I began to develop a miss at idle and at cruise and acceleration.
Changed spark plugs this week, coil was replaced this year, plug wires are only a couple years old. Injectors have been replaced with FIC injectors last year, all ohmed at 14k.

If i start the car, it idles fine, then begins to miss, and its worse with the a/c on. if i start to drive it goes away, then comes back. i can be driving under full load and it seems like one cylinder isnt even firing.
then after a while, if i floor it and run it through the revs, the miss goes away for a little bit, then comes back.

took it for a long hard drive just now, and the miss was a little better and happening occasionally now.

anyone have any ideas?

really appreciate it! cheers!

 

WHAT YEAR???

Did you look for vacuum leaks?

 

At night in a dark place, idling, look for sparks along each plug wire. Plug wires should not touch metal and if you have any missing or broken plug wire looms, replace them and dress the wires to not touch metal. Measure each wire resistance, they should be about 5000 ohms per foot. I had an open circuit brand new plug wire the last time I replaced them, so don't rely on newness preventing a defective plug wire!!!!!!!!!!!!!!!!!!!!!!

 

Sorry about that, its a 91. checked for vacuum leaks, apparently there are none.

JFB - a couple of wires seem to be touching some form of metal, but always have been.
ill check the wire resistance then, if thats the last option. thanks!

 

What about your fuel filter and fuel pressure? Might want to check them too.

 

If it is indeed just a Single Cylinder missing, You need to Isolate which Cylinder it is.

Couple of ways of testing for the weak or misfiring cylinder.

You could pull each Fuel injector clip off one at a time with the engine running. If You note any drop in engine RPM when an injector Clip is Removed , then that Cylinder is producing normaly power, or Contributing OK. When You Remove the Dead or Weak Cylinders Fuel Injector Clip, There wont much of an RPM Drop at all.

A tuneup tachometer hooked up the cars Ignition system makes it easier to see the Rpm drop. Or You can have a helper on Hand to Verify If You Corvettes Tachometer Readings drop or stay steady with each injector removal.

Method 2 is to remove One spark plug wire at a time with the engine running. Either off of the distribuator Cap Or at the Spark Plug. Again You are Looking for a drop in engine RPM or No RPM Change as each plug wire is disconnected.

You must reinstall each fuel injector clip or spark plug wire before proceeding to test the next cylinder. Or your testing will be invalid.

Method 3 is to track down someone with an Old SUN Diagnostic Ignition System Oscilloscope, Or a Snap On Counsler I - Counsler II Ignition Osciloscope - Scope.

Sometimes these Oscilloscopes are invaluable when testing for a hard to find cylinder miss. I own a Sun and a Snap On Counsler series Ignition Scopes.

It takes a little knowledge and training to read Primary and Secondary Ignition Patterns correctly using these machines.

Unfortunatley, These Oscilloscopes are no longer made, and not for many years.

The modern didgital oscilscopes are OK, But in my experience they dont work Near as Good as the the Old Analog Cathode Ray Tube Ignition Oscilloscopes when Diagnosing-Troubleshooting that hard to find Ignition Miss.

By the way, with some experience You can also see broken valvespings, sticking valvle guides on a close up - expanded Secondary Ignition Pattern on one of these Vintage scopes.

8 or nine out of ten times you can find what cylinder is causing the missfire using method 1 or 2 stated above. from there You can determine if it is an ignition problem, of fuel delivery-bad injector problem. Vaccuum leak- intake gasket. Or a mechanical problem, Burnt Valve, rings, worn cam lobe, weak-broken valve spring.

Hopefully you just have a worn out or fouled out spark plug.

Will try and help You more if You isolate what cylinder it is misfiring in, and still dont find the source of the misfire problem.

BR

 

Be sure and check for sparks along the wire in the dark!

