This 97 F150 4.2 is new to me, originally a very base 2wd white fleet vehicle. I picked it up for $1800 with 189,000 miles, runs smoothly and quietly, idles well, transmission shifts smoothly. Gets reasonable gas mileage.

Problem statement: the truck has an intermittent starting problem. The failure to start happens about 5-10% of the time. The motor will turn over fine, but but immediately stumbles while trying to start. Turn key off and then re-start attempt will be again followed by a bucking stumble. Some times the motor might try and run for 2-3 seconds bucking and stumbling. It is possible the problem happens more often when the truck is warm and has been shut off for 15 minutes or so. Once the problem happens it is very repeatable. It won't start again unless I wait about 30 minutes to an hour. Once it starts it seems to run very nicely. I have noticed some times when I go to successfully start the truck it might run roughly for the first 3-4 seconds bucking and sputtering a bit and then smooth out and not have an issue until the next re-start attempt.

The truck runs so nicely when it is running I doubt it is an issue with the plugs, air filter general tune up.

Could it be an intermittent fuel pump issue? The next time it occurs I will check fuel pressure.

Could it be fuel pressure regulator? Also tested by fuel pressure.

ECU? The truck runs very nicely most of time that seems like a long shot.

Crank sensor?

Bad fuel? I did add some dry gas.

Any advice would be great. It seems like a fairly common issue people have reported, I just don't see a common solution yet during my searches.

 

I would start by doing all the cheap/easy things and see how it goes. Like, Check the cold air intake and the whole intake assembly for blockage or clogged filter. Clean the MAF sensor wires with the special spray cleaner. Clean the throttle body/throttle plate area with the special spray cleaner. Run some injector cleaner through it.

 

Saxman,
Thanks for the reply. I agree, don't get crazy and start replacing expensive items without a valid reason. The air filter looks good, the throttle body and throttle plate look clean. I will clean the MAF this weekend.
Here is an update. After four days of driving the truck I have not had any more failure to start issues. All I have done so far is replace the accessory belt, tensioner, add a bottle of fuel inj cleaner and a bottle of dry gas. I will keep running it and give an update.

 

Been driving the truck daily for the last five days with out any starting issues. Possibly the dry gas/ injector cleaner helped. I did get a CEL today

P0174

http://www.aa1car.com/library/ford_lean_codes.htm

I have not gotten around to cleaning the MAF yet. Sounds like I REALLY NEED TO DO THAT! Also replace the fuel filter.

Will update with results.

 

Cleaned MAF, replaced fuel filter, cleared codes. Feels like it is running better. Will report back in 5 days if all is well.

 

Still getting P0171 and P0174 lean bank 1 and lean bank 2 and intermittent start.
The intermittent start only seems to happen when the truck is warm and has been sitting for 30 min or so.

Replaced MAF. (eBay $44 shipped)
6 Plugs NGK (eBay $17 shipped)
Both downstream O2 sensors (eBay $28 ea shipped)
Both head light Assemblies with bulbs (eBay $44 shipped)

Truck seems to be running better but still throws p0171 and 0174 after a few days.

I measured the fuel pressure running at idle. I see 28-30 psi
Driving down the road, if I floor it I watch the fuel pressure drop down to 15 psi.

If I shut the truck off and come back after an hour the fuel pressure might drop from 30 down to 25 or so, not much.

I did catch the intermittent start issue one time with the fuel pressure gauge installed. I would try to start and the engine would fire, but immediately stumble, sputter quit: if I let the truck sit for an hour it will start. The fuel pressure was reading 30 psi steady this entire time. Some time i notice if it fails to start, I can give it a shot of starter fluid in the TB it was stumble for a bit and then start and run fine. The strange thing with this intermittent start issue is that once it starts it seems to run fine. I wonder if there is a water leak into the intake some how? I'm not loosing coolant that I have noticed. Coolant, heat and temp gauge have been rock solid.

 

I forgot to mention I replaced the crank angle sensor. (Autozone $30)
No change

 

If I had an injector leaking it would still start and run on five cylinders. Plus my fuel pressure is not bleading off much at all.

 

One last data point. If I start the truck cold it will run and idle perfect. Some times after the truck is warm such as driving around town for 30 min to an hour, I notice the idle might get a little rough at times when sitting at a light. Almost acts like it wants to die. If I put it into neutral I can rev up the motor and it will clear out. It seems to me this is likely related to the intermittent start issue.

 

That pesky 171 and 174. Check all vacuum lines and pcv lines for leaks. You can spray some starter fluid around the intake manifold to have locate the leak. Those are some of the most common codes to come up on Ford obd2.

 

Yes that's good advice. I will spray some starter fluid around the intake manifold and report back.

 

Checked for vacuum leaks with starter fluid. None found. Vac hoses all appeared good, tested good.

 

Decided to replace fuel pump. $25 from EBay. 30 gal tank was full, so time to remove 8' bed to access the pump. Total job took a it 3,5 hours. Truck has been running fine for two weeks. No intermittent start issues, no more CEL.

My friend mentioned that it's possible that the alcohol in the fuel can form bubbles in the hot fuel rail of a not running motor. This can form a 'vapor lock' that is magnified by a weak or marginal fuel pump. Weak fuel pump was also apparently causing the lean bank error codes. Letting the motor cool off would allow the motor to start again.

Case closed

 

And Thank You for stopping by and posting the solution!

 

Great.....................