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Engine knocking on acceleration/ stall problems

C

cmoore1224

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Alright guys, I really need some help. I'm just 16 trying to figure out the problems on my first truck, an 85 K5 Blazer with the stock 305. One of my problems is the RPMs are very splotchy when I'm in park. Sometimes when I switch gear it just gives out. My main problem is that the engine has a knocking sound which I can get a video of and link up. It almost sounds like its supposed to be there but I highly doubt it! Please help because I really want this truck to last my high school years. Will have a photo up in a little

Picture album:

http://m1157.photobucket.com/albumview/albums/cmoore1224/Mobile Uploads/IMG_8857.jpg.html?o=0

Video:

http://m.youtube.com/watch?v=bd48dh5K35I

http://www.youtube.com/watch?v=WIwnd9clttY&feature=context&context=G2d5e63bAUAAAAAAAAAA
http://www.youtube.com/watch?v=xcy0dIwiUWA&feature=context&context=G2d5e63bAUAAAAAAAAAA
 
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timing could be off, its hard to tell from the video just when i can really hear the noise when you start to accelerate the video ends. I would start with checking for vacuum leaks around the carburetor, intake manifold, and all the rubber hoses that come off the carb and intake. you can mix up some soapy water in a spray bottle and spray it around the base of the carb and where the manifold bolts to the heads around hoses ect if rpms pick up when you spray then drop back down you have a leak in that area. carburetor may need to be rebuilt i think in 85 they used a ECU controlled abomination with an early idle air control setup or some sort of solenoid on the throttle linkage to control idle. based on what i can hear of your noise, sounds like an exhaust leak. find yourself a passenger to video for you so you can make it longer and safer!
 
I put up a longer video on the same YouTube account so you can check it out. It should be easier to figure out what's the deal. Thanks! Being just my mother and I, the help from you guys is tremendous on our wallet!
 
Sounds like an exhaust leak to me. cracked exhaust manifold, blown out gasket loose bolts on the manifold or loose clamps on your pipes. detonation from bad timing sounds higher pitched and is not as loud as what I am hearing. I don't believe your poor idle and noise are related I think you have 2 separate issues. To bad I live on the wrong coast or id come look at it.
 
Agreed, after listening to the video it really sounds like an exhaust leak. Try at idle and have a friend hit the throttle while your head is near the manifolds. I leak may not be present under light throttle, but when you give it some gas it will leak worse and get louder.
 
I would have to agree. it definitely sounds like an exhaust leak.

And i have to give you props on being in high school and working on your k5.
i thought i was the only one :D
 
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Thanks I try! Im gonna use the diagrams in the trusty Haynes Repair Manual and check to make sure
 
Oh and about the idle problems, could it be the fuel filter or pump? The truck sat up for about a year before I got it recently and that's one of the only things I haven't cleaned or looked at.
 
cmoore1224 said:
Oh and about the idle problems, could it be the fuel filter or pump? The truck sat up for about a year before I got it recently and that's one of the only things I haven't cleaned or looked at.
Click to expand...

not very likely its fuel. if you had a weak pump or clogged filter it would idle ok and have no power off idle takes less pressure/volume to maintain idle then when accelerating. I would change the filters as a maintenance item but i think your most likely cause is a vacuum leak and the carb should be rebuilt if it was parked with gas in it and sat for a year gas turns to a gel then varnish and can clog the tiny passages inside the carb. I'm also not a huge fan of the mid 80's computer controlled carbs so much went wrong with them i never owned one that worked correctly and i ended up taking all the emissions equipment off and putting a regular carb on. I do not recommend doing this if you have to meet any kind of emissions standards or have some kind of yearly inspection where you live. respect though for wanting to learn and turn wrenches its how i started out at your age, ask questions read the manual and take things apart. sometimes it works when your done sometimes it wont its all a learning experience.
 
Do you have a propane torch? Its a quick way to find a vacuum leak. Just set fire to all the hoses and then you will know where the leak is.............

Just kidding.

Let the truck idle, turn the torch on low but DO NOT light it. Let it blow around the vacuum hoses, base of the carb, places like that.
If you find a leak, the engine should speed up some when it sucks in the propane.

Turn it up a little and let it blow in the intake and you will see what I mean. Otherwise, you want it low so that when it speeds up, you will be really close to the leak.
If you put out too much, the leak can be a long ways away.

Careful squirts of carb cleaner or even WD40 will work too, but they are messier.
 
Im w/ blue 71 on this. Sounds like a vacuum leak or an issue stemming from the CCC (Computer controlled carb). All though with proper rebuild and setup tools the CCC stuff can run good it can also be very temperamental and really has no benefit over and regular qjet and dizzy.

For simplicities sake but semi-unrelated to your issue, especially since this is your first major mechanical undertaking I would look around for an earlier vintage non CCC qjet and purchase a cheap MSD crossfire stand alone HEI distributor.




The propane torch is a good trick. I usually just douche the motor with starting fluid :laugh:
 
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