An enigma insues...

[Paul Workman]

Originally Posted by bobbyhi

Paul, think of it this way - with all the experience, you could probably become a new mechanic for Marc....more power to ya buddy!!
get 'er running..

Oh sheeiiittt! I bet Marc has Pepsi squirting out hiz noze and all over the monitor screen when he reads that!

P.

[XfireZ51]

Originally Posted by Paul Workman

Sorry for the mystery...
Last winter (while you were still on on Pete's garage probation) therein was a discussion about the KS. At one time, Bob G bought a new knock sensor, suppose to be for a '90. However, when firing it up, the engine would hardly run. The ECM was taking out (I think he said) 20* of advance, and it turned out that the knock sensor for the 90 is different from those of later MY LT5s.

The key distinction was the ohms to ground: The 90 knock sensor is approx 98-101kΩ, whereas the KS resistance used on the newer cars is considerably lower (Pete and I measured 3.8kΩ on one such KS).

When I bought my new one, I gave the GM dealer the AC Delco part number that Jerry posted. However, when the parts guy says, "Oh, this is the old AC Delco number. I'll cross-reference it to the "new" number", I became dubious...

So, when my LT5 was running like a box of rocks, and there was no apparent reason for the change, I remembered that conversation about the KS. However, in spite of no knock counts or retard values, I put the old KS back in. But, it made zero difference in the way it ran, and the knock counts and retard values were still zero.

Shoulda 'splained my reasoning better, Dom. Sorry for the cornfuzion!

P.

Right. The ECM wasn't commanding any Spark Retard so "not likely" it was the issue.

[tpepmeie]

Paul, now that it's running again, can you share what was the problem? Would help the rest of us sleep easier if we know what to double, triple, check.

Todd

[LGAFF]

I think there was an old pull tab off of a Falstaff Beer lodged in the valve seat.

[Paul Workman]

I'll let Pete fill in the technical dimensions, were it not for his expertise I would have been in a "world of hurt". But, in a nut shell the machine shop entrusted to do the valve job, omitted checking the resulting valve height. Apparently, they don't do many/any DOHC engines - mostly LSx motors.

The unloaded lash (my word) within the LT5 lifter is about 120 thousandths. Preloading takes up about 75, leaving about 45 for expansion...and most first-time valve jobs are accommodated OK. However, we've reason to believe this wasn't the first time the valves had been worked on - after the fact - and all of the lash was gone on several valves.

The result was that those valves never fully seated/sealed: NO COMPRESSION! Furthermore, apparently there were several seats that were less than perfectly perpendicular to the valve - to the point lapping would not rectify it: they had to be re-touched. Once all that was done, Pete did a leak-down test to determine sealing integrity, and I did a baseline compression check of my own before bolting it all back together.

Perhaps Pete will answer specific questions regarding measurements and tolerances, but I'll say this: It doesn't matter how good this or that machine shop is, one would be well advised to fully understand the process being done to the point of knowing what to ask for and what to expect from any process, regardless. To that end, I think it is high time some sort of book be compiled with exactly this and so many more examples of what to do, questions to ask, and measurement procedures, be compiled. Hmmmm...I promise more on that later!

P.

[Pete]

Lifters have .120 fully loaded anything above that the lifter is totally plunged and will hang valves open,cams take up .075 from base circle.
It's .075 above the main journals so when you bolt down the cam it will push the lifter down .075 plus the factory has the lifter above the journal another .015 so a stock motor takes up .090 of the .120 you have .030 left in the lifter.
If you do one valve job should be ok if you do a second or third valve or even a crappy valve job the valve will sink into the hole which will bring the valve higher into the lifter bore,which means you will have to cut the tops of the valves to bring them withen tolerance.

So if the valve sinks in/comes higher into the lifter bore .035 your valves will hang open and will have very low compression #'s.
A leak down test will tell you if it's exhaust valves or intake valves.
If hieght checks good then i would say it's a bad valve job.

All of the above is an easy fix by just cutting the tops of the valves besides doing the job all over again.
Also be carefull when taking/cutting the tops of the valves you now have to make sure you don't take off too much you still need the valve stem to be above the retainer at least .020-.025 otherwise the lifter will knock the keepers out then you will have bigger iisues.

Most have worked on SBC,the LT5 is a finicky/different beast,not that it's hard just different.

LT5 is like a woman you have to work on them for a while till you find her sweet spot after that she will take good care of you and keep you very happy for a very long time. LOL

Hope i explained it all ok,if not ask away.

Pete

[Pete]

Originally Posted by Dynomite

Thank ya Pete for those technical details...appreciate it
Please bare with me a bit....I am just learning



So....you are saying the smallest diameter on the cam (all cams) is .150 larger than the camshaft journal diameter?


Either i don't understand the question or i'm understanding it wrong,the base circle (buttom of cam lobe or low side) is higher/taller then the journal by .075.


So...you are saying the uncompressed lifter sitting on a valve stem protudes into the cam .015 if the cam was just a round stock the diameter of the journal? Is not the lifter then compressed .075 plus .015 or .090 when you install the camshaft? I am thinking all camshafts installed with crankshaft set at 51 deg BTDC?

If you were to take round stock and bolted it down in the journals you will only compress the lifter .015.


Ok....if that be the case can I assume all valve stems (seated exhaust and seated intake) are suppose to be equal distance from the center line of the camshaft?

Yes.

If we take 1/2 the journal diameter, plus the lifter compressed (collapsed) thickness, plus .075, plus .015, plus .030, would that be the desired distance from the seated valve stem to the center line of the camshaft?

If i'm understanding it right,then Yes.

If that is the case....could I place a knife straight edge across all journals (inside journal surface closest to valve stem) and measure shortest distance to all seated valve stems which measurerment should be equal for all valves?

What would that measurement be to assure the .030 clearance for valve stem thermal extension or to assure we do not have the same issues Paul had?

You can use a depth gauge to measure the hieght of the valve,i don't have that info at hand.

Hope i got it right.
Pete

[LGAFF]

Pete, Its all Greek to me

[LGAFF]

Thats a Pete joke, he is Greek.....ok pretty lame, but had to do it

[tomtom72]

Originally Posted by Dynomite

Ya..sure

I get the feeling I am the only one that is not understanding this

I just want to check some things since I also had a burnt valve with zero compression on one cylinder

fwiw...no I haven't a clue also....

I guess gone are the days when you could just check the installed height of the spring assembled on to the valve stem with the valve in it's port/seat?