Didn't you just get this truck? I told you not to do it.
Misfire at idle 300 i6 f150
- Thread starter Orangepeel
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It’s an old truck it’s going to have issues. It’s still a cool old truck that I’m glad I bought. It’ll get ironed out and be great.
Doh! I said "timing light" but I meant "test light."
The iac on my cavalier sits above the intake throttle body and has a small hole. I took a long brass brush and cleaned out the buildup. You'd be surprised how much accumulated.
There was a ton of crud in the IAC channel in the intake of my daughters 4.0l Jeep. The valve was pretty sticky, too.
So the fact that your fuel trims clean up off idle is typically a sign of a vacuum leak. Spray around your intake manifold with water or brake clean and look for a change.
When I get a chance later at work i'll catch up
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Tonight I got some time to dig into it more and really focus on it.
Fuel pressure from the front tank was 30 rear was about 40 both running. I replaced the fuel filter and no change. Koeo front tank is around 25psi and rear is 35 or so.
Strange since the front pump and tank is brand new.
Pulled the vacuum line off the regulator and no change in pressures but it doesn’t leak fuel. So I’ve narrowed it down to fuel pressure but why I’m still not sure. The other day when I checked fuel pressure it was 40 and I didn’t go any deeper thinking that was an acceptable reading.
Fuel pressure from the front tank was 30 rear was about 40 both running. I replaced the fuel filter and no change. Koeo front tank is around 25psi and rear is 35 or so.
Strange since the front pump and tank is brand new.
Pulled the vacuum line off the regulator and no change in pressures but it doesn’t leak fuel. So I’ve narrowed it down to fuel pressure but why I’m still not sure. The other day when I checked fuel pressure it was 40 and I didn’t go any deeper thinking that was an acceptable reading.
Low fuel pressure can affect fuel trims as the ECU has to command extra beyond what it expects to need to. My manual says a 4.9 wants 50 to 60 psi key on engine off.
That said, poor fuel delivery usually exhibits better running at low fuel demands like idling and worse when you're accelerating.
That said, poor fuel delivery usually exhibits better running at low fuel demands like idling and worse when you're accelerating.
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Tonight I got some time to dig into it more and really focus on it.
Fuel pressure from the front tank was 30 rear was about 40 both running. I replaced the fuel filter and no change. Koeo front tank is around 25psi and rear is 35 or so.
Strange since the front pump and tank is brand new.
Pulled the vacuum line off the regulator and no change in pressures but it doesn’t leak fuel. So I’ve narrowed it down to fuel pressure but why I’m still not sure. The other day when I checked fuel pressure it was 40 and I didn’t go any deeper thinking that was an acceptable reading
Fuel pressure from the front tank was 30 rear was about 40 both running. I replaced the fuel filter and no change. Koeo front tank is around 25psi and rear is 35 or so.
Strange since the front pump and tank is brand new.
Pulled the vacuum line off the regulator and no change in pressures but it doesn’t leak fuel. So I’ve narrowed it down to fuel pressure but why I’m still not sure. The other day when I checked fuel pressure it was 40 and I didn’t go any deeper thinking that was an acceptable reading
yeah acceleration isn’t a problem sometimes full throttle it’ll spark knock though. So obviously some issues in that regard but it very much drives decent. I suspect fuel pressure regulator is to blame for the fuel pressure.
You are low on the front tank and borderline on the rear. Specs call for 30-45 psi at idle. The pumps are weak. Pressure should go up when the regulator's vacuum is removed with a healthy pump.. New means nothing if junk fuel pumps are used. OEM, Bosch and Delphi are the only good ones. In that order.
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Unhooking the vacuum line didn’t change anything. I used a Delphi pump so I would be surprised if it was bad.
Also fuel pressure specs seem to differ some depending on where I look.
Seems unlikely both pumps are weak especially when one is brand new. I guess it’s possible though. What about the fuel pressure regulator?
