So since I bought my 90 coupe about a year ago I have been trying to systematically remove all the reasons why I get a vibration and loss of power above 3-3500rpm. The vibration is worse with more power to a point but noticeable at all throttle inputs. So far I have had to do the following:

• AC compressonr bracing
• Reset the distributor to 6* advance
• Replaced the injectors
• Replaced the plugs
• Replaced the fuel filters x2

I am not sure but I may have to knock out the cats still due the amount of carbon building up in the oil in the course of a few hundred miles. Aside from a harmonic balancer and the flywheel I'm not sure what else would be the cause. The engine runs well at lower rpm but this lack of power and vibration is really starting to worry me.

 

Space387,
Hey,
I'm sure the pros will be along shortly, but in case you are in a hurry.
I would throw a fuel pressure gauge on it and run it up to 3500 rpm and see what, if anything, is happening to the pressure.
Maybe a weak pump or a regulator bleeding off early.
Good Luck!!

 

Good thought, I have had fueling issues in the past from rust in the tank. But I just got back and 40psi at idle, 50psi at WOT until about 5k and 37-38psi on decel. It is kinda reminiscent of when we first got the car running and found it was down 2 cylinders from bad injectors. Idles very smooth but give it RPM and it starts to feel like the car is coming apart.

 

It does sound like a miss to me... Did you replace the ignition wires when you did the plugs? Cap, rotor? Not to be a jerk, but are you sure the distributor cap and wires are in the correct firing order?

 

Engine vibration usually remove belt ,disconnect converter, motor mounts for grounding, C beam bolts. Look for a change in vibration.
The loss of power can cause a vibration mostly when the exhaust is very restricted or engine is misfiring.
i would persue the lack of power. Drop the exhaust at the manifolds and see what happens.

 

..... Take a look inside the distributor cap … make sure there is NO carbon tracking … also check that the carbon rotor button is intact … look closely at the upper inside of the cap to make sure that none of the coil hold down screws have pierced the plastic into the distributor … next check the coil ground strap … if you can , try a different coil … have your ignition control module tested (some auto parts stores will do this free) … make absolutely sure that the spark plugs are wired to the distributor per the firing order - 18436572 … 5 & 7 are easily swapped and won't affect idle too much when they are switched … lastly , do the night time fireworks check under the hood … you don't want to see any lightning going on around any of the spark plug cables … Good Luck ! …..

 

When my injectors went bad it was similar. Idle or very light throttle run ok but started to feel like crap with more pedal.
Once fixed was smooth...then discovered the cats took a beating and needed to be replaced, too. Car ran like new after that.

Edit: just saw you had new injectors, disregard!

 

What you are describing could easily be a EGR giving you a fit. When the EGR is not working properly you get a not so smooth idle and other problems in the lower part of the power-band. The EGR is controlled by a Vacuum Solenoid mounted near the thermostat. Be sure that the vacuum lines are hooked up properly and that the EGR is functional. Not having a working EGR makes the engine run like it has a vacuum leak. I was lucky because when I took my car in for it's last emissions testing the NOX was off the scale and this points to the bad EGR valve.

I just thought I would add this little note for you to check your EGR function. By the way, your car has at several u-joints in it, a u-joint going south could produce the vibration but that does not account for the loss of power above 3500 rpm. A catalytic converter can get blocked but that would affect the engine throughout the power band. A simple test is to pull your O2 sensor and start the car up, if there is a lot of back pressure it will force the air out of the sensor bung indicating a plugged exhaust or catalytic converter.

When ignition modules go bad they produce similar effects, a test will tell you if it is the culprit. The point made by C409 is a good one, carbon tracking happens way more often than a lot of people know. I had a MSD ignition and coil on my Chevy Pickup and I had to replace the distributor cap once a year due to carbon tracking. Turned out the Higher Voltage output ignition system was making the carbon tracks appear. I switched back to a GM coil and the issue went away. The folks at MSD said that the distributor cap size made the problem worse.

I don't know the mileage of your engine but doing a Leak-Down test might be in order, it would help identify any issues with the engine's Long-block. This test tells you more about the health of your engine and I do it to my cars when I have a reason to suspect something is wrong.

I still think the EGR is the "likely culprit", the EGR valves go bad regularly and then get plugged up with carbon. Check it thoroughly to rule it out as a suspect! If it is bad they are not hard to replace and are fairly inexpensive. A lot of folks tend to disconnect or disable the EGR valve, I am not a supporter of this idea. I keep my Corvette working like the engineers intended it to. If you do decide to disable it you will have to get a prom made to stop the EGR Codes from popping up.

Best luck in solving this minor issue! I hope that is something simple and easy to fix so you can enjoy your Corvette once again!

 

Good Afternoon All,
Fuel pressure seems good.
I'm going to agree with ctmmccloskey.
I would run down a possible ignition issue like, carbon tracing in the dist. cap, or a plug/wire breaking down under load.
Decades ago, when I was a dealership tech, I put new A/C Delco plugs in a brand new Vette (they used to flood/foul in the lot) and straight outta the box 2 of the plugs resistance were so high, it miss-fired down the dist cap wire lug to the cap under the wire boot.
I could hear the clicking and followed it.
sometimes it's the damndest things...
Good Luck!!

 

Anyone know what the plug wires resistance should be. I got ranges from 3k-7k ohmes across the different wires.

 

Resistance will vary by brand and fabrication. Calculations (advertised resistance) is generally done by the foot. Measure all of yours and determine value of each perhaps by comparing values to inch. Dependent upon the 'brand' they may actually publish values to expect for comparisons. There's much reading that can be done.

 

I know this is an old thread but I felt it worth closing out. After running the gambit on possible causes and upgrades over the years that vibration persisted. This past weekend I found the time to pull the trans and replace the throwout bearing and pilot bushing. I started to suspect these parts when the vibration got worse and more random in the rpm range. It was very similar to the effect you get with a worn out ball bearing, the faster you spin it the higher the deak to a point. It gets really bad then it finds a harmonic and spins freely again.

 

..... ??? Neither the throwout bearing or the pilot bushing are "in play" when the car is in gear .....

 

the throwout bearing is always in play. It is keyed to the fork and to the pressure plate.

 

When the caris in gear and the foot off the clutch, the throw out bearing is along for the ride.

 

well found the issue. I wound up having to to a rebuild on the engine over this winter. After having the block bored I had to have the crank balanced for the new pistons to find that the flywheel the last owner added, although the correct dimensions, was way out of balance for this engine. There was a large bore put in the rear weight of the crank to correct the issue. Upside is that now I''ll be running a 355 with AFR 195 heads and the TPIS Mini Ram.