Hey everyone, I've been trying to solve an issue I have been having with my truck stumbling under a load/acceleration. The truck seems to run fine under light acceleration, but as soon as I get into the throttle it starts to stumble really bad...almost a really rough miss. I have a long list of parts I have tested and replaced and a lot of tests I have run through but have not been able to figure this out. Some background: 88 F250 460EFI ZF5-speed with Hedman headers to a Flowmaster 10-series (dual in-dual out) with the EGR capped off, and S&B intake. Other than that, stock motor/truck. I bought this truck thinking it was running tip top.....and of course it wasn't. Started out by finding that the Spout Connector was never plugged in so the computer was not playing a role in timing advance etc.....he set the timing advanced to 30 degrees ATDC and drove it like that...so this has been my search, to find out why....


Parts replaced:
HEGO
Plugs Wires
Cap/Rotor
Ignition Coil
Fuel Filter
FPR
PCV (w/grommet)
MAP (twice)
IAC
ECT
ACT
New Vacuum Lines
EVAP Purge Solenoid
TPS
EECM (reman) -> The reman crapped on me after a few days, developed a constant fuel prime and would not let me check codes: will have another remanned unit tomorrow. Before I replaced the EECM, I checked codes and received all passing codes (11) so that hasn't helped me any.


I just purchased a "EGR Delete Plug" that relays a signal to the EECM that the valve is always close in attempt to rule out the timing advancement when the EGR is opened.


As far as fuel goes, I have a mounted fuel pressure gauge on the fuel rail that reads around 30-32 psi at idle and when I snap the throttle the pressure raises to around 42 psi. I have wired in a fuel/air ratio gauge as well that connects to the HEGO and relays the signal to my gauge. I've noticed that the truck runs lean most of the time and when I get my stumble to occur, it is running extremely lean. Only thing I can think of is the EGR playing a role in all of this. I don't know if it is advancing the timing too much, if its not getting fuel due to recycling the exhaust gases, I'm not sure (still learning). Im by no means a mechanic, but I am an engineer so I do understand concepts fairly well....I just lack the experience so please bear with me.


If anyone else has any ideas, please, I will take all advise!! Thanks in advance!!


PS: For those who enjoy reading, my previous thread may answer a lot of questions....but it is long as sh** now so I understand if nobody goes and reads it haha
https://www.ford-trucks.com/forums/1...0-460-efi.html

 

What is the static fuel pressure reading on the fuel rail after engine shutdown?

Manufacturer specifies an acceptable leak-down rate of not more than 2psi for 3 minutes.

 

It holds pressure fine, last time I checked it didn't leak down but I will verify again tomorrow when my EECM comes in

 

You really need to check the fuel pressure under load. Simply snapping the throttle is only a partial test. It does prove the fuel pressure regulator is working, but the full picture is under load and when the symptoms occur.

 

Yes I agree and I've been wanting to. Money is a little tight with Christmas around the corner but I plan on having an electronic fuel pressure gauge by the end of the month (along with vacuum and coolant temp). That's really the only other thing I can think of other than the EGR possibility

 

I forgot to add: when the truck is cold, it runs great. I cannot get it to stumble, it has good power and a good fuel/air ratio, but just as soon as it gets up to op temp it starts to stumble. My understanding is that before the motor reaches op temp it's getting additional fuel (compared to when warmed up) but once it gets to operating temp, it leans it out a little bit....is this correct?

 

Under open loop operation the computer is running the engine on default values. Once the engine is up to operating temperature the computer will go into closed loop under light/medium loads. This allows the computer to control the mixture

 

Okay that makes sense. Is there any way you can elaborate more on the strategy/process the EECM goes through as far as the EGR goes? It seems to me that the EGR opening up would cause the computer to send less fuel due to the unburnt fuel being recycled into the cycle, in turn, causing it to run lean. At the same time, I could see it throwing more fuel to compensate for the added air....just not sure of the strategys "thinking" process

Any clarification on these "thoughts" within the computer would be greatly appreciated

 

The EGR is not causing your lean stumble problem, the computer does make adjustments based on the presence/absence of exhaust gases in the combustion cycle but they are minor enough that you really would never notice on a correctly functioning motor.
What will have an effect is the thermactor system, so what condition is it in? There can be no leaks in the exhaust system upstream of the O2 sensor so if this system is no longer in use all places where it connected to the engine must be sealed up air tight.

 

This can be complicated by the fuel pump operating satisfactory when cold, but after some time it gets warm and pressue/flow suffer. I had a Tundra that would barely go up a hill at 15mph in the heat of the day; few gallons of fresh fuel and waited for the sun to go down, no issue for the rest of the trip (thankfully, we were hundreds of miles from home). Once home, fuel pressure looked good in the driveway, idle or snapping throttle. But put the gauge on the windshield and drive it for 30 minutes on a warm day, it would drop below 20psi under load, fall on it's face, and AFR surpassed 18:1. Could easily be misdiagnosed as a hot engine vs cold engine related issue when in fact the pump was just failing and it showed up under specific conditions.
Definitely verify fuel pressure under load before spending more money or running down other rabbit holes. No electronic gauge needed, just rent one at or use whatever you have.

 

Thanks for clarifying, I didn't think that it could be the issue but wanted to be sure.

As far as the thermactor....doesnt have one. Bought the truck without cats or thermactor, and I didn't install them back. I still have one solenoid that connects to the manifold and is capped on the other end (both are sealed tight). I was hoping I could just cap the manifold and remove the solenoid. I was also hoping to remove the EGR solenoid and vacuum reservoirs and cap that manifold vacuum, as well as the EGR vacuum (or buy a block off plate), plug this ebay EGR delete plug and call it a day.

 

See i thought about that as well. The guy I bought it from said he replaced all 3 fuel pumps but after what I've seen...who knows what's true. I will definitely see if I can rent a gauge long enough to reach outside of the motor bay...if not, I'm going to rig something temporarily (and safely).

Thanks for the input everyone, I really appreciate it

 

Well I got lucky and was able to sweet talk the girlfriend into an early Xmas present new electronic fuel pressure gauge should be here Thursday So now I will be able to get the fuel pressure while driving and hopefully figure something out

 

Well an update: got the new EECM today, swapped it and took it for a drive. It drove great, even after reaching op temp. Couldn't get it to stumble, I was getting constant ideal fuel/air ratio (not running lean like it was), everything seemed great......get to the house cut the truck off, let it sit for about 5-10 minutes, and went for another drive. Didn't run as well. It would stumble like it used to, run lean, and not sound good.

Sounding like a possible weak/tired high pressure fuel pump? I'll have my fuel pressure gauge Thursday so I'll be able to verify for sure

 

Just curious,

When you changed the fuel filter did you open it up? Did you pour out the fuel that was in it? What did it look like?