Twice (about two years apart) now I have heard some loud knocking come from my engine. The first was when I first starting the truck after it had sat for about an hour. This last time was after driving it for about 30 min. I had just crosssed a bridge and accelerated pretty hard, and when I let off the gas and started slowing down I noticed it. I think it was definitely in the motor, when I gave it more gas, the noise got more repetitious. The truck also started running really rough, the way the truck was running, reminded me of what it will due when it gets a bad vacuum leak.

This time I decided to try to get the truck home, and after I drove it a few yards, the noise quit, and never started again. I checked the oil, and it was low (over a quart). I topped the oil off, and drove it and was not easy on it, and I never heard any knocking again.

So, I am looking for suggestions as to what it might be? I was thinking if it was a rod, then it would keep knocking. Could a hydraulic lifter has lost it's oil pressure, and that caused the noise, also explaining why it ran like had a vacuum leak?

Thanks for any help!

 

A lifter makes a ticking, not a knocking. If it starts doing it again after a re-start, I would change the oil and the filter and try a different brand of oil filter. I would also get in a pattern of checking the oil every week, or every two weeks, depending on how much oil it uses.

Another theory is since it uses some oil, it may be building carbon up in the top of the cylinders, and it falls loose once in awhile causing a knock.

 

Thanks for the info. I use Penzoil 10W-40 oil, and motorcraft oil filters. I will definitely keep closer check on the oil level from now on. I have never noticed it burning oil.(Not claiming that it doesnt). I had figured the oil loss was due to leaks.

It was really odd that such a loud noise would suddenly show up, and then quit.

 

From my recent experience under the hood, I can surely say that many things can cause the system to 'act up'. I was having similar symptoms on my 86' f-150 5.0 HO and started working with the connectors in the engine bay after getting the diagnostic tool, and plugging it in and running the tests. It seemed with mine that the errors would change and not stay consistent on repeat tests.

I found a loose spout connector, poorly grounded connections to the distributor ignition module, fuel pump relay among others that I am not sure what their functions are. After cleaning the connections and adding some HQ grease to the plugs and receptacles the truck is running amazing!

If you can, invest in a diagnostic tool for your truck, they are indispensable.

Good luck! I hope you find the culprit!

 

Thanks for the advice. My truck has the computer removed, I replaced it with a Duraspark II ignition system, but from reading your post, I think it would be a worthwhile exercise to take apart all of the electrical connections and check them for corrosion, and apply some good dielectric grease.

 

I had a Mercury Comet with a 250 CID I6 that did that. Was really loud and definitely an engine knock. Made the noise at start-up, I immediately shut it off.... I re-started it and the noise was gone. I was told it was a "stuck" lifter that collapsed and rebounded on my second start-up attempt. Didn't really think that loud of a knock could come from the upper engine, but it is about the only thing that makes sense because of the way it suddenly appeared and disappeared. If your truck is still 'acting up', take off the valve cover and crank the engine over by hand. Make sure all of the valves are opening and closing about the same amount and the valve stems are even at the top of the compression stroke. You might have a bad lifter or a bent push rod. You can loosen up the rocker arm and pull the push rod and roll it on a sheet of glass to check straightness. Also look for broken valve springs when you have the valve cover off. All of these can make noise and cause it to run rough. Lucky for you it is super easy to replace lifters via the side cover on your 300! If the noise is gone, but it's just running rough, check the big rubber grommet for the PCV valve. I had the grommet in my 1980 300 I6 deteriorate and the engine barely ran at all, and smoked with stinky gas smelling exhaust. After I replaced it, the engine ran perfectly smooth and no more gross exhaust. It was like an open vacuum port on the carb. I never thought that seal would matter but sure as heck does! Good luck.

 

Thanks, Ford F834,

What you described with your Mecury, sounds exactly like what I experienced with my truck. I drove it to work today, and didn't have any trouble with it, but from reading your post, I think it may be a good idea to pull some covers, and just double check the pushrods lifters.. etc.

My PVC grommet is fairly new, so I think it is ok, but I will give it a closer look.

 

Sounds like oil starvation. In my experience Penzoil has been known for clogging up oil passages with varnish.

Now they might have changed the formula on Penzoil in recent years, but in decades past I've actually witnessed some of the damage it can cause. Especially if frequent oil changes were not the norm.

And for your info, ALL engines burn oil. Right from when they are new to when they wear out. It's all in the amount it burns that you have to worry about. A new engine may smoke a tiny amount on start up before it breaks in. A good engine may use a small amount between oil changes. etc... Why catalytic converters tend to plug up over time as well.

How much oil do you go through in say 3000 miles? If it's less than a quart, then you're fine. If you use a quart every 300 miles or something, then you could say there is something wrong. Either seals or other problem.

 

i have a 300 six in my truck and i have been told sometimes the timing gears will knock on the 300 i have even heard of this happening with new gears just my 2 cents

 

Here is a website for straight six nuts www.fordsix.com if you search there forum you will find alot of stuff about these 300 six timing gears knocking timing could be jumping around making it run bad

good luck