1993 F150 spark knock

Hello,
The specs: 1993 F150 , 2wd , long bed , E4OD auto, 194000 miles. Gas milage about 14-15 highway.
The spark knock problem started at 20,000 miles. Heavy pinging under hard
accellaration or FULL throttle. Also pings under at low RPM's ( nothing terrible but still more than normal.). Otherwise the truck runs fine.
It really hasn't been much of a problem all these years ( more of a minor annoyance ) because I usually drive with a very light foot and those situations where I MUST use hard throttle are rare. I can even use 87 octane.
The real problem is when towing. There is absolutely no way to avoid the pinging when towing. Usually I will have to slow to 20mph or so on the highway grades to keep the engine from blowing itself apart. Even using 93 octane doesn't help when towing. So therefore, I must get those problem diagnosed.
Heres what I've done so far:
1.Checked trouble codes : None 2.Replaced plugs and wires : I saved the old plugs. 3.Changed cooling system : I've ruled this out because pinging occurs even before the engine is fully warmed up. 4. Exhaust system is fine. 5.Fuel filter . Fuel pressure tested normal. 6.Cleaned out throttle body. 7.Replaced PCV valve, filter and air filter. 8. Timing is right on the money.
In effect, I'm stumped...
The engine is in good shape, doesn't burn a drop of oil. The old plugs looked a little light in colour perhaps suggesting a lean condition but I'm not certain what is a normal colour plug for this engine. The plugs are also even in colour ( no plug looks any different from the other). The truck used to get 20 mpg when new. It dropped to 14-15 when the pinging started at 20,000 miles and hasn't changed.
Any ideas ?
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Well, I don't have any really great suggestions, but you listed number 6: Timing right on the money.... well if it's pining maybe that's not so true..? :-)
My best suggestion at the moment, which is probably crappy:
You said even with 93 octane fuel, the problem persists. Is it equally as bad? or is it any minor bit better? And have you tried draning (or very very close to it) your fuel tank and filling it all the way with 93..... so that it's not mixing with lower octane fuel and therefore lowering the overall level.
Does it ping real bad in neutral when you rev it up?
Nic

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Base timing is 10 degrees with the spout disconnected. Many eons ago the ford dealer "diagnosed" the problem as base timing out out of adjustment. Well his "cure" wasn't so miraculous after all. Turn out all he did was retard the base timing away from factory specs. Trouble was, the truck ran crappy, took a big hit in fuel milage and failed the emmissions test. The ping was still there, though reduced somewhat. The emmisions mechanic put it back to factory specs, then it passed.

No, I havn't drained the tank but I have run it low before filling up. I even tried some of that octane booster additive with little effect. 93 octane definately helps, especially when under hard throttle. Helps only little when towing however. Ping is still heavy on those inclines .

No, it only pings when under a load .
By the way, I forgot to mention the motor is the 4.9 inline six fuel injected. At my old job we had a fleet of F150's with this motor. Real workhorses they were.
Thanks for your help..
John..
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Really? Some of our Ford work vans have the 4.9 in them (3 back bench seats and 2 buckets up front). They are the slowest beastliest vans we have. (We also have Fords with the 351 and GMC with the 350... both infinitely better than the 4.9 IMO) When you have the 4.9 vans loaded up with 11 to 14 people you have to keep them floored to go 70 down the interstate. And leave plenty of room to get up to speed.
But we never have had to have much work done to the 4.9. They are pretty good engines, but could use more power for an almost 5.0!
As for the pinging... have you considered the possibility of carbon buildup in the combustion chamber? My fords seem to have more of a problem with this than my other vehicles. (Pinging due to carbon buildup)
It seems to help a lot of I drizzle water down the intake with the engine running. Rev it pretty good. I got my '84 grand marquise to leave an actual PILE of carbon under the tail pipe using this method. I've been able to run regular unleaded in it since. And it was terrible with pinging before, even premium gas didn't help with the engine under load.
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The 4.9 engine is pretty low on horsepower, thats for certain. For a truck its a good engine because it gets its torque down real low. Thats one of the reasons why I've been able to live with this pinging all these years. There really isn't any reason to rev the engine to high RPMs. 0-60 times don't mean much to me. Work horse engines they are. Some of these at my old job had 300,000 to 400,000 miles. One ran with a dead cylinder spewing heavy smoke out the back, it ran like that for many years - We used to work these trucks hauling sand and rock salt and pulling utility trailers. The company mechanic sweared by those motors.
I have considered carbon buildup but most of my driving has been highway. Usually carbon buildup is a problem with older, carbureted engine that operate mostly in stop and go traffic and driving short distances without giving the motor a chance to warm up. Nonetheless I really don't have much to lose by running water or top engine cleaner through the engine. I still think this is a computer problem however. Possibly a bad sensor or a rusted sensor wire or ground SOMEWHERE causing a higher than normal resistence. God knows how to find something like that. Maybe the computer itself is due for an update.
John..

