73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Engine/Drivetrain => Topic started by: LTZ C20 on June 05, 2013, 11:36:17 pm
-
Alright guys, here we go. I've got miss fires with my 79 Chev Cheyenne 3/4 ton, sb350 with HEI. About a week ago I got really bad surging and miss firing out of the blue while driving, after a couple days of screwing with the ignition system I finally found that I had crappy spark plugs. So I got a new set of plugs in, drove the truck for about 2 days, all was well and the miss fires returned. Now I have gone thru and checked everything that I can think of. Just for precaution I got a full tank of gas and cleaned the fuel filter. Now, I have tested my coil, ohm'd my plug wires, cleaned my rotor and checked the cap for any signs of problems. All of these have checked out good with the exception of the plug wires. (Keep in mind this is with the brand new week old spark plugs.) I have headers on my truck so I have heat wrap around my wire boots at the plugs, there's enough heat wrap to cover the boot and wires completely around the header area. However I found the #8 cylinder wire boot had some cracking and was arcing to the head. I pulled that wire off, wrapped the boot neatly and completely in the heat wrap and used a zip tie to hold the heat wrap around the boot, insulating the cracked area. this works pretty good as there is no more arcing on #8. I found a second arcing at #6 wire, the heat wrap was leaning against the header, I repositioned the wire and rotated the heat wrap around so that there was air space between the header and plug wire. That seamed to solve the seconding arcing problem. The miss fire used to happen constantly, at any speed, being steady cruise or acceleration and at idle in Park or Drive. After all of the above mentioned tinkering, the miss fire has stopped AT IDLE but is still present as soon as throttle is applied. Now on to the coil, the coil I got used but tested working properly a year ago, I tested it again 2 days ago with two different ohm meters, both of which registered as having failing ohm readings. Would this mean that because the coil is used and worked then could be failing now? I have heard it's hard to kill a coil but is possible. I followed the testing instructions in my Chilton's manual, what they called the secondary test is what both my ohm meters provided me with failing readings. So am I just really looking at a bad coil that doesn't operate under load but is still working at idle?? This truck is my only vehicle and chasing miss fires while having to drive it everyday sounding like a straight 6 and lacking power is irritating, I'm trying to fix this properly on my own so I don't have to throw it in a shop. I was thinking about getting a new coil, putting new boots on all plug wires at the spark plugs and getting better quality heat wrap. Is this a good idea? Does it sound like I'm doing a good job of trying to fix it? Thanks for the input.
-
replace plug wires, re-check firing order and get a new coil, some fail only when hot some fail cold and hot. but i have headers and dont use heat wrap, havent had a problem since i been running this motor for 10 or so years. granted wires arnt that old upgraded a couple years later but never had a problem just dont them touch hot things or rub on other things
-
Ok, I was hoping someonee would say that. I'm fairly convinced the coil is crapping out. I will order a new coil and wires today at work and see what happens tonight. Also, I have already rechecked the firing order and timing multiple times, both are correct.
-
order? any parts store will have in stock. you dont need performance parts if you do want performance again parts store have it.
-
Yea I ordered the ac Delco original gm parts, they were here by the end off the day. Tonight I got the wires and coil on, it's running alot better, doesnt miss at idle, steady cruise or normal acceleration. But from a stop up to speed and on hard accel it's still misses, it's not a hard miss enough to be reduced power and slow accel, it powers thru the missing no problem but you can still here and feel it. As soon as you let off enough to cruise at a steady speed it goes away. Sounds like it's got one cylinder not running right or not firing. Its safe to say its definitely drivable but not not running smooth and proper like it should. My buddy has a 79' GMC 3/4 4x4 and that thing runs soooo good. Not a single problem. So it's irritating not getting this running properly.
-
Miss on acceleration is normally ignition, so the things to check first are 1) problem with one contact in the distributor cap 2) problem with one spark plug wire, such as cracked boot etc 3) bad plug (they can have cracked bodies new right out of the box) 4) arcing down the plug body due to water, oil, dirt.
Easiest way to check the distributor cap is visual inspection. Do any of the eight contacts look different?
Easiest way to check the wires is to use the longest wire from the old set, and sub it one wire at a time until the problem goes away. No need to loom it, just disconnect each existing wire at the distributor and plug and plug the old (but working) one on, one at a time, and test.
