Most of the time when you are trying to accelerate and it is dogging?

RJWALCH

MEMBER
2005 FORD F-150
166 MILES

History: 5.4L FX4. Have replaced Cam Phasers, O2 sensors, plugs, injectors, and throttle body. (Some of these were done for other reasons, but all within the last 40k miles) Engine has no mods, chips, intake or exhaust modifications. Runs reasonably even, but intermittently it has a mostly flat accelerator where you push it but little happens. Sometimes it is ok and accelerates normally.

I have run the full gammet of tests at the dealership and they can't figure it. Computer says everything is fine. Also ran injector flow test and was right in the middle.

Two notes here: firstly, most of the time when you are trying to accelerate and it is dogging, if you let up on the throttle just a little, it begins to pull harder. Also, there is a very notable increase in overall power using 93 octane gas almost immediately. I'm thinking there is a sensor failing somewhere, but I am at a loss as to which one, and why does the 93 octane gas make such a dramatic improvement? It's almost as if the timing is off or something.

I'm going to replace the MAF sensor tonight to see if that is the problem, but I have already replaced a lot and I have not impacted the issue at all, except with the 93 gas.

Other than the O2 and MAF sensors, what would evenly impact the entire engine?

7 Replies

HMAC300

MECHANIC
48,601 POSTS

Check yoru vacuum at idle, if it's 15" or below suspect a catalytic converter problem. also check crank sensor for ohms, it may be to hoigh causeing this as well. also when was the last time the fuel filter was changed. there is a tsb published in 2009 for a lack of power but it's the torque converter clutch tip in at 2nd gear and it's an unintended tip in. Please go over these guides to help find the issue.

https://www.2carpros.com/articles/engine-has-low-power-output

and

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

Please run down these guides and report back.

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Thursday, May 24th, 2012 AT 4:49 PM

RJWALCH

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Thank you for the prompt reply, I will check it this weekend, but the cats are most likely ok. Once you are out of the powerloss range, above 3500 rpm, it will wind right up to 5k without even thinking about it. Trans and torque converter were just rebuilt for a broken low/reverse band, and it made no change at all (other than the fact it will go backward now).

Fuel filter is only about 40K miles old, and the fuel pump control box in the rear was also replaced due to corrosion.

I can't help but think the 93 octane gas is the clue. Would the crank sensor cause early or late timing?

I'll check the crank sensor, do you have any idea what the reading should be? I hadn't considered that.

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Thursday, May 24th, 2012 AT 11:55 PM

HMAC300

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I can't get to the ford site to get resistance but I thinkit's like 400-600 ohms. You could always go to an auto parts store and check a new one with an ohm meter. Normally they would allow you to do that especially if you buy parts from them all the time. Who rebuilt the trans? Because the tcc swithc inadvertenly applies and there is a check ball that needs to be taken out to cure it. 93 octane is only for high performance motors and really sholdn't make a difference with that vehicle. When a crank sensor goes bad it screws up the cam and engine timeing. The pcm really doesn't know where to go with it. Also if you have asomething near the knock sensor that is banging it is going to retard the spark and when you let off the gas, it may quit rattling and let the spark go wher eit is supposed to be. So check by that for a loose shield hitting the block etc. But other than thati cannot think of anything else, especially if it's intermittent like you say. Maybe you could do a freeze frame and see if it picks soemthing up.

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Friday, May 25th, 2012 AT 12:04 AM

RJWALCH

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Trans was rebuilt by the dealer, believe it or not, they have a very good tranny guy. Moderate price, but if it goes as long as the original I'll be happy.

I'll put an ohm meter on it this weekend and see what I get. Would it cause late or early timing if bad?

I know what you mean about the 93 octane. It should actually make the situation worse, but it was unreal. Not only was the change in power almost immediate (almost as soon as it hit the engine) but it added about 1 mpg on that tank. The only reason I can think of that that would happen is if the timing is too early. Could the pcm be confused?

Unfortunately, I don't have the capability to get a freeze frame. I'd love to find the diagnostic software for my laptop and a cable, but I cannot find anything that is not associated with a chip. Do you know where I could get it? I also have an 01 Mercury Grand Marquis so I could also use it there.

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Friday, May 25th, 2012 AT 12:27 AM

HMAC300

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When teh tcc applies it drags the engine down when it s' not supposed to giving you a feel of loss of power. If the kock sensor is activating it will retard your spark cause it hears what it thinks is a spark knock. Any auto parts tore have these scanners as long as you get theo ones over $100 they will tell you if it has freeze frame or not. All cars since 95 or 96 have the same connection so it will work on ANY OBD2 system. Mine is an actron, cheap but works ok but won't do alot of other diagnostics. Some have abs capability as well.