4.9L Hesitation..
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Junior User
Joined: Apr 2014
Posts: 65
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From: Cranbrook, BC, Canada
4.9L Hesitation..
I've read all over these forums and many others. But I can't seem to find a solid answer or root to the problem I seem to be having. Long story short, I have this brutal hesitation which produces a massive loss of power at acceleration. Brand new distributor, cap, plugs, fuel filter. The only thing I haven't replaced is the plug wires because they're only a couple years old so I was hoping that wouldn't be it. Any ideas? Need any ideas ASAP so I can root a problem, any types of sensors or anything? She is my daily driver, and looks too good to have her parked in the garage! Thanks guys! 
'93 F-150 4.9L
4" Lift w/ 33's
'93 F-150 4.9L
4" Lift w/ 33's
Posting Guru
Joined: Feb 2002
Posts: 1,712
Likes: 1
From: Atlanta, Georgia, USA
It's most likely one of the electrolytic capacitors inside your PCM that control the EGR circuit. If it's leaked out it tends to cause that exact problem. There's several outstanding write-ups on this forum- you could probably search "PCM capacitor leaking" in the search box and find them pretty quickly.
The good news is that if you have a talent for solder and some patience you could fix it for less than $2. The bad news is that it's very meticulous work. You might have luck with a remanufactured PCM. AutoZone sells them from $100-$250 and generally has them in stock or close by, but there's no guarantee that a remanufactured PCM will be any better than the one you have.
The PCM sits in a pocket in the firewall just under the brake master cylinder. It comes out by bending the wheel well cover a little bit.
Good luck! -TK
The good news is that if you have a talent for solder and some patience you could fix it for less than $2. The bad news is that it's very meticulous work. You might have luck with a remanufactured PCM. AutoZone sells them from $100-$250 and generally has them in stock or close by, but there's no guarantee that a remanufactured PCM will be any better than the one you have.
The PCM sits in a pocket in the firewall just under the brake master cylinder. It comes out by bending the wheel well cover a little bit.
Good luck! -TK
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Junior User
Joined: Apr 2014
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From: Cranbrook, BC, Canada
I'll go run codes now and post what I come up with. If it happened to be the pcm before rooting to that issue could I test the egr plug for power? Because if there's power there it should be fine, correct?
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Junior User
Joined: Apr 2014
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From: Cranbrook, BC, Canada
I was just out and did the KOEO test. After talking to a journeyman ford mechanic last week, before doing the test I disconnected the - terminal for 20 mins to reset everything. Now the only code I'm throwing is 111, which in my book is a system pass. Wtf! I also tried the KOER test, but got nowhere with that it blinked 3 times and that was it. Last time I had run codes it was throwing all sorts of wacky crap... Where should I start?
Cargo Master
Joined: Nov 2013
Posts: 2,168
Likes: 4
From: South Central,MO
You erased all the codes. Now you need to drive it a while to make it act up so if it is throwing codes it will store some. And it wouldn't hurt to check the fuel pressure while you are driving it around.
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Junior User
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From: Cranbrook, BC, Canada
Makes sense.. haha. I will drive it to work tomorrow and report back with codes. Thanks appreciate the help!
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Junior User
Joined: Apr 2014
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From: Cranbrook, BC, Canada
Update:
So I ran the codes after struggling to drive it to and from work today, and this is what I came up with:
- 111 (System Pass)
-172 (Heated 02 sensor fault/lean bank)
When I bought the truck it had the cat cut off and different exhaust put on and the 02 sensor hanging there so it's never been hooked up. (Unless that piece that's supposed to sit on the cat is the egr tube I haven't looked into it). But four years later it's never caused me any issues so I can't see that being the problem? Should I replace the 02 sensor and see what happens? Can it really make the truck run that bad? Let me know! I'm in Canada, too, so my Autozone is called Lordco haha. If I ever figure out how to upload a picture here I'll throw one up!
