351 SEFI slight knock on hard accel

[74BroncoCO]

I need a little diagnosis help. I just moved down here to TX and this is something new since the move as I don't remember this in CO. Anyway it seems under hard accelaration the engine knocks a little. It's high frequency knocks like the valves ticking or something. It's not loud, but has me worried. The engine oil is probably almost a year old, but has less than 500 miles on it. It's 10w40 Pennzoil. I bought new oil (10w30) and was going to change it until I discovered it only had 500 miles on it. there isn't any smoke, it seems to run fine and have enough power, but I am not pushing hard because of the ticking. Thanks for your help!

J.D.

 

[AZMike]

Probably just pinging in the TX heat. Try retarding the timing or putting some super in the tank and see if it goes away. As long as you're not sounding like a diesel, you're not doing any harm to the motor. An occasional ping/knock is nothing to be too worried about.

 

[musetech]

That's pre-ignition. Usually due to the blend of fuel or ignition timing. Nothing oil related. Fill up at a different gas station, you may have just filled up with cheap gas. That is most likely the case seeing how you have just moved. If that doesn't work go to a higher octane. With SEFI its not likely a timing issue as that is computer controlled, other than base timing.

Don't take it lightly as it can cause major damage to the piston.

 

[DavidCCTX]

In the larger cities in texas (Houston, DFW, San Antionio and maybe others) the fuel is oxygenated i.e. 90% gasoline 10% additive (i.e. ethanol or mtbe) to help reduce pollution. This effectively leans you out 10%, while modern sefi theoretically adjusts for this by burning more fuel, I find that my Toyota Tacoma also knocks when i fill up with oxygenated fuels (i live in corpus christi and we have 100% gasoline here.)

David

 

[Skuzzlebutt]

When you come down in altitude you need to retard the timing, or increase octane. The denser air increases compression.

 

[74BroncoCO]

I will try an octane boost to finish out this tank. Thanks for the help guys. Makes me feel a little better that it's nothing real serious. As many miles as I put on my bronco a year, surely I can afford to put premium or at least mid-grade in it.

J.D.

 

[74BroncoCO]

I filled it up with premium and it seems the knocking is gone. Can someone
explain to me what was actually happening? Why does it knock with lower grade fuel? I might try mid-grade next time and see what happens. Thanks for your help! Oh, the SEFI is from a '95 F-250.

J.D.

 

[braxton357]

Just preignition. The higher the octane, the higher it's resistance to detonating/burning. Pinging is when the fuel ignites before the spark plug does or it ignites on more than one front. Hot valves, wrong timing, cheap gas, and high compression can all do it. I'm not sure about your 95 351w, but I know the earlier efi versions didn't have a knock sensor...so the computer doesn't know it's happening.

 

[AZMike]

What Braxton said ;D Higher Octane will prevent pre-ignition - which is where the knock/ping is coming from. Didn't see the fact that you had SEFI in my 1st reply, but there are a couple of things you can try:
- Complete tuneup
- Set base timing
- Avoid gas stations that consitiently make your motor ping
- Try some detergent - I use Techron every 6 months or so

The detergent will not only clean the fuel system, but will help to clean out any carbon deposits inside the combustion chamber. This is a common source of pigning. The carbon inside the combution chamber will get hot enough to pre-ignite the fuel. I have a 2001 Corvette that pings on hard acceleration whenever I'm down in Phoenix in the summer time - it even has knock sensors in it, and I run nothing but super unleaded (only 91 here in AZ)! It's just the reality of driving in a place that has crappy gas with a motor that's pushing the envelope to begin with. I don't ping at all up here in Flag, or in the winter time down in the valley.

Another way to help remove the carbon deposits is what is known as the 'Italian Tuneup'. Basically, run the piss out of the motor to get it good and hot inside the chamber - this will blow out a lot of the carbon buildup in there. That method is really fun with the Vette! I like to do it coming home to Flagstaff from Phoenix - there's lots of steep uphills that get the inside of the motor good and hot. This method will probably be a little scary in a Bronco though

 

[Huntersbo]

My 1982 351 W F-150 was deiseling-I hope it is ok. I will probably pull the heads to see. It ran fine on 87 octane until we had the water pump repkaced an the mechanic set the timing straitht up and I had knock problems and gradually lead to a diesel sound and coolant squirted everywhere. So I will tighten everything down, pull the heads to see the pisons, compression check, mabe a valve job and never buy an emsision controlled truck again.

 

[74BroncoCO]

Thanks guys! I have used the Italian tuneup method before, maybe it's time to take my bronco out, although it seems I can only get it up to temp on the trail. If I am headed down the highway, she runs real cool. Anyway i will look into detergents and maybe try the Techron. Thanks again guys, I am sure glad to be a part of this board!

J.D.