Ok I read through that whole other post that was linked above and from that Im getting that I should replace the knock sensor and get a Bosch dist cap with the brass contacts. I went to the Bosch web page and they don't even list a cap for the V8. Anyone have a part number or know where I can find one?

 

I think the other posts at the other site talked about if the resistance of the knock sensor was too high, replace it. I wonder about the Bosch stuff too...

 

You`ve had it on a scan tool, have you looked at cam timing or advance? The Cpi systems runs from the crank and cam sensor to set up the SFI....if the cam sensor gets more that -6 or +6 degrees out of sync the engine will begin to miss and pop but idle will be okay, I`ve replaced many a 96-99 vortec intakes for coolant leaks and if the "distrubtor" aka cam sensor get mis aligned, it will cause this, only way to see the cam timing is to have the scan tool on there and adjust the sensor to get it to "0" timing. Some of the vortec`s actually allowed a adjustment and some were lock down with a locating tab. this was the first thing that came to mind, eliminate this possibility before buying any more parts.

 

My scan tool didn't allow you to see cam timing. All of the elements I posted are what I could view on my scan tool. It sounds like it really needs a timing chain, and dizzy gear, but I'll be physically inspecting those items before I buy more parts.

-Tom

 

simply twisting the distbutor "cam sensor" & test driving in a trial and error pattern will tell the tale

 

My Vortec seems to be running much better now. The fact that it was weather related should have clued me in that it was related to the secondary ignition system. My problem was probably mostly just bad arcing inside the distributor cap.

You can see Chevy tried to make a cool design by rerouting the pins so they are in cylinder order on each side, but it makes the cap more complex and the conductor traces are running across the top of the cap.

I only have a cheap, generic scan tool that shows just ignition advance, I don't see any readout for the cam timing sensor. It is just a notebook PC with a cable.
BR-3 OBDII Scan Tool:

http://obddiagnostics.com/


Tom 400 sounds like he has a tougher problem than I did! I wish him the best with his Vortec.

Thanks all, signing off.

 

Hey I was lookin at my hold distributor cap and I had an idea. It seems that the plastic just doesnt block the arcing enough, so why not coat it with something that would. So I was lookin at my shelf of stuff and Exhaust paint jumped out because right on the can it says ceramic coating. So what I'm thinking about doing is taping off all the terminals insided and then spraying a couple nice thick coats of the ceramic paint on there. Any thoughts?

 

bad idea. The best dielectric that is readily available is ........ air. Only a vacuum or inert gas is better. Basically, think of anything that is in between two conductors as anything that can conduct(weakly), so anything solid has more molecules than a liquid, which has more than a gas. A perfect vacuum is devoid of any molecules which is impossible to reproduce.

what would perhaps help is to better vent the distrbutor, like the lt1 optispark, to draw a light vaccuum to draw away ionized air.

 

Welp...

The distributor gear was indeed, badly worn, even w/exclusive Mobil 1 oil chages, every 3k. But replacing the Dist. gear didn't fix it. It made it a LITTLE bit better but it's still definitely there. This sux.

The next step is to borrow a boroscope and take a good look at the cam gear. Then have someone hand-turn the crank back and forth while I watch the cam gear, and see how much slop is in the timing chain.

Question: how much vertical "slop" is allowed in the distributor shaft? Mine has about 5/32" which allow considerable rotor "retard" IMO.

-Tom

 

I am new to this thread, but if your dist. gear was worn wouldn't you suspect the cam gear to have some wear too? If you have a ton of miles and are going to replace the timing chain/gears, I would go ahead and do the cam too.

Just my $0.02. Good luck!

 

Generally the cam is made from much harder material than the easily replacable dist gear. I'll be using a boroscope to check the cam's gear though. At the same time, I'll have someone turn the crank pulley back and forth while I watch the cam turn to see how much slop is in the timing set.

Question: how much vertical "slop" is allowed in the distributor shaft? Mine has about 5/32" which will allow considerable rotor "retard" IMO.

 

That sounds about normal to me...did you ever experiment with small distrbutor adjustments to adjust cam timing?

my snap-on scan tool "mt 2500" shows the cam timing

 

Kind of. Here is the root of the problem:

Excessive timing system wear and the resulting "slop".

In my case I had (and still have some) wear in 3 places that was/is affecting the cam timing and rotor timing.
1. Timing chain. I backed the engine up to TDC #1, then using a Boroscope to watch the cam gear, had someone turn the engine forward until the cam starts to turn. At this point, the timing mark on the balancer has moved about 2" ahead of the TDC mark on the timing cover!! That is some SLOP!
2. Dist gear was very worn out. This accounted for several degrees of rotor retard.
3. Dist. shaft end play. As I said above, my shaft has about 5/32" of vertical slop. Obviously, the load of the oil pump will cause the dist gear to "climb up" the cam gear, until it hits the thrust washer and starts to spin. This would account for another several degrees of rotor retard.

