Ive got an 86' coupe that ive had since 2008. It runs great but I have continual problem with what I can only describe as surging while driving.I first noticed the "surging" while cruising anywhere from 1000 to 1400 RPMs- from 1400 to redline it runs great with no surge or hestitation. Ive talked to many forum members about this problem and ive got many good replys and ideas, I have replaced the injectors w/
Bosch IIIs and it ran better but still had the original problem so this summer I took the vette to a speed shop that specializes in corvettes and they were able to reproduce the problem and did a complete scan of the engine and fuel system and spend over 4 hours of shop labor and the only thing they could find was a sticking throttle position sensor which they replaced, when I got the vette in 08' ,it only had 28,500 miles on it and right now only has 33,ooo so its not like it has been beat on:
this is a list of things that have been replaced:
Cap,rotor& wires( all original)
Sparkplugs, injectors,fuel filter
timing set @ 8 degrees
new air filter,oil

Once again I am asking for any suggestions on what the surging could be caused by??

 

Is your tourk converter locking an unlocking?

 

If the shop that you took the car to was good and duplicated the problem, then it might be in other areas other than the engine. Perhaps transmission if it is a automatic. The trans might be locking and unlocking at the lower RPM's.

Other than that....the "usual" talked about items...
vacuum leaks....check all hoses including the break booster hose.
EGR valve.

 

Do you have it only when driving

 

I just went through an intermitent problem that drove me crazy. Turned out to be two partially broken wires. They were only held together by the insulation on each wire. The wire inside was actually broken. One on the throttle body position sensor wire harness and the other on the ignition harness to the ignition coil.

To check for broken wires, with the engine running shake the wire harnesses connected to the TPS and ignition system. If the engine stumbles or speeds up, you'll know there is a problem with one of the wires. Then you just have to start shaking individual wires to find the broken one.

Just another thing to try....good luck

 

8 degrees timing is too far advanced and it will cause bucking on level roads at constant speed in the rpm range you say you have problems.

 

Ive ran it @ 6 degrees and 8 degrees ,it just runs better @ 8)- also it it
4+3 transmission

 

If it ran better at 8 degrees, your post wouldn't be here. My 87 4+3 was returned to me by my mechanic with about 8 degrees of advance and it would buck on level city roads at constant speed. I set the timing back to where it is supposed to be and it stopped.

 

Mine does it too, nature of the beast as they say.

 

coil ???

 

my 86 never did that. Once, while it was in park, it was surging, and I unplugged the ignition module over by the heater, and it stopped doing it. I replaced the module, and it never came back. cleared the code, and all was well.

 

"The manual shift cars with too much base timing are inclined to do that" is how i should have worded that.

 

Why are you cruising at 1k rpm? Drop it down a gear, problem solved.

 

Clean your MAF and throttle body. You put OIL after AIRFILTER in your post. May have oil in MAF and throttle body.

Also are you using high octane gas. With 8* you should be using 93 octane.

 

Exactly.

 

From what I've read on the forum, the timing ring on the harmonic balancer can shift. This may be more of an age thing than a mileage thing. If the outer ring has slipped even a little your timing will be off. I'd try different timing settings and see how it runs. Also you might try hooking up fuel pressure gauge and taping it to the windshield so you can see it while your driving. I've seen pics on the forum of guys doing this. Hope you find the problem.

 

Is it engine chatter (bucking) or surge? I run mine at 8* and have no surge or chatter at reasonable rpm.engine smooth at all rpms. I do run only 93 octane.

 

I always run 93 octane and have tried it @ both 6degree and 8 degree with out chatter or knocking , it just excellerates better @ 8 degree