Chevy Car Questions & Answers - Performance Q&A

Kevin McClelland Sep 1, 2009 0 Comment(s)

Factory tappet preload is one turn from zero lash. You can try to go to this tighter setting, but this probably won't help much. What Chevrolet did in the later years to quiet down the valvetrain was to install covers over the valve covers. This kept down the injector clicking and the tappet noise. They're also on the LT4 '96 Corvette; this also damped the high-pitch ticking. If you can run down a set of these covers it will also help with the noise. Give the valve adjustment a try with the tighter preload. The standard EO-IC procedure works just fine with this camshaft. Hope we've been some help.

What's That Smell?
Q: My '95 LT1 Camaro has an OBD-I computer, and the engine has been rebuilt. I've got ported stock heads, a Comp Cams 218/224-duration, 0.565/0.570-inch max lift cam, a ported MAF, a 52mm Edelbrock throttle body, 32-pound Siemens injectors, midlength Pacesetter coated headers/Y-pipe, and a Hooker after-cat with a straight pipe in place of the cat. My problem is that even before I rebuilt the car, it always smelled rich. Even with the stock exhaust and cat on it. I was told the car has split BLM readings when the computer was scanned. It has a computer tune on it for my modifications. I've also heard that the split BLM readings can be caused by the EGR hole on the throttle body not lining up with the cast hole on the front of the manifold (I think this hole leads to the emissions passages in the floors of the ports in the manifold). The car is mostly rich at idle and low-speed or part-throttle driving. Have you ever heard of this problem? Thanks.
Mike Topczewski
via email

A: Several things could be contributing to the rich-smelling exhaust. First, without a catalytic converter unburned hydrocarbons are coming right out the tailpipe. This is the rich smell. As you stated, you had the converter on it earlier and it still smelled. Was the cat functioning properly? Was the cat up to temperature when it smelled this way? Unless the converter is up to temp, it can't eat up the bad stuff in the exhaust. Let's look at the other factors you listed.

The BLMs you refer to are the Block Learn Memory fuel trim tables in the calibration. You have short-term and long-term BLMs. These fuel tables are adjusted by the O2 sensors based on the air/fuel ratio it is monitoring. BLMs are stated a few ways: 128 is right in the middle of the fuel trim window for the cell it's running in. Below 128 the cell is adding fuel, and above it's leaning out the mixture. It's not uncommon to have a slight difference between the banks of the engine. Any more than 2 percent, we'd be looking for a reason. That said, if you are to trust your BLMs with aftermarket components like headers, ported MAFs, and aftermarket injectors, they must be up to OEM quality. If your headers have an air leak, they will cause an imbalance in the block learns. Unless the fuel injectors flow exactly the same, again the block learns will be off. And if the BLMs make no sense at all, we've seen the O2 wires swapped from bank to bank-this really screws up the mix and the controller is trying to fuel the wrong side of the engine!

If the block learns in these types of ranges, the controller is fueling the engine at 14.7:1 at idle. The fact that your throttle body isn't lining up with the cast hole in the front of the manifold isn't the EGR circuit. This hole in the manifold in some models is for the idle air control. If this didn't line up at all it would not allow idle air to flow from the throttle body into the manifold. This is probably not the problem, because if you have idle control, you are getting idle air to all eight cylinders through the passages in the manifold.


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