A friend called me regarding a problem with his timing on his 90 L98 Vette. The initial timing appears to be fluctuating from 6 to 30-40 degrees while watching it with a timing light. The engine noticeably sound different when the timing starts to move. The problem is intermittant.
He replaced the ignition module and the cap. Anyone had similar experiences. I know others here on the CF have replaced the stock distributor with a MSD 8366 unit, but not sure the reasons.
Thanks in advance.

 

A loose/corroded reluctor ring will do what you describe.

 

After looking at the GM service manual, the only parts described are the module, pickup coil, magnet and pole piece. Not sure which piece is the "reluctor ring".

 

Sounds like a bad balancer. The ring on those are known to go bad over time, allowing the timing readings to be inconsistent.

 

I know anything is possible , but the balancer is a new ATI unit.

 

How old is the distributor? If the shaft bushings are worn out this can cause erratic timing. Also disconnect the "set timing" connector and see if the timing is still erratic. Doing this will disable the EST from the ECM, so if the timing is still going all over the place then its something mechanical in the distributor. Another thing to check is the distributor gear itself should you have to remove it. Also check for end play, up and down movement of the shaft as well. If the shims are gone and you have excessive up and down movement it can also cause the timing to jump around.

 

A bad connection at the EST bypass connector would do that.

 

Thanks for the great suggestions. The distributor is a 1990 stock unit. The distributor has <20k miles, because it's in a road racing car.
The idle is erratic when checked with the EST disconnected. The distributor has recently had a composite gear installed , because a new roller cam installation. The work was performed by a notable engine builder.
My suggestion was to replace the distibutor. He has located a spare HEI dist. that he is going to try.

 

balancer
worn timing chain
distributor


the list goes on...........

 

The balancer and timing chain are new. So, the possibilities are not endless.

 

Let's back up to square one.

When did this condition begin? Was it after some modification was done to the engine and the condition DIDN'T exist before the mod?

You mentioned intermittent; What are the conditions when it occurs: Open Loop. Closed Loop, Cold Start, Idle, Driving, etc.?

Did he make any other changes to the ignition system?

Jake

 

Jake, those are some great questions, but I'm getting the info secondhand.
He had a new motor built. The distributor gear got completely chewed up , after only a few laps.
The motor was rebuilt and the distributor got shimmed the 2nd time. The problem started when he got the motor back the 2nd time. He does not have any diagostic software (ie Diacom) to help determine the trouble. The erratic timing has occur when the motor is cold and also after being taken for a test drive.
Maybe I can get the owner to reply to this thread.

 

Contrary to what the oil pump companies will tell you, high oil pressure will cause accelerated distributor gear wear.

Incompatability between the dis. gear and the cam gear will cause it too. From what I've read CompCams has a new gear that's suppose to take care of that problem and GM offers a Melonized gear that's meant for roller cam engines.

I've had a distributor gear wear so badly that the engine actually shut down as I was driving down the road. I went Melonized.

But to jump from 6 to 30 is a LOT! You may be able to get an idea of how much backlash there is between the two gears by removing the distributor cap and trying to turn the rotor back and forth to see how much slop there is.

Crane recommends cutting a groove in the distributor housing just above the gear to allow pressurized oil to flow to the gears. The diagram of how and where to cut the groove is in their catalog and may be on their website too.

Did he, by any chance, change coils?

Keep us posted.

Jake

 

Composite gear? That is the first place I would check out. I know that Comp says that they are good, but I have personally never used one and I can't say how well that they would work. Definitely that is one place to look.

 

I certainly appreciate everyones help in trying to diagnose this problem with timing. Yesterday afternoon I witness the timing jump after the car was warm / throttle blipped etc., then back to idle. I had set the timing at 6 degrees with the est disconnected then tested again only to see the timing at 14 then 21 then back to 6. Ten minutes later I tested again and the timing was dead on 6 degrees throttle response and idle were dead on perfect. Very frustrating!! New Accel cap and rotor have been installed, using Accel super coil. The motor is NEW less than 1 mile on this engine.
I understand that when the EST is disconnected and the timing is set that you take the ECM out of the equation.
With this car being a road race only (not street legal car) it is critical this problem be fixed prior to getting it to the race track. Timing changes like this at 6500rpm could certainly damage this new motor.
The distributor is the original unit.
The distributor gear seems to be fine. (composite gear) . It has been shimmed and used specifically for the cam installed, again by a notable engine builder. Timing chain is new. ATI balancer is new.
I am thinking at this time that the original distributor is the culprit. I would like to replace it with a MSD HEI unit with the large cap and the coil on top of the cap. I cannot find one of these units without the mechanical advance in any catalogue or MSD on line web site.
Thanks again for everyones help and I would appreciate any other info or suggestions you might have.
Thanks.

 

Whoa. Isn't it the job of the ECM to play around with the timing? That's why we disconnect the EST to set it, right?

My 90 (only 37K miles) does the same thing when the EST is connected, and with no ill-effects I know of. Does anyone have hard info on the proper range of fluctuation?

 

You wrote that he's changed coils. What are the colors of the wires on the coil and the pick-up coil?

The 85 thru 89 have a polarity issue when those two units aren't properly matched and become incompatible. The wire colors determine if they are or not. You can also use the twisted wire check procedure.

I don't know if the same concern applies to the 90 though.

Are you running a ground spade connector?

Jake