A common complaint, especially when temperature start to drop is, “My car runs poorly cold, or my car is hard to start cold”. Both of these complaints are generally caused by a problem with cold start enrichment. In order for an engine to start and run cold, it requires more fuel than when it is hot. This condition where the engine needs more fuel cold is called cold start enrichment.
If the car does not get the proper enrichment when cold, it will have trouble starting, or run poorly until the engine warms up. These complaints are mysterious at first, but once you learn the common causes, you can often cure them with little to no trouble.
Start with the easiest things. When the car is cold, Check the coolant level. Make sure the coolant is full, and if you can’t remember the last time you replaced the radiator cap, toss a new one on. When the car is cold, it relies heavily on the engine coolant temperature sensor. If the coolant level is low, the coolant temperature sensor can send false information to the computer, causing a hard start of cold start poor running condition. Similarly, plug a scan tool such as this one
This is a relatively inexpensive scan tool made by autolink. It pulls codes as well as reads and displays your basic drive-ability data. I would recommend this to anyone who wants to repair any modern car (1996 or newer). I have one, and I use it all day long.
Watch the engine coolant temperature sensor from start up, all the way through to when the car has reached normal running temperature. You are looking for the sensor to be way out to lunch. If it reads 200 degrees Celsius when it is more like 30 degrees, there is your problem. If the computer doesn’t know it is cold, it won’t add the proper enrichment.
If the vehicle is old enough to have a cold start enrichment injector, also make sure that is working. This injector will generally be somewhere in the intake manifold. Start the car cold and listen to make sure the injector is clicking. If the enrichment injector is not working, then you certainly will not get the desired cold start enrichment.
Next I would grab a can of break clean, and clean the throttle body, idle air control valve, and the Mass Air Flow Sensor (MAF) if equipped. Often times the throttle body will become very dirty, and will not allow enough air to pass into the engine. The engine will have trouble maintaining an idle until the car reaches normal operating temperature. Furthermore, if the throttle body is extremely dirty, it may allow so little air to pass by that the engine will flood (too much fuel in the cylinders, thus the engine won’t start). Either way, a dirty throttle body is easy to cure, so why wouldn’t you clean it?
The hardest thing for an engine to do is to sit and idle cold, so the idle must also be raised in order for a vehicle to idle cold. If the idle air control valve is dirty, no matter how much it opens, it may not allow enough air to pass by in order to raise the idle enough to keep the engine running while cold.
Lastly clean the MAF if equipped. Just because they get dirty, and a dirty MAF always means trouble.
Still having trouble? Next I would check for a vacuum leak. If the engine has a vacuum leak, the computer will not be able to achieve the proper cold start enrichment. Start the car and let it reach normal operating temperature.
Plug in your scan tool and watch your short term and long term fuel trims. A properly running vehicle will add or subtract fuel within 5%. If you watch your trims and the computer is adding fuel both short term and long term, particularly in large quantities (10-25%), you most likely have a vacuum leak (assuming your MAF is reading properly).
To check your MAF, flip the throttle several times while in park or neutral, you should be able to get 60 – 90 grams per second, depending on the size of the engine. Also, most engines idle at 3-6 grams per second. Make sure your intake air temperature sensor is about right. Confirm your barometric sensor is about right.
If the MAF data all seems about right, take a can of break clean, and spray the engine while watching the fuel trims. Hit vacuum hoses, the gasket for the intake manifold, anywhere where air could enter the engine. If at any point you see the fuel trims reverse, and start to pull fuel, you have found a vacuum leak. You see, the engine would be pulling in more fuel (the break clean), the computer would see that fuel via the oxygen sensors, and adjust accordingly. Repair the vacuum leak, and you may just have fixed your hard cold start, or poor cold running condition.
Another common cause of a hard cold start, or poor running while cold is dirty intake valves. Over time intake valves become coated in a black gooey substance. Generally it will be worse if you let your air filter get dirty, and if you use lower grade fuel. This black gooey substance causes two problems, it soaks up fuel, and causes intake valves to stick. Both problems cause all sorts of havoc on a cold engine. The extra fuel the computer is trying to send to the cylinders is absorbed by the goo, thus robing the cylinders of the cold start enrichment they need. Also, when valves stick, it just isn’t good.
You can toss a bottle of BG’s 44K in the gas tank, and suck a bottle of decarb through a vacuum line. These two quick fixes may be all you need. A full blown injector purge may be the ticket. However, I have often seen valves so dirty that you have to remove the intake manifold and manually clean them. So, if you find a vacuum leak at the intake manifold gasket, remove the manifold, replace the gasket, but also go in the and clean those intake valves.
Lastly, and particularly on certain Hondas, you may have a problem with valve clearance. Yank the valve cover off and do a quick valve adjustment. I’ve seen several Hondas have the exhaust valves too tight. These cars would die when coming to a stop while cold, but run perfectly when warm. Re-set the valve clearance and the problem vanishes.
Those are your main causes of cold start enrichment failure. I suppose a fuel pump could be weak, and not be able to deliver enough fuel. Or a fuel pressure regulator could be going out. Generally speaking, any other cause of poor running/a hard start cold would also cause problems while the engine was warm.
See also: Check engine light causes