off a tooth

 

Pictures I promised everyone:


The rotor base plate is press fit onto the top of the distributor shaft itself. This should not be a loose connection ever. On the used distributor that I have, it is solid and would prove to be quite difficult to remove if I tried.

The shaft rides in two sleeve bearings, one at the top of the distributor body and one at the bottom. On this distributor, the bottom bushing has some play in it which is likely due to the amount of use. It may be allowing for some deflection while running which could further modify timing. The distributor itself does not offer up much, if any, resistance to turning. However, the oil pump does take power to turn and being driven by the distributor shaft. This deflection can change the engagement between the cam gear and the distributor gear, modifying timing possibly outside the bounds of what the PCM can correct for. Combine this with wear on the drive gear and it can definitely become a problem.

 

this is very good to know. it would explain why my truck started up and ran around town with the distributor un-pluged! i forgot to plug it back and it ran like that for a few weeks, lol. mine stumbles and hesitates with loss of power and fuel economy right now... is it at all possible that the rotor base can become magnetized somehow and then send the wrong signal when it passes through the cam sensor? the cam sensor has magnets in it that a north and south i think. however, i know they call it the hall affect. what you think?

 

I was having the multiple misfire problem not too long ago also. I did all the common tune up stuff and it was still present (although much less often). I got a new distributor from Summit Racing and took great care to make sure it was installed properly. It runs like a champ and idles smooth, but my diagnostic code changed to P1345. I verified that I am not a tooth off on the distributor by moving it a tooth in either direction. (1 tooth clockwise it won't start and chugs oddly. 1 tooth counter clockwise it starts but idles very lopey. 2 teeth counter clockwise it won't start and is chugging oddly.) Everything bolts in tight and there is very minimal play in anything.

Is it possible to be 180 degrees off on the distributor? Maybe Im not saying that right... is it possible I used the decompression stroke when I aligned my marks? Is it possible I got a bad sensor in the distributor?

Like I said, the truck runs fine. Aside from getting very, very poor fuel economy (maybe I'm just used to my gas-sipping Civic) it runs like a champ.