Engine Dying need some expert diagnostic help please.
Freshman User
Joined: Sep 2002
Posts: 28
Likes: 0
From: Gulfport USA
Engine Dying need some expert diagnostic help please.
I have a '77 351M in my 71 F250 and has started to develop some problems that I need some help diagnosisng. I plan on doing a complete rebuild next spring so am looking to just getting it "runnable".
So here are the symptoms...
Starts a little hard but then runs fine running around town. Get it on the highway up at 70 and about 5 to 10 mins later starts popping something awful. I originally thought it was an exhaust leak but after getting it home today I found it's actually backfiring through the carb. It does this only at Higher RPM's ( will hold 45 MPH np but try to accelerate and it develops again) and only after it is run at higher speeds / rpms for a while. Possible related possibly whole different issue, I noticed when it started "popping" the heat suddenly went from warm to ice cold. My thoughts were this could be one of 3 things, bad cam lobe ( though I'ld think that should be constant) bad valve(s) or timing? Going to get a timing gun ASAP and check that..... but any thoughts or help would be greatly apreciated
N3
So here are the symptoms...
Starts a little hard but then runs fine running around town. Get it on the highway up at 70 and about 5 to 10 mins later starts popping something awful. I originally thought it was an exhaust leak but after getting it home today I found it's actually backfiring through the carb. It does this only at Higher RPM's ( will hold 45 MPH np but try to accelerate and it develops again) and only after it is run at higher speeds / rpms for a while. Possible related possibly whole different issue, I noticed when it started "popping" the heat suddenly went from warm to ice cold. My thoughts were this could be one of 3 things, bad cam lobe ( though I'ld think that should be constant) bad valve(s) or timing? Going to get a timing gun ASAP and check that..... but any thoughts or help would be greatly apreciated
N3
Posting Guru
Joined: Dec 2002
Posts: 2,003
Likes: 2
From: chicago burbs
Engine Dying need some expert diagnostic help please.
sounds like you might have to really baby it till you're ready to go in.orville reddenbacher in the air cleaner.i can't see it from here,but my guess is an exhaust valve not opening,causing all the stuff to burp back up when the intake opens again.(i drove a work truck that did this- 305sbc)a combination of everything being worn,flat cam lobes,lifters dished out on bottom and collapsing/won't hold pressure,oil pump/pressure spring not what it used to be.OR for some reason an intake valve is sticking open.not likely on a well worn engine.i never bought into the first scenario when i first heard of it but have come to believe.not sure of any way to diagnose or prove it other than tear down.an oil pressure gage may give some insight,and measure the actual cam lift at the valve w/a dial indicator.i'll bet you find at least one hardly moving.what are your rebuild plans?any tweaking?shouldn't be hard to find another engine to prep while this one almost still runs-
Freshman User
Joined: Sep 2002
Posts: 28
Likes: 0
From: Gulfport USA
Engine Dying need some expert diagnostic help please.
"an
>oil pressure gage may give some insight,"
Well this truck has an actual gauge not an idiot light for Oil pressure and assuming ( I know may not be the safest assumption) the sending unit and gauge are operating per norm it reads midline all the time.
"what are your rebuild
>plans?any tweaking?"
Basically I want to tear it down to the block and replace all seals, bearings, and basically all moving parts. Give me a known good starting point if ya know what I mean. As far as tweaking no real plans for anything major, will probably go with a non retarded timing chain, and maybe a little higher performance cam, but as I said nothing too stellar.
"shouldn't be hard to find another engine
>to prep while this one almost still runs"
Well actually I found a whole truck 69 F100 with a 6 banger I'm working on franticly to be my driver while I do the 351 and a bunch of body resto on the F250. It's been hit in the fron so have to do whole front right fender ( inner and outer) , radiator support, grill lights etc. I start school again in Jan ( after a 7 year break in the USAF ) and will be majoring in auto mechanics. So I figure doing the rebuild will be a good way to put what I'm learning into practical aplication....
Anyhow thanks for the reply and more input thoughts are always welcome.
>oil pressure gage may give some insight,"
Well this truck has an actual gauge not an idiot light for Oil pressure and assuming ( I know may not be the safest assumption) the sending unit and gauge are operating per norm it reads midline all the time.
"what are your rebuild
>plans?any tweaking?"
Basically I want to tear it down to the block and replace all seals, bearings, and basically all moving parts. Give me a known good starting point if ya know what I mean. As far as tweaking no real plans for anything major, will probably go with a non retarded timing chain, and maybe a little higher performance cam, but as I said nothing too stellar.
