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4.9 skipping time

The 4.9 in my truck is skipping/jumping timing. I can set the timing and get it idling good, but when I try to drive it the timing jumps way off. Timing gears are good, dots lined up, distributor hold down is tight. This same engine/ignition combo has worked for a couple years, so it isn't new parts not matching. Any ideas? I haven't checked the distributor drive gear roll pin yet, could this be cam "deflection" from worn bearings (132k on motor)? The outside of the dist. isn't moving, this is all internal.
 
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11
is the chain stretched out?
 
That was my first thought,but it has gears instead of a chain and the gears look great. Turns out the guilty party was the pin on the distributor gear sheared. I had thought that it was ok, because when it quit I pulled the cap off and tried to turn the rotor-no go. The rough edges of the broken pin still had enough grip to defy a cursory twist.
Replacing the pin fixed it all, but the question now is why did it shear that roll pin in the first place? Thanks for the reply.
 

blacksnapon

Moderator
Staff member
That was my first thought,but it has gears instead of a chain and the gears look great. Turns out the guilty party was the pin on the distributor gear sheared. I had thought that it was ok, because when it quit I pulled the cap off and tried to turn the rotor-no go. The rough edges of the broken pin still had enough grip to defy a cursory twist.
Replacing the pin fixed it all, but the question now is why did it shear that roll pin in the first place? Thanks for the reply.
Lots of times its an oil pump with little pieces of valve seals or debris in it.
 
Is this a common problem for the 4.9? I wouldn't think so since I have driven this truck for years without this happening.
Since I haven't seen the oil pump pickup, I can only assume that there is a screen on it. Granted it was probably a pretty small piece.
 

SuperCab

Moderator
Staff member
10,068
547
Montana
All I can say is that the only 300/4.9 I ever had did this exact same thing.

Just like you, I thought it was the gears, and just like your truck, the gears were in mint condition...

I always suspected the shear pin on the gear, but when I was gonna sell the truck, I told the guy about it and he thought he could fix it, so I just knocked $50 off the price and he took it. Never knew what the problem was...
 
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1
High volume oil pumps, using thick oil etc. can do this too.

High volume oil pump = harder t turn the oil pump = more load on the roll pin

Thicker oil = harder to turn the oil pump = more load on the roll pin
 

radialarm

Clown of Death!
If the distributor is not slipping in the holddown then it has got to be the dist drive gear pin. I had a 240CI I6 that ate those pins all the time. I got to where I carried spares in the ash tray. I could change one and be back on the road in 15 minutes.
 

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