I have an 86 with the L98 motor. At higher RPM the motor really starts to chatter. I would say it starts at 4500 rpm and on.I thought it might be timing, butI am having trouble setting it right.
Might it be because I am using cheap gas? Are those early cars set up for 93 octane?
It sounds like the top of the motor is floating for lack of a better word. I have ad have had LS1, LT1's and the likes. none have this problem. is it normal for L98 motors?

 

Floating the valves? What kind of "Higher RPM'S"?

 

Mechanic thinks it might be lifters because of spark advance module malfucnction. Don't know exact RPM. Tach is inop, becauseI have an HEI distrubutor I think.I would guess 3500-4500 RPM. this sucks!!

 

If you think it's in the lifters or heads, put a compression gauge on, and measure the squeeze in all 8 holes. Check to see how long each will hold pressure too, like each one at 15 minutes, an hour, etc., ...

 

I have a '90 with same engine and similar problem. Had good tune-up put on it few months ago and still get valve clatter under hard accelleration. Have been burning Premium gas but
also add a bottle of STP Octane Booster every 3rd or 4th fillup. Seems to help but I have
been advised here in past that I am only putting band-aid on problem. ????

 

Did the person who referenced the band aid suggest anything else we could be doing?

 

I went back and checked my previous posts. After tune up was recommended a timing check and new EGR valve was recommended here at this forum. Both accomplished but still have
that valve clatter/knock on occasion.


(Always gotta be something ya know.)

 

Sounds like the ESC (Electronic Spark Control) is not working or either EGR system is not working. Both will cause detonation. The EGR uses a solenoid usually right behind the thermostat housing that controls vacuum to the EGR valve. Usually, ESC will set a code if not working properly. Does your 85 have a tube off the right hand exhaust manifold that goes to the intake and also has a temp switch in the tube? [8D]

 

I don't know, I will check about the tube. I seemed to have fixed most of my problem filling with 93 octane gas. My mechanic said myEGRis stuck open and that might cause it too. He also said I need a new MAF... he said alot!

 

If the EGR is stuck open you would have idle problems as that is a big vacuum leak @ idle. MAF would most likely cause stumble or poor idle conditions,