I sold to my brother a 1978 Ford F-100 Ranger XLT with a 351m C6 transmission a few years back. It had about 110,000 miles on it. It progressively had a hesitation issue that I chalked up to an accelerator pump diaphragm problem or similar gas issue. I drove the truck intermittently. I sold it to him a couple years back. He drove it in town, but not much outside of town.

I informed him of the problems, but he needed a vehicle. My brother, who is not mechanically inclined, drove the truck for over 6 months and later complained that the truck had started to get a growing hesitation at half throttle. It would be fixed by pumping the pedal once and it would then be able to surge past the hesitation. It seemed to lose power beyond the half throttle. It had gotten so bad and that it didn’t work so well pumping the peddle so he quit driving it.

The truck used to run terrible before I put in a new timing set. To drive it where it needed to be worked on, I had to advance the distributor and increase the engine to a fast idle and move it at 5-10 mph to get it there. When I pulled the timing cover off, you could slap the chain around on both sides, it was that bad. At that time, it would run bad at all speeds but always fell flat after about a third to half throttle. Before I replaced the timing set, the truck would backfire out the carb at about 1/3 to half throttle. I had thought the timing chain was going out so we decided to replace it. With the original cam in there, we put a new timing set/chain on. I advanced the timing set 4 degrees to get better low end torque.

I started the truck and it is fine at idle (or at least idling well enough), but the engine consistently fires out the carburetor at 1/3 to half throttle. It stumbles at any speed beyond that. If the peddle is maintained beyond 1/3 to 1/2 throttle, it will continue to backfire out the carb and immediately bog down. When in drive, the truck will move in idle to 1/3 throttle without losing power, but once it is more than that throttle, it will give heavy backfire out the carb and bog down.

The motor is at 114,000 miles (I don't think it rolled to 214,000). I've checked my wires and they don't look crossed. I installed a new fuel pump when installing the timing chain. The carburetor is fairly new. I checked the accelerator pump diaphragm by looking down the intake with engine turned off and saw healthy two squirts of gas. I checked the power valve by blowing on it. No leak.

I pulled out a vacuum gauge, and I get 16 in. when at about 12 BTDC, and have to advance it to about 20 BTDC to get about 19-20 in. The thing is, it doesn't hold consistent. It wavers one or two inches. Maybe I need to check again, but I think it will slowly move back and forth with a slight tick one way or the other within that 2 inch range. I tried to set the idle mixture to rule out that vacuum gauge symptom but it didn't change anything.

I pulled the valve covers off and had the engine cranked over while I looked at the rocker arms and there weren't any moving less than any of the other ones (none sticking). Everything was coated in carbon, however.

Could all the lifters/cam be equally worn on a 351M with only 114,000? I really don't know what to do at this point. Any advice is greatly appreciated. Thanks.

 

The vacuum reading suggest a bad/poorly seated valve if it flicks downward when that particular valve is moved by the cam. A fast "flick" of the needle indicates a bad valve where a slow "flick" would indicate a poorly seated valve. Valve guides could be questionable, too.

Have you checked for vacuum leaks?

However, with the backfire through the carb and poor performance tells me the cam/lifters are pooched.

Next time you have the right valve cover off, yank #1 cylinder's exhaust lifter and #2 cylinder's intake lifter....they're the easiest....and note from where they came. Looking at the lifters feet, put a straight edge across it or use the side of the other lifter. You're looking for a slight crown in the lifter's foot.....or a downward crown - which I think you'll find. That means it's new cam/lifter/valve springs, etc time.

Side note: Today's oil is death for flat tappet pushrod engines....there's not enough zinc and phosphorous in it and it'll cause premature wear on the cam/lifters. An additive to the oil (ZDP or ZDDP) is needed, or an oil that has higher levels of zinc already in it.

 

As far as vacuum test, I sprayed carb cleaner around possible leak areas. Then I smoked a cigar and blew through the brake booster vacuum line. No discernible leaks that I could find.

Yeah, I'm just starting to learn about how nowaday oil is wrecking flat tappets - the hard way. Got a 1971 Mark III and an '80 Chevette that I'll be adding some ZDDP to from now on.

Any suggestions on cam? The engine is stock and I would like to avoid replacing too much if the cam is too aggressive. This isn't a show piece. Also, what is recommended to be replaced alongside the Cam? I know lifters, but do springs and pushrods need replacement? Never replaced a cam, to tell you the truth.

 

Let's start with the basic troubleshooting of the cam/lifters. Have you pulled a couple lifters and tested their respective feets?

Without testing/troubleshooting I'd hate to have you throw parts at her without knowing the cause of the illness.

That being said, replacing a cam, lifters, timing gear/chain, et al, isn't difficult...just time consuming to be able to get it out and in. Engine in the vehicle - pushrods, lifters, radiator, hoses, timing cover timing gears, fuel pump and eccentric....all have to be removed.

Not to scare you off about the work involved....but we can yak about cam choice, etc, after you've ascertained the cam is shot....or on it's way out....but a small RV type cam will work best for your application.

 

No, I haven't pulled the lifters yet. I'll let you know what they look like. I did the timing set a couple weeks ago. Like you said, not hard work but time consuming. Now I wish I did the cam while I had the timing cover off. But you live and you learn.

 

I know the general rule is disdain for resurrecting a dead thread, but if I created it, heck I figure I can bring it back to life. We had pulled the front lifters and saw that the lifters were substantially crowned. Did a vacuum test on all spark plug holes and did not see any odd vacuum issues that would indicate a valve issue. We put the old girl in storage after that and haven't touched it since, but I figure it is about time to get to work on it - 2 years later...

The number one problem I am concerned about is the oil light goes on in that truck when it warms up and remains on at idle when warm. Goes away once the accelerator is pressed down but comes back at idle. I know the 351m always had problems with oil pressure issues, but I have also heard that the 351m can run a long time with those issues. Getting down to brass tacks, if the cam bearings are shot, what damage is done by installing a new cam, push rods, springs, and lifters with bad cam bearings? Will there be metal shavings floating around or will the install be just a waste of time (run as long as the motor was destined to run anyway)? Don't mind installing if it doesn't do much harm. I saw that crate motors aren't too expensive anyway if it throws a rod. I would appreciate your thoughts. I can post a new thread if that is needed. Thanks.

 

I have a '77 f150 with a 351m, it had about 230k on it before it blew a main bearing. Same low oil pressure issue at idle and would come back up when you took off. After taking the thing apart there was more damage than I initially thought. Besides the main being shot, the reset would not have made it another week. The P.O. installed some street rod cam in it and the lobes on 3 cylinders were opening just enough to run. I assumed it was sluggish because of the size of the cam. I pulled the motor and had my motor shop build it a little beefier than stock, nothing crazy. Put a RV/Towing cam in it and a straight up timing set. It is like a totally different truck, way more power, oil pressure holds, and gets about 12-13 MPG, I was getting about 8-9 prior. So yes if you are having the oil oil pressure and your cam is smoothing out, it might be time for a rebuild and that motor might have more miles on it than you think. My rebuild cost about $2800, I pulled and installed the block.

 

So, installing a new cam in a motor with worn cam bearings would damage or not damage the motor? I got everything but the cam and springs and pushrods. I just don't want to blow the motor up doing it.

 

I'd rebuild the carb & replace the fuel filter before I would stick a cam in it. The "fairly new" carb could have sucked in some dirt.

 

I don't think its the carb or the fuel filter. Rebuilt the first and then swapped it for the new one. Fuel filter replaced each time. Same exact symptoms. The lifters pulled were concave. It's the cam.

So, is there harm in installing a new cam in a motor with worn cam bearings?