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Bob, Auto Tech
Category: Chevy
Satisfied Customers: 4087
Experience:  Plus 40 years GM, over 30 certified with Chevrolet, ASE certified Master Tech. (Expired, Retired)
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1993 Chevy S10 Blazer: a rough idle..operating temperature..vibration

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Have a 1993 Chevy S10 Blazer. It has a rough idle, have tried many things to solve it but have not found the problem yet. It started with a rough idle when at operating temperature but have progressed to rough idle all the time, and now I can feel a rough vibration when underway as well. There is also a strong rich mixture smell from the exhaust as well. Recently replaced parts include, plugs, wires, distr cap, rotor button, fuel pump relay, ignition coil, fuel pump/sending unit assembly, air filter, fuel filter, pcv, egr and maybe more. Any suggestions?
Hello, sounds like you have pretty much covered all the bases. About the only things I see that you have not addressed would be a possible vacuum leak, which you may be able to locate by pinching off vacuum hoses and seeing if the engine runs better when one is shut off, which would mean the part you just turned the vacuum off from was leaking, or by spraying carb cleaner around the intake manifold and other areas that may be suspect and if the motor speed increases the area you just sprayed has a leak. The other thing you have not checked would be the injectors, maybe one is stuck shut or open, but that is a little hard to check so we will leave it until last, as you may need a class 2 scanner to test them by doing a balance test on them. But if you can pull the plug wires one at a time with the motor running to see which cylinder is not doing its job, pulling a wire off a good cylinder will make the truck run much worse and when you pull the wire off a cylinder that is not working the motor will not change at all. Once you know which cylinder has the problem I would do a compression test to see if there is a mechanical problem inside the cylinder, like a bad valve or broken piston ring etc. Let me know what you find and we can pick it up from there. Thanks
Customer: replied 10 years ago.
Reply to Bob's Post: Yes, I have tried changing the CMFI assembly that I had acquired from a salvage yard with no change in symptoms for the better or worse.

I have been thinking of getting a compression check and perhaps having the Chev dealer put it on an engine analyser.

There has not been anything in code since this has been going on which was the fall of 2006, around the same time that I had my fuel tank replaced. Could there be a restriction in the new fuel filter that they had installed at that time. What about the strong fuel mixture smell in the exhaust?
Another thought is the converter has been gutted and is basically a shell at this point, could there be back pressure issues to consider?

I really think a compression test and a scan of the computer is in order. If the truck has a burnt valve or something similar, the compression test should show it. And if there is a problem with the exhaust being restricted, it should show on a vacuum gage or on the analyzer. I don't think from your description that it has that kind of a problem. I am more inclined to think it is a problem with a single cylinder. And anything that makes it miss on one cylinder will change the fuel mixture and make it smell in the exhaust because it is not burning all of the fuel in all of the cylinders. The idea with the dealer and the compression test sounds like a good one to me.Let me know what happens. Thanks
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Customer: replied 10 years ago.
Now that you mention it Bob, we have noticed that one spark plug is a brownish color as opposed to the ashy color of the other 5 spark plugs and when I had the air intake dome off to change out the CMFI I noted that one of the intake ports looked a different color.
I think you are right, it sounds as if something is going on with one of the 6 cylinders.

Let me know and if you find a cylinder with low compression, I will tell you how to do a leak down test on it to see if it is related to a valve or maybe even the rings. Thanks