Exhaust temps per Cylinder - Impala Tech
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post #1 of 2 (permalink) Old 10-01-2014, 08:40 PM Thread Starter
BA.
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Exhaust temps per Cylinder

So my poor Impala has been on a slow death spiral for about 2 months now. Each time I drive it somewhere (granted - not very often) it seemed to get a little worse. Slightly rough idle, and a noticeable but small backfire (poof) out the exhaust. (both at idle and during acceleration) Not super consistent, but, you can't miss it and it wasn't cool to drive.

Long story short, I was suspecting the worst out of the Blueprint 383 because that's just my luck,...BUT as luck would have it, it appears to be just bad plugs. (NGK R5672A-9 with only 1500 miles. Blueprint suggested them in their documentation)

During and after the course of checking things, I've noticed that 2 of my cylinders have what I would call a significant exhaust temp difference.
Even granting a good amount of lenience, I think they are oddly cold.

This is a standard Blueprint 383 small block chevy, I think about 10-1 CR, and torquey roller cam with something like 224/232 duration, alum heads and 1 5/8" headers.
Six of the cylinders were between 650 and 750, measuring about 2"-2.5" off the head.
Number 5 was only around 400-450 and Number 8 was way down at 300ish.
These numbers are with the engine running around 2500rpm at best guess.

Since these plugs are brand new I hope to get a good read on them, but, I'm wondering if I should get a brand-name intake manifold or am I worried for nothing?

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post #2 of 2 (permalink) Old 10-02-2014, 11:52 AM
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Allan the symptoms of a carb malfunction (out of adjustment) and an ignition malady are very similar. Attempting to diagnose over the internet is just a guess and try again and again different checks. Since only two cylinders are affected about the only way I can think of a carb could be at fault would be if you had the wrong jet on the side and quadrant that is experiencing the problem (I'm guessing too rich). Definitely in the secondaries if at all (you don't have a problem with fuel pressure over powering the needle and seat do you?).

All carbs are designed to be able to close the needle and seat with seven psi of fuel pressure as a maximum. If anything goes wrong, such as a surge in the electrical system causing voltage to spike pushing more fuel at a higher pressure will cause it to flood running very rich. For this reason I run a half inch fuel line with a three eighths return line at five psi of pressure. A half inch pipe supplying gas at five psi can fill the fuel bowl a lot faster than the gas can be sucked out through the 88 size jets. I have never run out of gas with my Aeromotive 1000 fuel pump (I had in the past with a Holley Blue pump even feeding an 850).

Like I said I suspect an electrical problem. Time to round up the usual suspects. Plug wires and the distributor cap are at the top of the list. You need to do more than just check continuity with the plug wires you need to check resistance while moving the wire around (you move it because the problem is intermittent, it could be a bad connection at the terminal or a break in the wire). If the wires are carbon impregnated fiberglass (aka radio suppression wire) then they are immediately suspect as the carbon that is the conductor gets converted to carbon dioxide gas by the high voltage going through it with time. They literally burn up and have to be replaced at least once a year. I much prefer a spiral wound copper core conductor that uses the twisting of the conductors to suppress a lot of the static that solid metal wires generate (If it is good enough to suppress cross talk in communication and data cables it should work with a spark plug).

Carbon tracking is another issue that crops up inside distributor caps. It is caused by moisture inside the cap (endemic in Florida that is so humid if you slow down mold grows on you as soon as you sit down). Additionally there is a possible problem with the carbon button that transmits the secondary ignition surge from the coil wire to the rotor. If the carbon cracks it can dissipate the charge as it reaches that crack while rotating. For that reason I select the bronze button included with MSD distributors.

The bronze button is consumed just like points pitting and has to be replaced periodically (about the same time that I replace the bronze distributor gear required to ride on my forged steel billet cam).

The distributor cap itself can be an issue if it is not centered on top of the distributor. The air gap will change with the distance off center from the distributor shaft. Note that the firing order is a clue here as the rotor passes over 1-8-4-3-6-5-7-2 as cylinders 8 and 5 are on opposite sides of the cap.

Good Luck

Big Dave
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