1992 F350 FI300-6 Auto C6

Truck idles well. Runs good going down the road and pulls good.

If you just barely open the throttle it stalls.

When you come to a red light or stop sign it stalls.

Parts changed between previous owner and me;

Wires
Cap and Rotor
Plugs
Air Filter
Fuel Filter
Crankcase Filter
PCV Valve
TPS
Fuel Pressure Regulator
IAC
Ignition Module
Fixed all vacuum leaks, Swapped out MAP sensor no change so I didn't replace it. There are a few other emissions vacuum switches replaced along with an air injection tube for the cat.

I'm not that familiar with Ford Fuel Injection systems, so I don't know where to go next other than the PCM which I think is going to hard to find.
Any more suggestions?

 

im thinking possible weak fuel pump. i am also having this problem, but seems to be less serious than yours. after driving for a while, if i come to stop, my truck feels like it misfires a little at idle, and if i barely give it gas, it stutters until i give it more throttle, then evens out. seems like maybe 1/4 throttle and less, i have the problem, over 1/4 throttle feels like smooth rpm acceleration. im also running ac so this may add to the problem... i will update if i figure it out

 

Thanks I don't have a fuel pressure tester to check this out.

I had not really thought of that because it runs so well at highway speeds.

Unfortunately the truck had two tanks but the previous owner removed one because it was leaking, or I could just switch between tanks to check this.

 

it could be the switch yes. a fuel pressure tester is very cheap. harbor freight has them. just make sure that you get the one for FUEL INJECTION. not mechanical fuel pump style that only read to about 15psi.

it makes sense that you probably have low initial pressure because when you press the gas pedal, you get more air immediately. if there is not enough fuel (quick enough) then the truck will be lean and die. you have to have a good air/fuel ratio to run

 

If I disconnect the TPS

Of course the idle goes up and then the chk engine light comes on.

But it no longer stalls out and it's driveable

 

It does seem like right off of idle it would have too much air. I know when carburetors get old they start to leak air through the shaft for the choke plate.

Are throttle bodies the same? Could the throttle body itself just be worn out?

 

Bought a fuel pressure tester.

42 psi when running with vacuum connected to fuel regulator.

55 psi with no vacuum.

When I plug the the vacuum line for the fuel pressure regulator and leave the pressure @ 55psi I can slowly
open the throttle and it will not stall out.
But if I snap the throttle it still stalls out.
Through all this the pressure stays at 55psi and never falters.

 

I would remove the throttle body, clean the bores and blades with mild detergent, hold it up to light and check for any gaps between the bores and blades. That could be your problem if the blades aren't sealing off when closed. Though I would think you would have a high idle rather than a stall. But it is worth checking.

 

Timbersteel thanks for the suggestion. I am wondering about the throttle body itself it does have 180,000+ miles on it.
I also notice a hitch when I open the throttle body by hand.

 

Your fuel pressure is perfect. I would clean the throttle body . a bad tps sensor can cause alot of strange problems. They are cheap and and easy to change. As soon as I changed mine, my throttle response was awesome
.

 

Changed the TPS today
No change still stalls.

 

A stall at throttle tip-in is indicitive of a vacuum leak, had the exact same problem on my '89 that took a while to find. How much vacuum does your motor generate at idle.. it should make 18-20 in Hg. Besides vacuum lines there are one or 2 vacuum canisters that could be the source of a problem, on older trucks there was a metal "juice can" on the passenger inner fender that would rust out on the bottom, that was changed to a plastic resovoir at some point but have a look for it. Also check that the vacuum lines are connected to this can correctly, I ended up with them reversed on my current truck so the can was doing nothing, and if you pull the lines off just after shutting down the motor it should release vacuum and sound like opening an air valve. There may also be a can for cruise control behind the rad support and vacuum also powers a diverter valve in the HVAC system in the dash, if you put the system on the windshield and it switches to the floor or dash all by itself when the motor is under sustained load(low vacuum situation) that indicates a broken vacuum line in the dash. Another source for vacuum leak is the brake booster, a problem there is usually pretty obvious but as a test disconnect and plug the big line going to it and then crack open throttle to see if it makes any difference.

 

I was thinking tha same as you.
I did have a vacuum tube that had melted through and I replaced it.
I took a can of starting fluid and sprayed each vacuum connection found a few small leaks where previous owner had used fuel line for vacuum lines and there was not a good seal.
There is no surge or change in idle when I push on the brake pedal.
I had not disconnected the brake booster though.
I will check behind the dash.
Most of the vacuum lines look good or have been replaced.
I will get the book and try to trace out all the vacuum lines.
The vacuum reservoir is plastic and there is no second one for cruise control.
I even climbed under it and checked the vacuum line for the modulator valve.

 

Thanks for the input I'll keep this in mind as I keep looking.

 

can you get a vacuum gauge? that would be very helpful. im not sure how much vacuum your truck should have at idle. maybe someone on here does?