 

thanks for the responses.

i checked at night for the sparks and nothing...
i took the car for a drive today, and no problem for a long time. it was occasionally missing when i was stopped or a couple times i was accelerating i could hear that miss noise from the exhaust.

the car is also pretty weak on power, going uphill and flooring it takes a long time to rev without a downshift.

i just had the valve seals replaced, as they were worn and oil was causing a cloud of smoke on startup. the miss started to happen before the valve seal change, so i thought oil had contaminated the spark plug. but have changed out the spark plugs and still had this problem.

could the oil leaking into the cylinders have caused anything?

 

No, Oil leaking past the Valve Stem Seals will usually do NO More than Foul Out the
Spark Plug(s).

You Really Need to Isolate what Cylinder(s) are mifiring. It Is A Very Important Step in the Troubleshooting Process in Your Car.

If all cylinders are Misfiring, then it might be the EGR leaking/sticking or premeturaly opening, leaning out the intake mixture. That would cause a stumble or miss under different part throttle conditions.

Also check the Ignition Base Timing while You are at it troubleshooting. The distribuator may have been loosened during previous work and not fully tightned down. And the ignition timing is way off now.

Also Be sure to check the Spark Plug Wire Routing - Firing Order Starting off the #1 terminal off of the HEI Distibuator Cap. From there going in a clockwise direction, 1,8,4,3,6,5,7,2 - Check the routing to spark Plugs and they coincide with the correct cylinder. Drivers side = 1,3,5,7 from front to rear. Pasenger side= 2,4,6,8 from front to rear.

Often Cylinders #5 & #7 are mixed up by car owners and by mechanics in a rush. The Chevy Engine runs and drives, But allways has a miss at all engine speeds. It still runs because the #5 & #7 cylinders fire 90 degrees apart.

Get out there and make these checks YOURSELF !

Goodluck,

BR. - ASE Certified Master Mechanic & L1 Advanced Driveability

 

One way I have heard of isolating a missing cylinder is with a laser heat gun. About 20.00 bucks at your local auto supply. Go along each cylinder next to the head with it and the one with the lowest temp is your missing cylinder. I've never tried it but it sounds logical.

Good luck!

 

OK, have checked all the wiring, its correct.
no arcing at the spark plug wires in the dark.

miss is now intermittent.. so strange. all plugs are sparking, removed each one and plugged in a test spark plug.

checked timing by disconnecting the EST plug. timing was at 6 degrees, but was not extremely stable (not sure if this is normal or not) as the engine was not very steady due to the miss.

The EGR was replaced last year, could this have failed that quickly? any advice on how to check the egr?

thanks again

 

I changed my plugs about 2 months ago and my engine did almost exactly what yours is doing. I knew everything was right so I talked to the service tech at the local chevy dealership and he said to double check the plug wires (2 swapped). He said that if 2 are swapped that the computer will shut them down and it will still run.

The 2 that I was certain were right, 6 and 8, were swapped.

 

Hi, Yes it is possible that the EGR Valve Could have failed. But before calling the EGR Valve Bad-defective............. You Really Need to Isolate what cylinder or Cylinders are missfiring. Try my advice I stated earlier.

Removing a fuel injector clip or a spark plug one at a time and listening for an Engine RPM drop. If there is no change, then You found the Cylinder that is not working as it should. It may be a fouled out spark plug. Or a defective fuel injector. Even though You changed out the fuel injectors in the past.

This is the process I use troubleshooting an Engine miss on cars and trucks with electronic fuel injection daily with vehicles without OBII systems(built prior to 1996). I Diagnose and repair all makes and models daily. C4 Corvettes included. It works and nearly Foolproof. Just listen carefully as You do this test.

Do this and let me know what you find , what cylinder or cylinders are missing.

BR

 

OK i tried pulling the fuel injectors, and the only one that didnt seem to make a difference was #3. then again the engine was vibrating pretty badly already and it was quite hard to tell, but it seemed not to do anything when that fuel injector clip was pulled.

its so wierd as the problem goes away completely, and then comes back as if nothing is wrong.

thanks guys ill keep hunting for the problem, ill try new plug wires in the meantime.

