73-87chevytrucks.com
73-87 Chevy _ GMC Trucks => Fuel Systems and Drivability => Topic started by: Stewart G Griffin on February 04, 2008, 10:58:30 pm
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What things could cause a momentary "cutting out" and sort of a "sneezing" sensation (and thus a slight "bucking") from the engine?
NOTES:
stock quadrajet and quadrajet manifold
crate 350 (260hp version)
stock exhaust manifolds
Everything is basically stock.
The "cutting out" and "sneezing" (and thus a very,very slight "bucking") does not happen too often and it's only for a split second per each occurance. It tends to usually, but not always, happen in groups of 3 or 4 at a time.
Tonight was really the first "normal" drive i've given the truck since i've sorted everything out since i got the truck. The only thing i've changed since getting the truck is switching to a regular ignition from the "ESC" ignition system that came stock on 83's. The distributor in it now i bought from a guy about 5 yrs ago and is of unknown origin/age/mileage. So, i'm thinking that ignition may have something to do with it?
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Sounds like a plug wire or connection causing a miss. You can do a quick test with a squirt bottle filled with water. Mist the wires thoroughly and road test. If it becomes worse narrow it down to the source.
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how fresh is the gas? perhaps a liitle derbis causing a hiccup?
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I let my fuel filter get so clogged one time it caused my truck to buck (or sneeze) :-[. But I'm sure you've already thought of that.
As far as Vile's suggestion about going over all the electrical connections, don't forget to check the battery cables. I replaced a loose terminal on the negative cable of a friend's car. The engine was running rough at times and the computer was throwing a code for engine misfire. Cleared the codes after fixing the terminal and it never came back. Engine runs noticably better.
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ok, i think i've solved the problem; It turns out that the #2 plug wire was not on all the way and i think that was the problem because it runs pretty smooth now. It is suprising how one little thing could cause driveablility problems. i had the same thing happen on another car--a 85 monte carlo. After i had finished changing plugs, i apparently did not put #5 wire on snug enough. It eventually fell off and it caused a rough idle, but no missing/sneezing/cutting out. Perhaps the reason was because on that car, the distributor is computer controlled?
2) The fuel wasn't real fresh because the truck sat for probably 4-5 months; i filled up last night with some fresh gas.
a) The fuel filter will have to be changed regardless because i've had the truck for about a yr and a half, the filter hasn't been changed since i got it, and i don't know the complete history of the truck.
3) As far as debris, that may have also been the problem(or became a problem as a result of the popping/sneezing) and there is a little bit more to the story and it is something i want everyone to be aware of and i will post more as soon as i can get my pictures up.
Thanks again everyone who helped.
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Well, as Paul Harvey says, now for the rest of the story............
What happened was, apparently, when the engine backfired, it blew(burned?) chunks of the foam out of the air cleaner. i suspect the foam was burned and blown upwards because i noticed some of the foam sticking to the underside of the hood.
(http://i223.photobucket.com/albums/dd31/sencraig/IMG_0234.jpg)
You might notice that the holes are not centered. This is because i purposely oriented the air cleaner pointing towards the direction of the left, front headlight so i more easily adjust the idle speed and return spring etc. (see my other post QUADRAJET 101 for more info) This is not the best quality picture, but the mesh grille is still there it's just the foam that was burned thru.
This is another reason why i don't like this type of air cleaner. The first reason why i didn't like it is because, while it's a nifty design, it injests hot underhood air. i was only using it because it was easier to adjust the carb while i needed to iron out some driveability issues. And, the only reason why i had it was because it came with another car i bought and used to own. i have since switched back to the stock air cleaner assembly and i like it.
i just wanted you all to be aware about this type of air cleaner apparently cannot take a backfire very well.
i hope the engine did not injest some of that foam
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WOW, I thought by now everyone who had one of those allready had trouble & threw them away. That is a perfect example of a poor design being sold over & over for MANY years creating havoc along the way. Not to pick on you, but that is what makes sorting out what a guy has going on for a problem tough without seeing the vehicle. If you would have started your first post like "I have one of those stupid cheesy little tri-angle foam air cleaners, does anybody think it could be the problem to..." We may have been able to get it sorted out sooner for you. :) My brother put one of those on a car 20 years ago, the first time he nailed it, down the foam went,coughed,caught on fire,stuck the carb open a little,ect.ect. Lorne
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i like the idea of the compact design and the re-usable foam (not re-usable anymore though), but the first problem is, along with any open element design, is that we are breathing in HOT, underhood air. The second problem is, i've read that this low profile design causes some sort of turbulence and actually restricts air intake somewhat.
