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1997 - 2003 F-150

Limp mode under load


Old 10-11-2017, 10:03 AM
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Join Date: Aug 2017
Location: Silver Springs, FL
Posts: 7
Limp mode under load

This will be a long, drawn out scenario of my truck because it's been driving me insane and I have not been able to come to a conclusion on it and need some help. It could be one issue or a thousand giving me this issue, so help me scooby doo this please!

Truck is 2001 Ford F150 screw 4x4, 5.4L miles

75mm throttle body
airraid spacer
Pacesetter long tube headers and y pipe
no cats
K&N intake
accel coils
New plugs within last 2000 miles

I've been noticing lately that with my ac on and accelerating it will feel like I'm not putting any power to the ground. Will take forever to accelerate to the speed limit. It'll growl like I'm pulling a house behind my truck, but I won't be going anywhere. I've also found that if i get annoyed and floor it it'll rev up to around 3500 plus rpm and back fire, and make all sorts of racket like it's in some sort of weird limp mode. Returning back to normal when I return to partial throttle. My ac will also stop blowing out of my main vents and will blow from defrost and floor vents during this.

It rarely happens with the air conditioning off, but once in a blue moon i notice it without having it on.

Sometimes i feel like my transmission is just giving out and not putting power to the rearend, other times it feels like the engine is confused and trying to scavenge for timing and can't get it right.

I do have a header gasket leak, I've got the parts to fix with new stage 8s to eliminate that issue, but I can't figure that would be my only issue.

I've seen my egr vacuum line collapse under hard acceleration, would that essentially limp mode under load?

I'm sorry if I'm not touching all my bases, I'll answer questions as they come.

It is not tuned to run without cats currently, my upstream O2s are connected, rears are not.
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Old 10-11-2017, 10:25 AM
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Join Date: Jul 2004
Location: Easton, Pa.
Posts: 5,711
I don't know how to answer you without possibly sounding like an insult but you have a mess with those modification listed.
The TPS will not track properly with the larger throttle body.
The spacer does nothing.
There is a vacuum hose leak going to the A/C control as the reason the A/C function changes etc.
No cats etc.
Accel coils are not the better ones you could have used.
How can you be helped with all this going on?
See why you have a mess?
With mods, you have to take full responsibility for the results.
If your running all this on a modified program, your on your own.
Again sorry if its not what you want to hear but rethink what you have.
All this seem not worth the investment you made looking for more power.
Good luck.
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Old 10-11-2017, 10:44 AM
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Join Date: Aug 2017
Location: Silver Springs, FL
Posts: 7
Well with the factory TPS i did have issues with the throttle body, matched with correct one it corrected some of my issues when i first did this. I've been batting with having it tuned, because a lot of my issues almost simulate a starve for fuel or improper timing to be honest from my terrible flashbacks to a 4.6.

I've never had this much issue with a lack of cats giving me this issue, but I could remedy that with a proper tune. This isn't my pavement princess, so a motor pull is in it's future to freshen things up.

I've never had a problem with accel coils, run them for years in my dirt car and strip car and they've always done me well. Are they not up to par with the 5.4? I just went with what I was happy with on prior vehicles.

Vacuum leak for the ac has been found at least, was hissing at me today when i was under the hood. So that'll no longer be an issue. So I guess this comes down to the lack of cats possibly or something wanting to grenade as far as timing goes? I'm a pessimist, so don't worry about any kind of insults. Lol. Quite a few years as an infantryman made me fully aware that criticism should be taken as it comes.
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Old 10-11-2017, 04:02 PM
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Join Date: Dec 2006
Location: Joplin MO
Posts: 36,271
Accel COILS were excellent. Their COIL ON PLUGS have not been worth a hoot. Can't beat stock Motorcraft here, same with the plugs.

Coils: DG508
Plugs: SP479

You need a good custom tune from a reputable shop with all those mods. Without one, it's going to run like crap.

Fix the exhaust leaks and replace those collapsing vacuum lines. Check thoroughly for more vacuum leaks. Read out the OBD2 codes, if it has anything besides P0420 and P0430, fix it. Connect those rear O2's, they of course will trip those 2 codes with no cats but not connected they may effect the way it runs. Replace the fuel filter and throw some Techron in the gas.
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Old 10-15-2017, 12:14 AM
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Join Date: Jul 2004
Location: Easton, Pa.
Posts: 5,711
On the ACCEL coils, you have not run them long enough and under the conditions the truck runs at to see the possible issue with these coils.
Heat cycling can eventually cause shorted turns then low output, then misfires begin to occur without codes set to tell you which one. At this point you need to use a Scanner to find the faulty cylinder coil.
As far as limp mode, no not unless the motor overheats and the CHT sensor detects it to puts the PCM/ motor into Limp mode to protect the engine from seizure damage.
You messing with a computer system you need more understanding of.
BTW, nothing wrong with the 4.6 engines.
We are running a 4.6 on a big road course at 130 mph at 6400 rpm in a 97 Mustang 6 speed/3.73 geared.

Good luck.
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