1985 corvette engine miss
Thread Starter
Cruising
Joined: Aug 2006
Posts: 14
Likes: 0
1985 corvette engine miss
Ok Guys I am about ready to take the vette to a chevy dealer which would be easy accept on the wallet and besides this is suppose to be fun RIGHT. I have cleared the codes on the ecm which i only had one code 15 .I changed the cooling temp sensor and then the male plug to the sensor and it cleared . hooray, The car was running rough idle before that and it never changed by clearing the code. It also had a miss like a plug or bad wire. I changed the plugs and checked each wire with a plug tester where you ground the test plug with each wire connected to see if you get spark . All wires checked out. The car continues to miss and is actually starting to backfire a little just sitting there idling. When you drive it it runs like it is running on 7cylinders loss of power
Have I dug deep enough for next step a dealer ? Ireally would like to find this on my own and learn something !! Any help would be great tks Randy
Have I dug deep enough for next step a dealer ? Ireally would like to find this on my own and learn something !! Any help would be great tks Randy
Le Mans Master
Joined: Aug 2006
Posts: 7,698
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From: Just the Tip... Just to see how it feels...
1 thing I'm certain about: DO NOT GO TO THE DEALER 
Perhaps a cap and rotor replacement may be in order?
Did you do a compression test when you changed the plugs?
Perhaps a cap and rotor replacement may be in order?
Did you do a compression test when you changed the plugs?
Le Mans Master
Joined: May 2004
Posts: 5,866
Likes: 60
From: Hellinois
Finalist 2020 C4 of the Year - Modified
Originally Posted by zo1boy
1 thing I'm certain about: DO NOT GO TO THE DEALER
Perhaps a cap and rotor replacement may be in order?
Did you do a compression test when you changed the plugs?
Perhaps a cap and rotor replacement may be in order?
Did you do a compression test when you changed the plugs?
Once you try what zo1boy suggested it may be time to look under the valve covers for, (A) broken spring, (B) bent pushrod, (C) a valve that isn't opening very far which could indicate flat cam. Good luck, I hope it's a quick, easy fix for you.
Thread Starter
Cruising
Joined: Aug 2006
Posts: 14
Likes: 0
Originally Posted by zo1boy
1 thing I'm certain about: DO NOT GO TO THE DEALER
Perhaps a cap and rotor replacement may be in order?
Did you do a compression test when you changed the plugs?
Perhaps a cap and rotor replacement may be in order?
Did you do a compression test when you changed the plugs?
Le Mans Master
Joined: Mar 2004
Posts: 5,107
Likes: 171
From: Dublin CA
When you changed out the wires, any chance that you crossed the wires. Recheck the firing order to be sure that you didn't switch a wire to a different plug. Sometimes you can cross a wire that will be close enough to the firing order the car will run but backfire at or above idle.
Thread Starter
Cruising
Joined: Aug 2006
Posts: 14
Likes: 0
Originally Posted by John A. Marker
When you changed out the wires, any chance that you crossed the wires. Recheck the firing order to be sure that you didn't switch a wire to a different plug. Sometimes you can cross a wire that will be close enough to the firing order the car will run but backfire at or above idle.
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Thread Starter
Cruising
Joined: Aug 2006
Posts: 14
Likes: 0
Originally Posted by intruder85
how do Icheck the module .what do i need to do? thanks Randy
Ialso checked for loose sparks . shut the lights off and in the dark looked around for any grounding . looked ok!!
Race Director
Joined: Jun 2005
Posts: 17,640
Likes: 1
From: Lenoir City TN
St. Jude Donor '08-'09-'10-'11
Originally Posted by intruder85
how do Icheck the module .what do i need to do? thanks Randy
Please post your fuel pressure results, this will help.
Please post each cylinder compression results as well.
Pull each spark plug at examine the tip this will tell you as well if you have a problem with one particular cylinder.
Thread Starter
Cruising
Joined: Aug 2006
Posts: 14
Likes: 0
Originally Posted by rspreng86
Some auto parts store will check them for you but I am not sure what ones. Make sure you track the plug wires 1-8-4-3-6-5-7-2 for the firing order. The module is not expensive but I do not like throwing money away at problems until you do all the trouble shooting.
