I hope this thread helps those with this issue and or the info can be passed on to those who need it!! Good Luck!
SYMPTOMS:
1-Misfire #8
2-White ash on plug
3-CEL on (not flashing)
4-Some power loss
5-Low curb idle pressure (@ operating temp)
6-Truck in gear *D* stopped with foot on break, once break ped released truck shudders
7-Stops once gas is applied
8-@ 40mph and @ low rpms will shudder again with noticeable power loss

Symptoms not present include but are not limited to..not overheating, no smoke from tailpipes, no visable leaks or on ground, not smelling any fumes/antifreeze,

Recent work I've done: plenum gasket and pan, plugs (NGK), wires, cap, rotor, air filter, replaced IAC, throttle body cleaned thoroughly, air change temp sensor, upstream o2 sensor (denso), new cat, fuel injector o-rings, valve cover gaskets, fresh oil & antifreeze, seafoam via oil, gas and bb vac, crankcase breather, PCV, back to 195* t-stat from 180*, bypass hose, battery, terminal block & brush assembly (alternator)...after CEL came on I replaced the #8 plug and after two starts and a few miles the CEL went off..a few starts/miles later it came back on..it was steady/never flashed...

Plan to do: replace #8 fuel injector..test results..If it doesn't work things I could do TPS, MAP sensor, CSP sensor, compression test cylinders dry/wet, test FI with an ohm meter (results not totally reliable tho), swap injectors, hydrocarbon leak test (via the radiator), leak down test, head gasket, trans filter/fluid change, add torque converter shudder treatment, go back to OE plugs...

I do believe some symotoms aren't related but I sure as heck have been known to be wrong ..Also, keep in mind it's a 1998 with over 266,000!!

These lists might not be complete, I'm 1/2 asleep, so please excuse me in advance

I'll post anything I forgot, if needed..and will also post the results of completed work...

 

With as much work as you've done, I would start rechecking things.

Also, was it doing this before all the work?

 

Yes it was doing it before the work started..

 

I'm thinking maybe cracked valve seat(s). Rather common.

 

I should have found chunks in the IM or on the belly pan when I did the plenum if that's the issue? And, if that's the problem there should be piston damage as well correct?

 

So an easy (& free) test to do before I go ahead and replace the head gaskets.. is pull the oil dipstick and if the 'oil' looks tannish-white that's a good sign the gasket(s) are blown and coolant is mixing with the oil..but if it's not and just because it doesn't look strange in color doesn't rule out the fact that it's blown..another (free) test is to check the radiator..with the cap off and the rad topped off, I'll have someone crank the engine while I look for bubbles or even for coolant to splash out..I shall do this at a safe distance and NOT when the engine/coolant is all hot! duh!!! If either/both happen there's a good indicator that there is combustion gases (hydrocarbons I think) trying to escape. Now, just like the first test, if they're no effects this still doesn't rule out a blown HG...Now I do know there's a more formal test for the hydrocarbon gas leak. It's called a 'combustion leak tester' (cost approx $55 on amazon..little more/less elsewhere).. It's a VERY simple test to do and it comes with enough stuff to do the test a hole bunch of times and on any ride so it can be shared with family & friends. With that all said and what I plan to do is buy and use a compression tester (actron $26 @ advanced auto)...more about this in the next post...

 

No and No,The valve seats crack between the valves.

 

You're talking about 'seats' and not 'seals' correct? And, when I do the compression test for this issue, it should be done 'wet'? Then also do the test dry one (and I'd do the dry type first). I think if there's no change in compression between the two tests pionts to a valve issue (and/or head gasket)..Thanks for your help!!

 

Dry test first, then add a teaspoonful of oil to each cylinder (one at a time) and re-test. Post your results.

 

I think if there's no change in compression between the two tests it points to a valve issue (and/or head gasket)..and if the wet test is much higher that points to a piston problem...I shall post the results!