 

Do a compression test on Cylinder #3. You wil need a GOOD Cylinder compression guage. Such as Snap On, MAC, Matco, or Craftsman Brand. Cylinder #3 and all the other cylinders should be around 120 to 150 PSI mininum after 7 or 8 revolutions of the engines crankshaft using the ignition key to crank the engine over.

You must disable the cars ignition system 1st. Pull the Distribuator power connecter off of the GM HEI unit. Or go on the passenger side of your Corvettes dashboard and remove the Ignition fuse of Fuel injection fuse. That wil prevent the car from Firing up as You crank the starter motor over as you check the #3 cylinder.

After checking cylinder #3 , check some other cylinders too. A few more on the drivers side and the passenger side.

Write down the actual readings you get checking each cylinder. They all should be within 10 to 15 PSI if the the engine is mechanically sound.

Also, Have You removed the # 3 spark plug as seen the condition of that spark plug ?
Is it fowled out - black in color ? Or is it Bone White in color. That will help indicate what is happening inside the engine, Lean or Rich condition.

Thinking that maybe You have had a fuel injector fail. Even though you have replaced all of them in the past With FIC units. Alcohol in fuel and dirty fuel still causes Havic.

Check the engine cranking compression in cylinder #3 and a few other cylinders. Checking all 8 cylinders is preferable. That way you can determine if the engine is truly mechanically sound or not.

BR

 

So, finally got the new wires installed, and that seemed to do the trick.

the only thing is wierd now, theres a serious loss of power under wot, will check the fuel pressure later and see what could be wrong, this only happens if the a/c is on, can the a/c compresser really take that much power away?

thanks for all the help, so happy the car is at least running good now!

 

Hi,

A Check of the Fuel Pressure with the KOEO ( KEY ON ENGINE OFF) Or with the car running and the Fuel Pressure regulator vacuum line Off- The fuel presure should be 40.5-47 PSI.

With the Vacuum line connected to the fuel pressure regulator while the engine is running, You should see a 3-7 PSI lower fuel pressure than the earlier check.

Leave the Fuel pressure guage hooked up yet and shut the car off. Then note the fuel pressure. It should not drop more than 20 PSI over 20 minutes than what You had before.

If so, then you might have a defective fuel pressur regultor. Or a bad fuel pump pulsator, defective fuel pump, or leaking hose connection to the fuel pump inside of the gas tank.

More test wil be required.

When is the last time you chnged out the fuel filter ?

They should be changed every 2 years if you drive the car alot.

The fuel filter might be plugged up with dirt, especially if it is the original 1990 Fuel fliter the car was built with.

BR

 

Hi,

I changed the fuel filter around 3-4 years ago, i will check the pressures as you have stated next week, thanks again for the help, it is much appreciated!

The loss of power seems to come and go, and is felt more the hotter the car is.
Also, at wot, seems like something is restricting there are short bursts of power, and hesitation comes and goes throughout the rpm band. maybe it is a clogged fuel filter as you said.

cheers

 

it is not necessary to have a pulsator at the fuel pump, I dont have it ...

 

you could be experiencing something other than a miss. 1st place I'd start looking after verifying your fuel pressure is your Throttle position sensor voltage should be .54 V at idle and a steady increase as the accelerator is pushed steady (up to 5V), next check your knock sensor, yes knock sensor, it sounds funny, but I had this happen to me and it didn't throw any codes at all. Use a digital volt meter to check it, unplug the connector, use the dvm set on 2v and insert the + in the sensor, and ground it on the exhaust, It should read something. Tap on the exhaust and it should jump, if not replace the sensor. Another quick check for knock sensor is to just unplug it and run the car, You get a service light, but if it runs great...it's the sensor that's retarding the spark.

I'm sure you have already, but did you try pulling the codes?