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BassFantasizer 11-21-2013 01:42 PM

Fuel Trim
I've posted a couple of times about a lean running 5.4. I've got a bunch more information so I'm starting a new thread keying on the problem.

When I start the truck, the Short Trim on both banks behaves reasonably. Mild variation from about -2% to +2%. The Long Trim immediately goes to +25%. Revving the engine to about 1,700RPM reduces the LT to about 12%, but it immediately shoots back up to 25%.

After a few minutes, both banks throw lean codes and the Check Engine light comes on.

This started more than a year ago when the O2 sensor on the right bank failed. The sensor was replaced and all was normal for a couple of months. Then the lean codes started showing up.

I've replaced the fuel pump, filter, and fuel pressure regulator, yet the problem persists.

Any 5.4 guys out there with an idea, I'm all ears. I'm out of ideas.


projectSHO89 11-21-2013 03:38 PM


When I start the truck,
Ignore anything before the PCM switches to its closed loop operation for a fully warmed up engine.

You have two sets of each trim, one for each bank. You have to monitor both banks and both trims for both banks to get any insight into the mixtures.

Did you ever check the infamous PCV hose and it's various elbows? It accounts for the huge majority of this set of symptoms.

BassFantasizer 11-21-2013 03:40 PM

Yeah. The PCV hose was actually swapped before the O2 sensor. The engine's been tested with the infamous "brake cleaner spray test" and there's no sign of a vacuum leak.

projectSHO89 11-21-2013 08:24 PM


The engine's been tested with the infamous "brake cleaner spray test" and there's no sign of a vacuum leak.
And how was that test executed?

BassFantasizer 11-21-2013 08:53 PM


Originally Posted by projectSHO89 (Post 13766193)
And how was that test executed?

I don't know. A mechanic friend tested for vacuum leaks and determined that there weren't any.

Got a suggestion?

projectSHO89 11-22-2013 07:39 AM

Well, there is a way of doing that test that doesn't work (spray and listen) and one that does (spray and watch a scan tool's live data of the O2 voltage or the STFTs).

Depends on which one he did.

BassFantasizer 11-22-2013 08:31 AM

My guess is that he did the first.

If the weather allows, I'll hook up my tools at the house and rerun the tests this weekend.

Thanks for the suggestion!

RBlac 12-05-2013 09:08 PM

Bass, what did you use for spray? Did you find anything? One of my long term trims seems off in comparison to the other.

BassFantasizer 12-06-2013 08:07 AM

Hi RBlac,

I've been on the road (with a different vehicle!) and haven't had a chance to test it yet. If the weather holds, I've already informed the boss (wife) that Sunday is "vehicle maintenance" day. I'll let you know what I find.

projectSHO89 12-06-2013 08:19 AM

Several things work well. Brake cleaner, carb cleaner, ether, unlit propane torch.

Good ventilation is important!

RBlac 12-06-2013 05:22 PM

thanks guys. :-drink I was thinking of something like that.

BassFantasizer 12-08-2013 08:35 PM

Well this went an entirely different direction than I had expected.

I replaced the fuel pressure regulator and all of the lean issues seem to have resolved. I've put about 2 hours on the truck since then, with absolutely no indication of a problem.

Two different Ford mechanics told me that the pressure regulator had tested OKand in good-natured fashion laughed at me when I told them that I was going to change it. "That's a waste of money. The regulator tested out fine". (3 fuel pumps and 3 filters later, I replaced the $30 part.....)

Apparently, they're not just pass/fail as long trim now varies from -3 to +3, where it was locked in at +25 before.


RBlac 12-08-2013 10:16 PM

Good to hear. Thanks for sharing back the results.

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