Author Topic: 1986 K5 Blazer, Running Terrible  (Read 4860 times)

Offline 86K5Silverado

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1986 K5 Blazer, Running Terrible
« on: April 25, 2016, 03:32:06 PM »
Alright so here's the rundown. The vehicle is a 1986 Chevy K5 Full size Blazer with a stock 305, Rochester Quadrajet 4BBL and zero engine modifications. About 3-4 months ago I was driving on a my local parkway home from school when I noticed that the gas gauge was practically one E. Seeing this i decided I drove the truck pretty hard in efforts to not only get to work on time but the the nearest gas station. In any case, after I got off the parkway, the truck was running a bit rough than usual. Mind you, this truck always ran well but not perfect. I just figured it was because the fuel pump had sucked up some sediment from the tank and clogged the fuel filter. Keeping this in mind, that weekend I replaced the fuel filter in the carburetor and called it a day. Truck ran fine for a week or so after but then started to develop the problems it has now. When you first start it up, it idles real rough, almost like its barley running for a few seconds until I give it a few shots of fuel and it smooths out. After the truck warms up to operating temperature, I'll pull away from where it's parked and give it a gentle acceleration like I always do. The truck will run fine for a few minutes but then it starts to screw up big time. From a stopped position, I'll give it the same gentle acceleration and the truck bogs right down. I have to take my foot off the pedal and practically floor it in order to get going at a decent speed. Once I'm moving, the truck shifts hard form second to third (automatic transmission) and just has no power whatsoever. I'l have my foot to the floor and be doing only 25mph. If I'm out on the parkway where the problem first began, the truck can barley make it up the hills with my foot to the floor. Now here's where the confusing part comes in. Some days the truck will run like it always did before with no problems and be like brand new. Other days, it will run like a genuine P.O.S. It's not weather conditions because it's run great and poor in rainy cold conditions and ran great and poor in warm and sunny conditions.
What I've done to the truck after this problem is such...
-Replaced Fuel Filter
-Installed In-line filter between tank and fuel pump
-Installed brand new fuel pump and fuel line from fuel pump to carb
-Rebuilt Carburetor(new accelerator pump, new gaskets, new automatic choke housing, new needle and seat, adjusted float level)
-Replace distributor cap, rotor, plug wires, and plugs
Also to note is that there is a ticking that I hear on the drivers side that has always been with the truck since I first bought it. It's gotten increasingly louder since this problem has come up. I feel it's most likely an exhaust leak but my father who is an experienced mechanic seems to believe it's slop in one of the push rods. One more thing to note is that recently the truck is running pretty rich on the bad days. I pulled the plugs in cylinders 2 and 4 too see if they're fouled up. 4 was fine but two was coated in oil. Don't know if this has any relevance but figured I might include all that I can.

I know that was a lot of info but I've tried all that I can and all other threads I've been looking at haven't helped me much. I'm only a 19 year old college student thats mechanically inclined but I'm the most experience in working on these trucks. I appreciate all the input I can get from my elders. Let me know what you guys think and all replies are much appreciated.
Thank you,

Offline blazer74

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Re: 1986 K5 Blazer, Running Terrible
« Reply #1 on: April 25, 2016, 03:56:28 PM »
What do you have for exhaust? Possible loose baffle in muffler blocking exhaust sometimes.

Ticking could be an exhaust leak or a loose valve.

Could still be crap in the tank moving around blocking the sock at times.

Runs like crap, does that mean missing or just no power?

Offline 86K5Silverado

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Re: 1986 K5 Blazer, Running Terrible
« Reply #2 on: April 26, 2016, 05:03:54 PM »
I have a new stock exhaust setup that I put on about 7-8 moths ago from the catalytic converter back. Cat is still the original. Running like crap means no power. I just replaced the vacuum advance today and ignition control module about 2 weeks ago and saw no improvements.

Offline XrayTedd

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Re: 1986 K5 Blazer, Running Terrible
« Reply #3 on: April 26, 2016, 05:43:07 PM »
Catalytic converter could be plugged. May be time to change it specially if you think it's the orig. Also you can pull your valve covers and see if there is any differences in your push rods just to eliminate those.

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Offline haroldwca

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Re: 1986 K5 Blazer, Running Terrible
« Reply #4 on: January 23, 2022, 07:46:04 PM »
An intermittent problem can be the most difficult to diagnose.  What you have described could be the result of a vacuum leak.  Air may be entering the intake stream in some manner other than through the carburetor.  These trucks have a lot of vacuum hoses, and age does them no kindness.  If you have not already done so, I would inspect all the vacuum hoses.  Pay particular attention to the hard plastic lines.  They can break, but their stiffness may hold them in position so that the break can go unnoticed.  If this inspection does not reveal a problem, you might try the spray can method.  Start your engine, then spray brake cleaner, WD-40, ether, or some other flammable liquid around all possible ports of entry.  If a vacuum leak is present, the engine will surge when you spray the area near the leak.  (My choice would be brake cleaner, because it evaporates cleanly.  WD-40 will leave a residual mess.)  WARNING: Spraying any flammable liquid into the engine compartment can cause a fire if there is a source of ignition.   However, this method will reveal a vacuum leak, if one exists. 

Just throwing this idea on the table.  Once, while re-installing the carburetor on my '85 Silverado, I over-tightened the vacuum line to the brake booster.  This is the steel line that attaches to the rear center of the throttle body.  I remember hearing a small cracking sound as I gave the fitting its last quarter turn.  Not long after, my truck began running intermittently poorly, and even stalling on a few occasions.  This may or may not have anything to do with your issue, but more information is always a better option.  Hope you find your problem.  Good luck !

Offline JohnnyPopper

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Re: 1986 K5 Blazer, Running Terrible
« Reply #5 on: January 24, 2022, 03:58:14 PM »
How old are your ignition components?

If they are old, change out the Dist cap, rotor, wires, and plugs.

If they're not, look for something that might be shorting out.

Look for your wires shorting out on some steel/iron part. You'll see gray residue where it is happening.
1957 Apache 3100 235 Inline 6, 3 on the tree
1973 C-20, 3+3 454 4BBL TH400  Water Injection
1978 K-10, 350 4BBL TH350 NP203 M.M. Part time Kit/Hubs
1980 C-10 under construction