Engine noises
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Tuned
Joined: Sep 2003
Posts: 348
Likes: 2
From: Northern Michigan
Engine noises
I'm new to Fords. Recently bought a 79 F-250 with a 351M mated to a manual 4-speed. In a nutshell, it runs like do do. It has 68,000 original miles. It was used to haul a camper back and forth from Florida to northern Michigan on I-75. Driven one winter. Normaly I don't think I would have bought it cause I'm a GM kind of guy (family and in-laws have members that work or have worked for GM with white and blue collars).
I'm pretty sure that I have the choke set right. I installed a new EGR valve to help the pinging @ part throttle. The air pump is operational. I can find no vacumn leaks. Exhaust pipe is solid. Haven't touched the distributor other than to check operation of vac advance and hose it with PB-blaster as a first step to attempt to adjust timing.
The biggest complaint I have right now is that when I am decelerating under engine braking conditions, I get some pretty loud pops and bangs out the exhaust.
This truck is pretty darn nice and I hate to have people turning their heads because they think they are being shot at.
I'm pretty sure that I have the choke set right. I installed a new EGR valve to help the pinging @ part throttle. The air pump is operational. I can find no vacumn leaks. Exhaust pipe is solid. Haven't touched the distributor other than to check operation of vac advance and hose it with PB-blaster as a first step to attempt to adjust timing.
The biggest complaint I have right now is that when I am decelerating under engine braking conditions, I get some pretty loud pops and bangs out the exhaust.
This truck is pretty darn nice and I hate to have people turning their heads because they think they are being shot at.
Elder User
Joined: Jan 2003
Posts: 851
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79 OEM 351M driveability problems
Another thing. Since your truck has a 4spd, Does the truck have duel exhaust or some kinda fre flowing exhaust? If so that could be where ya gettin the pop on motor braking. My 78 with a 400M with a 4spd does this after duels where added. My 86 F150 with a 302EFI does this after duels added. Then my 79 Bronco with a 351M with auto don't after duels added. Every Standerd truck or car with duels or a free flowing exhaust does the pop.
Post Fiend
Joined: Jan 2003
Posts: 10,323
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From: Montana Territory
79 OEM 351M driveability problems
First of all, you aren't backfiring. The popping you hear is related to an absence of backpressure. I'm not sure how it works.
You should not have an adjustable choke. Unless you are talking about your idle set screws. In which case you basically only have to adjust them until the motor will run at idle for the conditions present. You start by turning them all the way in and then backing them off 1/4-1/2 turn at a time. Good luck and welcome to FTE. :X-22
You should not have an adjustable choke. Unless you are talking about your idle set screws. In which case you basically only have to adjust them until the motor will run at idle for the conditions present. You start by turning them all the way in and then backing them off 1/4-1/2 turn at a time. Good luck and welcome to FTE. :X-22
Logistics Pro
Joined: May 2003
Posts: 4,238
Likes: 21
From: Northern Illinois
79 OEM 351M driveability problems
does the motor run good other than the popping?
if it is all original it could be time for a timing chain. ford used a phenolic camshaft gear and they wear rather easily. put a socket on the crank and turn it back and forth and watch the distributor rotor to see how long it takes to turn
if it is all original it could be time for a timing chain. ford used a phenolic camshaft gear and they wear rather easily. put a socket on the crank and turn it back and forth and watch the distributor rotor to see how long it takes to turn
Thread Starter
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Tuned
Joined: Sep 2003
Posts: 348
Likes: 2
From: Northern Michigan
79 OEM 351M driveability problems
Well I put new plugs and wires on it today but haven't had time to run it much. The choke is indeed adjustable by loosening three screws and rotating the assembly. The guy that had it before me had it adjusted so that it never closed but also had the idle screws open all the way rich (there is about 300 degrees of adjustment). I did drive it a little and noticed a flat spot at about 1/4 to 1/3 throttle. Also noticed that when idling ~ 1000-800 rpm engine speed fluctuates between those two numbers about 2 seconds per cycle. Not sure if I can get it to idle lower without stalling. @ WOT the engine pulls strong on all 8 cyls.
I think I'll remove the belt from the air pump just to see if it still pops. That should let me know if air flow is stopping upon deceleration.
Thanks 4 replies guys.
I think I'll remove the belt from the air pump just to see if it still pops. That should let me know if air flow is stopping upon deceleration.
Thanks 4 replies guys.
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Tuned
Joined: Sep 2003
Posts: 348
Likes: 2
From: Northern Michigan
79 OEM 351M driveability problems
well I removed the belt from the air pump and it solved the popping exhaust problem.
Pulled the carb, gave it a bath, and removed the caps on the idle screws so I could adjust them more. Presently @ 3 turns open and the flat spot @ part throttle is gone. 3 turns seems like a lot but what do I know. It still does the loping idle though.
I never did check the timing chain slack.
Just barely pings. I can live with it.
The idle is just about like a governor that is set to sensitive on a gasoline engine generator. It's got me baffled.
Pulled the carb, gave it a bath, and removed the caps on the idle screws so I could adjust them more. Presently @ 3 turns open and the flat spot @ part throttle is gone. 3 turns seems like a lot but what do I know. It still does the loping idle though.
I never did check the timing chain slack.
Just barely pings. I can live with it.
The idle is just about like a governor that is set to sensitive on a gasoline engine generator. It's got me baffled.
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Senior User
Joined: Jan 2003
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From: Addison,Texas
79 OEM 351M driveability problems
Originally posted by palmrose2
well I removed the belt from the air pump and it solved the popping exhaust problem.
