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Rough Running Engine - James on Engines #2

Posted by: James Dunst

rough-running-enginesThrough the Contact Us forms on the Bell web site, we get all kinds of inquiries about fuel problems and engines from the general public.  Some of them describe mechanical problems that people are having – problems that, while common across the industry, require some further investigation before the right advice can be given.

In this series, Bell’s resident master mechanic James Dunst discusses the most common mechanical problems that he gets asked about. He discusses the causes, what (if anything) can be done about them, and any further questions that a mechanic would probably ask in seeking to properly diagnose a fix such a problem.

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The Common Problem: Rough Running Engine

A properly-operating engine should run smoothly and without excess noise.  If your engine starts operating in a manner that gives you the perception of “running rough”, it is likely due to a handful of common causes. A mechanic would investigate these common causes first in order to rule out a simple explanation for the engine’s unsatisfactory operation.

The common causes for rough running in an engine are:

  • Engine miss
  • Vacuum leak
  • Dirty fuel injectors
  • Carburetor problems
  • EGR valve
  • Ignition coil
  • Spark plug wires

Before looking at each of these, let’s take a moment to consider how the mechanic should diagnose the cause of the problem.

Checking the computer for diagnostic codes

When diagnosing an engine with a rough idle, there are certain things that must be done prior to replacing parts or performing service. If you have a 1981 or newer vehicle, it will most likely have an engine management computer. The computer monitors the functions of a number of engine compartment components. When a problem develops in a system that is monitored by the computer, codes which relate to the particular system will be set and retained in the computer’s memory. When this happens, the check engine light on the dash will illuminate. If the light stays on continuously this is called a hard fault and is the easiest problem to find because the problem is there right now. If the light comes on and goes out, the fault is intermittent, meaning the problem was there momentarily but has cleared up.  Both these conditions will set codes that are retained by the computer for later investigation.

In most vehicles these diagnostic codes will remain in memory for fifty engine warmup cycles. If the problem does not reoccur, the codes will be erased by the computer. Steer clear of a mechanic or shop that replaces parts without doing the proper diagnostics first. Checking for computer codes first is important because it may lead you to the rough running problem.  And this is what you should expect a competent mechanic to do if you present them with this kind of problem.

Mechanical reasons for a rough idle or engine miss

High mileage vehicles with over 100,000 miles are the most likely to start having engine miss problems (it can be less in some vehicles), and it is advisable to have a compression test performed to rule this out. If you have low compression on one or more cylinders, this could be indicative of a more serious condition that would need to be fixed immediately.

The normal procedure for a compression test is to remove all the spark plugs, insert a compression gauge into the spark plug hole, and then turning the engine over about three times. Record the reading and do this for all the cylinders. If there is more than a 20% difference in any of the readings, a further test would be required to determine if the variance problem is due to an issue with the piston rings or the valves. 

How can a mechanic (or you) tell which is the cause? The next step would be to squirt a small amount of oil around in the cylinder that had the low compression reading, and repeat the compression test on that cylinder.  If the compression rises from the previous test reading, the problem is due to the piston rings. The oil helps to improve the seal, thus compensating for the compression that was lost from worn piston rings in the first reading. That’s why the compression would be higher in the second test. 

If no increase is seen in the compression the second time around, the problem is most likely a burned valve.  So now you have a better idea of which kind of mechanical fault is contributing to the problem.

If either problem is found, disassembly of the engine would be required to repair the problem. Special attention should be paid when removing the valve covers to make sure the valves are all moving the same amount of downward travel. Some engines have issues where the camshaft lobes will wear down and prevent the valve from opening which can also be the cause of the misfire.  At this point, a good mechanic will be able to take the problem on and fix it. It may not be a cheap fix, however.

Rough idle caused by a vacuum leak

Most engine compartments have a maze of vacuum hoses which can wear out from use at any time.  What we mean is they get brittle and hard over time. It’s just one of those things that happens during the life of the engine. If any of these hoses spring a leak, a lean air/fuel condition will result, causing a rough idle from the resulting engine misfire.  The misfire could be in one cylinder or in multiple cylinders, depending on the size or location of the leak.

Vacuum leaks can also be caused by leaking intake manifold gaskets, vacuum brake boosters or vacuum supply tanks.   How can you tell if the problem is caused by a vacuum leak? When driving a vehicle with a small vacuum leak, you will notice that everything feels normal at higher speed or RPM, but runs rough when the engine is at an idle. The engine could seem to rev up and down on its own at idle, but it’s more likely that it will simply rev up abnormally and stay there at idle.  Either way, that’s a big indicator of a vacuum leak.

A mechanic will seek to get  confirmation of a vacuum leak by looking for a lean code set in the engine computer.  This makes sense because a leak in a vacuum hose means more oxygen is entering the system than is needed. That results in a lean reading – not enough fuel, too much air.  The engine can’t run properly if the air/fuel mixture isn’t right.

Once a mechanic has narrowed down the problem cause to a vacuum leak, they will want to determine where the leak is located at.  It would be helpful if you can pinpoint it yourself. Who knows, you may be able to fix it at that point.  When attempting to diagnose the source of the leak, the first thing you do is to listen for a hissing sound in the engine compartment. Many times it can be a simple thing like a vacuum hose that has become dislodged – in this case, all you need to do is reinstall it. If you suspect the leak is caused by a component like a vacuum brake booster or vacuum supply tank, you can pinch off the hose to the suspected component with a pair of needle nose plyers. If the component tested is the problem, the idle will smooth out.