One way to check which cylinder it is is to disconnect one plug wire at a time and test. You should have TWO misses under acceleration on all but one, which will still have only one miss under acceleration. Pull that plug and replace it.
-
Ok so here is the new scoop, I got a new cap and rotor also. So far I have a new cap, rotor, coil, wires, and plugs, all Ac Delco O.E. parts. I had a loose contact in the old cap. The truck is running alot better, only misses and accel from a stop and hard accel from a steady speed. I'm gonna have to check the new plugs for a problem like Rich just said. However, I'm starting to wonder if my problem is not the ignition but my fuel system. Maybe I have a fuel pump not working right under load only or the pick up in the tank is clogged and not enough glow under load also. My other thought was that I have a valve that's burnt or hanging up and I should look at doin a compression check also. Any ideas on if my fuel system is bad or not? The pump is only a year and half old. Any ideas as too compression not being proper? Also I adjusted the idle mixture, it was running a little rich also, running evenly now, just got that miss.
-
Ok so I just did pulled one spark plug wire at a time from the distributor and start the truck and see if I have 2 miss fires in stead of 1. I couldn't hear any change. For all 8 cylinders, every time I pulled a wire off and started it, then revved it to listen for a miss and hesitation the sound never changed, it always sounded like 1 miss instead of 2. Did I do the test wrong or does that mean it could be my fuel system, my friend mentioned something, that maybe my accelerator pump is not working right??
-
Gotta do it with the motor running, pull a wire and listen for a change as you drop a cylinder, keep doing that all the way around if you pull one and there's no change then that's the cylinder that has the issue, if all cylinders cancel when you pull a wire then look elsewhere..
-
Kevin, connect a vacuum gauge to manifold vacuum and post the results (i.e., the vacuum reading and how the needle is acting - steady, bouncy, etc). How does the oil look?
-
What is your timing set at? Have you checked to see if your vacuum advance is working properly? Which port is your vacuum advance plugged into? Have you checked that your centrifugal advance is working properly?
-
Well I tried pulling it with the motor running, I had big heavy welding gloves on, insulated pliers, and I got the wire off but when I tried to put it on it shocked the crap out of me. I knew I would bare handed but not with big gloves and pliers. So I also reset the timing again cuz it was off a little. My timing is set at 10 degrees, my vaccum is comming from the vac advance port on my carb, like the carb instructions told me, I'm pretty sure the vac advance is working, is misses on accel, as soon as u let off a little and cruise or accel slowly it's fine, if you get into it then it misses again. I found my distributor hold down was bent up so it wasn't holding fully, I flipped it over so the bend pushed down on the flange and it holds proper now. I've gone thru everything I can think of. I put a stethoscope on my fuel pump and it doesn't make a weird noise, I did notice however that when the truck is shut off, my clear fuel filter drops down, the fuel drops low, then when restarted it rises back up and u can see the flow thru it, is it not supposed to drop down when off? Also, my oil is black but not thin or running, it's pretty fresh oil. And I'm not sure if this is connected but I have a short down at the starter wiring harness, it makes my lights stop working sometimes, is that a source of battery power to the dist? Could that cause insufficient power?
-
I use plug wire grips but even with insulated pliers you shouldn't get zapped, might want to check the wires they could be arcing??, easy way to do that is run it in the dark you'll see where it's at..
-
Oh, man. 10* BTDC is not advanced enough, and running the vacuum advance on ported vacuum will make it even more retarded when the throttle plate is closed. When you get on it from idle, the distributor is in the wrong position and takes time to catch up. Reset the timing to 16* BTDC (with the vacuum advance disconnected and the port plugged). As long as you don't hear knocking when you, say, floor the motor from low speed heading up a hill, you're good. You shouldn't hear any knocking, but if you do, back it off to 14* BTDC. Then move the vacuum advance line to the lower port ("non-ported" vacuum), I don't care what the instructions say. The lean mixture at idle burns slower, and so needs more advance, but the pollution control weenies moved the vacuum advance above the throttle plate to pass idle emissions tests. Which didn't really help real emissions, but there you go, they were playing the game by its own rules. That's fine, but it makes the engine run crappy. Prior to the emission controls era, the vacuum advance was ALWAYS taken from below the throttle plate. On ported vacuum, the idle will be rougher, throttle applications from idle will catch the distributor in the wrong position (fully retarded) causing a transition stumble until the advance catches up, and the advance will be all wrong when decelerating with your foot completely off the throttle, causing incomplete burning and ejection of unburnt mixture.