Pulled the carb, gave it a bath, and removed the caps on the idle screws so I could adjust them more. Presently @ 3 turns open and the flat spot @ part throttle is gone. 3 turns seems like a lot but what do I know. It still does the loping idle though.
Just barely pings. I can live with it.
well I removed the belt from the air pump and it solved the popping exhaust problem.
Pulled the carb, gave it a bath, and removed the caps on the idle screws so I could adjust them more. Presently @ 3 turns open and the flat spot @ part throttle is gone. 3 turns seems like a lot but what do I know. It still does the loping idle though.
Just barely pings. I can live with it.
He adjusted the fuel system to run rich. So your base timing is probably advanced too far. Pull it back to say, 12 (or, if it's at 12, 8) and set the choke to index or 1 rich. You'll have to adjust the idle screws, because it will be way too rich. (I didn't think idle limiter caps came around til 80).
Then hook the Thermactor back up, and make sure the vacuum circuits are hooked up right. The bypass valve is supposed to spill when you're decelerating.
ash
['I think that's right.']
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Thread Starter
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Tuned
Joined: Sep 2003
Posts: 348
Likes: 2
From: Northern Michigan
79 OEM 351M driveability problems
Still haven't driven this truck much but have come to the conclusion that the pinging is nonexistent until the temp gauge gets half way through its range. The previous owner said he replaced the t-stat and I'm wondering how hot this bugger is set to run. It warms up to just below the "hot" range and stays there. Mabey I can help this truck by installing a cooler t-stat.
Where on the un-numbered temp gauge do most of these trucks run?
Where on the un-numbered temp gauge do most of these trucks run?
Posting Guru
Joined: Aug 2000
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79 OEM 351M driveability problems
Palmrose, I would start by installing a 180* thermostat, if your present one is hotter than that.
To cure the pinging, you can go up a couple of carb jet sizes. These engines were designed to run extremely lean from the factory, and bigger jets will enrichen the mixture to a more "normal" range.
The "flat spot" may be in the distributor advance curve. You can change the advance springs with a kit from Summit, to give a smoother curve.
My Bronco also had pinging and a flat spot, and these changes made mine run MUCH better.
To cure the pinging, you can go up a couple of carb jet sizes. These engines were designed to run extremely lean from the factory, and bigger jets will enrichen the mixture to a more "normal" range.
The "flat spot" may be in the distributor advance curve. You can change the advance springs with a kit from Summit, to give a smoother curve.
My Bronco also had pinging and a flat spot, and these changes made mine run MUCH better.
Posting Legend
Joined: Sep 2001
Posts: 30,255
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79 OEM 351M driveability problems
They should run about halfway up, about the "R-M" region. You can stick a thermal probe on the t-stat housing and insulate it with a rag so that the engine fan does not blow air over it or you can stick the probe in the top radiator tank and add 10F to the reading for a close approximation of your real temperature.
Thread Starter
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Tuned
Joined: Sep 2003
Posts: 348
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From: Northern Michigan
Temp gauge hangs out in the "L" region.
There is definately some slop between the distributor rotor and the crankshaft. I don't know what effect this really has.
I may change jets. Does anyone have recommendations for 500-1500' 0-80 degrees F.
For the life of me, I can't figure out what would cause the idle to jump around at such a regular interval. Timing chain whip? If so, why only @ idle? When I slowly increase engine speed the variation stops around 2300 rpm.
There is some tapping coming from the right side valve cover that doesn't start until about 5 minutes after start up (quiet when COLD). It looks like it has never been removed. The left side valve cover has been, evidenced by the black silicon pushed out around the sides.
There is definately some slop between the distributor rotor and the crankshaft. I don't know what effect this really has.
I may change jets. Does anyone have recommendations for 500-1500' 0-80 degrees F.
For the life of me, I can't figure out what would cause the idle to jump around at such a regular interval. Timing chain whip? If so, why only @ idle? When I slowly increase engine speed the variation stops around 2300 rpm.
There is some tapping coming from the right side valve cover that doesn't start until about 5 minutes after start up (quiet when COLD). It looks like it has never been removed. The left side valve cover has been, evidenced by the black silicon pushed out around the sides.
Last edited by palmrose2; Sep 26, 2003 at 05:36 AM.
Lead Driver
Joined: Nov 2000
Posts: 6,620
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From: Albuquerque
You might still have a vacuum leak. Also the EGR "plate" under the carb has passages that funnels exhaust fume though the intake passages, the EGR valve and into the intake to be mixed with air and fuel. These can burn though and give you EGR function at idle when not called for. Especially if you plate is aluminum instead of steel. It will definitely screw up the idle.
Thread Starter
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Tuned
Joined: Sep 2003
Posts: 348
Likes: 2
From: Northern Michigan
update
I finally changed my timing chain as there was about 10 degrees of slop between the crank and the distributor. Still have loping idle.
I removed the stock flex fan and shroud and installed a 180 degree thermo in place of the 195. Eventually I will install an electric fan. I love the room in the engine bay and the way it more easily revs. While I was playing I changed the high output heater core. It was partially plugged. Also flushed the block and rad. Lots of gunk.
I removed the stock flex fan and shroud and installed a 180 degree thermo in place of the 195. Eventually I will install an electric fan. I love the room in the engine bay and the way it more easily revs. While I was playing I changed the high output heater core. It was partially plugged. Also flushed the block and rad. Lots of gunk.