What would cause this?
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New User
Joined: Sep 2009
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From: N.W. Georgia
What would cause this?
I have a 91 f-150 with the 302 and 5 speed. The other day it started bucking a little under load like it was missing so I researched on here and I pulled the codes 31,32,33. I tested the EGR (it was ok) then I tested the EVP and it was out of specs so I replaced it. Just so happened I picked up a set of plugs because it had never been done since I bought the truck. Replacing the EVP did nothing but when I pulled the number 8 plug, all the electrode was gone and some was partially melted to the top. All the other plugs looked fine. I researched on here and seen where people mentioned problems but could not find what causes it. Thanks for all your help.
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Joined: Aug 2009
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if its missing its either spark, fuel, or compression (easiest tested in that order) Take a spark tester, and take every plug one at a time WITH TRUCK OFF put on the tester, then start truck.....best way to see if its a "good" spark is to stick a screw driver in then hold it near a ground, not touching though....trust me...if you get bit youll know it so be careful to not touch the metal part of screw driver if its a good bright spark your fine.
Next would be fuel. You should have a schrader valve <basically what a vlave stem on a tire is> Put on a fuel pressure gauge and see whats happening if its below about 30? id look into it more.
Last is compression, hardest of them all, if you dont know how to do it yet, i suggest you get somone qualified since it involves disableing the engine so it wont start. However if it has a lot of mileage that could be something to look into also.
Hope i helped. lol i know its a lot to take in
Next would be fuel. You should have a schrader valve <basically what a vlave stem on a tire is> Put on a fuel pressure gauge and see whats happening if its below about 30? id look into it more.
Last is compression, hardest of them all, if you dont know how to do it yet, i suggest you get somone qualified since it involves disableing the engine so it wont start. However if it has a lot of mileage that could be something to look into also.
Hope i helped. lol i know its a lot to take in
Senior User
Joined: Aug 2009
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However after reading your tranny post you should have no problem doing any of these, lol
Does your truck have a MAP sensor of some type? might check into that, that causes bucking under load in some cases....if not can you give some more details? :P
Does your truck have a MAP sensor of some type? might check into that, that causes bucking under load in some cases....if not can you give some more details? :P
Senior User
Joined: Jun 2009
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From: Mt. Clemens Michigan
you dont have to disable the motor for compression test just make sure its in neutral and jump the starter relay on the pass fender. i usually put a socket in the throttle linkage to hold it open a little.
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New User
Joined: Sep 2009
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From: N.W. Georgia
it seems to be running fine "now" but I bought the truck used and wanted to make sure there was not some underlying problem that would be causing it. What boggles my mind is why its just one plug? Seems like something like it running lean would cause problems to more than one. Think maybe a bad injector? Which I don't know if the computer would show a code for that.
Lead Driver
Joined: Nov 2002
Posts: 9,557
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From: Concord, NC
The PCV vacuum port attaches at the rear of the intake manifold, dumping the majority of the oil and crankcase gases into the number 8 cylinder. Later models re-routed the PCV to the center vacuum tree to better distribute the crankcase gases and oil vapors between all of the cylinders. It is a good idea to re-route the PCV vacuum line on the older trucks to help with this problem. If this is indeed what caused the issue with the #8 plug, hopefully you have caught the problem before it caused any serious damage.
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New User
Joined: Sep 2009
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From: N.W. Georgia
I will definately check into that when I get back. Where would I find the center vacuum tree? and is there a spot to add another line. Sorry for the questions and this maybe easily seen under the hood but I'm 400 miles from the house at work right now
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Lead Driver
Joined: Nov 2002
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From: Concord, NC
What I did on my truck was get a longer vacuum line for the power brake booster and hooked it up where the PCV valve normally attaches, then routed another section of vacuum hose from the PCV valve to the vacuum tree on the top driver side of the intake manifold.
Cargo Master
Joined: Jul 2009
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From: Des Moines
I thought the PCV valve was supposed to feed into the intake, behind the air filter? Even 70's model vehicles routed crankcase gases this way. There's only one hose leading from the PCV in my 91 F-150 and it leads to the intake. Or was adding a second hose supposed to be better for the engine?
FTE Legend
Joined: May 2008
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From: In my own world
ford has a tsb about #8 plug being cooked because of the pcv. they recomend removeing the plenum, drilling and taping a new hole in the center, and putting a plug in where the fitting is now.
Fleet Mechanic
Joined: Feb 2009
Posts: 1,672
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From: Nixa
I found my spark plug like that when I changed them. I didn't know what caused it, now I do. I put the new plugs in and checked them some 200 miles later, and it was still good as new. So i wonder why it did it the one time, and not since.
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