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the CEL will come on but...Make sure it is working, no cloged passages ect. Or...It is possible since you drove it real easy you have major carbon build-up in the cylinders. You can clean this by slowley adding water to the throttle bore while at a high idle. A spray bottle is the safest way. Remove the intake hose at the throttle body. Run it at high idle while spraying water into the engine. spray as fast as possible without killing the engine. If the engine bogs alot, slow down till it smoothes out and continue. BUT check the EGR first. At idle open the valve manually by pushing up on the diaphram. The engine should stumble if the passages are open. If it doesnt you have a clog somewhere. If it does stumble, with the engine warmed up to normal operating temp, feel the diaphram under the valve to see if it opens while you rev the engine a bit. Check back with your results.....GL
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something else?
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Initially I had elomainated the EGR system as a cause of the spark knock because the ping occurs at full throttle in addition to hard part throttle. The EGR valve is supposed to close at full throttle. However I just checked the egr valve just in case, and yes, manually pushing the diaphram causes the motor to stumble and stall. Also the diaphram does move when the engine is revved .
Concerning the Cylinder carbon buildup:
About a year back I had a professional cylinder cleaning cleaning done to no effect. To be honest, I didn't think this was the problem because almost all of my driving has been highway. Also the ping started at 20,000 miles. Isn't this a little early for carbon buildup? I also cleaned out the throttle body fairly recently. It had a light black coating but nothing substatial.
This is definately pinging. Higher octane fuel quietens it somewhat. John..

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It might be time to have a look at the knock sensor. A scan tool would be in order to see what is going on with sensor and the timing while going down the road.
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Many years back I replaced the knock sensor as a shot in the dark ( it was cheap enough ) .
Unfortunately I don't have a scan tool.
John..

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You don't need one. With a 12 volt test light or volt meter and a jumper wire you can get the codes from your truck.
Go to www.batauto.com and look at the trouble code section. Fords have a better scan them the GM for the do it your selfer, IMO.
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I've already checked the trouble codes. None were reported. I think scott was talking about reading live sensor values while driving , and reproducing the ping. This equiptment is very expensive. Unfortunately most mechanics don't have this equiptment and the ones that do ( and qualified to use it ) are hard to find.
John..

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(oops... guess I should finish the thread before adding my 1/2 cent?)

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had 20k when it first started? If not check them again. How did you check the exaust? Does it have TBI or port injection?
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Scott, All of those things have been done recently. Other things I havn't mentioned were done way back, with no fix. From memory, Replaced, MAP sensor, oxygen sensor, knock sensor, cap/rotor. The ping hasn't changed since 20,000 miles. The cause has been a big mystery. Thats why it never got fixed. As I said, its only a minor annoyance as long as I drive with a light foot . Any heavy accellaeration will cause it to ping ( including full throttle ) . While towing it is very difficult to drive with a light foot. The pulling power is there . Except for the ping, the truck runs beautiful, though gas milage dropped considerably when the pinging started.
BTW I just took it to the shop to have it checked on an engine anilyser. It passed all tests ( including EGR ) . WTF....
Is there an updated computer or something?

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