Black oil almost always means running rich, leaving carbon in the cylinders that gets swept by the rings into the oil. What kind of carburetor do you have and how many miles are on it? If it's the stock Quadrajet, it may be leaking gas at high vacuum past the fuel-well plugs, a known failure mode. These can be sealed up with epoxy if you know what you're doing. Get the Rochester Carburetors book by Doug Roe and see explanation on page 80, pics on page 81. It can also have the idle mixture set too rich. It can also be that the engine was modified (headers, high-performance manifold) without re-tuning the carburetor.
-
Oh, man. 10* BTDC is not advanced enough,
Well, maybe maybe not all depends on how the curve is set, total is what's critical need to set that first initial is what it is.. ;)
-
He's probably running the stock distributor with 20* of centrifugal advance. If not he should set the total advance to 36* once all the centrifugal advance has come in.
-
Ok I'm confused about everything Rich just said. I don't know how many miles are on my carb but it was professionally rebuilt 1 year ago, it's a 1400 edelbrock performer, it's not a 1401 or anything, it's a straight 1400, I also have the matching performer dual plane intake and long tube flow tech headers. Rich if you could give a more "watered down" explanation. Not that there's to many fancy words just info overload. And from what I've heard you don't wanna run really any higher than 12* BDC?? My vaccum advance only has vaccum when pulled open by the carb, it's labels vaccum advance port. I'm also not sure what you mean by upper and lower vaccum ports, all the ports on my carb at at the bottom, none are near the top, all are under the throttle plate, which is the same as the butterflies at the top of the carb right?
-
OK, you got the Edelbrock, so I can help you a bunch, that's what I have.
The plate at the top of the carb is the choke plate. The throttle plate is down in the carb venturi. See this page for the explanation if you want, but mostly for the pic. You want the vacuum advance connected to the 3/16" manifold vacuum (driver's side port), NOT to the 3/16" timed vacuum (passenger side port). See in the pic how one is a little lower than the other? http://chevellestuff.net/tech/articles/vacuum/port_or_manifold.htm
Do you know what metering jets and rods you have in the carb? It should be something like: seondary jets .094; primary jets .094; rods .068/.052. Do you have those numbers, or can you ask the rebuilder which ones he put it. I will tell you that the Edelbrocks come from the factory set richer than heck. You are supposed to tune them on the car, but most people don't, so the factory has to tune them so they won't run lean (and burn the valves under load like pulling a trailer on the highway) and burn the valves no matter what engine they are in.
I can tell you what jets and rods will work on your setup, because it's the same more or less as mine, but I want to know what's in there first. I may be able to set you up without having to split the upper and lower halves of the carb.
-
That's also the EGR carb. Is the EGR set up on your engine?
-
Ok lemme go over this and try again.
-
I just looked at the picture and realized they are slightly higher and lower, I also just realized thanks to Rich that the right is for emissions and I don't have any emissions. So I should switch ports. As to what metering rods and such I have no idea and they guy who rebuilt it probably doesn't remember haha. So today I should try switching vaccum ports, advancing timing to 16 and then drive around and romp on it to see if it stumbles?
-
He's probably running the stock distributor with 20* of centrifugal advance. If not he should set the total advance to 36* once all the centrifugal advance has come in.
Yes, 36* is a good number and should be all in by 2800-3000, if it can handle it 2500 is even better.
-
I just looked at the picture and realized they are slightly higher and lower, I also just realized thanks to Rich that the right is for emissions and I don't have any emissions. So I should switch ports. As to what metering rods and such I have no idea and they guy who rebuilt it probably doesn't remember haha. So today I should try switching vaccum ports, advancing timing to 16 and then drive around and romp on it to see if it stumbles?
Not stumbles. Knocks. Sounds like there's loose ball bearings banging around in the engine. But otherwise, yes, you've got it.