Please post your fuel pressure results, this will help.
Please post each cylinder compression results as well.
Pull each spark plug at examine the tip this will tell you as well if you have a problem with one particular cylinder.
Please post your fuel pressure results, this will help.
Please post each cylinder compression results as well.
Pull each spark plug at examine the tip this will tell you as well if you have a problem with one particular cylinder.
Le Mans Master
Joined: Feb 2006
Posts: 8,753
Likes: 9
From: Louisiana
This could be a fuel problem.
1. I would check the connections on the fuel injectors (making sure they are clean/tight and use a test light on the terminals of the connectors while the engine runs and feel the top of the connector while connected to the injector.
2. Pull the spark plugs and check their condition
3. Do a fuel pressure leak down test
4. Ohm each injector.
5. At night, disconnect the hood lights and look for sparks around the engine bay while the engine runs.
1. I would check the connections on the fuel injectors (making sure they are clean/tight and use a test light on the terminals of the connectors while the engine runs and feel the top of the connector while connected to the injector.
2. Pull the spark plugs and check their condition
3. Do a fuel pressure leak down test
4. Ohm each injector.
5. At night, disconnect the hood lights and look for sparks around the engine bay while the engine runs.
Thread Starter
Cruising
Joined: Aug 2006
Posts: 14
Likes: 0
Originally Posted by RRT vette
This could be a fuel problem.
1. I would check the connections on the fuel injectors (making sure they are clean/tight and use a test light on the terminals of the connectors while the engine runs and feel the top of the connector while connected to the injector.
2. Pull the spark plugs and check their condition
3. Do a fuel pressure leak down test
4. Ohm each injector.
5. At night, disconnect the hood lights and look for sparks around the engine bay while the engine runs.
1. I would check the connections on the fuel injectors (making sure they are clean/tight and use a test light on the terminals of the connectors while the engine runs and feel the top of the connector while connected to the injector.
2. Pull the spark plugs and check their condition
3. Do a fuel pressure leak down test
4. Ohm each injector.
5. At night, disconnect the hood lights and look for sparks around the engine bay while the engine runs.
Drifting
Joined: Nov 2000
Posts: 1,428
Likes: 1
From: Jacksonville FL
Originally Posted by intruder85
how do you perform item #3
A good indicator for leaking injectors is after shutting the car off and going to restart it within 10-15 after it has been shut off it will take a few more seconds for it to restart than it does if it is left overnight.
Normal start <1-2 seconds of cranking
Leaking injector start >5 seconds of cranking.
If you are experiencing these conditions, depressing the accelerator pedal while cranking will help evaporate some of the puddled fuel from the leaking injector(s) and help to restart the car.
Hope this helps.
Mike.
Intermediate
Joined: Jan 2006
Posts: 31
Likes: 0
From: Enid Okla
I have a 85 and did the same thing, hunted for a long time changed everthing I could, even replaced injectors. Finally found it (the miss) had a broken injector wire. Pealed the plastic sheild off each plug, checked each plug, found the wire inside was broke, the outside looked fine but broke inside. Good Luck
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Can you tell if the backfire is in the exhaust or the intake? Exhaust backfires are often from a malfunctioning air pump. Intake from extra fuel, Vapor Canister satuated or a broken switching valve allowing fuel into the plenum at all times.
You can isolate a single cylinder with a grounded test light and some 1 inch pieces of 1/4 inch vacuum hose. Place a piece of hose between each plug wire and the Distributor and then touch each piece of hose with the test light to kill the cylinder. Any that don't have the same RPM drop is the problem. Once isolated, troubleshoot that cylinder - plug, wire, injector, valves, etc.
You can isolate a single cylinder with a grounded test light and some 1 inch pieces of 1/4 inch vacuum hose. Place a piece of hose between each plug wire and the Distributor and then touch each piece of hose with the test light to kill the cylinder. Any that don't have the same RPM drop is the problem. Once isolated, troubleshoot that cylinder - plug, wire, injector, valves, etc.
