finished a heads and cam swap and the engine ran fine for a little while but now at 3000rpm if you get on it the engine pretty much just dies right out... smooth acceleration does not cause it... the engine also seems to have a "load" on it when revving in Nuetral... then it will fix and be fine again... enough for the perf shop to get dyno tune on it... then it will just do it again... changed ignition module, crank pos sensor, and coil now... have msd ignition.. could it be a bad msd box??? or is it just the opti??? i did replace the opti about 5K miles before i did this swap but i know it can still be bad


thanks
jeremy

 

make sure the rockers are only about 1/16 past zero lash as too much preload does some goofy things.

make sure there are no exahsut leaks, clean maf facing correct direction, good plug wires, no leaks between MAF and TB, etc.

Start with these and if you have anything on this list, fix it bfore moving on to other things. Colud be opti but alot of other things can cause it as well.

After fixing these things, get some datalogs and see look at the MAF, knock retard, compare L/R O2's and BLM's and the TPS reading. This will tell use a lot more about what might be going on.

Lloyd

 

What kind of opti did you replace it with?

 

Lloyd ill check those things that youlisted but when i took it for a lot of these things.. i guess i can try a Maf sensor i have had these go bad on my GN but those always go bad on them... i replaced the opti with an Autozone one so not sure what the hell it is... sometimes the car runs fine tho... crazy damn car... i hate electrical problems...

 

any of you guys know how to diagnose a bad maf??? i know for GN's they have a tap test... does this work on the LT1 maf sensors also???


thanks
jeremy

 

1/16th really isn't necessary unless a specified lifter calls for such preload. All (unless otherwise specified) OEM lifters do just fine with 1/4 to a full turn past zero lash. With that kind of adjustment a hyd. roller valvetrain must be spot on or certain damage may ensue. Basic rule of thumb is 1/2 turn past zero lash.

 

the lash is ok its not the problem... the problem comes and goes but its mostly there... smooth accel is not a prob but when u get on it it dies at around 3k rpm.... what are the symptoms of a bad maf sensor???

 

if the problem comes and goes, it would not be the lash doing it.

I have seen lashing the valves 1/8 to 1/16 help alot of people with stock lifters that have problems at higher RPM's. It the rockers are RERALLY tight, it can effect the idle and everything else.

Some people can suggest other lash settings that they have seen work well.

A plug gap that is too wide can do goofy things as well. I have seen .030-.035 gap fix alot of stuff too.

Lloyd

 

Perhaps those goofy things you've seen w/ different lash settings had something to do with the cam profile?
I've never seen any idle issues with anything ranging from the Comp Extreme cams to the LPE cam I ran to the 305 and 306 cams run over the years with an adjustment of 1/2 turn past zero lash. Only as of late have I read about such things...

 

CC 306 and GM 847 have different cam profiles for sure but I have seen it help both.

 

I run my gm847 just a hair over 1/8" past zero, I would agree that looser is better than tighter with big cams.

 

Yes... "loose is fast" but a 1/2 turn past zero lash should have absolutely no affect on idle quality or drivablity as long as there is a competent person doing the valve adjustment.