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Stewart, Did you see the picture I posted of mine in the PERFORMANCE section under "cold air intake"? A K&N filter is all you need as far as re-useable,good flow & no HOT air,ect. It is relatively easy to make & should be dirt cheap. Have Fun, Lorne
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Yes i did and i like it. i also have more questions about the truck, but i'll get to those later.
Probably my next step, after i sort out the speedometer gear problem, is to get some dual cold air intake system in place. Although, i'm not against the high-end, top-dollar aftermarket stuff, i prefer to try to be frugal and to utilize stock type equipment first. It's more fun.
Since i can't weld right now, i know that 84 trans/camaros had a dual snorkel type setup. And, also 83-88 monte carlo SS's also had a similar setup. Repro or NOS might end up costing the same or more than aftermarket, so i'm actually leaning towards the ramairbox kit.
Actually, come to think of it, i could find another snorkel at the salvage yard and bring it to "my" welder and have him do it; He seems to get off on making hot rod parts as i had him make custom engine mounts another time.
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Stewart, You could also screw the snorkel on, leaving a flange from the donor air cleaner base. If you look at the photo I posted of my brother's Mustang that is how they are. Ford did this originally,so that is what he chose to do. Yes you are correct, the fun is in spending the money wisely, it is the ONLY limiting factor. If you have lots, you can buy/have built whatever you think of, if your wrong, you start over buying/having built the new plan. Continue on, Lorne
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Well, it turns out that it isn't the rest of the story. (My car problems are sort of like horror movies---the villan isn't dead yet!)
The truck started bucking/sneezing/missing again today, so it turns out that the #2 plug wire loose really wasn't the whole story. And by the way, i've got the stock air cleaner assembly installed so we don't have to worry about burning the bottom of the hood.
It was intermittent; It only did it about 5-6 times and i put about 140 miles on it today. So i feel that it is most likely a plug wire(s) going bad and needing replacement. Also, the truck was idling rough most of the time---like a cylinder was off.
Anything else it might be?
1) Your recommendations for plug wires?
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Could be a plug with a cracked insulator too perhaps?
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Is that the white, porcelin part of the plug?
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Yeppers.
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If you are pulling your plugs why don't you read them for us and let us know what they look like. Are the insulators a nice caramel color, or are they blistered and light gray, or maybe charcoal black? This could help the pros here diagnose your problem. Check them all to tell if only one cylinder is off.
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Always use the highest quality ignition wires, caps, rotors etc. No need for MSD or anything unless you have a high perf ignition system but just stay away from the cheapies.
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Air Cleaner/Air Cleaner housings Pt.2
NOTE: i'm going to address the ignition issues in a tic, but to continue with the discussion on aircleaner housings i (and yes i do work, but i came down with a very bad cold),
i was doing some reading and was somewhat shocked/suprised to discover that the stock air cleaner assemblies are actually designed to maintain the incoming air to the carb at 100F!
This isn't good, is it? As far as performance, mpg, and general driveability?
i never knew what the circular thing on the snorkle was until yesterday. Apparently, there is an air door inside the snorkle that regulates the amount of cold air vs. warm, underhood, air.
Would it be beneficial to disable this mechanism so as to provide only cool, outside air? NOTE: i am emmisions exempt.
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Stewart, I don't know where you live or if you use your truck in cold weather, if not, then yes, you don't need it there. If you do drive it in the winter, simply remove the hose from the exhaust manifold in the warmer months. The way most of them work is a tube brings warm air from the exhaust manifold up to the air filter housing,aiding in early drivability.The valve on top (door in the snorkle) is pulled open from vacuum once the heated air triggers a heated vacuum switch inside the filter housing allowing outside temperature air to enter. The pieces you see on my set-up are from when the truck was driven year round, none of which work now. I put a freeze plug in the lower portion where the hose goes on & the pull rods,doors,ect. are gutted from inside the snorkles. Also many air filter housings have a shield/deflector right inside of the snorkle which also can be removed. Lorne
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All this time i thought i was getting completely cool air (on the cars that had the ductwork intact), but no! i know the factory did it for emmisions reasons, but what is the physics behind feeding the carb 100degree air as opposed to as cool as possible air?
