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Generation 7 1980-1986 F150, F250, F350

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  #1  
Old 10-12-2016, 01:02 PM
Liberty Blacksmith Liberty Blacksmith is offline
 
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460 Backfire Problem f250 4x4,4 spd.

Here's the run down on what I've done since buying this truck at the end of Sept. Being an old Motor Head am sure this will be fixed but need some pointers if anyone has been through this before.

1986 Ford F 250, 460 CI V-8, 46K miles (original), non catylitic converter, Duraspark II, non-computer

Symptoms: Runs rough at idle, back fires slightly through carb when give it gas.

Plugs, appeared dark, electrode eroded, replaced plugs.

Plug Wires, couldn't get resistance value with tester, replaced wires. Cylinders 5 and 6 wires were crossed.

Replaced cap and rotor. Noticed weak spark on test plug. Tested coil, within stock values; replaced with MSD coil 40K volts, still weak spark.

Replace electronic pick up in Distributor, no change in spark or running.

Siphoned 15 gallons of suspect gas from both tanks, replaced with fresh non-ethanol gas.

Replaced Holley 4160 with Edelbrock 1406 4 bbl. Rejetted for altitude, jets, metering rods. Now idles without backfire, within 25 RPM on test meter. Re-check timing.

Tested Duraspark II unit, not within value. Replaced with Checkers Auto brand. Spark much improved.

Compression test, all 100-103 #'s. Re-check timing, reset idle tuning with vacuum guage.

Pulled right side valve cover, cycled engine with remote starter, all rock arms and push rods appear okay.

Verified rotor pointing at #1 plug @ TDC, re-checked wires, okay.

Point: starts with hesitation, then turns over and fires. Feels like distributor off a tooth. Timing is where it should be, vacuum hose on Timed Port on carb not full vacuum. Vacuum is 14-15 Lbs. Needle fluctuates, jumps in that range. Standard should be 17 Lbs. There are miles of vacuum lines.


Note: Have not checked cam lift yet. Friend remembered Ford had a run of bad cams in the mid-eighties (news to me). Was told when I bought the truck it had a "bad lifter". Did remove oil filter and put in half a quart or more of Marvel Mystery Oil.

No valve lifter noise can be heard in the engine compartment but you can hear a similar noise in the cab of the truck, fairly pronounced.

Next step, going to pull the left hand valve cover and check rocker arms and push rods for function.



Any shortcuts anyone know of??
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  #2  
Old 10-12-2016, 06:38 PM
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crazycanuck crazycanuck is offline
 
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Your compression test numbers look a little on the low side, is that wet or dry test. Sounds like the rest you have covered, so with the new ignition system you still say it has weak spark? When diagnosing older trucks like this, I have a standalone ignition system built that hooks to the battery and a remote switch to eliminate any truck wiring, to determine ignition or mechanical issue. Also the reason I do this is I was once diagnosing a issue with a buddies car, and with the car not running, all the electrical was testing perfect, but start it , running rough and most fire. Turns out there was a chafed wire that when not running wasn't touching anything, but the vibration while running it was momentarily touching metal.

Your on the right track, pull the valve cover, check movement of all rockers, possible stuck or sticky intake valve? If a noise is there, then most likely cam or lifter problem, something else to look at, is possible extremely stretched timing chain giving you issues.. good luck and keep us updated
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Old 10-12-2016, 08:07 PM
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Could be a sloppy timing chain issue ,losing compression out of valve
timing ,an causing back fire when the ignition hits that cylinder!

I don't know ,its a wild guess!...
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Old 10-12-2016, 09:25 PM
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Did you check the fuel pump and filter?

Could just be some contaminates in the line somewhere.
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Old 10-13-2016, 02:50 PM
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Quote:
Originally Posted by cbaker65 View Post
Could be a sloppy timing chain issue ,losing compression out of valve
timing ,an causing back fire when the ignition hits that cylinder!

I don't know ,its a wild guess!...
second cam timing and chain.

erratic intake vacuum points to a valve train issue. bad cam, poor cam timing, overhead (valve lash) set to tight, worn valves or poor sealing of valves.

compression seem rather low. a leak down test might might help show bad valving.


sounds like everything you touched is spot on.
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Last edited by Lorddurock; 10-13-2016 at 02:53 PM.
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Old 10-14-2016, 12:46 PM
Liberty Blacksmith Liberty Blacksmith is offline
 
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Put a clear fuel filter in by the carb, can see the gas flow, appears adequate. Seems like there should be other filter(s) on the frame rail but I didn't notice.
Thanks all for input.
Not too worried about compression since spec is that all should be 75% of the others, seem like a pretty wide range.
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Old 10-14-2016, 12:59 PM
Liberty Blacksmith Liberty Blacksmith is offline
 
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It was dry test, tend towards your suggestion of sticky valve/lifter. Will pull valve cover this weekend and see. Anxious to get it resolved before the snow flies.
Thanks,
Frank
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Old 10-14-2016, 05:02 PM
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Quote:
Originally Posted by Lorddurock View Post
second cam timing and chain.

erratic intake vacuum points to a valve train issue. bad cam, poor cam timing, overhead (valve lash) set to tight, worn valves or poor sealing of valves.

compression seem rather low. a leak down test might might help show bad valving.


sounds like everything you touched is spot on.
Quote:
Originally Posted by Liberty Blacksmith View Post
It was dry test, tend towards your suggestion of sticky valve/lifter. Will pull valve cover this weekend and see. Anxious to get it resolved before the snow flies.

+1 on Lorddurock said ,valve timing or sticky valve ,an also a collapsed lifter
or even a flat lobe on the cam ,those will cause a spudder an popping backfire
out of the carb.
Maybe in for a top end rebuild!...

Thanks,
Frank
You just got to figure it out ,an isolate what it is!
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Old 10-25-2016, 10:28 AM
Liberty Blacksmith Liberty Blacksmith is offline
 
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After pulling the left side valve cover (a real job removing all the vacuum lines and plumbing) found a collapsed valve lifter on the last lifter. Due to the heater box, could not use my lifter removal tool. Took the intake manifold off, there was no valley pan, gaskets only. Does this mean someone has been into the engine before? I thought valley pans were OEM.
At any rate, now I am going to test the cam lift with a dial indicator directly off the lobs. Want to make sure all are okay before I replace all the lifters and button it up.
Thanks all, for your help and suggestions.
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Old 10-25-2016, 02:25 PM
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Quote:
Originally Posted by Liberty Blacksmith View Post
After pulling the left side valve cover (a real job removing all the vacuum lines and plumbing) found a collapsed valve lifter on the last lifter. Due to the heater box, could not use my lifter removal tool. Took the intake manifold off, there was no valley pan, gaskets only. Does this mean someone has been into the engine before? I thought valley pans were OEM.
At any rate, now I am going to test the cam lift with a dial indicator directly off the lobs. Want to make sure all are okay before I replace all the lifters and button it up.
Thanks all, for your help and suggestions.
Yea ,that would cause a backfire & a pop sputter back through the
carb along with a flat cam lobe.

I had a flat cam lobe on a 350 chevy that cause a pop sputter backfire
out of the carb.
But with your case ,its a collapsed lifter!..
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