Off idle hesitation, spark knock, code 24, replaced TPS...

[schardein]

So my truck has pretty bad "valve train rattle" or "piston knock" that lasts 10-20 seconds on startup.  It also does it a little during "float" condition while cruising and is quite noticeable when accelerating at cruise speeds.

 

I've done some minor engine mods including aftermarket exhaust manifold, newer style intake, 1996 throttle body with MAP mounted to it, 4 hole injectors with no appreciable change to the noise.

 

Recently, truck started having an off idle hesitation.  Then I noticed some mild spark knock, and the transmission wanted to stay in a lower gear longer.  I would accelerate and then lift off the pedal to get it to shift.  Making it upshift would lesson the spark knock.

 

Driving to work two days ago, valve train rattle seemed a lot worse, and actually sounded more like spark knock, (pre-ignition, detonation).  Then it threw a check engine light which showed a code 24.  I was actually happy about this as it hopefully narrowed down my troubleshooting.

 

Replaced the TPS with a spare last night.  WOW!  No more off idle hesitation and it most definitely cleared up ALL the spark knock.  Running great again.  Shifting is also back to normal.

 

I can't remember if I left the 1996 TPS on the throttle body when I installed it, or swapped on the original 91.  But the spare 91 TPS sure solved it.

 

I still have the valve train rattle on startup, and I hadn't realized how bad the spark knock was until it was fixed.

 

I ordered a case of WIX oil filters which arrived yesterday.  Hoping the next oil change shows an improvement on the valve train rattle at startup, but I fear the motor has just had a hard life/sustained some excessive wear in the past.  We'll see.

[HOrnbrod]

All the 91-96 TPS's are all the same, so the one you took off had a problem. You might try cleaning it up with some throttle body cleaner and try it again.

 

How's the 96 MAP working for you? I have that too, but on an aftermarket 62mm bored throttle body.

[schardein]

All the 91-96 TPS's are all the same, so the one you took off had a problem. You might try cleaning it up with some throttle body cleaner and try it again.

 

How's the 96 MAP working for you? I have that too, but on an aftermarket 62mm bored throttle body.

Good to know the TPS are the same.  I'm thinking I left the original 96 on it, because the bolts seemed really tight, tighter than I would have put them on had I switched it out.

 

No issues with the 96 MAP.  I wasn't expecting any changes one way or the other, just wanted to clean up the engine compartment a little and eliminate the potential failure of the vacuum hose in the future.

 

I did have a problem with a high idle shortly after the 96 TB swap, fixed by replacing the IAC with a spare.  Hate to admit, I can't remember if I kept the 96 or swapped my previously cleaned up 91 IAC during the swap.  I really do keep a maintenance log!  Just didn't keep good notes on the TB swap.  Plus I've done the same mods on my XJ.

[schardein]

It just occurred to me, the junkyard 96 XJ I pulled the TB from had like 290,000 miles in it.   Guess the IAC/TPS having issues shouldn't come as a surprise.

 

It had been catastrophically rear ended.  Other than that, it was in pretty good shape and looked to have been reasonably maintained.