Also, you probably have the stock jets. You should pull the rods and get the numbers off them. Take off the air cleaner cover. Put a rag in the bore of the carb so you don't drop any parts into the engine. In the top of the carb, on either side, are two torx screws. Back them out about two turns, and the little metal plates rotate out of the way to expose the rods and step-up pistons. They'll jump up in the holes when you rotate the plates out of the way. Pull the rods and pistons out and use a magnifying glass and strong light to read the numbers stamped on the rods. Should be four digits, like "6852". Put the rods and pistons back, hold them down while you rotate the little plates back into position and snug down the torx screws. DO NOT OVERTIGHTEN THESE; stripping the holes would be bad!
Tell me what those numbers are and I'll tell you if they sound close or are way off for your engine setup, as well as what to replace them with.
-
If it's a 1400 it's calibrated for economy, if it's all stock it comes with the Metering Jets - Primary .098, Secondary .095; Metering Rods - .073 x .047; Step-Up Spring - orange (5" Hg)
-
If it's a 1400 it's calibrated for economy, if it's all stock it comes with the Metering Jets - Primary .098, Secondary .095; Metering Rods - .073 x .047; Step-Up Spring - orange (5" Hg)
If he has those settings, I'm running slightly richer in the cruise circuit and quite a bit leaner in the power circuit (.092 with .065/.052). With .095 stock jets, the better rods would likely be .070/.057 or .068/.057, given his setup is so much like mine. And 8# springs with the vacuum he's likely pulling, or the power circuit will come in too high.
I wonder how well his idle mixture is set up? You have to be pretty rich to turn the oil black.
-
If it's a 1400 it's calibrated for economy, if it's all stock it comes with the Metering Jets - Primary .098, Secondary .095; Metering Rods - .073 x .047; Step-Up Spring - orange (5" Hg)
If he has those settings, I'm running slightly richer in the cruise circuit and quite a bit leaner in the power circuit (.092 with .065/.052). With .095 stock jets, the better rods would likely be .070/.057 or .068/.057, given his setup is so much like mine. And 8# springs with the vacuum he's likely pulling, or the power circuit will come in too high.
I wonder how well his idle mixture is set up? You have to be pretty rich to turn the oil black.
I have the 1400 on mine and it's out of the box and it's right there wouldn't want to go any leaner but pulls very hard, but of course every motor is different and requires different tune up. But to pollute the oil like his you'd have to be dead fat from idle all the way up. One thing that'll make them run real fat is if the rods are sticking.
-
Well, you'd have to be rich from idle on up at least through the cruise circuit. Depending on how you drive and what the commute is, that might be enough. But he's not rich in the cruise circuit, so I don't know where the black oil is coming from. You could be right about the rods sticking, as long as they were sticking in the up position. That would run richer than a cob at cruise.
Kevin: you can check if the rods are sticking by opening up those little plates and seeing if the pistons are free to move up and down freely. You should be able to bounce them up and down with you finger (once you get the rod through the jet). If they're sticky, a shot of carb cleaner into the pistons may be enough, or you could pull the pistons out and clean them off, and clean the cylinders with Q-tips wetted with carb cleaner.
-
Edde's are just a glorified Carter and I remember way back in the day it was common for rods to stick open, had to filter the fuel real good. Only other thing I can think of at the moment would be floats sticking open???..
-
Thanks for all of the info guys! It's alot for my brain to take it but I think I got it. Today I will set the timing to 16* and switch ports, tonight or tomorrow if I get a Lil time I will pull the rods out, get the numbers, clean everything and put em back in. Ide love to have this running alot better. Hopefully should be a big up in horse power right? Plus it seems like lately, past month, I've been goin thru alot of fuel, that should be due to all my parts being way out of whack?? Thanks again, I've learned alot
-
If the rods are sticking in the up position: 1) it will run crappy, 2) it will turn the oil black with carbon, and 3) mileage will suffer.
Sound familiar?
And they do gum up.
So that's certainly something to check on. I was surprised how gummy mine were after just a year when I recammed the engine and switched rods to get the mixture right.
