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85 corvette missfire

 
Old 03-07-2019, 09:01 AM
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cvettefan85
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Default 85 corvette missfire

hello I am new to forums so I hope I am posting this right, I need some help. I bought a super clean 85 corvette with 67k miles about two weeks ago, when I purchased the car I noticed it had a miss in it and no check engine light so I figured it would just need a tune up to get rid of it but not the case, so far I have replaced all 8 fuel injectors, tested all the connectors with noid lights I pulled the lower intake since I already had the upper intake off to replace all the intake gaskets and I replaced the EGR valve, air charge sensor, water temp sensor and idle air control valve since I already had it apart, I replaced any vacuum lines that looked bad, and since I was in it that far I figured heck ill throw a timing chain in as well, I also did a compression test and all 8 cylinders were between 160-165 psi, I spun the motor over to check for flat cam lobes or busted valve springs but everything looked great, I reassembled everything and installed new distributor cap and rotor, plugs and spark plug wires i started it but it does the exact same thing except now I have a code 22 for tps sensor so I have one ordered. at this point I don't know what else to check, any help will be greatly appreciated
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Old 03-07-2019, 04:19 PM
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Originally Posted by cvettefan85 View Post
hello I am new to forums so I hope I am posting this right, I need some help. I bought a super clean 85 corvette with 67k miles about two weeks ago, when I purchased the car I noticed it had a miss in it and no check engine light so I figured it would just need a tune up to get rid of it but not the case, so far I have replaced all 8 fuel injectors, tested all the connectors with noid lights I pulled the lower intake since I already had the upper intake off to replace all the intake gaskets and I replaced the EGR valve, air charge sensor, water temp sensor and idle air control valve since I already had it apart, I replaced any vacuum lines that looked bad, and since I was in it that far I figured heck ill throw a timing chain in as well, I also did a compression test and all 8 cylinders were between 160-165 psi, I spun the motor over to check for flat cam lobes or busted valve springs but everything looked great, I reassembled everything and installed new distributor cap and rotor, plugs and spark plug wires i started it but it does the exact same thing except now I have a code 22 for tps sensor so I have one ordered. at this point I don't know what else to check, any help will be greatly appreciated
Could be a $15 fuel filter or O2 sensor. You probably could have saved a lot of time and money by diagnosing rather than changing everything

Last edited by Cruisinfanatic; 03-07-2019 at 04:19 PM.
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Old 03-07-2019, 05:03 PM
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40 psi of fuel pressure and no code for the o2 sensor, I replaced the stuff I did because I already had everything apart anyway
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Old 03-09-2019, 11:13 PM
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Time to purchase a Field Service Manua (FSM)l that is specific for your year of Corvette. You want the full manual and not the generic Haynes-type book. You can find the DVD version or the paper version on eBay or Amazon or post up a "need" here in the C4 Parts for sale/wanted section.
This will provide you with a "runs poorly" or "no start" flow chart. This product will save you a huge amount of money with your first fix.
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Old 03-10-2019, 07:44 AM
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since you're throwing parts at the problem, I don't see where you mentioned plugs, cap and rotor, PCV valve, and wires. i'm guessing plugs are a given, and maybe cap and rotor, and PCV valve, but what about plug wires? question on the mis-fire - need more specifics. is the misfire just at idle, or across the whole RPM band? you're not mis-diagnosing that mis-fire for a rough idle caused something like a worn motor mount or something else - just saying. second note when you say 40psi on the fuel pressure - more details. 40, steady state running pressure, sounds a bit high to me for an 85. 85's tend to run a bit lower FP than other model year L98's. my 85 runs in the 32-33 psi range. I personally don't think slightly higher FP would cause a mis-fire though - just noted. I see where you've checked the injectors with a noid light, but have you check spark using some sort of in-line diagnostic tool? on the fuel filter and O2 sensor, AND FSM. filters and O2 sensors are a periodic maintenance items and should be replaced accordingly. the FSM is a must. I have my suspicions on this one. good luck....

Last edited by Joe C; 03-10-2019 at 07:54 AM.
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Old 03-10-2019, 08:33 AM
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My first question is have you replaced the plugs, wires, cap, and rotor? My 86 had a misfire when I bought it and I went though the ignition and fuel system. I replaced the plugs, and wires, and when I got to the cap, I found that the grounding tab inside of the cap was not bolted to the cap, but merely just laying in the cap. I properly secured it and the misfire was gone after fastening it. I was figuring it would just be plugs and wires as the previous owner had recently replaced the cap, but it turned out to be that little ground in the cap.

In your case, I would start with the ignition components as you have been through everything else. Adjust the TPS to .5V, key off and keep us updated with what you find.
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Old 03-10-2019, 12:53 PM
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Default Miss fire

Originally Posted by Joe C View Post
since you're throwing parts at the problem, I don't see where you mentioned plugs, cap and rotor, PCV valve, and wires. i'm guessing plugs are a given, and maybe cap and rotor, and PCV valve, but what about plug wires? question on the mis-fire - need more specifics. is the misfire just at idle, or across the whole RPM band? you're not mis-diagnosing that mis-fire for a rough idle caused something like a worn motor mount or something else - just saying. second note when you say 40psi on the fuel pressure - more details. 40, steady state running pressure, sounds a bit high to me for an 85. 85's tend to run a bit lower FP than other model year L98's. my 85 runs in the 32-33 psi range. I personally don't think slightly higher FP would cause a mis-fire though - just noted. I see where you've checked the injectors with a noid light, but have you check spark using some sort of in-line diagnostic tool? on the fuel filter and O2 sensor, AND FSM. filters and O2 sensors are a periodic maintenance items and should be replaced accordingly. the FSM is a must. I have my suspicions on this one. good luck....
sorry but yes all the ingnition stuff has been replaced as well, plugs, wires, cap, rotor , pcv valve,
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Old 03-10-2019, 02:51 PM
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Have you checked/set base timing?
At what RPM does it idle in "N" when warm?
Do you sense the "miss" hot & cold?
Is the manifold vacuum solid and "green"?
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Old 03-11-2019, 10:12 AM
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Paul Workman
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Originally Posted by Cruisinfanatic View Post
Could be a $15 fuel filter or O2 sensor. You probably could have saved a lot of time and money by diagnosing rather than changing everything


Replacing parts is NO substitute for troubleshooting. In fact-
- statistically speaking, the period in a new part's life when failure rate peaks is actually when it is first installed! ("Infant mortality" is term given in statistics for this phenomenon. Infant mortality is why aircraft parts are required to successfully endure a minimum "run-in" period before they're certified as a replacement part).

OP: Establishing a new time line, i.e., "resetting the clock" is not a bad idea - so don't get me wrong. However, any time a par is installed, its performance should be verified (measured). In 50 years of playing with cars and electronics, I've lost count of the times ch*t failed "right out of the box".

As for your missing issue, FUEL - AIR - SPARK. One of these is missing. And, the best tool in my box is a scanner that not only will read (OBD-I) codes, but also has an "event key for storing dynamic readings both before and immediately after the "event" occurs. A scanner and an FSM and you'll be well on your way to fixing most anything.

Good luck.
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