Warming it up

So I get the idea . . . a car runs better when properly warmed up. But . . . why? I was driving my 12 year old Son to a soccer game yesterday morning, 7:30 am, about 35 degrees. The soccer car is an old accord, carb’d with a lot of miles. He made the simple comment “Boy Dad, she doesn’t like the early morning cold, does she?” . . referring to the somewhat rough running and stumbling the old accord does for the first minute or two. Soooooo . . . why DOES a fully warmed up car run better than a stone cold one? Rocketman

Carbs have three modes: cold, warming up and warm. None are optimal and at this point in time 20years+ way out of adjustment likely.

Any fuel injected car runs very well in all conditions since the modes are essentially infinite and no adjustment except by computer.

I know when I go to drive my Chevelle, I’ll let it sit for atleast 5 minutes before I drive it off. Even then it can be stubborn and not want to go when I switch from reverse to drive(yes I do come to a full stop before I do that). Luckily I live in a small town so when it stalls on me, it isn’t as bad as if I lived in Columbus or Cleavland.

So you think that a stone cold injected car will run just as well as a car that has warmed up fully? Rocketman

In short the engine is designed to run best when at a specific operating temperature. When cold not all the fuel gets burned because the cylinder walls are cool and they cool the air fuel mixture next to them so it does not burn completely. Also at different temperatures things expand and contract with the temperature so they may not fit as well as at operating temperature. When cold the oils (engine gears wheel bearings etc) are thicker, stealing energy.

Adjustments made by the car counter some of these things, but not all.

Modern cars are usually better and are designed to warm faster and cool more precisely to maintain that ideal temperature. I will add that the biggest motivator at improving modern cars was pollution control requirements.

So just like you and me, it works better when at the right temperature.

The modern fuel-injected car has sensors all over it that assist the computer in selecting optimum fuel/air mixture and ignition timing for a cold start and for the warm-up phase. Not so for an elderly carb’ed car, which can only approximate those settings.

In addition, engine tolerances are spec’d for the final operating temperature. You have gaps (rings, valves) until the engine completes its warmup. This is true even for modern cars.

Rocketman, my 87 Accord behaves in the same manner. The worst conditions for a cold engine seem to be on a cold, rainy morning when the car hasn’t ran in like 3 days. I found that simply putting it in gear and going warms the car up more effectively than letting it just sit there and burn gas, stewing in the cold air. Honda literature suggests only letting the car warm up in extreme cold temps. Also, cold oil is all thick and gooey and doesn’t lubricate as effectively as it does when it’s warm. That’s why I avoid freeway driving until the needle has at least started to move. Is yours the one with over 400k on it?

Yep . . . and I know the driving characteristics of the cold engine well, believe me. But sometimes I just like to throw out a theoretical question like this one, becasue of the wide and diverse background portfolio we have here . . aren’t the answers great so far? My accord just turned 440,000 miles and warms up better when driven slowly at first. But I park it in my driveway facing in, on an incline, so that when I start it I drift backwards out of my driveway onto the rural road which my driveway connects to, then go downhill for another 1/4 mile until I reach the main road . . so it idles on startup for about a minute before I actually put it into gear . . .and that where it runs a little “rough” and alway has, for 440,000 miles. BTW, I can cure this by simply turning on the AC after startup, which brings the idle up from 500-750 to about 1500, but puts load on the engine which I don’t think is healthy on a cold startup. BTW, on a cold and rainy morning . . pump the gas thre or four times and then hold it down halfway when you crank it . . . it starts right up. Rocketman

Actually, most modern cars don’t use many sensors at all when they’re warming up. Most fuel injected cars are still using open-loop mode (data taken from pre-written tables in the ECM) until the car is “warmed up”.

Yes they just idle faster till warmed up. There is never any roughness or hesitation in my last three fuel injected cars, 1988 Jetta GLI 16v, 1995 Honda Civic, 2004 Subaru WRX.

Fuel vapourizes and atomizes better at higher temperatures, that’s all.

Venturi pressure differences in carburetted engines can actually cause carb icing - ask any pilot, he’ll tell you.

With carb engines the fuel lambda mixture is a fog at running temperature, when cold it’s more like a precipitate and does not ignite or burn well.

The same applies to injection engines, but the injectors make a better job of atomizing the fuel during the spray injection, it’s more like an aerosol than the fuel droplest released by a carb into the inlet manifold.

I happen to be a pilot and i’ve experienced carb icing, scary stuff. The engine in the plane almost completely quit on me. Not fun when you’re only 3000 feet above the ground and hostile terrain. Why don’t cars have “carb heat” ?

Some of them have. Or at least, something like what I assume a plane’s carb heater is. GM for a while was using something called an EFE grid, which stood for early fuel evaporation. It was a ceramic grid with heating elements that fit between the carburetor and the intake manifold. Unfortunately it worked terribly and leaked like a sieve. Obviously the aviation industry has come up with something better, but I’m sure there’s a reason it hasn’t been adapted to automobiles.

Carbureted cars and throttle body injected (TBI) cars do. It?s typically done with a valve in the intake duct that pulls warm air off an oven mounted to the exhaust manifold. Some vehicles also used a heating plate mounted between the carb or TBI and the intake manifold. I have an aftermarket 4-barrel TBI unit on my Chevy, and it will ice up and stall when idling if it?s cold and damp and I don?t use the thermostatic air cleaner (I modified mine to be manually controlled with a switch, from the factory it was controlled with a thermostat)

Ported fuel injection doesn?t usually have a problem with icing on cars because they have a dry manifold, so there is no fuel evaporating from the manifold and carb, which removes heat.

More important than the heated intake air duct was the exhaust gas crossover port found on most older V8 engines. It allowed exhaust gases to swirl up around the metal around the base of the carburetor. That was done for two reasons. First, as Scudder mentioned, the warmer manifold enabled the fuel to better vaporize and atomize (meaning less cold weather hesitation and stalling). Second, it provided the heat for the choke-pulloff coil - which relaxed the choke as the engine warmed.