Hi i Have a 1991 ford f150 inline 6 5 speed 4x4 with 170,000 miles and in the morning it cold starts fine every morning and then if i drive it for a long time and i turn it off and then try to turn it back on right after i turn it off it will hesitate to start like the battery is weak but it always starts. and i had everything tested and the alternator failed so i replaced that but it still does it sometimes. I am wondering if anyone else has this issue or if anyone has any idea what might be causing this.

 

your saying it is hard to crank or takes a long time to start.

imho i would start by cleaning all the cable end contact places for the positive and negative cables. battery connections too.

 

Could be your timing. Try to start it warm with the coil wire unplugged from the coil and see if that makes it spin over normally. If so, then your timing is too far advanced.

If that is not the issue, then it either your battery cables or your starter.

 

How old is your battery?

 

My 4.9L 6 did same thing, except it wouldn't start for about 45 min after I drove it. My water temp gauge read between the R and M of "NORMAL". Replaced broken temp sensor and now restarts fine and gauge reads between N and O.

I have pics/situation in thread about '87 4.9L Computer on page 2 of topic threads.

My anecdotal experience would *suggest* either you are running hot (above my pay grade as to why) or your computer *thinks* you're running hot (sensor) and doesn't want you starting it up and destroying the engine.

Additionally, in a 302 I had a problem with the exhaust manifold (sans heat shield) heating up the starter, causing drag when trying to do a hot restart and eventually leading to premature starter failure.

 

The ECU in these trucks cannot perform this function, and even if it could, it would have no way to cut back voltage to the starter so that the engine turns slower than normal.

 

Good to know and fair enough. As I mentioned here and more detailed in my Computer Question thread, anecdotally fiddling with the broken sensor elicited changes in engine idle, gauge readings, and ultimately the truck restarting when the gauge read lower temp from sensor, even though actual water temp *probably* was the same (it also *could* be cooler due to the sensor activating cooling system adjustments as well, to be fair, although the fan ran continuously in all scenarios). Since my warm-up drive was the same, short 2 mile route, I doubt I heated the engine to nearly seize in one instance and be fine the next, which is the reason I *anecdotally* believe water temps were likely near the same and the only variable was the sensor reading. I could be wrong. It's been known to happen.

My kinda informed layman's thinking here is this is heat causing drag on the starter.
Diesel-electric train locomotives have dynamic brakes that intentionally function this way. To assist brake power, they push heat onto the electric traction motors on the axles, creating drag, and slowing them down (same principle, different method, as a Jake brake in a semi).

My anecdotal experience and semi-informed knowledge of heat vs. electricity make me think check/replace water temp sensor and/or weak starter and/or heat shield on exhaust manifold is bad/missing if the 4.9L is so equipped (honestly haven't ever needed to look in mine, but will when I get home).

I do appreciate the correction and apologize for misrepresenting the ECM's "thinking."

[Edit: Just so I'm clear...if the truck overheats, the ECM will not shut the engine down???]

 

Not in 1991.

 

Replacing the water temp sensor solved my issue and I have no idea why if the ECM doesn't prevent an overheating start. I'll just go with it and be happy. And obviously as the one previous poster said, it wouldn't make your starter drag even if it did have at capability (my starter never dragged).

I looked at my 4.9L and it doesn't have a heat shield, doesn't look like it ever had one, and doesn't look like it needs one. The manifold clears the starter by quite a ways. I suggest having your starter checked. I'm not saying replace it and find out the new one does the same thing, I'm just saying check it to see.

It may be fine for cold starts but may not have the oomph to deal with the extra drag from latent heat between the exhaust manifold and engine block.

 

Newer vehicles have Automatic Shutdown Circuitry and programming that shut the engine down for overheating, coil & plug failures, transmission failures etc., just not in 1991, just codes.

I would be replacing my 16 year old starter, and if my battery was older than 3 year, I would replace that as well,

Timing could be an issue if someone changed the base timing, or if the PCM has corrupted programming.