-
Okay guys here is the scoop. I set the timing back to 16° and I switch vaccum ports like Rich said. I pulled the metering rods out of the cylinders and they were pretty dirty, they were covered in this blackish gray stuff that has this really foul smell that guessing is the carbon buildup. So I got the metering rods and the springs and the cylinders and those little metal parts and they're all clean and back again, I fired it up and let it run for a couple minutes while I put my tools away and then I power braked the truck and it didn't feel like it was missing it just felt like it was being held back by the brakes. So I haven't driven it yet but I will after work on my way home and I'm hoping that all is well and it should be running pretty good it sounded better when it was idling.
-
Sounds promising. Dirty rods together with retarded timing will screw up a lot. Surprised it ran as well as it did!
-
Yea! Well now I'm getting really irritated. It power braked pretty good at lunch earlier, but when I drove it home it was still doin it not as bad. Then I drove to my friends and it's getting pretty noticeable. I checked the timing again, yesterday set it at 16 like Rich said, today it was at 14, the. I revved it, and back at idle it was back at 16 then moved to 18 and then to 17. It's like something is forcing the distributor over or pulling it out of time. My timing clamp bolt it tight and it seems to be holding so I don't get it. It sits at a number then if you rev it and let it go back to idle it resets at a different number. After all this work I'm getting pretty aggravated at this.
-
Sounds like the advance is sticking, pop the cap and check the weights they could be crudded up, make sure they're clean and moving freely.
-
Sounds like the advance is sticking, pop the cap and check the weights they could be crudded up, make sure they're clean and moving freely.
X2, when was the last time you adjusted the valves? Prob bout time
I'd pull the valve covers off and inspect for flat cam lobes. Or push rods not spinning
-
Ok so, I pull the cap off, then what I'm I checking??
-
Under the rotor you'll see two weights and springs, take them off and clean everything and put a film of grease on the little round pad the weights rest/slide on, make sure they move freely their full travel.
-
isnt there supposed to be little plastic bushings also?
-
Hmmm, I think the newer ones have the plastic bushings?, I got a bunch of them and none have them but they are all early 80's on back.. But if it does have the bushings they probably need to be replaced by now..
-
See the third picture in this article.
http://www.chevyhiperformance.com/howto/97438/
-
That one don't have the bushings on the weights, only bushings used on those are on the limiting pins to shorten the curve.
-
Hey, I was wondering if maybe a video of the truck idling or driving would be of any use? So you guys can hear it? Also, an old hot rodder at my work said I should check the timing chain variance. Pull the cap off, set the motor at TDC and then rotate the crank with a wrench. He said if there's more than 2* I could have valves not opening fully or at the right time? How's that sound to you guys?
-
As long as the lobes on the cam aren't flat the valves has to open, only thing a stretched chain does is retard cam timing and if anything will give a little more top end and won't cause eratical behavior.
-
Ok I just tried to clean those, the had a little crud on them. I cleaned em best I could, I didn't realize they came off without pulling the distributor. So tonight I'll try again. I did try and push the apart but they seemed to not expand very far, maybe a 1/4 or 3/8 inch?? They seemed like they were moving a little bit then stopping against something underneath them, is that normal? I thought they didn't go that far but if there supposed to go further then I'll try and pull them off completely tonight. The cleaning I did just now didn't seem to do anything.
-
There is a little c clip that hold them on. Just slide it off, pick them up clean them and the base they sit on. This may not b the issue. Take the valve covers off . Turn the engine on let it idle then see if all valves and rockers are moving the same distance. Also check for pushrods that aren't spinning. If you have a close to or all the way flat exhaust lobe on the cam that will b the miss you are having as the engine will inhale but not b able to exhale...
-
Ok, that makes more sense, so how far are the weights supposed to fold out when I do it by hand? Also, if the timing chain is stretched then it won't let the cam be in time with the crank so wouldn't the valves be moving slightly off from when they should? Causing a miss right? The engine is running almost perfectly, it's got that miss on accel from stop and hard accel from cruising. If the cam lobes are flat wouldn't it miss all the time?
-
Not unless the valves were too tight... Your case is diff. But it is worth the check.
-
Ok guys I've gone over everything and I've done a timing chain variation check, I haven't pulled the valve covers. I'm gonna drop it off with a guy my friend knows, guy like you guys, a new set of eyes to just start fresh and check it out. This guy worked on my buddies truck since his dad bought his 79 gmc back in 83. Guess we'll see what happens. In just tired of struggling with this stupid motor. Haha