Capturing atmosphere in today's vehicles

Keeping the air moving through intake manifolds keeps your customers rolling.

Drivability intake manifolds EGR automotive repair automotive maintenance vehicle maintenance automotive aftermarket With pistons traveling up and down in their bores and atmospheric pressure shoving air in every time an intake valve opens, every cylinder has to get an equal amount of everything if an engine is to run right. Intake manifolds are engineered to do exactly that.

During the past 20 years, intake manifolds have morphed from cast iron to aluminum and then to composites and plastics. Equal amounts of air, fuel and recirculated exhaust gas (EGR) are the order of the day for today's emission-friendly engines. Purge vapors from the carbon canister and crankcase ventilation gases (PCV) also must be distributed evenly when they're flowing. When engines had carburetors and the air/fuel mix was handled right above the throttle plate(s), the intake manifolds to which those carburetors were bolted had their runners engineered with careful precision.

Cylinders nearer the point where atmosphere rushed past the venturi had to have their runners snaked and stacked in such a way that every hole got an equally stoichiometric mix of explosive gasoline and air. That being established, any kind of air or coolant leak along the way tended to foul up the balance and cause the engine to run badly, particularly at idle. And on those units, EGR was fed into the mix right underneath the throttle plate, just as it is on most EFI platforms today.

With the valves and pistons doing their intricately timed dance to create low pressure between the throttle plate and the cylinder's point of entry (the intake valve), engineers have found more than a few useful purposes for that force we call manifold vacuum. In other words, because manifold pressure is so consistently lower than atmospheric pressure, we can use the naturally occurring imbalance between these two forces to do work.

In the 1940s and '50s, vacuum was harnessed to drive the wiper motor. German car manufacturers liked using piped vacuum to operate the door locks, but that meant there had to be a good tight vacuum reservoir that could keep atmosphere at bay for days or weeks at a time. On an aging vehicle, that tends to be a pipe dream. Vacuum has been the force of choice for power brake assist for decades, as it has been for cruise control and HVAC air distribution doors.

Because manifold vacuum tends to go away when the throttle plate is in its wide-open position, check-valved vacuum reservoirs have been the order of the day for a very long time. Emission control system engineers still choose to use manifold vacuum muscle to get things done.

Finding Normalcy

Manifold vacuum readings on a healthy engine should be a smooth and solid 18 to 22 inches of mercury (Hg) at idle. But late ignition or valve timing can cause manifold vacuum to be less than optimal. Every technician should have a vacuum gauge in his or her arsenal.

Valves that don't seat can cause a bouncing needle on the gauge, and EGR flow (which should never happen at idle on a gas burner) also can negatively affect manifold vacuum. That's important, because many manifolds (even MAF equipped platforms) have a Manifold Absolute Pressure (MAP) sensor, which also provides important information to the PCM.

Because the PCM uses manifold absolute pressure to modify fuel delivery (speed-density systems), low manifold vacuum can trigger the PCM to expand fuel injector pulse width to a place it should never be at idle. The engine will run rich, and sometimes technicians fail to perceive how a base engine problem can cause what seems to be PCM or sensor related.

Case in point: A couple of weeks ago, I took in a 1995 Chevy pickup with a rough, rich idle and only 13 inches of manifold vacuum at idle. That's seven to nine inches lower than it should be on a healthy engine. The vacuum would drift up to about 17 inches at off-idle with no load (higher rpm). We found that somebody who knew just enough to be dangerous had tried to Band-Aid the situation by advancing the timing, which usually increases manifold vacuum and engine speed at idle. Their idea was that if the timing was retarded, the engine vacuum would be low. So without a timing light, they advanced the timing, which did nothing to correct the problem.