"shouldn't be hard to find another engine
>to prep while this one almost still runs"
Well actually I found a whole truck 69 F100 with a 6 banger I'm working on franticly to be my driver while I do the 351 and a bunch of body resto on the F250. It's been hit in the fron so have to do whole front right fender ( inner and outer) , radiator support, grill lights etc. I start school again in Jan ( after a 7 year break in the USAF ) and will be majoring in auto mechanics. So I figure doing the rebuild will be a good way to put what I'm learning into practical aplication....
Anyhow thanks for the reply and more input thoughts are always welcome.
Elder User
Joined: Feb 2001
Posts: 829
Likes: 0
From: Porterfield
Engine Dying need some expert diagnostic help please.
I noticed when it started "popping" the heat suddenly went from warm to ice cold.
Gee have you checked the antifreeze level in the radiator.. Could be over heating .....
Gee have you checked the antifreeze level in the radiator.. Could be over heating .....
Freshman User
Joined: Sep 2002
Posts: 28
Likes: 0
From: Gulfport USA
Engine Dying need some expert diagnostic help please.
Yes shazam I had actually just filled the radiator and replaced the cap the afternoon before this began happening, could it possibly be a thermostat? I have a new 195 one I'll be installing today guess we'll see if that helps. As I said in my first post not sure if teh 2 problems are even interconnected, but trying to supply all available info so I can get at least educated guesses.
Thanks
N3
There is joy in work. There is no happiness except in the realization that we have accomplished something.
-Henry Ford
http://home.earthlink.net/~eqbauldor/index.html
Thanks
N3
There is joy in work. There is no happiness except in the realization that we have accomplished something.
-Henry Ford
http://home.earthlink.net/~eqbauldor/index.html
Hotshot
Joined: Oct 2002
Posts: 18,488
Likes: 22
From: The Big, Oregon
Engine Dying need some expert diagnostic help please.
It's either your timing chain or distributor advance. I had this happen on my 78 351m. On mine, though, it was actually chewing the gear off of the distributor, and if I adjusted the distributor, it would run ok for a while, but not long enough to get anywhere. The first time it broke down, it was the timing chain though.
Jared
Jared
Trending Topics
Freshman User
Joined: Sep 2002
Posts: 28
Likes: 0
From: Gulfport USA
Engine Dying need some expert diagnostic help please.
Jared how did you fix ? You said it was chewing the gears ioff the distributor, did you swap out the whole distributor, the timing chain and gear?
Thanks for all the input folks at the very least giving me things to eliminate so I know what it isn't.
On a side note I changed out the thermostat It's producing real heat now so I'm assuming the old one was stuck.
While I was there under the hood went ahead and did the weekly fluids check. I found the oil to be 2 quarts low...not good so I fill it back up on the dipstick and now the oil "gauge" reads low all the time. Why would it read normal range when oil fluid is low and low range when fluid level is normal? ( if that made any sense).
As well my wife won't let me buy manuals for it ( guessing someone bought me them for x-mas ) so if I'm missing stuff that should be there and leaning on the boards harder then I should there's the reason.
N3
There is joy in work. There is no happiness except in the realization that we have accomplished something.
-Henry Ford
http://home.earthlink.net/~eqbauldor/index.html
Thanks for all the input folks at the very least giving me things to eliminate so I know what it isn't.
On a side note I changed out the thermostat It's producing real heat now so I'm assuming the old one was stuck.
While I was there under the hood went ahead and did the weekly fluids check. I found the oil to be 2 quarts low...not good so I fill it back up on the dipstick and now the oil "gauge" reads low all the time. Why would it read normal range when oil fluid is low and low range when fluid level is normal? ( if that made any sense).
As well my wife won't let me buy manuals for it ( guessing someone bought me them for x-mas ) so if I'm missing stuff that should be there and leaning on the boards harder then I should there's the reason.
N3
There is joy in work. There is no happiness except in the realization that we have accomplished something.
-Henry Ford
http://home.earthlink.net/~eqbauldor/index.html
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
Top 6 Best Deals Available on New Fords & Lincolns Right Now
Brett Foote
This Hennessey Takes the Expedition Tremor's Off-Roading Capability to the Next Level
Verdad Gallardo
Top 10 Fords at 2026 Carlisle Ford Nationals
Joe Kucinski
3 Best / 3 Worst Parts of Modern Ford Ownership
Brett Foote
10 Amazing Upgrades That Solve Common Ford Truck Owner Headaches
Pouria Savadkouei
Every 2026 Ford Engine Explained
Brett Foote
10 Ugly Ford Trucks That We Still Kinda Love
Joe Kucinski
10 Things Every Truck Owner NEEDS (2026 Edition)
Michael S. Palmer