Engine running lean ...

[1968 Pontiac]

I changed the plugs and wires on the weekend for the first time since I bought the car. When I removed the old plugs I found that both front plugs on each side showed that the cylinders were running lean. The rear two plugs on each ide were fine. Is there a design flaw in the intake on my 91 Caprice, or could something else be wrong? The car runs pretty good, but seems to bog sometimes when you try to accelerate hard, either from a stop or on the highway.

[Blue Bowtie]

O.K. Since no one wants to touch this, I'll give it a stab.

The intake is designed to support a firing order which alternates fuel/air feed from one side of the TBI to the other. Passages are also split so that a vacuum leak from one fitting or device would not affect all four cylinders on one end or another, but alternate through the firing order.

There are two possible scenearios which might cause your symptoms. First, the last places that coolant travels before exiting the thermostat housing and going to the radiator to be cooled are the front ends of the heads. If coolant flow is restricted somewhat, and the front four cylinders were seeing hotter chamber temperatures, you might be able to detect that on the visual inspection of the plugs. Normally, coolant flow is sufficient to prevent these kinds of hot spots, but if the water pump impeller is eroded, slipping, or there are restrictions and sediment in the system, those symptoms MIGHT occur.

Second, and more likely, is that a leak at the front edge ot the throttle body is creating a lean condition for BOTH sets of intake plenums and runners toward the front end of the engine only. The oxygen sensor should detect the average EGO level, and adjust fuel mixture overall so that the average mixture satisfies the sensor. That would leave the front four cylinders a bit lean, and the rear four cylinders a bit rich. Actually, it would be "front two" and "rear two" cylinders, since the O² sensor is only used on one bank.

I'd suggest investigating the TB mounting, gaskets, any spacers, and the TB itself for possible cracks or warpage. It's also a good excuse to thoroughly clean the TB, IAC passages and IAC stepper motor, and check the injector inlet screens for debris. You can get a TB overhaul gasket set (for the injector pods and mounting) for nearly nothing.

[1968 Pontiac]

Thanks Blue, the second suggestion sure does make sense. I'll let the forum know how I make out. We have to keep in mind that TBI is just a 'smart carb'.