We reset the ignition timing to zero with the ignition bypass disconnected, but the engine still ran like a three-legged dog. This was a high-mileage Chevy 350, and I had two wrench-eager students. I put them to work uncovering the timing chain, which turned out to have quite a few degrees of stretch. The tough old 350 got a new timing chain, and when we restarted it, the engine vacuum was instantly in the green on the gauge at about 16 inches of mercury. But that was still too low and the engine still didn't run quite right. Off with the valve covers!

A dynamic valve adjustment brought the manifold vacuum to a nice smooth 19 inches. What was puzzling was that this fairly simple-to-fix 350 had been to several other shops. All of them had done things to the truck, but none of them had taken measures to straighten out the problem. Low manifold vacuum numbers should have led them to the same conclusion we reached.

Here's some simple math. If you take manifold vacuum and add it to manifold pressure, you get barometric pressure. The vacuum/pressure split happens in the manifold and can be confusing until a new tech grasps the math aspect of it. And remember, the MAP sensor on a lot of vehicles also measures prevailing barometric pressure — when that BARO reading turns out to be a lie (and it can when a MAP sensor goes bad), everything fuel trim related then is thrown out of kilter.

Managing Atomized Fuel

Fuel dropout and puddling tends to occur on centrally fueled platforms (carbureted, TBI, etc.) that pipe the air-fuel charge through curvy passages en route to the combustion chamber. In regard to volumetric efficiency, those old manifolds were not our friends. But as far as fuel fallout goes, now that the fuel injectors are mounted in the manifold very near their respective intake valves, the fuel charge has little if any traveling to do before being swept into the chamber.

And that's a good thing. Be that as it may, the tendency of vaporized liquid to condense when it comes in contact with cool metal drove engineers to design manifolds with an exhaust passage that travels through the intake when the engine is cold so as to heat the part of the manifold where condensation is normally the worst.

Mid-1980s Ford Escorts actually had an electric grid under the carburetor to neutralize fuel condensation on cold days. And don't forget those dandy thermostatic air cleaners! They were designed with a vacuum servo and a flapper that would close when the incoming air was below a certain temperature, thus drawing warm air through that rather fragile foil-and-paper hose connected to the sheet metal stove sheathing the exhaust manifold.

Interestingly (and mysteriously), that same foil hose and stove design remained a part of the air intake system for a while even after multipoint fuel injection became prevalent. Since the 1960s, engineers have crafted creative ways to warm the manifold right where the carburetor or throttle body delivers its fuel mist.

Engineers originally designed a fuel pressure regulator that would change fuel pressure in response to manifold vacuum. A diaphragm was crafted with a calibrated spring, diaphragm and a small valve and seat to route excess fuel pressure back to the fuel tank. With a manifold vacuum line connected to the nipple feeding the chamber above the diaphragm, the pressure would rise when vacuum was low so as to keep fuel delivery pressure constant.

It took about 20 years for some automotive designers to recognize just how insignificant the injector pressure differential was in the grand scheme of things, and in the mid-1990s, that realization brought us the gift of returnless fuel systems.

Intake Manifolds & EGR

For years, EGR always was delivered right underneath the throttle plate for even distribution, then in the 1990s Ford and Nissan decided to route the EGR system's recirculated exhaust gasses on some of their engines through passages in the manifold to be delivered right near the fuel injectors. Not only would this more evenly distribute those gasses, but the hot exhaust feed at this point would presumably turn the already atomized fuel mist delivered by the injectors into a vapor for better flame propagation during firing events.

The problem with that arrangement is that when some of those ports clog (and they typically do), the clogged ports deliver no EGR at all to their cylinders. The one(s) that have yet to clog get too much EGR, causing dreadfully evident misfires, and that experience can lead an unwary technician to replace spark plugs and coils, and do compression checks trying to determine the reason for a road-speed skip and a P030x misfire code.

OEM Intake Manifold Enhancements

Engines with multi-point fuel injection need the same thing carb and TBI platforms need: equal amounts of everything to every cylinder. Because the fuel is delivered right to the door of the combustion chamber (that door being the intake valve) by those little atomizers we call injectors, the air is pulled past a dry throttle plate to be collected in an intake chamber we call a plenum that has runners of equal length leading to each cylinder.