The regulator actually keeps the pressure from going too high, so if it's not leaking , it's ok. . That's why the pressure should increase by disconnecting the regulator's vacuum. You can check for a weak pump by disconnecting the regulator's vacuum and monitor the fuel pressure as you goose the throttle. If the pressure drops a lot (10 psi or greater) then springs back up, the associated pump is weak.
I go by what my factory manuals state. For 1987-1994 300's the idle pressure should be 45-60. On 1995-1996 the pressure should be 30-45 psi at idle. 1996 was the last year that beast engine was made.
You have a lean miss as indicated by your highly positive long term fuel trims. Don't pay much attention to the short term trims.The computer is adding a lot of additional fuel to maintain proper air/fuel mix. Low fuel pressure, stuck EGR, exhaust leaks upstream from O2 sensor, major vacuum leaks or bad data from a sensor.
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The IAC can cause problems, especially if carbon is building up inside of it.
This sounds like a vacuum leak…
Delphi fuel pump = major sketchy.
Still, I'd run it off a top end cleaner at the fuel rail. You can just use gas rather than cleaner of course. This way YOU can run it at ANY pressure you want. You may still need to block off the supply line to the rail or the FPR may just rob you of any pressure beyond what it deems acceptable.
This way YOU control the pressure and can verify your theory.
I'm not entirely convinced that 25 to 35psi would cause any problems at idle.
Still, I'd run it off a top end cleaner at the fuel rail. You can just use gas rather than cleaner of course. This way YOU can run it at ANY pressure you want. You may still need to block off the supply line to the rail or the FPR may just rob you of any pressure beyond what it deems acceptable.
This way YOU control the pressure and can verify your theory.
I'm not entirely convinced that 25 to 35psi would cause any problems at idle.
This would be much easier in person to diag.
Another thing is when warmed up, what do your upstream and downstream o2 sensors do.
You also sound like you have a couple issues. I think chasing your fuel pressure at this time is a red herring but another way to verify suspicions of a fueling issue is to crack the whip on the truck (preferably going up a long hill so you can hold the pedal to the mat for a bit) and watch what your upstream O2 sensor does. It should peg out and stay there the whole time you're wide open. If it starts to drop or cycle again then you have a fueling issue.
Still, if the truck is not setting any codes then I would first rectify your positive fuel trims at idle. Those are very high and i'm surprised the pcm even has control still. The way your trims are reacting is a classic vacuum leak symptom. If you can see your IAC duty cycle that can also confirm a vacuum leak as it will be very low since the leak is adding the idle air on it's behalf.
Another thing is when warmed up, what do your upstream and downstream o2 sensors do.
You also sound like you have a couple issues. I think chasing your fuel pressure at this time is a red herring but another way to verify suspicions of a fueling issue is to crack the whip on the truck (preferably going up a long hill so you can hold the pedal to the mat for a bit) and watch what your upstream O2 sensor does. It should peg out and stay there the whole time you're wide open. If it starts to drop or cycle again then you have a fueling issue.
Still, if the truck is not setting any codes then I would first rectify your positive fuel trims at idle. Those are very high and i'm surprised the pcm even has control still. The way your trims are reacting is a classic vacuum leak symptom. If you can see your IAC duty cycle that can also confirm a vacuum leak as it will be very low since the leak is adding the idle air on it's behalf.
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I have confirmed with a smoke test and the old carb spray method that there aren’t any vacuum leaks. At this point the fuel pressure being low is an obvious issue. I’m going to rectify that before chasing anything else. High fuel trims can mean to much air or to little fuel. Low fuel pressure would indicate not enough fuel.
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Correct me if I’m wrong, I am currently looking at a wiring diagram and trying to fully understand the system. What about the fuel tank selector valve? It says it’s on the left frame rail. Should I check and make sure I have good voltage there? Is it possible it’s restricted?
These dual fuel tank systems really suck! Way more complicated than most!
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