1994 Garbled at WOT

[GuyzVette]

I'm starting to get a little frustrated, and could use some advice as to what to check next....

1994 LT1 M6.

Coming off a fresh engine rebuild because I burned up some rocker bearings, everything was running smoothly. Just recently, though, I've noticed that at wide open throttle, the engine gets to be what can best be described as "garbled". It runs really smooth at normal driving, but at the high RPMs starts to show the problem. And at interstate on-ramps, if you get on it, it is really bad.

So, plugs and wires are all new. Since the problem, I have replaced the fuel pump and in-line fuel filter. I've put a fuel pressure gauge on, and driven it, trying to tell if there's an inconsistency, but it's hard to tell. It looks a little suspect.

So here's my question:
Next stop is going to be to take it to a dealer for a diagnostic. Does anyone have any idea of anything else to try that is easy for a not-so-greasy mechanic? Or is it time to get an expert to analyze the problem?

Thanks in advance for your help!
-Guy

 

[Toms007]

I hate to cry wolf, but this sounds like an optispark issue...are you getting an SES light?. Was the opti new on the rebuild?

 

[GuyzVette]

No, not a new opti. Just a rebuild on the engine internals. I was worried that it might be the Opti, but I was hoping for something else.

Don't get any SES lights.

Is there any test at a dealer or something where they can hook up to it and see if it's bad?

 

[Hib Halverson]

No, not a new opti. Just a rebuild on the engine internals. I was worried that it might be the Opti, but I was hoping for something else.

Don't get any SES lights.

Is there any test at a dealer or something where they can hook up to it and see if it's bad?

I'm gonna guess this is not a problem which will turn on the engine light...at least, not yet.

Rebuilt motor....new valve springs?

New plugs, wires, old distributor...what about the coil? New...old? What?

fuel filter and fuel pump...does the engine pass the fuel pressure tests listed in the factory service manual?

How many miles on the engine at the time of overhaul?

In the end, while a faulty Optispark is possible, I think I'd diagnose a little more before I replace the distributor on GP.

My Karma ran over my Dogma

Is that as bad as my friend running over my dog with his Karman-Ghia?otfl

 

[GuyzVette]

Not new valve springs. 2 new rocker arms, new crank, all new bearings, new rings, new timing chain and small gear (they had the wrong large one in stock, couldn't get the new large one, so used old one, that was in good shape).

Same coil.

I will need to look in the manual, and go through the numbers for the fuel pressure again, this time have someone else look at the numbers while I'm driving. Is there a test sequence to follow, like RPM levels and gears, and the pressure should be X?

115k miles at the time of the overhaul.

Thanks for your help!
-Guy

 

[So Bay Vetter]

This may be a stupid question but, if it's doing it a high RPM are you sure it's not the high rev limit kicking in? Also an OBD code scanner is always a good thing to have around. If nothing else it'll help with whether the mechanic is being honest with you.

 

[Paul Higg]

You may also want to consider cleaning your throttle body and checking the TPS voltage. You would not believe what a bad throttle position sensor can do to a Vette. I know from first hand experience. That little three wire variable resistive circuit has WAY too much control over things in a Vette in my opinion.

 

[So Bay Vetter]

Found this on the Grand Sport FAQ Page:

"Yes, the LT4 comes factory equipped with an engine rev-limiter that functions by temporarily cutting off the fuel flow at a pre-set RPM. The stock setting is 6411 rpm. The "resume" rpm after fuel cutoff is 6397 rpm (at which point fuel starts flowing again). While effective, the rev-limiter can be quite disconcerting and somewhat violent, especially if you "bounce off it" in the middle of a high speed shift. Those that install a higher lift, higher revving camshaft, suggest reprogramming the PCM to move the rev-limiter up a bit, to 6800 RPM or so. Doing so will get it out of your way while still providing some level of protection for your your engine from excessive over revving. "

 

[John Robinson]

Recheck the valve lash settings. Some of the valves may be adjusted to tight causing the valve to hang open at high RPM.

 

[G Winter]

Good Call John. One other thing, I have seen where valve guides were replaced and fitted to tight. When engine warms up the valves can be a little slow to close. I have seen it so bad that they would even do it at low rpm when full operating temp.

Glenn

 

[Hib Halverson]

Recheck the valve lash settings. Some of the valves may be adjusted to tight causing the valve to hang open at high RPM.

Valve lash?!

I hope you mean preload, which is actually the opposite of lash.

Preload is more difficult to adjust because it's difficult to use any kind of measuring device.

To set preload, you do it with the valve closed and the lifter on the base circle. Back off the nut until you have lash, tighten it until just where it is at zero lash then give it half a turn more.

Note that with the lifter dry, it's really hard to feel zero lash but someone with good tactile sense can do it.

 

[John Robinson]

I stand corrected I meant to say that if there is too much preload it will hold the valve open. All I can plead is Another Senior Moment. Sound and feel are the best tools you can have sometimes.
If you do not mind a little oil mess you can start the engine and back off the adjuster until you can hear the lifter clacking then tighten until the clack just goes away and give the adjuster a quarter turn more. This may not be as precise as some of the other methods but it will get you close.

Valve lash?!



I hope you mean preload, which is actually the opposite of lash.

Preload is more difficult to adjust because it's difficult to use any kind of measuring device.

To set preload, you do it with the valve closed and the lifter on the base circle. Back off the nut until you have lash, tighten it until just where it is at zero lash then give it half a turn more.

Note that with the lifter dry, it's really hard to feel zero lash but someone with good tactile sense can do it.