Since i originally purchased a used 2012 F150 I’ve had a constant battle with negative fuel trims. anywhere from -12% to -20% LTFT at idle or cruising. During the year long tune up process I continued to troubleshoot the issue.

Intake Valves

I performed a port induction service, blasting the intake valves and stems with walnuts to remove carbon build up thanks to the nature of direct injection.

Clean valves allow air to enter the combustion chamber easier. While a needed service the engine needed, this didn’t have any effect on the trims.

Fuel Injectors

The next possible cause for negative fuel trims can be “stuck” injectors, putting more fuel into the combustion chamber than the PCM calls for. I inspected the original fuel injectors which seemed quite dirty. Although I didn’t have any other symptoms, like flooded cylinders or dropped fuel pressure after the engine is turned off, I replaced them with remanufactured ones. I hoped this would help, but as the logical part of my brain expected, the fuel trim issue remained.

VCT System

Next was the VCT system. I monitored the error PIDs like VCT_EXH_DIF1 to see what PCM was calling for and what the phasers were actually doing. They all seemed to have around 1 degree of error while idling.

While driving around I noticed periodic high and low spikes. I read in the service manual that spikes like this are normal.

I wasn’t able to find any definitive information about how many degrees of error were acceptable so I assumed these were fine.

I removed the valve covers and inspected the vct solenoid screens for debris. There was some minor build up that I cleaned off. I tested the solenoids with 12V to ensure they were turning on and off correctly.

Oil Catch Can

I installed a catch can to help filter out oil and fuel from entering the intake from the PCV system. While it does filter out quite a bit of liquid, it also had no effect.

I also performed the “weep hole” mod, an attempt at getting condensation that builds up inside the intercooler to drain instead of being sucked up into the intake.

There was a considerable amount of fluid, but no changes in the fuel trims were noticed. This issue mainly manifests with hesitation and misfires while accelerating quickly, when rapid airflow picks this liquid up.

Exhaust Manifolds

The 1st and 2nd generation 3.5 EcoBoost engine is notorious for warping the exhaust manifolds. Mine had a pretty bad exhaust leak on both sides. Exhaust leaks can fool the upstream O2 sensors into thinking their is a rich condition by letting unmetered air into the exhaust pipe. These sensors are used by the PCM to adjust the fuel trims to compensate for rich and lean conditions.

While there are third party manifolds that claim to fix this issue by using all the bolt holes present on the cylinder head, I went with the revised manifolds from Ford, made from stainless steel to prevent the warping.

I went ahead and replaced the turbo coolant and oil lines as a preventive measure for the notorious leaks they can have. I did not get a picture of it, but the turbo oil supply line screens were spotless, no gunk was present preventing the turbos from getting oil. Unfortunately this repair had no effect on the trims either. I was stumped!

Some Time Passes

I had thrown in the towel for fixing this issue. I couldn’t find any information online about this. No facebook group or forum post had similar issues. After a few months passed I came across this blog post which had nearly identical symptoms! A stretched timing chain can cause intake/exhaust valve timing issues. I purchased a pressure gauge and attached it to the intake manifold. Sure enough, low vacuum pressure!

Like a kid at Christmas I ripped the passenger side valve cover off and shoved a borescope down to the timing chain tensioner. It was extended to the maximum position. The engine had a stretched timing chain!

Unlike the 2nd generation EcoBoost, the 1st generation is not known for cam phaser issues. Almost all the information online centered around cam phaser rattle at start up. I assumed since I did not have that issue my timing system was operating normally. The 1st generation engine _does_ have issues with stretched timing chains because it’s nearly 5 feet long!

I took the timing cover off and inspected the engine:

The timing chain stretch was confirmed. I ordered the parts and performed a timing job. New chains, phasers, guides, oil and water pump were installed. I did keep the original VCT solenoids because they were working correctly and aren’t known for issues. All genuine Ford parts, which ended up actually being cheaper than most aftermarket kits.

Buttoned everything back up, reset the KAM, and took the truck for a spin. After getting the engine warmed up the trims were much better! At a warm idle the VCT errors were also smaller!

Next project will be replacing the worn out engine and transmission mounts.