I bought my son a 2003 FX4 Level II almost a year ago. The original 4.0 V6 SOHC had 198k, so we pulled the engine, sent it to a machine shop, and reassembled it with all the new parts you’d expect (bearings, pistons, oil pump, chains, tensioners, guides, etc). The engine is now reinstalled and it has just over 300 miles on the rebuild. We have a couple issues I’m hoping to get some help with.

The engine sounds great when the oil is cold, but once it warms up, there is a rattle at idle that sounds like it’s coming from the rear chain. If the rpm’s are increased, the rattle goes away. I suspect a tensioner issue but wanted to hear some thoughts. Is this common with brand new tensioners, chains, and guides?

Also, we are getting a cylinder 3 misfire at idle (P0303 & P0316). The coil and wires have tested good and it has brand new Motorcraft spark plugs and brand new injectors from Napa.

Been losing sleep about these issues after spending a lot of time and money on this project. Any thoughts??

 

Any warranting on the reman engine?

 

There’s warranty on the machine work, but we reassembled the engine ourselves so…

 

Thoughts
Were Motorcraft tensioners used? I've read here it makes a difference.
What oil (weight) did you use? This is a shot in the dark regarding your rattle.
Motorcraft oil pump or aftermarket?
Also, is there not some "red plastic pin thing" involved with oil delivery to the tensioners? I remember seeing pictures but can't recall specifics.

Sorry, nothing more definite.
Was the original engine exhibiting any symptoms?

 

Original engine was not making this rattle or showing any of these symptoms.

This evening we replaced the RH/rear tensioner with a Motorcraft tensioner and primed it before installing. No change.

We ran Lucas SAE 30 break in oil for the first 300 miles and could hear the rattle. Just changed the oil to Motorcraft 5W-30 synthetic blend. No change.

A new oil restrictor tube is installed in the LH/front head. No restrictor tube in the RH/rear head.

We have a neighbor coming over tomorrow night to help us check compression and oil pressure. Been losing sleep over this and anxious to get some answers.

 

UPDATE: The neighbor came over tonight with his fancy computer/scanner and ran a bunch of diagnostic tests. Everything passed and seemed to be okay.

Next, we checked oil pressure and are getting near 60 PSI at 1000 RPM with cold oil. We’re getting over 40 PSI @ 1000 RPM when the oil warms up. Around 30-35 PSI @ idle with warm oil. Whew… I don’t think we have an oil pressure problem. I plan to buy a Motorcraft tensioner for the front/LH side, prime it, and install it tomorrow. I will report back with the results. 🤞

 

We primed a new Motorcraft tensioner and installed it on the front/LH side. No change.

It does seem like the noise is coming from a timing chain. But everything I’ve read says rattling chain guides are most commonly heard during cold start and warm up. This engine sounds PERFECT during a cold start and while warming up. But the rattle slowly creeps in as the engine reaches operating temperature. It can only be heard at idle and goes away when RPMs are increased.

I’m stumped. Probably need to pull valve covers and take a closer look. Thoughts??