Jumpy Timing Line/Thoughts? Kinda Long
Thread Starter
Burning Brakes
Joined: Oct 2004
Posts: 898
Likes: 1
From: Pearland Texas
Jumpy Timing Line/Thoughts? Kinda Long
I've been experimenting with the timing on my car. If any has been following my threads I finally got the fuel injectors and intake manifold gasket replaced, (along with other things). I am presently trying to solve a late ignition timing concern. My vaccum gauge is showing 15 inches. I tried to run the timing up to 10 BDC. But when I connect the EST wire it adjusts it to almost straight up under the water pump area. Also the line is jumping erraticly back and forth. Is that correct ? At 6 BDC the line jumps between 10 and sometimes 12. I' am not sure how the ECM is supposed to adjust. I have done all the vaccum leak checks possible, (except smoke), carb cleaner, WD 40, water, propane, nothing seems to effects the engine idle speed. I turned the dist. cap with the vac. gauge on the brake boost line to watch its affect as I turned it. To get the reading to normal (17 in) I could'nt advance it enough. Set it at 7 BDC. Runs good, slight lag in the gas mileage dept, idles good, except it has a ever so noticeable miss. The exhaust kind of sputters and the tach will show 900 rpm, then 700 rpm, 1000 rpm. Doesn't flucuate visibly, just notices its there. Ive included pictures, (Note vacuum lines look good) This is a lot info, Its been a long process figuring this one out. Help !!!
Melting Slicks
Joined: Dec 2004
Posts: 2,537
Likes: 768
I'll be interested in finding out your outcome. I also have a 1990 six-speed coupe and the engine just recently started idling slightly rough. Sometimes it idles smoothly. But I usually notice a rough idle when I'm sitting at a traffic light. It has 102,000 miles. It was running fine until I installed an inlet fairing that was supposed to increase the horsepower. I removed the gadget and sent it back to the distributor, but the idle is still slightly rough at times. I haven't done any of the testing that you described. Good luck. I hope you solve the problem. Maybe I can learn something from you.
Safety Car
Joined: Dec 2004
Posts: 4,088
Likes: 0
From: Poop land
You are supposed to set base timing with the EST wire disconnected. I just noticed you also have all those pesky vacuum lines from the EVAP and EGR. You vacuum leak might be there somewhere around those vacuum lines, causing an unstable idle. Plug the 2 ports on your TB (one on the pass side, below the PCV port and small one under the TB) and see if that temporarily smooths out the idle.
Team Owner
Joined: Dec 2000
Posts: 90,675
Likes: 304
From: SE NY
Cruise-In II Veteran
Yes the timing should jump up when you connect the EST as the ECM is then in control of ignition timing.
Since you have hunting idle if plugging the various vac ports doesn't locate a vac leak:
-clean the TB bore & plates w/ TB cleaner
-remove the IAC and clean the pintel & bore under the TB
-reset base idle
To search for that miss clip a timing light in turn onto each wire to see if missed flashes on one or more wires coincides with the engine miss.
Since you have hunting idle if plugging the various vac ports doesn't locate a vac leak:
-clean the TB bore & plates w/ TB cleaner
-remove the IAC and clean the pintel & bore under the TB
-reset base idle
To search for that miss clip a timing light in turn onto each wire to see if missed flashes on one or more wires coincides with the engine miss.
Thread Starter
Burning Brakes
Joined: Oct 2004
Posts: 898
Likes: 1
From: Pearland Texas
Yes the timing should jump up when you connect the EST as the ECM is then in control of ignition timing.
Since you have hunting idle if plugging the various vac ports doesn't locate a vac leak:
-clean the TB bore & plates w/ TB cleaner
-remove the IAC and clean the pintel & bore under the TB
-reset base idle
To search for that miss clip a timing light in turn onto each wire to see if missed flashes on one or more wires coincides with the engine miss.
Since you have hunting idle if plugging the various vac ports doesn't locate a vac leak:
-clean the TB bore & plates w/ TB cleaner
-remove the IAC and clean the pintel & bore under the TB
-reset base idle
To search for that miss clip a timing light in turn onto each wire to see if missed flashes on one or more wires coincides with the engine miss.
Burning Brakes
Joined: May 2006
Posts: 857
Likes: 1
From: Morrisonville Illinios
Just a thought that you might want to check out the harmonic balancer. It might be sliping (vulcanized joint failing) which would really throw the marks off. Had it happen on ours. Right after we put the new engine in. wro87
Thread Starter
Burning Brakes
Joined: Oct 2004
Posts: 898
Likes: 1
From: Pearland Texas
Heater/AC vacuum line
Actually no, the gauge I got is new, harbor freight , first time it was running bad the mechanic I took it to said it was pulling 6 inches, posible cracked manifold, (pulled it, looked fine, replaced gasket). Car has 40K and I got 15 inches first time I hooked it up. Did a lot of setting, still searching for a vacuum leak. The gauge was cheap, (I know,,,you get what u pay for) needed something quick, and the local autoparts sources didn't have anything. Does any one know how hard it is to access the vaccum line that is linked to the heater/AC ?
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Every 2027 Corvette Engine Explained
Joe Kucinski
Designer Imagines A Corvette That Looks More Like a Corvette Than the Corvette
Verdad Gallardo
10 Ugly Corvettes That We Still Kinda Love
Joe Kucinski
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Thread Starter
Burning Brakes
Joined: Oct 2004
Posts: 898
Likes: 1
From: Pearland Texas
Wouldn't it waiver even if the EST wasn't connected? It is steady on the degree I set it. Until I reconnect the EST and then it goes wacky.
Thread Starter
Burning Brakes
Joined: Oct 2004
Posts: 898
Likes: 1
From: Pearland Texas
Are there any signs of this to check for ? Its hard to see from the top with the accessories in place. Will ramp it and check underneath. I noticed their is alot of crud build up setting on it. Had to poke a stick with a rag to clean it up.
