Hi all,
I have a 98 5.2 270,000kms 4x4.
The truck runs very well, no error codes or reliabilty issues, tail pipe smoke or over heating.
I recently used it for a lot of highway km's - about 30,000 over 5 months and noticed excessive oil consumption, worse and worse gas milage and very mild spark knock at mainly highway speeds - slight uphills that sort of thing, nothing that you would think would really strain the motor. If you look at the tailpipe it is black with a thin layer of soot, to me its running rich. Anyway, so I replaced the plenum gasket no problem - Hughes upgrade - and I expected all to be well, but although I aint burning engine oil any more I am still getting spark knock, power sucks, it was never great but it really doesnt have any get up and go - I know these engines aren't fireballs, but I had a 1970 D100 a few years ago with a 318 and it would run circles round this truck.
So to sum up, Replaced plenum, pre cat oxy sensor, removed and cleaned rear sensor on throttle body, new plugs, new air filter, oil change. And I am pretty sure the cat is fine, at idle it sounds fine at tail pipe, and smells normal - the engine will rev without any issue.
Now, one thing that I remember about that old D100 I mentioned earlier, I had the timing chain replaced when I owned it and the amount of extra response from the throttle was VERY noticable afterwards, could I have a timing gear that is worn?

Just to put this in perspective, I have owned this truck for over 6 years, and last summer pulled a 2 tonne boat about 160kms over hills etc without any knock. But now it can't pull its own weight without protesting!!


Any ideas as to what could be causing the rich running and mild spark knock?

 

Could be a possibility of a loose timing chain, timing kit's are fairly cheap and most of us go with the double chain setup.

One thing i would suggest doing is seafoaming the hell out of it, when the plenum blew it could have gunked up your intake system and the seafoam will clean out alot of that old burnt on oil and carbon.

Just my 2 cents.

Not to mention plugs,wires,cap,rotor

 

spark knock is caused by a few things -
vac leak, associated lean fuel mix, and associated higher cylinder temperatures.
low octane fuel;
carbon build up and its associated higher temps;
timing too far advanced.

you might have a vac leak in your intake gaskets.
your timing chain may look like a wet noodle.
you might have carbon build up.
you might benefit slightly from a cooler plug. autolite 3923 instead of 3924.
you might benefit slightly from higher octane fuel.
you might benefit from a combustion chamber cleaner or seafoam.

the sudden onset of spark knock is an indication of a new problem, and when its pinging, it doesn't have any power because its firing out of time. its also very bad for the engine.

 

Thanks very much for all your advice. I know its gonna be one of those issues that will take time to pinpoint that's why I came to the experts for a sound starting point. I will look into your suggestions and post my progress. Thanks again!