Basically, I'm fully bought into the theory of spark jumping to the wrong post in the cap. You factor in full "mechanical" timing advance, then "vacuum" advance at cruise, then even more advance during EGR (when the symptom is the worst)...then you back the rotor up some 15* or so... Now the system is throwing a spark when the rotor is pointed somewhere between the cylinder it SHOULD be firing to, and the last cylinder that it went past. That makes it about "6's" as to which post the spark will actually jump to.

I "fixed" it (tricked it, really) by retarding the dist housing as far as I could (until the cap hits the intake plenum) which was essentially the same as advancing the rotor. It ran PERFECT w/EGR functioning, to work this morning.

In conclusion, I need to replace the timing chain, shim the dist shaft, and then "clock" the dist housing properly.

Incidentally, I also sometimes get a P0305 code (cylinder #5 misfire) only at 4500+ RPM in 3rd gear, but it never actually is missing; it pulls like a freight train. I bet you anything that it's because the cam is "bouncing" radially fore and aft relative to the crank w/in that sloppy timing chain, causing the ECM to “think” there is a misfire when there isn’t.

 

I have some more info.


Since I posted last my truck started running alot better. Well then our nice spring time weather went away and it got cold again, and the truck started running like crap again. Over the course of a few days the temp went up and down and I came to the determination that <60 deg runs like crap, >60 runs great. You can tell that the sound of the truck is completely different (it has straight pipes) so you can really hear whats going on.

Anyways today on my way to work I rolled 199,000 miles and right about 199,003 my service engine light comes on and the truck sounds like crap, and theres no power what so ever. So at lunch I went to the parts store and had them read the code. Miss fire on Cylinder #8. I look that the distributor and sure enough one of the ears where the cap screws in is cracked and the cap is a little loose. The tech at the dealer must have messed it up when he set the timing on the new distributor.

Anyways I decided that Im not going to install another piece of crap stock design distributor. Did some searching and it turns out that Accel makes a billit aluminum hi performance vortec distributor. It looks just like the stocker but the base is all aluminum instead of that fiberglass/plastic stuff that cracks. Through summit it costs $265 which is about what a stocker costs at the dealer. and a summit double roller steel timing chain set is another $40. Hopefully this will solve all the problems.

Oh yeah if your interested in that Accel distributor it doesn't show up on summit when you search by vehicle, you have to search the accel web page to get the part number (I think its 60116) and then put that into summit and it comes up.

 

Mine is still running perfect, w/no SES light.

I think you're wasting money on a $265 distributor. I think you need a timing chain, like I do, a factory dist. that's shimed correctly and timmed properly, and your problems will go away.

Since this thing has a crank sensor, a cam sensor and a rotor, it's all about getting the three of those items "in synch".

 

I agree that $265 is alot for a distributor but the one I have is already cracked and the GM distributor is just as much. The one I have now I got on ebay for about $85 a year ago and its no better than the stocker, at least with the Accel one I wont have to worry about the base cracking anymore. I would like to get another 200,000 miles out of this thing if I can, lol

 

That can't be the last post. What ever happened, any updates? Did you replace your timing chain, Dist gear? I have a 97 c1500 with the vortec motor, it has 147000 miles on it, and I have the same exact symptoms. A lil over a year ago I had to replace the fuel pump, made the mistake of buying the cheap one, replaced it again about 2 months ago. What a pain, need an access door in the bed. Some time in between that time the ignition module went. Now I have this problem, I've checked all vacuum lines, intake for leaks all is good. New cap, rotor, plugs and wires. Still bad, eventually the truck developed a small rattle, right away I knew that was the cat, took a quit look and she was glowing. Fixed that as any self respecting redne** (do people really find that insulting, I don't) would, hollowed it right out. Still have the same problem. With the cat have being clogged, I decided to check my compression, and make sure I didn't damage my valves or pistons, every thing is good even numbers across all 8. When pulling the plugs 6 were good still looked new, 3,5 had heavy carbon deposits. I've been told by a couple of mechanics that the plastic lines and poppets on the cpi are known to clog and that I should replace that, but like you I don't want to go throwing money away on a guess, especially when my 87 TA needs a motor, paint, exhaust, interior, etc, etc. This truck was supposed to be the parts getter, not the project.

 

I have still not replaced my timing chain, and it is still running great. Just turned 180,000 miles on my way to Lake Powell last weekend, and got 14.4 mpg, while towing my boat to boot. It's running awesome.

Basically Greezemonkey was right; it was the orientation of the dist housing reletive to the rotor. I needed to move mine to accomodate the timing chain stretch and dist gear wear (which I did replace). I SHOULD replace my chain, but unfortunately my truck is at the bottom of my free-time-to-work-on-stuff-list. The 'Vette, Boat, sleds, jetski, and Dodge Stealth TT come first. Mostly because the truck is so damned reliable w/nothing other than oil changes.

