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1980 - 1986 Bullnose F100, F150 & Larger F-Series Trucks Discuss the Early Eighties Bullnose Ford Truck

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Old 08-08-2010, 11:28 PM
Jackson Jackson is offline
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1986 f-150 overheating / backfiring

Thanks in advance for the help....1986 f-150 4.9l carburated...Bought the truck for $900 and it's in pretty decent shape..runs strong..body straight..interior nice...steering and suspension tight...transmission seems good...It'll need a clutch soon.
Anyways, the water pump wasgoing out...wobbling a bit...As I was driving on the highway, it happened....The truck also stalled and wouldn't start for a while..I replaced water pump, thermostat, all hoses and belts...It was still hard to start, and when it did start, it would die out...A little gas into the carb and it started right up...It would also backfire a little when started. Got it started drove it and the truck ran very hot...It's now idling rough, but runs great at highway speeds...except when quick accelerated, it backfires...Is this a timing issue??? Confused???
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Old 08-09-2010, 12:04 AM
DLP Performance DLP Performance is offline
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Replace the spark plugs & do a compression test while you're at it. You may have hurt a cylinder.
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Old 08-09-2010, 09:50 AM
Jackson Jackson is offline
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Could this be reason for overheating?? I'll post compression results.. I also have a fairly substantial vaccuum leak coming from the power brake system...when I depress the brake pedal the air noise stops an the truck idles better..Vaccuum leak coming from where brake pedal arm comes through the firewall.
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Old 08-09-2010, 09:58 AM
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Your brake booster is bad, causing the vacuum leak. If you pinch the hose to the brake booster, it should run better also, but you won't have power brakes.

If it's back-firing through the exhaust system, I suspect you have carb problems, and it's dumping too much fuel into the engine. Raw fuel entering the exhaust and hitting hot exhaust pieces will ignite and cause a back-fire.

Your overheating problem might be something else like a clogged radiator.

I believe you bought a truck that was run and never taken care of, and you will discover several problems with it that will need worked out.
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Old 08-09-2010, 10:00 AM
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Vacuum leak will cause lean condition which will cause overheating. The booster will need to be replaced.
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Old 08-09-2010, 11:55 AM
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Does the entire booster need to be replaced or is there a seal of some sort that can be replaced. Overall, the truck seems to be in pretty good shape for its age...I guess time will tell..Thanks for the info...I'll replace the brake booster first and see if that takes care of all issues. Again thanks
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Old 08-09-2010, 12:15 PM
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You'll have to replace the whole unit. They're $85 or so "Remanufactured" from the parts houses.
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Old 08-09-2010, 08:47 PM
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You really should do the pair, booster and master cylinder ... The typical failure for the booster is brake fluid leaking from the M/C piston.

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Old 08-10-2010, 08:48 AM
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Originally Posted by Jackson View Post
Does the entire booster need to be replaced or is there a seal of some sort that can be replaced. Overall, the truck seems to be in pretty good shape for its age...I guess time will tell..Thanks for the info...I'll replace the brake booster first and see if that takes care of all issues. Again thanks
I would recommend replacing both the master cylinder and the brake booster as a matched set whenever you need to replace the brake booster. IMO it is just cheap insurance. I have run into issues when not doing that.
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Old 08-10-2010, 10:11 AM
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I've never seen a leaking master cylinder cause a booster to fail. I've seen a booster fail & suck fluid past a worn out master cylinder, though. However, if the back side of the master cylinder is the slightest bit damp you might as well replace it now while it's loose. They sell a booster/master cylinder combo also. It saves you about $10 that way, opposed to buying them separately. Remember to bench bleed.

Then again, you don't want to spend a ton of money on the brakes if the engine is burned up. You might want to clamp off the vacuum supply to the booster & test drive. Don't forget, your brakes won't stop very well w/o power assist. Be careful. That way you'll know if the vacuum leak from the booster is or is not the culpret for the overheating.
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Old 08-10-2010, 12:32 PM
Jackson Jackson is offline
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OK so I replaced the brake booster....Master Cylinder looks newer...no fluid apparent at or on booster....Truck now idles much better....Replaced plugs...Old plugs looked pretty decent...no fouling, just normal wear...did a compression test...Interesting results...

cyl 1...70
cyl 2...90
cyl 3...110
cyl 4...115
cyl 5...115
cyl 6...115

Test drove truck ..... about 2 miles from house stalled out at stop light...seemed like a vapor lock...started after about 5 - 8 minutes....lifted hood and there is still a whitish vapor coming from around the carb area... Drove truck back to house...so truck was running a total of about 15 - 20 minutes or so....Factory gauge was about to the m or a ... a little high... I wanted to run it up on the freeway to see what the cooling system did. Thanks again for all the comments and tips.... What now???
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Old 08-10-2010, 12:58 PM
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Cyl 1 is dead. Cyl 2 may as well be dead. The rest are pretty well worn out.
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Old 08-10-2010, 02:53 PM
Jackson Jackson is offline
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What's strange though is that the truck does not at all run like a truck that's going out. It runs strong..No smoke...no plug fouling... could the compression issues be from a bad head gasket caused from the overheating issues??? Before the overheating occurred, aside from the vaccuum issue, The truck ran well, just idled a little rough because of the bad power brake booster
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Old 08-10-2010, 02:59 PM
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Is it pushing air into the cooling system?
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Old 08-10-2010, 03:07 PM
Jackson Jackson is offline
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I'm not sure....A mechanic friend has a "sniffer" to detect combustible gases in the cooling system...I'm going to have him run that check and see....I'll post what I find.
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Old 08-10-2010, 03:07 PM
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