Misfire at idle 300 i6 f150

freakazooid52 said:
{snip} So I looked on rock auto and bought Delphi and Carter pumps even though they have a bad rap sometimes they actually list psi pump output which was more than 4.9 needs so it will self regulate at fuel rail as designed. Installed Delphi units and all was well no cross feeding and proper in spec fuel psi

It sucks re doing fuel pumps but think you need to buy non part store unit that list psi output. I'd try just replacing saddle tank pump first as it's easier to do than rear tank. Take driveshaft out and you have all room in world to drop tank
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According to Posts #71 and #73 OP already had a new Delphi in the front tank?
 
D60 said:
According to Posts #71 and #73 OP already had a new Delphi in the front tank?
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I didn't see that post thanks. I'd still venture to say try another fuel pump as part quality is terrible these days. I wish ford still supported these trucks

Edit: here's a quick bullet point from HP10265 delphi pump from rock auto (fits 94-96 4.9L) that I've used in my saddle tanks
-Integrated fuel pressure regulator maintains pressure at fuel rail

Best way I've found to block off these dual tank system to check flow/pressure/cross feeding problems is with fuel quick disconnect block off nubs. I'll link a example below. Pretty sure the obs uses 5/16 and 3/8 line. Could be another thing to check before replacing a potentially weak new pump. System is simple. Voltage runs pump , pump makes pressure, fpr regulates pressure via return flow. I know some fuel pumps will have internal regulator maybe that's causing his low psi? If that's the case pump may be new but bad

https://www.amazon.com/Quick-Connect-Adapter-Fitting-AN807-01A/dp/B07G3JVNMT/ref=mp_s_a_1_4?crid=2HVLHGRADKCON&dib=eyJ2IjoiMSJ9.Lrzt16dcWsPt3IFo5WKjE6RW_1GHbtu6454wCA36Cu6dC3kOsPhyqDMsaOKcAH1qcdYQqVylvpkqS8-z064MVNiwiWvnUt6QZT6Qsk4VlOdEXpTWHEMPZe196OjxMvkgVmw90N7sW4dcU9sb8wGAgODZaveVJnRmPhJwAwTUvLcckCMujt2C27z-RzXbdf_Mr1ggbnxtPe9m6Q6dRxbT7A.m1qNv_8f5bjifh28SUf88mt2PJXgbU3yPZP5gWHjffQ&dib_tag=se&keywords=ict+billet+fuel+block&qid=1755701458&sprefix=ict+billet+fuel+block,aps,115&sr=8-4
 
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Another set of pumps and they both 46 KOEO and 36 running. The minute the pumps aren’t running pressure drops to 20psi and stays there. Pulled injector rail and they aren’t leaking so bad new fuel pumps? Crazy that you cannot get decent fuel pumps.
 
Orangepeel said:
Another set of pumps and they both 46 KOEO and 36 running. The minute the pumps aren’t running pressure drops to 20psi and stays there. Pulled injector rail and they aren’t leaking so bad new fuel pumps? Crazy that you cannot get decent fuel pumps.
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Your saying you've tried two sets of new pumps and same psi each time?

V8 and 4.9 use same pump# only difference is regulator on fuel rail. Looking like new pump has internal regulator that's not working or wrong for your application ie aftermarket catering to v8 and forgetting about 4.9L, or your regulator is faulty, or voltage supply to pump is weak or varying. Need to start breaking down system and verify what works.

Can also take return line off tank your using and see what amount of return flow there is, if it's a small trickle it would point to weak gpm/pressure on output side
 
freakazooid52 said:
Your saying you've tried two sets of new pumps and same psi each time?

V8 and 4.9 use same pump# only difference is regulator on fuel rail. Looking like new pump has internal regulator that's not working or wrong for your application ie aftermarket catering to v8 and forgetting about 4.9L, or your regulator is faulty, or voltage supply to pump is weak or varying. Need to start breaking down system and verify what works.

Can also take return line off tank your using and see what amount of return flow there is, if it's a small trickle it would point to weak gpm/pressure on output side
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Regulator is new and passes any test you can throw at it. Voltage is battery voltage to both pumps. When I’ve pulled the return hose to check fuel coming back it’s been quite a bit of fuel coming back definitely more than a trickle.
 
Orangepeel said:
Regulator is new and passes any test you can throw at it. Voltage is battery voltage to both pumps. When I’ve pulled the return hose to check fuel coming back it’s been quite a bit of fuel coming back definitely more than a trickle.
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You sure you've got the right regulator? Sounds like a V8 regulator. They will fit on either rail difference is in diaphragm on inside

I've had good luck with standard ones PR208 is what you need.
 
Can always just throw a new ignition module at it. Like said here before those tfi modules just do weird things as they age. The standard brand ones still usa made or get part # and search eBay for nos one. Lots listed
 
This seems to have really gotten out of hand.

Fuel trims (high at idle, near 0 elsewhere) do not suggest a fuel pressure problem. Even if technically out of spec, it's not the cause of the rough idle.

Everything points to air. I've seen numerous posts questioning the IAC, not sure if it's been removed from the equation? Have all vacuum diaphragms been tested to make sure they hold and don't leak?
 
emmett442 said:
This seems to have really gotten out of hand.

Fuel trims (high at idle, near 0 elsewhere) do not suggest a fuel pressure problem. Even if technically out of spec, it's not the cause of the rough idle.

Everything points to air. I've seen numerous posts questioning the IAC, not sure if it's been removed from the equation? Have all vacuum diaphragms been tested to make sure they hold and don't leak?
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Yes iac is functioning. Vacuum system was smoke tested more than once.

The misfire is a lean misfire. With the fuel pressure being low and nothing else obviously wrong it seems foolish not to focus on the obvious concern. Even if it’s not the entire concern. It’s obviously not right . Obviously a lack of fuel pressure will cause a lean misfire.
 
GSCJR said:
An intermittent stumble at idle sounds ignition related, not fuel related; just sayin. How do the compression and engine vacuum numbers look?
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175psi compression vacuum is steady at around 20psi.

Ignition is working as designed.

It has a slight miss at idle and spark knocks at wot.

The other day a poster on here claimed the fuel
Pressure from ford said it should be 30-45 or right around there. I asked to see the source because that’s not what I’m seeing but that was to no avail.
 
Orangepeel said:
The misfire is a lean misfire. With the fuel pressure being low and nothing else obviously wrong it seems foolish not to focus on the obvious concern. Even if it’s not the entire concern. It’s obviously not right . Obviously a lack of fuel pressure will cause a lean misfire.
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I think your truck has gone woke and is identifying as a carbureted 300. I had a 1965 F100 with the 300(first year). The single carb and long intake manifold made fuel distribution ummm... challenging. An idle mixture rich enough to idle smoothly and lean enough to to pass smog didn't exist.

Ed
 
Orangepeel said:
175psi compression vacuum is steady at around 20psi.

Ignition is working as designed.

It has a slight miss at idle and spark knocks at wot.

The other day a poster on here claimed the fuel
Pressure from ford said it should be 30-45 or right around there. I asked to see the source because that’s not what I’m seeing but that was to no avail.
Click to expand...
20 psi. Is it supercharged? Roots, screw, turbo?
 
edhackett said:
I think your truck has gone woke and is identifying as a carbureted 300. I had a 1965 F100 with the 300(first year). The single carb and long intake manifold made fuel distribution ummm... challenging. An idle mixture rich enough to idle smoothly and lean enough to to pass smog didn't exist.

Ed
Click to expand...
🤣🤣🤣🤣 I have jokingly considered ripping everything off and carbureting it. All the emissions crap is in the way anyway!
 
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