If your truck idles fine, and pulls smooth and hard at anything above 35% throttle, then you don't have an injector issue, so you're right not to spend monay there. My injectors/poppets are original w/180k and work perfectly.

How do you get around generating a cat efficiency SES code w/a gutted cat??

 

Well thats the weird thing about the truck, it never gives off any trouble codes, never even seen the check engine light come on. I wondered if it worked for a while, so I decided to unplug something and check, it came on an produced the proper code. Other than that, it's never come on. Well I am gonna go adjust my dist, see what happen. Well no difference, i really should be checking the cam and knock sensors I guess. I really think it is a timing problem. We have had this truck for about 8 years now, it was always taken care of. Then my dad decided to give it to my brother, that is really what is wrong with the truck, he only knows one speed. WOT!!!! He beat the bag out of the poor thing and getting him to change the oil, impossible. I guess it will stay parked for now I'll run through the sensors and if I can't come up with something, I'll take the heads and scrap the truck.

 

well if it melted a cat your way lean or way rich....have any way to get a data stream out of it to see what the o2 numbers look like? I have had problems with the CPI`s clogging, generally though just idle and above are usually affected. Expalin again exactly what it is doing.

 

Well the truck idles a little high around 1200 rpm. Engine runs extremely rough in the low to mid rpm range I'd say about 1600 to about 3000. I'm starting to notice now at WOT, it pulls hard but there is a lil bit of a surge to it. I'm burning through gas like it's nothing. Checked the compression yesterday and it was even in all 8 cylinders, when checking the plugs which are about a month old 6 of the 8 still looked new cylinders 3,5, had some pretty bad carbon deposits compared to the others. The truck has had a complete tuneup recently cap rotor plugs air and gas filters pcv, new fuel pump about 3 months ago second one in the past 2 years. I've checked all the vacuum lines, and the intake itself for leaks, all is good there. This problem started out of no where, it wasn't something that started out and increased in severity. I'm wondering if it could be a clogged or bad injector(s)
Couldn't that be the cause of my 2 fouled plugs. I don't know, guess I really just don't want to spend the money on the new injectors, so I want to be sure it's not something else. Why can't it just have a carb, so simple and easy to deal with.

 

Well those symptoms are nothing like what I and a few others were describing. I doubt that any of the information posted thus far in this thread will help you...except that MAYBE you could have injector issues.

Well it's not that hard to change it over...but you'll not be getting 20+ highway with a carb in that truck...like it should get now.

 

Tom`s right, you have something going on with the fuel injector most likley, try introducing a little extra fuel under a part throttle brake torque (carb cleaner into a small vac port while an assistant brake torques the engine to where the problem is most noticeable) this is an easy way to tell if it`s wanting more fuel.

 

Don't add more fuel. Your fuel pressure regulator diaphragm is ruptured and dumping fuel into the engine unmetered.

I had the same issue, very rough idle, although my setup (97, 3/4 ton reg. cab) threw a 'too much fuel' code. I was prepared to change out the injector spider, but it turned out, it is a regulator repair. The regulator is inside the manifold, and when it ruptures, it just adds fuel very haphazardly, causing stumbling, rough idle, and it was causing surging on the highway. Gas mileage goes out the window....not that it was great to begin with.

 

well, I have seen that too but they defintely do not idle at 1200rpms with a open fuel line inside the intake, besides this is a V8 CFI not a V6..the lines on this are external as well as the regulator. Lean or rich the computers stategy is to keep enough air vs. fuel via the throttle by pass valve.... it will open the steps during a lean and rich condition to maintain desired idle speed. Adding fuel to it is only a diagnostic step to solving the problem, there is a sequence to diagnosing such problems and having had ASE certifications for over 18 yrs now I think I now where to start Having the data stream would be the first step but, it is not an option obviously.

 

Greeze,

I'm not arguing that you are wrong about all of this, but the vortec v8 fuel regulator is indeed inside the manifold.....I've already taken 3 of these manifolds apart. No v6 here.

the regulator is a known problem and covers most of his symptoms, it's worthwhile checking into if he won't be doing any datalogging diagnostics.

it's a great idea to throw a fuel pressure guage on this system as well, since the operating pressure has a very narrow range of proper operation.

 

Yea the fuel lines and regulator are both on the inside of the intake. I just wanted to verify something, the pcv system pulls out of the pcv valve and pulls fresh air in on the opposite side of the motor off the intake tube right. I have alot of vacuum on both sides, take the oil fill cap off and it sucks your hand right to it with a good amount of pressure same thing with the dipstick. I have a friend with the same motor you pull the pcv valve and the truck stays running fine mine stumbles and stalls. I also found that 2 of the injector lines are leaking, so that needs to be fixed whether or not thats the whole problem well I won't know for a couple of weeks.

