I've got a 79 f100 with the good old 300 CID. I have a problem with knocking at low rpm and under load, and there is a major lack of power. Timing is set at 6btdc and mechanical and vacuum advance are functional. Egr is not working, but I've disconnected and plugged the vacuum lines. Is it possible that I could have a vacuum leak causing this, maybe on the intake, or some burnt valves? Any help is appreciated... If I received shop wages for all the time I've spent on this, I'd be rich.

 

I'm guessing pre-ignition. probably carbon buildup. try dumping a bunch of octane booster in the tank and see if it improves. if it does iv had some success with sea foam down slowly fed down the intake to remove carbon.

 

Is the octane booster just to identify as to whether it's a vacuum leak or burnt valves? what's the pupose of it? I will try it though.

I have ran some seafoam in the spray can directly in through the carb, and no noticeable improvement. anywhere else I should put it in?

 

what can happen is carbon buildup on the pistons and cyl heads can basically raise compression ratio's and cause a per-ignition knock, where basically the fuel lights off before the spark plug fires and it will cause a knocking sound. raising the octane level of the fuel will make it less likely to knock if that's what is causing your knock. octane booster is cheap so its a good place to start trying to figure out the problem

 

to check for vacuum leaks with the engine running spray some carb cleaner around area's you suspect might have a vacuum leak and listen for a change in how the motor runs.

 

Also, I forgot to mention that on startup, it idles quite rough and the sound resembles a diesel engine

 

when you shut the engine off does it try to keep running or chug a couple times after you turn the key off. or does it shut down like turning off a light switch.

 

Shuts down like a light switch.

 

some compression numbers would be helpful. if you don't already have a compression tester they are fairly inexpensive or they might even have one on a rental program at a auto parts store. a leak down test will help you use compressed air to preasurize each cyl on there compression stroke and will tell you if you have bad rings or valves ect.

 

ok, I'll see if I can get that. I just checked the timing again when I got home from school, and figured out that the diesel noise gets worse as I retard the timing, but the knocking doesn't stop. I test drove it with less advance and more advance, still knocks. I found an article that said " If pulling timing out of an engine doesn't suppress or eliminate knock, then most likely your problem is rooted in carbon deposits or a hot spot on the piston or combustion chamber. At that point you may want to remove the carbon chemically without dismantling the engine." (The Causes of Internal Engine Knock, and How to Eliminate it. By Ray T. Bohacz)

Will the octane booster keep the fuel from igniting until the spark if carbon deposits are an issue?

By the way, I really appreciate your help. I drive my truck to school and back, so I want to get this fixed ASAP so I don't ruin her.

 

Sounds to me like you need to sea foam that thing. I've done it several times and it works. Buy yourself the can of liquid not the spray and make sure the engine is good and hot. Take the vacuum line off and ease it into the sea foam. You might. It be able to let it drink all at one and probably have to speed the engine a little for it to handle it. I usually let it drink half the can and shut it off immediately. Then let I sit for like 3 hours and go beat the crap out of it. It will most likely fix it the first time but 2nd time might be needed. It will smoke like its on fire just a warning.

 

Ok, well when I pull a vacuum line the engine dies. Is this normal? If so, How do I get the seafoam into the line?

Edit: Ignore this post. This happened a while ago and I've adjusted timing since, and the idle just speeds up a bit now, which I believe is normal.

 

Ok, so I took a vacuum line off, and the engine stayed running, so since last time I took a vacuum line off, I've changed something that fixed that. Anyway, I just ran 1/2 the seafoam through her,and she smoked a fair bit. Will start it up and do some drifting in a few hours, and see how she smokes!

 

Ok, little update. I did the sea foam treatment (1/2 can in through a vacuum line) and let it sit for 3.5 hours. Drove it hard, and there was a bit of smoke, but not a crazy amount. got back home and did the second half of the can, in through the vacuum line, shut off right away,and let sit for 5 minutes. Test drove again, and a bit of smoke but nothing crazy. Both times test driving it I tried going up a hill in 2nd gear and accelerating, and there was a bad knocking both times,and a general lack of power. There is also octane booster in the gas, and it was still knocking bad, but only at lower RPM in 2nd and 3rd. Didn't notice anything in 1st, but it's an insanely low ratio. Also, when flooring it from low speed, the truck would nose over most of the time no matter where I had the choke. It just dropped back to an idle, and sputtered in and out. This was really bad when it was cold, and it still happened when it was at operating temperature and the choke was all the way in or part way out. Lastly, I still noticed a pretty big lack of power, with 2nd not wanting to get up and go like it used to, and it was hard to break the tires free in any gear other than 1st (I'm in Canada, we've got snow all over the roads). So in short, I don't think the sea foam really helped much. I did notice more black deposit coming out of the exhaust pipe than usual, but there's always some that comes out when I start it up.

 

Engine really misses bad and knocks when I goose it and it's cold. Worse than before I ran the seafoam through I think (but then again, it wasn't -20 C here yesterday... just -7 C or so)