Stumble on deceleration
Thread Starter
|
Post Fiend
Joined: Mar 2011
Posts: 8,501
Likes: 4
From: Lynchburg, VA
Stumble on deceleration
Ive having trouble with my 1979 F150, 351m, c6 with 125,000 miles on it. When im in first gear and let off the gas, the vacuum drops down around 10hg and the engine nearly stalls. If Im in second or third, and let off, no problems, until it shifts back down into first.
Basically, Im cruising at 40, let my foot off the gas and apply the brake. vacuum goes from 14-16hg up to 20-22. Then, as my speed gets down around 15-20mph, the vacuum drops off and engine sputters, but then comes back after a few seconds.
Ive taken the carb back off (had rebuilt in June) and blown all the passages out, no difference. Checked the float height, right on. Replaced the trans modulator, no difference. Checked for loose/broken/cracked vacuum lines, none. The timing is set to what the sticker on the hood says and I did a tune up less than a thousand miles ago. All the emissions parts are intact, although I dont know how to verify if theyre working.
I need to figure out what the heck is wrong, its driving me crazy. This is my daily driver, I use it to get to school. Does anyone have any insight? I posted something similar in the '73-'79 forum and came up cold. Any help would be GREATLY appreciated. I can get a video if need be.
Basically, Im cruising at 40, let my foot off the gas and apply the brake. vacuum goes from 14-16hg up to 20-22. Then, as my speed gets down around 15-20mph, the vacuum drops off and engine sputters, but then comes back after a few seconds.
Ive taken the carb back off (had rebuilt in June) and blown all the passages out, no difference. Checked the float height, right on. Replaced the trans modulator, no difference. Checked for loose/broken/cracked vacuum lines, none. The timing is set to what the sticker on the hood says and I did a tune up less than a thousand miles ago. All the emissions parts are intact, although I dont know how to verify if theyre working.
I need to figure out what the heck is wrong, its driving me crazy. This is my daily driver, I use it to get to school. Does anyone have any insight? I posted something similar in the '73-'79 forum and came up cold. Any help would be GREATLY appreciated. I can get a video if need be.
Thread Starter
|
Post Fiend
Joined: Mar 2011
Posts: 8,501
Likes: 4
From: Lynchburg, VA
What do you mean by hanging up? Its new, btw. The PO had removed it and adjusted the timing to prevent pinging. I replaced it and reset the timing. I was sorta wondering if the valve that activates the egr was closing too fast. No idea how to test that though, and not sure where to get a new one.
Thread Starter
|
Post Fiend
Joined: Mar 2011
Posts: 8,501
Likes: 4
From: Lynchburg, VA
About 4,500. The PO said he rebuilt the bottom end and replaced the timing gears and chain a year or so before he stopped driving it about 3 years ago. It was running good for a few months. Hardly a hickup driving it home from Virginia where I bought it. I recorded a video of the vacuum gauge readings on my way home to school. Im in the process of uploading it to YouTube...
Thread Starter
|
Post Fiend
Joined: Mar 2011
Posts: 8,501
Likes: 4
From: Lynchburg, VA
Ok, video is up. Sorry, its pretty crappy but hopefully the annotations help clear it up. One stumble @4:00, and another smaller one around 4:50. It stalled out as a result of the stumble the other morning pulling out of the driveway and happened again this afternoon pulling out of the school parking lot.
Vacuum gauge - YouTube
Vacuum gauge - YouTube
Thread Starter
|
Post Fiend
Joined: Mar 2011
Posts: 8,501
Likes: 4
From: Lynchburg, VA
Ok, just went up to the hardware store and while I was there I talked to two of the guys about the problem Im having. Ones a Chevy guy, but at this point I'd listen to a Daewoo mechanic. Both of them suggested advancing the timing from the 6* its set at now. I'm gonna try setting it at 8 or so and see how that does.
Trending Topics
Posting Guru
Joined: Nov 2008
Posts: 1,937
Likes: 2
From: wyoming
kinda out their?
If you could was thinking odd situations. like a worn guide in distribtor and during deceleration the extra clearance might cause some gear to cam backlash. Or make sure you vacume advance is moving smoothly internal. Or like you are thinking on the low vacume your egr is sticking and causing a lean condition. another is are your springs on your carb holding throttle shut. another is make sure your carb bolts are tight and carb isn't shifting around and sucking air. IF you have rubber plugs on vacume ports make sure they are in good shape and not cracked on the ends and sucking air into manifold. Check your intake mainfold bolts are tight. ran across something similar before and it was a loose carb situation. just shooting from the hip, watched the video and difficult to tell. 