In 1989, Ford released the SHO Taurus with a hotrod Yamaha-built 3.0L, which had a more complicated set of intake runners. Ford's prototype engines originally had chrome plated intake manifolds, but to shave money off the cost of each engine, Ford decided to dump the bling for a bead-blasted aluminum look.

The fancy manifold plumbing on the SHO was designed around the principle that long intake runners give better idle quality, but short runners are better for power and speed. With little throttle-like plates in every runner, the length of the intake airstream could be changed on the fly, and it's great fun to experience the resulting surge of power. At 4,000 rpm during acceleration, the SHO's PCM changes the position of those plates and the power curve takes on a whole new profile.

The Intake Manifold Runner Control (IMRC) system spun off some new forms, migrated to lower cost, more mundane platforms, and took on other names as it evolved. Split Port Induction appeared on Escorts, and other engines began to utilize IMRC, but lower performance engines move the plates at 3,000 rpm instead of the SHO's gutsy 4,000.

As the 21st century dawned, it occurred to engineers that there were simpler and cheaper ways of enhancing power with manifold plates. Ford pickups with modular V8s got a two-level manifold that could be tuned very easily by changing the angle of a single blade in the front of the chamber. That was called Intake Manifold Tuning.

While hot-rodders had been porting and polishing heads and intakes for years to make the air passages smooth and throaty, some late model Mustangs with modular engines use an array of fancy little PCM-controlled plates that choke the incoming air flow during low throttle conditions to give the air more velocity. When the throttle is wide and the pipes are thundering, the air is traveling faster anyway and changing the angle of the plates removes the restriction.

Searching and Finding

Intake manifolds don't just carry the air-fuel charge, they also carry hot coolant, house the thermostat and Engine Coolant Sensor (ECT), and serve as a mounting point for all manner of small components and wire harness routing anchors. Sometimes intake-to-head gaskets blow and either dump coolant outside the engine or give it egress to the sacred oil-drenched innards of the crankcase.

One of the most egregious offenders in the latter category would be the GM 3.1 to 3.4 platforms, which have milked up many a crankcase because of blown gaskets, however those are great money-makers.

Those plastic intake gaskets with the silicone port liners tend to yield to the constant onslaught of hot coolant with fair regularity, and it's not unusual to see a V6 or a V8 dumping voluminous amounts of coolant where it ought never go. Crown Victorias with plastic intakes were the target of a Field Service Action about a decade ago because their plastic intake manifolds were splitting and losing coolant.

Then there are the air leaks caused by malformed silicone gaskets that can cause a rough cold idle and nasty positive fuel trim numbers as soon as Closed Loop happens and the HO2S becomes a player. One Friday I was visiting with some old buds at the Ford dealer, and one of the line guys had a 4.0L Ranger that would start and die. A smoke test (which is wonderful for this) revealed a huge hole burned in the bottom of the upper intake by a steady stream of hot exhaust gas.

Then there was the Toyota with a misfire on number one cylinder. A brief shot of carb spray at the front runner smoothed the skip and a smoke test revealed the problem for what it was. The paper gasket had failed and had opened up a channel for outside air to come in.

Conclusion

Keeping an engine physically healthy and emission-friendly centers on you being able to keep harmful elements out of the carefully filtered airstream (like unmetered air, dirt, oil, coolant and foreign objects) and keep necessary elements (like coolant, EGR and the air charge) contained.

An engine with a breached intake manifold is either out of commission or deteriorating rapidly. Our job is to repair those breaches and to reason why they happened.

Richard McCuistian is an ASE-certified Master Auto Technician and was a professional mechanic for more than 25 years. Richard is now an auto mechanics instructor at LBW Community College/MacArthur Campus in Opp, Ala. Email him at [email protected].