6th gear shakes ?
 

Ok why is it that shifting into 6th at 65mph , makes everything shake like its coming apart ? It kinda clears up around 80mph ,is it low rpm's due to 2;73 rear gears ? Not going fast enough for proper gear meshing ? ok open for answers !

Re: 6th gear shakes ?
 

Is the shifter rising during your shifts ? 6th gear is normally used after 70 mph. Wait a minute 2:73 gears ?

Re: 6th gear shakes ?
 

Have you checked to know for certain what the differential gear ratio is?

Easy enough to do.
(1) raise both rear wheels off the ground,
(2) Transmission in neutral,
(3) rotate rear wheels, while watching pinion yoke, until the one of the 2 yoke joints is looking straight at you from the side,
(4) mark the tire at 12:00,
(5) rotate the wheel, counting the revolutions of the tire, until the pinion yoke has completed 1 complete revolution & no more/less.
(6) Observe the clocking of the tire mark, i.e. 9:00 for example.
(7) Multiply the clock position (example: 9:00 x 0.083 = 0.747 & add the number of complete revolutions (example: 0.747+3.0 = 3.75) and you have the differential ratio.

You don't mention the tach reading when you shift into 6th gear. What is the rpm @ 65mph when you shift?

The shake is probably due to the miss you've inquired about in your other post.

Re: 6th gear shakes ?
 

I will go test it to get the rpm , gear ratio is definitely 3:73 , I was told it was 2:70 . I know that there is some kind of electronic box , I assume is to correct the speedo , near the battery . But 3 and 3/4 driveshaft turns to one complete tire rotation answers that !


ok its twisting 4300 rpm at 65mph , how accurate that tach is , I have no idea !

Re: 6th gear shakes ?
 

Is the car still running OE 315/35-17 or some other combination? With the KEY-ON engine NOT running does the TACH park at 0 ?

What year is this car?

Re: 6th gear shakes ?
 

Its a 91 .Factory tire size front and rear . Yes the tach stays at zero key on , engine off.

Re: 6th gear shakes ?
 

Do a drive in 4th (4th 1:00 ratio) gear and record speeds at a few different RPM - try 1000, 1200, 1500, 1700 and 2000. Maybe just use 3 or 4 of those.

Now maybe do the same drives using the SPEEDO at 40, 50, 60 and 70 and record the RPM display. If you have a GPS maybe record also the GPS display at those speeds and the effective calibration of the DRA/ERA could be determined.

**************
Using a 3.73 and a tire diameter just a little short of the advertised 25.68 I might expect

1000 - 20+, 1200 - 25-, 1500 - 30+, 1700 - 34+, 2000 - 41-, 3000 - 61+, 4000 - 82

I used a calculator that I've used before for just random comparisons. I did no math but I believe I have in the past. I didn't use a calculator to go MPH to RPM, didn't figure it necessary if you do the TACH to MPH display and the MPH display to GPS.

** I didn't take into consideration that I believe the ZR-1 TACH is 1K, 1500 and 2K incrementally so you might just do those three and I stretched the expected to 3K and 4K.

You could use the C68 display to confirm the cluster tach operation OR a scanner

Re: 6th gear shakes ?
 

OK, given that you have now confirmed 3.73 gears in a follow-up post, there is no question that you should be able to accelerate smoothly from 65mph in 6th gear.

NOT a gear meshing issue.

I suspect a spark problem.
May be be coils or wires.

Check spark;
Suggest you use a spare spark plug to test with. Best done in dim light
Engine off remove one plug wire
Insert spare plug in plug boot.
Ground plug
Start engine
Gradually increase distance of plug from ground & observe spark color & length.

Good spark is blue in color & will jump 1/2"~3/4"
Weak spark is orange in color & will not jump far from ground
Bad spark is obvious

If you have weak spark, note the cylinder number.

Repeat for all 8 cylinders

I'm not an advocate of measuring resistance of paired cylinder plug wires & the coil all together. It serves no real purpose since the plenum has to come off for either wires or coil. With the plenum removed, you can ohm the plug wires individually and check the coil independently for meaningful results.

If all 8 cylinders have good spark, then your problem is most likely fuel injector related.

Re: 6th gear shakes ?
 

Well spark isn't an issue ! I did change the O2 sensors which made somewhat of a difference .Now only #8 seems to foul -sooty . I'm wondering if the injectors need cleaning , the car is only driven a few times a year ?

Re: 6th gear shakes ?
 

Is the fuel fresh or treated with fuel stabilizer?
Do you keep the fuel tank full when not being driven?

Understand you have a 1991 ZR-1. Original 89~92 model ZR-1 injectors are venerable to damage from gasoline/ethanol blended fuel. Unless you use only 100% gasoline (no ethanol), it is likely that your problem is fuel injector related. Cleaning the injectors will not remedy injectors damaged by ethanol.

The correct fix is to replace all 16 injectors with modern injectors manufactured with components that are compatible with ethanol.