It would seem like doing things such as raising compression, advancing timing, suppling coolest air etc. would help the engine run better thus lowering emissions, but apparently not?
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Stewart, It is best to not try to think why or talk logic with anyone with basic knowledge of engines about what went on & why during the '70's-early '80's with the manufactures of vehicles. I don't have all of the answers, but know enough that the smart guys weren't making the decisions for themselves. There are allot of fators that go into vehicle rules & regulations,thus what & how it gets built. Also you have to look at the big picture, the manufactures have to make the vehicle operate for Jonny Q Public wherever he goes with his vehicle regardless of the temperature, ect. Where did you find the 100* air information? I'm not saying that is wrong, it's just not something I've ever heard or can't see how it is possable with say a stock air cleaner assembly on a '86 GM truck. There just isn't any "controller" for that to happen. Lorne
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The thermostatic air cleaner is designed to aid in EFE (early fuel evaporation)
Cold fuel in a wet intake manifold, be it carbureted or TBI will partially condense back into liquid fuel. Liquid fuel will not burn resulting in high HC emissions, fouled spark plugs, cold start stalling, washed cylinder walls, failed catalytic converters (especially in carburetted vehicles).
Once the engine warms up and the thermostatic vacuum switching valve turns off the inlet from the stove pipe the air is drawn in from the hose in front of the radiator support.
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The 100degree air info is in the Doug Roe book, How to fix,tune and install Rochester carburetors. i don't know the page # because the book is in my appartment. There is also a book called "How to fix your Chevrolet" by Bill Tobolt that says the same thing. Apparently, the air door will stay partially closed/open while the engine is running to maintain a 100degree intake temperature at all times!
Shocking isn't it? And apparently every Gm aircleaner from the 70s/80s with the circular thing on the snorkle does this. The door is never completely shut unless the engine is not running or outside temps are 100 or above!
Roe's book states that a 100degree air intake temp causes a leaner mixture which reduces emissions. i guess that makes sense; Colder air equals denser air which, i guess, would bring in more fuel.
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Ok, I'll do a little research on that. I have the Roe book. I guess part of my dismay to what you said simply is: What if it is 104* in Arizona? How does the hot engine keep the incoming air at 100*? Just making you think a little. Lorne
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It's designed to maintain between 85°F and 115°F. Again it's purpose is to maintain the best consistancy ensuring complete combsution, longevity of the engine, the oil, the catalytic converter and emissions standards. Their primary concern isn't if you can cram 10 more molecules of oxygen per gulp for tire spinning purposes.
That's why you rarely see any hotrodders running around with a functional thermostatic air cleaner.
Different emissions calibrations also had different specs...here are the specs from an 86 C20 with heavy duty emissions.
The TAC system is designed to improve carburetor operation and engine warm-up conditions. It achieves this by keeping the air entering the carburetor at a temperature of at least 100°F or more.
Carburetor air temperature is controlled by a pair of doors located in the air cleaner snorkel, which channel either preheated or under hood air to the carburetor.
Preheated air is obtained by passing under hood air through ducts surrounding the exhaust manifold, causing it to pick up heat from the manifold surface. The heated air is then drawn up through a pipe to the air cleaner snorkel.
Underhood air is picked up at the air cleaner snorkel in the conventional manner.
The two air mixing doors work together so that as one opens, the other closes and vice versa. When underhood temperature is below approximately 86°F., the cold air door closes, causing the hot air door to open. Hot air from the exhaust manifold stove is then drawn into the carburetor. As the underhood temperature increases, the cold air door begins to open until the temperature reaches approximately 115°to 130°F, at which time the cold air door is fully open and the hot air door is fully closed.
The doors are controlled by a vacuum motor mounted on the air cleaner snorkel. This motor, in turn, is controlled by a sensor inside the air cleaner which regulates the amount of vacuum present in the vacuum motor according to air cleaner temperature. Whenever manifold vacuum drops below 5---8 inches, depending on the unit, the diaphragm spring in the motor will open the cold air door wide in order to provide maximum air flow.
The vacuum motor and control door assembly in the left snorkel on outside air induction units does not have a sensor and is controlled only by manifold vacuum. This snorkel remains closed until full throttle is obtained. With manifold vacuum at 6-8 inches, the door will open, allowing maximum air flow.
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What i'm wondering is how would disabling that mechanism affect mpg? It seems to me that anything that helps the engine run better also, generally, helps mpg----like advancing the timing a little.
