Ok so let me begin completely from the beginning. The truck is a 1989 F250 4x4 with a 351 auto, and only 24,xxx miles but hard miles used a hospital truck. I purchased the truck about two months ago, and drove it an hour home. After further inspection I noticed the truck had a knocking noise coming from the engine. After reading a bit I figured to try and adjust the timing and see what happens. I retarded the timing and the knock disappeared and the engine seemed great. Now I had to replace the valve cover gaskets, exhaust manifolds, spark plugs, hook up the cat, and finish the rest of the exhaust system. Now flash forward and I have done all of the above. Strange enough the knock has come back once again. If I retard the timing quite a bit the knock seems to disappear, but the engine has absolutely no power at all. Then if I adjust the timing back to where it should be the knock comes back, and it noticeable to my ears. When the knock is there it also isn't a consistent rhythm however it sounds like a hammer hitting metal. I've been assuming the worse that the engine is taking a crap, but it just doesn't make any sense to me that if it was a rod knock that the timing would effect how noticeable it is. I've been thinking that possibly it could be a combustion issue that's causing the engine to knock? I pulled two codes from the truck today which were 32 and 41. 32 says that the EGR isn't responding to the test, but I just cleaned out the EGR valve about a month ago. 41 is saying that the truck is running lean. Which I'm guessing it could be a vacuum leak or bad fuel filter? I'm simply trying to rule out that the engine may be shot. If anyone has any advice they could give it would be greatly appreciated. Not really sure where to go from here. Don't want to dump anymore money into the truck if the internals of the engine are shot. Thanks in advance
-Sal

 

A couple of basic questions.

1. Is it full of oil? (not condescending you, but internet diagnosis can not assume anything)
2. If it is full, how is the oil pressure?

Retarding the timing reduces the pressure build up that makes an engine turn, so whatever is worn will have less stress placed on it as well. It makes perfect sense that ignition timing affects how a rod knock sounds.

A plugged fuel filter will show itself with reduced fuel pressure, especially under load. A vacuum leak is fairly easily found by spraying carb cleaner on the vacuum lines. Some folks use an unlit map gas torch.

I haven't messed with egr too much, so I don't know what to do for that at the moment. I do think you are wise for thinking the engine is gone. Service vehicles get ignored by employees. They just get in and go.

 

The engine is full of oil. Did an oil change not to long ago and no metal shavings were present in the oil. After the change the oil has still been clean. Not sure on the oil pressure as the stock gauge doesn't read accurately. Yeah unfortunately service vehicles really get beat on by the employees it's really quite a shame.

 

Stock gauge is essentially a dummy light anyways, and is useless to determine oil pressure. You need to use a mechanical test gauge.

 

Timing will very definitely affect the intensity of noise coming from a given cylinder with mechanical problems.

I'd first pull out the stethoscope --rod knocks are loudest at higher RPMs, bearings tend to be noisier on deceleration---wrist pin knocks are very rare

If you can't discern the location of the problem with the stethoscope, then finding the cylinder with a problem is done by shorting out the plug wires one by one..........If the noise is changed when the plug wire is shorted to ground, you can figure the problem is in the reciprocating bottom end parts. (piston, wrist pin, connecting rod or connecting rod bearing)

Just a start.............good luck with the old truck !!! (who in their right mind doesn't like old trucks ?)