So Annoyed With My Solenoid

I have a ’79 Firebird Formula with a 400, four-speed, and air conditioning. Whenever the clutch for the A/C kicks in and out, the engine feels like it has a miss. I know the A/C compressor robs some horsepower, but there must have been something to help compensate for the drop in rpm.

Via Internet

Rocky Rotella responds: As the air conditioning compressor cycles, its clutch engages and disengages regularly. It isn’t uncommon to find engine idle speed drop 100-200 rpm as the clutch suddenly energizes.

Many GM Divisions fitted its Quadrajets with a solenoid that increases idle speed to compensate for the compressor load in those conditions. The solenoid was fastened to the driver-side of the Quadrajet’s main body using two bolts. It contained an adjustable plunger that pressed against the throttle linkage. It was connected to the main engine harness by a single light-green wire with a single black stripe.

I recommend verifying that the idle-compensation solenoid is still attached to your Quadrajet. If it isn’t, you can likely find one on any similar vintage Quadrajet, regardless of GM make, at your local salvage yard. You can then adjust the plunger using an appropriately sized open-end wrench to achieve the reasonable engine speed when the solenoid is energized and the compressor is loading the engine.

You didn’t mention if your Formula’s 400 is stock or modified. Sometimes the rougher idle associated with an aggressive aftermarket camshaft can cause the engine to shake or shudder if idle speed drops too low and/or too much load is placed upon it at that point. Whether your 400 is stock or modified, I strongly suggest ensuring it’s in an excellent state of tune, and the carburetor and ignition system are operating ideally.

In the end, you might find that setting your 400’s hot idle speed between 800 and 900 rpm, and installing and/or adjusting the idle-compensation solenoid to achieve a similar amount under load should correct your issue.

Oily TPI

I was changing the EGR valve on my ’90 5.0L TPI T/A and noticed an oily film in my intake runners. What could be causing this?

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Ray T. Bohacz responds: It is very common to find an oil film, along with carbon, in both the runners and the plenum of a TPI engine. The oil is from the vapors that are picked up by the PCV valve and end up falling out of suspension in the air stream. The carbon is from the EGR circuit, and when it mixes with oil fumes from the PCV, can really gum things up. The long TPI runners (about 25 inches) make for a large surface area for the deposits to form and impede airflow.

Though this is a normal occurrence, the question that comes to my mind is, how great are the deposits? I’ve seen both carburetor and especially TPI engines with a good deal of accumulation, and this is usually the result of poor maintenance (oil changes), an incorrect flow rate PCV (aftermarket generic type), and/or not being cleaned with carburetor spray for many years and miles.

It sounds as if the engine is running now. Hopefully you cleaned off the oil film before putting it back together. If you did not, it isn’t a problem. Remove the throttle body for access to the plenum, and give it a good washing with carburetor cleaner spray. Also clean the throttle-body (both sides of the throttle plates and bores), and put it back together with a new gasket.

On cars with a MAF sensor (yours is speed density and uses a MAP sensor), it’s a good idea to start the engine with the sensor off. Often the engine will backfire from the carburetor cleaner, and if the MAF is in place, it will usually damage it and break the hot wire-sensing element. Running with the MAF off will set a code. Simply shut the engine off, disconnect the battery ground cable for 30 seconds, and reinstall the MAF—all will be fine.

Cool Coolant

I’m in the process of a major engine build and I remember an article in your magazine about a special coolant that not only keeps it cool, but increases power. Unfortunately I cannot remember the issue or the product. I know it cannot be mixed with regular coolant, so before I add anything to my engine, now would be nice to know the name of the product. I hope you can help.

Jim Novak
Via Internet

Ray Bohacz responds: You are referring to Evans NPG+ coolant. The Evans coolant itself does not make horsepower. It allows the engine to be tuned for higher power, since it removes more heat from the combustion chamber. The coolant does not boil until 370 degrees F with no pressure. When used in a Pontiac, it offers the opportunity to run a more aggressive timing curve and leaner air/fuel ratio, all while protecting the engine and keeping detonation at bay.

The best time to install it is with a fresh engine, since it is a direct pour-in after blowing out the heater core with air pressure. If the engine already has traditional coolant in it, it will need to be flushed out before the Evans product can be installed. The company offers a Prep Fluid for this, which you will not need.

Not only does the product lower the cylinder-head-metal surface temperature, but it is also a lifetime coolant and removes all water from the system. No corrosion can occur since there is no water. I speak from experience since all of my engines, even my farm tractor, have Evans NPG+ in them. You can contact Evans for more information at (888) 990-2665 or .

The first thing is to change the original 3.08 ratio to at least a 3.55, or better yet, a 3.73. The 3.73 will offer a great mechanical advantage for the original 200hp engine, making the 3,400-pound-or-so car much quicker.

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