Then again, i heard that leaning out the carb as much as possible, but not to the point of damaging the engine, is best for mpg.
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I just couldn't resist workin' over Stewart, nice guy & for some reason I like to bust his stones. I honestly didn't know those units (termostaic switch) were that sensitive. I allways knew when the vehicle warmed up some, it opened the door & the next time you looked when it was cold, it was closed. Thanks Stewart & Vile, You allways have room to learn! Lorne
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Blows your mind, doesn't it? All this time we thought, after the engine warmed up, that we were getting completely cold air from the outside. Not so, Homer! And this started in 71.
It's like we are all a bunch of suckers, and GM is evil.
Anyways, it's on pg. 160-163, Chapter 9 in the quadrajet book. i found chapter 9 to be particularly fascinating.
But i'm still unclear about whether completely cold air vs. pre-heated air is ultimately better. Specifically, my goal(s) is/are, maximum mpg without sacrificing engine durability/reliability. Also note that i am emissions exempt. While i do care greatly about the environment, one truck is not going to destroy it.
We know that in a racing/high-perfomance situation, cold air and, if possible, rammed cold air is the best.
But for street/Daily driving situations, i read that warmer air aids vaporization which helps fuel distribution for each cylinder in a 4barrel v-8 setup. Also, warmer intake air=leaner mixture.
My theory is that for a multi-carbed setup, ideally 4 two barrel carbs, that completely cold air would ultimately be better for mpg. While colder air=richer mixture, you simply would not have to give it as much throttle to maintain 65 mph?
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Just out of curiosity. If mpg is the ultimate goal, why aren't you considering EFI more seriously? TBI kits are pretty cheap and they are going to offer better fuel economy potential than any carb setup. They are definitely less $ than an exotic 4x2 setup. Also keep in mind how popular late model engine swaps are with these trucks (tons of help available, no need to feel intimidated by this). You can score a complete used 4.8L or 5.3L engine for real cheap nowadays (of course with this swap its all the little "conversion costs" that really get you).
On another note you seem like the type of guy that might be interested in "lean-burn (http://franzh.home.texas.net/lean.html)" engines. I read somewhere that GM experimented with this in the 70's (?), but one of these never made to production. Honda makes some engines that run at about 22:1 A/F I've heard. There's tons of info available with one quick google search. Its a very interesting concept, I think.
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I believe you are referring to the PRE-combustion chamber 3 valve per cylinder design which all ended up with cracked heads, warped and blown head gaskets. Good in theory bad in real life. I do agree however Stew that you should consider a TBI swap. You can buy a bummed out suburban, blazer or pickup for $100-$200 bucks and just swap everything in.
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i am considering efi, and other things, seriously. Specifically the ramjet system from gmpp. But the problem, generally, with switching to efi is that after crunching the numbers, it doesn't make a whole lot of sense; i think with most aftermarket systems, i calculated that you would have to do about 60,000 miles just to break even. Considering that i don't put anymore than 300 miles per week, on average, on the truck AND if you factor in the interest of just leaving the money in the bank/other investment, as opposed to spending it on a fuel injection system, i may never see a return on investment (ROI). My calculations are based on the prices of aftermarket EFi systems to be at least $1000.
Just out of interest, i pay about $50-51 per fillup and i think my tank is 16 gals.
1) but are you saying that tbi units can be obtained cheap?
a) How do these units compare to other units in terms of mpg? Vs. Multi-port etc.?
2) The newer engine swaps, i think they are called "LS?" interest me too, especially displacement on demand. But, these aren't really straightfoward, bolt-in swaps are they? To me, what makes a swap economical is not only the result, but the time it takes to actually do the swap.
3) The other thing i really like is the idea of diesel. But here again, as in #1, it would probably take a long time to see a break even point and a return on investment. Some people say that the 6.2 has reliability problems; i don't know anything about them. i'm thinking towards a cat C7 or smaller (possibly a 4cyl) but that's probably mega $$$. i think diesels only make sense if you either drive alot or will keep the truck a long time or both.
There really is no free lunch when it comes to mpg.
i think stuff like headers, electric fan, tonneau covers, tires etc., stuff that doesn't cost a whole lot is where you'll see mpg gains that will pay off.
i think aerodynamics will be the key to getting good mpg with these trucks; It's not a disaster--the windshield is raked back pretty steep if you look at it.