So I ran the codes after struggling to drive it to and from work today, and this is what I came up with:
- 111 (System Pass)
-172 (Heated 02 sensor fault/lean bank)
When I bought the truck it had the cat cut off and different exhaust put on and the 02 sensor hanging there so it's never been hooked up. (Unless that piece that's supposed to sit on the cat is the egr tube I haven't looked into it). But four years later it's never caused me any issues so I can't see that being the problem? Should I replace the 02 sensor and see what happens? Can it really make the truck run that bad? Let me know! I'm in Canada, too, so my Autozone is called Lordco haha. If I ever figure out how to upload a picture here I'll throw one up!
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Junior User
Joined: Apr 2014
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From: Cranbrook, BC, Canada
That's probably the cheapest place to start. But yeah I agree that's why I looked here! Not going to throw money away on parts when I don't need them all, thanks for the input I appreciate it! I'll let ya know how it goes when I throw the set of plug wires in tomorrow!
Logistics Pro
Joined: Mar 2005
Posts: 4,698
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From: Mexico, Missouri
The O2 sensor should be just before the cat. It sounds like it needs replaced. But....
If you are running lean, that could be a huge part of your problem. It sounds like you are and you should check fuel pressure. The 4.9 is very sensitive to fuel psi. It runs around 50 psi.
When was the O2 sensor last replaced? It's only good up until certain point and then it becomes lazy. It directs the PCM to either widen or narrow the injector pulse to lean out or too rich up the fuel delivery at the injector. If its getting screwy info from it, but good data from the TPS, MAP, ECT, ACT, then yes, the O2 sensor sounds like its failing.
An O2 sensor is really subjective to extreme variables and if its been 4 years, it probably wouldnt hurt to replace it. But if your fuel psi is low, then you're out $$'s for a new O2 sensor and then the $$'s for then problem due to low fuel psi. I know. I've been down exact same road thinking it was O2 and it was the front fuel pump was weak.
If you are running lean, that could be a huge part of your problem. It sounds like you are and you should check fuel pressure. The 4.9 is very sensitive to fuel psi. It runs around 50 psi.
When was the O2 sensor last replaced? It's only good up until certain point and then it becomes lazy. It directs the PCM to either widen or narrow the injector pulse to lean out or too rich up the fuel delivery at the injector. If its getting screwy info from it, but good data from the TPS, MAP, ECT, ACT, then yes, the O2 sensor sounds like its failing.
An O2 sensor is really subjective to extreme variables and if its been 4 years, it probably wouldnt hurt to replace it. But if your fuel psi is low, then you're out $$'s for a new O2 sensor and then the $$'s for then problem due to low fuel psi. I know. I've been down exact same road thinking it was O2 and it was the front fuel pump was weak.
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Junior User
Joined: Apr 2014
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From: Cranbrook, BC, Canada
Yeah ill take a look after work! I replaced the spark plug wires with new NGK's. It seems to run awesome before it gets right up to normal running temp. Then it acts up a bit but not as bad. I guess I'll go buy a fuel pressure gauge tomorrow and try that?
Logistics Pro
Joined: Mar 2005
Posts: 4,698
Likes: 47
From: Mexico, Missouri
The fact that it runs great when cold leads me to believe then it's O2 sensor related. The PCM runs in Open Loop until the coolant reaches around 173F on SD vehicles and somewhere around 135F on MAF (my Speed Density 302 went Closed Loop at 173F and with it being converted to MAF, it goes Closed Loop at 133F). In this Open Loop, it uses preset fuel and timing tables built into the PCM to run well and for good start-up. When it enters Closed Loop, that's where ECT, ACT, TPS, MAP, O2 sensor start sending or otherwise producing a signal the PCM can use to obtain optimal performance.
My bet is O2 sensor, but checking fuel psi is a good start and wise invesment if you plan on keeping the truck as a daily driver.
My bet is O2 sensor, but checking fuel psi is a good start and wise invesment if you plan on keeping the truck as a daily driver.