 

Your PCV valve isn't working correctly; it sounds like you have a healthy vacuum leak right through it. I.e. it's not restricting flow as it should. Replace that $4 part and report back.

-Tom

 

I forgot to post back on what I found with my truck eventualy.

Timing chain was stretched beyond beleif. With the cover off I watched the gears as I turned the crank. I turn the crank CCW until the cam just started to move, then turned CW through all the slack until the cam moved. I had over 40 deg of slop/slack in the chain. When it was in the middle of the slack you could pinch the chain all the way in until it would touch both gears.

I replaced it with the GM single roller timing chain from the zz4 as recomended further up in this post. Got it all back together and the truck runs better than ever. No more miss, hesitation....

I would recomned that anyone having issues with this to at least check the slop in the timing chain.

 

Indeed. Which is what I said here in post #63

 

I never doubted you a bit Tom. after I read that post I realized that was the one thing that wasn't new in my system and I was just dumb for not thinking of it.

Even so I never would have belived the chain could/would stretch that much without breaking and the engine would still start and run. After seeing that chain I was just glad it didn't cause something else to get damaged. If I get a chance I will take a picture of the original 200k mile timing set so you guys can see just how bad is was.

 

I have to post my story and thank you all! I have a 99 silverado, 4.3L Vortec V6. Light throttle misfire 2-3000 rpm, P0300 code once in a while. I had troubleshot every sensor possible(tps, maf, all ignition stuff, etc...) and they all checked good. I didn't believe it was the injectors because it ran ok WOT and idled perfect. I datalogged and found that under steady throttle/load on flat ground, everything would stay steady and then all of a sudden the timing would go from like 38 degrees to 18 and the truck would hickup/misfire. You guys gave me the idea to check the dist gear. It was very badly worn on 180 degrees of the gear. I don't know what could have caused this, but I replaced it and the misfire is gone. I'm thinking that the timing chain is probably stretched out pretty bad as you guys have suggested.

Anyway, a suggestion to anyone out there with a light throttle misfire, check the dist gear first!

Thanks!

 

That's cool that this thread has help some folks.

 

Holy cow, this thread saved me some money but I did spend some on 4 oxygen sensors, plugs, wires, cap, and rotor before finding this on a search. I just had to join this forum to tell my story.

In Dec. I had a code to replace the cam pos. sensor so I did. Shortly after that I developed a miss at cruising speeds and my mileage got terrible. I replaced all the parts listed above and even physically tore into the intake and cleaned every injector.

Still no change until I read this post and realized I may have moved the dist. when changing the cps. Well, last night in 2 min. I found out the bolt holding the dist. down was not very tight and after moving the dist. towards the intake and tightening the bolt, I went on a 15 mile test drive.

ALL is solved now. Unbelievable. Isn't it something little most of the time. The part that had me stumped was that it idled great and if you put the hammer down, she ran great thanks in part to all the new parts and clean injectors, LOL.

At least I found it now and it only cost me about $300 in parts.

Thanks to all for your efforts.

 

Now that is cool.

Same here. It was wierd, but of course, now it makes sense. Good job on the fix!

 

Thanks, Tom, I had hoped you would see my response. I didn't even put in that I have 1997 Sub. 5.7 Vortec with 192k on it. I am ready for the next 50k now hopefully.

 

Yep. Chagne the oil and those last for ever. Mine has 192k, and one of my workers has a '97 with over 260k....runs perfect. I wish you could still buy the same truck, brand new, now.

 

It was my catalytic converters!

Many thanks to all who contributed to this thread. You scared the mess out of me and at the same time educated me.

My 1999 Suburban with 5.7l Vortec was having very similar symptoms. It could run @ idle just perfectly. No DTCs.
When RPM was increased approximately 2000 or more, it would sputter and lose power then DTCs set were P0300, P0101 - sometimes. Other times it could run up to 3000RPM smoothly. The longer it ran only DTC P0300 would set.
After much fussing with wiring, fuel pump, various sensors, and sorcery for hours - I noticed the freeze frames showed vacuum was about 12 inHg. Seems it should be much higher.
Now it was time to deploy my shade tree mechanic training by romping on the throttle. Eventually the sound of empty cans falling out from under the truck occurred. The engine suddenly began to run more smoothly, but would stiil set DTC P0300. The freeze frame vacuum was about 18 inHg.
Finally the vehicle was drivable. A run for several miles seems to have further destroyed the cat cores and now no more DTCs and much more power. Vacuum is now about 22 in Hg @ 3000RPM.
Vacuum is the only measurement I really understand from the freeze frames. They rarely lead me astray. I have a spare ECM for comparison tests.
Now to find new cats. Groan.

Please consider Occam's Razor - please Google it if an explanation is needed.