FTE Stories
Ford Trucks for Ford Truck Enthusiasts
Top 10 Most Expensive Ford Trucks Ever Sold on Bring a Trailer
Joe Kucinski
2027 Ford Super Duty Buyer's Guide (Every Model, Engine, & Package)
Brett Foote
Top 10 Ford Truck Tragedies
Joe Kucinski
AEV FXL Super Duty - the Super Duty Raptor Ford Doesn't Make
Brett Foote
Lobo Vs Lobo: Proof the F-150 Lobo Should Be Even Lower!
Michael S. Palmer
Ford's 2001 Explorer Sportsman Concept Looks For a New Home
Verdad Gallardo
10 Best Ford Truck Engines We Miss the Most!
Joe Kucinski
2026 Shelby F-150 Off-Road: Better Than a Raptor R?
Brett Foote
2027 Super Duty Carhartt Package First Look: 12 Things You NEED to Know!
Michael S. Palmer
Posting Guru
Joined: Nov 2008
Posts: 1,937
Likes: 2
From: wyoming
another scenerio?
your carb might be sloshing fuel into intake and possibly causing a rich situation? your carb float internal does it have a spring to help keep float up. Its a light spring assisting the float stay in the shut situation. if float is sloshing during decel could be flooding carb. Next question is can you set a rubber hose on fuel vent and route away from butterflys. are you sure you don't have water in fuel. If I remember correctly the jets are in the btm of the fuel bowl and things just get into the jets and cause strange things don't think this is the problem but look for water or something in fuel bowl. interesting situation and would irritate me also. could block off egr temporary and see if problem goes away, scary simple. head scratcher situation!
head scratcher situation!
Thread Starter
|
Post Fiend
Joined: Mar 2011
Posts: 8,501
Likes: 4
From: Lynchburg, VA
your carb might be sloshing fuel into intake and possibly causing a rich situation? your carb float internal does it have a spring to help keep float up. Its a light spring assisting the float stay in the shut situation. if float is sloshing during decel could be flooding carb. Next question is can you set a rubber hose on fuel vent and route away from butterflys. are you sure you don't have water in fuel. If I remember correctly the jets are in the btm of the fuel bowl and things just get into the jets and cause strange things don't think this is the problem but look for water or something in fuel bowl. interesting situation and would irritate me also. could block off egr temporary and see if problem goes away, scary simple. head scratcher situation!
head scratcher situation!
. Odd thing is, that was June and it ran fine without it for several months.No water to be seen. Been running a can of seafoam through the system BTW. I just sprayed all the carb passages out a week ago, no change. Im thinking its NOT carb related, at least not directly.
I advanced the timing to around 10* BTDC and it seemed to be running alot better, I even gained an inch or two of vacuum. But then I got greedy and tried to mess with the timing again and its running crappy again. Im thinking maybe the timing just needs a little more tweaking along with some idle mixture adjustments.
Posting Guru
Joined: Nov 2008
Posts: 1,937
Likes: 2
From: wyoming
vacume good.
what are your vacume readings? once you get your best vacume readings your very close and see where your timing at also. I know your doing this and just record your readings. Definetly look at your vacume advance and make sure not sticking internal. suck on vacume hose to diaphram and see if any odd movements. good luck and drop a message on your findings.
Thread Starter
|
Post Fiend
Joined: Mar 2011
Posts: 8,501
Likes: 4
From: Lynchburg, VA
what are your vacume readings? once you get your best vacume readings your very close and see where your timing at also. I know your doing this and just record your readings. Definetly look at your vacume advance and make sure not sticking internal. suck on vacume hose to diaphram and see if any odd movements. good luck and drop a message on your findings.
The vacuum advance is definitely fine. I forgot to reconnect it after I last adjusted the timing and realized something wasnt right within 30 seconds of driving it. Im going to be out of town for a week starting tomorrow so this will have to be put on hold.
Elder User
Joined: Jan 2011
Posts: 831
Likes: 1
From: Tanilba bay NSW Australia
Another poster had something similiar to this. It came down to the vac mudulator on the trans. maybe it is holding it is second for too long. He got it adjusted and it is now apparently fine.
Thread Starter
|
Post Fiend
Joined: Mar 2011
Posts: 8,501
Likes: 4
From: Lynchburg, VA
Ill try messing with that. Looking at the dashpot and throttle though, Im wondering if the 2 throttle return springs are just so tight that the dashpot cant resist and gets forced closed. I think Ill try to get a new dashpot sometime soon...
EDIT: Just ordered a new pot from Rockauto for $13 including shipping
EDIT: Just ordered a new pot from Rockauto for $13 including shipping