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Old 09-30-2013, 02:20 PM   #1
drazenv
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84 vette TBI misfire

Having problem with two dead cylinders #8 and #7.Checked compression is 155psi,manifold vacuum is only 12hg,plugs look OK and the car starts fine but idles rough.Took the car to a diagnostic center and they told me that the intake manifold is bad at the rear of the manifold.Replaced the upper manifold gasket to no avail still same problem,Any ideas would be appreciated
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Old 09-30-2013, 04:03 PM   #2
jdl
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Re: 84 vette TBI misfire

I guess this is obvious, did you check spark and fuel at those two cylinders? If your computer controlled, check for applicable trouble codes.
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Old 09-30-2013, 10:18 PM   #3
drazenv
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Re: 84 vette TBI misfire

I have spark and new plugs and yes I do have spark.Trouble codes showed lean condition which translates into the rough idle and the diagnosis by the shop.Pulled the vacuum hose of the MAP sensor and the idle evened out which would be caused by the ECM richening the mixture since the MAP output went to 4.5v.The cause has something to do with the two cylinders being across from each other or just being at the rear of the engine I can't believe it is the intake gasket because I just took the intake off and the gasket looked good.
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Old 10-01-2013, 04:30 PM   #4
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Re: 84 vette TBI misfire

What were the actual codes? I'm lazy, I guess I could look it up. Did you check fuel pressure? Usually at idle, the tps and map sensor should read close to the same. If your vacuum is down, something is going on?

A vacuum leak will show up at idle but not higher rpms.
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Old 10-01-2013, 07:54 PM   #5
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Re: 84 vette TBI misfire

Spray some carb cleaner or get a small propane tank and shoot some around the cylinders at the manifold mating surface see what happens.
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Old 10-03-2013, 03:04 PM   #6
drazenv
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Re: 84 vette TBI misfire

I just put the intake gasket on still the same problem,I really didn't think that was the problem but the tech who did the diagnosis told me that that was the problem.I can see why he got fooled because when i scan the propane wand at the back of the intake the rpm picks up just like before.Does anyone know if there are any vacuum lines running at the back of the intake manifold it would save me some time trying to diagnose this problem
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