Lastly, if you suspect the intake manifold gaskets are leaking, you can take a product like WD40 and spray it along the edge of the intake manifold while the engine is idling. If the gaskets are leaking, the idle will smooth out or change when spraying the WD40.

Rough idle caused by dirty fuel injectors

If dirty fuel injectors are the cause of the problem, you can expect more than just a rough idle.  Dirty fuel injectors are perhaps the biggest contributor to poor gas mileage.  If the injectors are restricted, the engine’s lack of performance will be even more noticeable when accelerating the vehicle, which causes a higher fuel demand. 

Diagnosing this condition is best done by a repair shop with an exhaust gas analyzer. Restricted injectors will cause high carbon monoxide and hydrocarbon readings. This is because anything that interferes with an ideal fuel spray causes an incomplete burn of the fuel, and this will generate excessive carbon monoxide in the exhaust. We know this because complete combustion gives water and carbon dioxide as byproducts.  Incomplete or improper combustion means the fuel isn’t burning completely and you’ll get both elevated carbon monoxide and higher hydrocarbon readings as a result.  You get higher hydrocarbon readings from the exhaust because when an engine misses, raw fuel enters the exhaust system and creates high hydrocarbon readings. Restricted injectors will cause both of these conditions.

If dirty injectors are indeed the problem, a mechanic will use an injectable cleaning concentrate to clear them out. This is a problem that really lends itself to prevention. Using an injector cleaner gas additive to prevent problems before they happen is the best way to keep this kind of problem from happening in the future. 

Rough idle caused by carburetor problems

When we start talking about carburetors, you know we’re thinking of both older vehicles with higher mileage and small engines. One of the indicators of a carburetor problem is a considerable amount of black exhaust smoke when the engine is up to temperature.  A properly-working carbureted system shouldn’t produce volumes of black smoke – this would be a sign that something is amiss.

The first thing to check is the choke to make sure it is fully open when the engine is warmed up.  If the choke is open, the likely problem shifts to an internal issue requiring a carburetor rebuild.  A damaged float might be an example of this.  In many cases, the floats in the carburetors have been damaged from the ethanol in today’s fuel.

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Another area of concern is the throttle shaft in the base of the carburetor. Over time the throttle shaft will wear the carburetor base housing at each end of the shaft, causing a vacuum leak. You can check this in the same way I explained earlier in this article by using WD40. Just spray it at both ends of the throttle shaft and see if the idle straightens out or changes.  Should this be the problem, the repair for this condition is a carburetor replacement.

A word of warning when purchasing rebuilt carburetors.  Before you buy, check for wear or play in the throttle shaft.  Some of these rebuilt carburetors have just as many miles on them as yours does. Excessive play in this area means you want to avoid that particular piece. 

Rough idle caused by the EGR valve 

The EGR Valve (Exhaust Gas Recirculation Valve) is an emission control device that allows exhaust gas to enter the intake manifold.  These started becoming common on both gas and diesel engines sometime in the 1970s. They function to reduce harmful emissions from the engine.

There are three times in a engine’s operating cycle than an EGR valve is never supposed to operate:  1) at engine idle, 2) at wide open throttle or 3) when the engine is cold. In all three areas, it would have a negative effect on the way the engine runs, especially when the engine is idling. 

With respect to our problem of rough running, an EGR valve can be a cause if it becomes fouled with carbon.  This causes it to stick open, causing a rough idle or engine stall. 

As explained earlier, in many cases the computer may set codes which can lead you to the EGR valve as the prsoblem.

Confirming a stuck EGR valve can be as simple at giving it a light tap on the side of the valve, which may free it from its stuck-open position. In most cases, dirty EGR valves can be cleaned which usually resolves the problem. If cleaning is not an option, a replacement valve would be required. Again, this is a simple problem that’s inexpensive to fix – most people can replace an EGR valve themselves.  So if your engine’s unsatisfactory performance is due to a dirty EGR valve, you’re going to get off cheap. 

Rough idle caused by spark plugs, spark plug wires or ignition coil problems

If you have a rough idle that is caused by either the spark plugs, spark plug wires or ignition coil, you will also feel it when accelerating. If these parts are bad enough to cause a miss at idle, it will continue to miss while driving. If either one of these items is starting to go bad but not missing at idle, a jerking sensation will usually be felt when accelerating the vehicle under a load. So paying attention to how the vehicle feels at different point of driving is what will help diagnose this. 

Catalytic convertors 

It is important to note that anything that causes a miss in a vehicle with a catalytic converter needs to be repaired immediately. If you have a cylinder missing, it will be sending raw fuel back into the exhaust system. Unburned gasoline going into a hot catalytic converter is a perfect recipe for a fire. This condition will cause the converter to glow red and may cause a fire hazard if parked over combustible materials. Many grass fires have been started in this manner. Even if you escape a fire, continued operation of a vehicle with this condition can damage or destroy the catalytic converter requiring replacement. And that can be pretty expensive. 

Rough idle in vehicles with Gasoline Direct- Injected engines

Drivers of direct-injected gasoline engines have been complaining about a rough idle and a check engine light that has been illuminating on the dash. These engines are experiencing a problem with carbon build-up on the top side of the intake valves.  Excessive deposits in this location can disrupt the air fuel flow, causing an engine to misfire. These engines inject the fuel directly into the combustion chamber, eliminating the ability for the detergents in the gasoline to wet the top side of the valves and keep them free of carbon. 

This carbon build up comes from the PCV gases in the crankcase and from EGR exhaust gases entering the cylinder through the intake valve. Most of the vehicles experiencing this problem have more than 30,000 miles but some have been seen with less. The check engine light comes on because the engine is missing at idle, causing a P300 random misfire code to be set in the computer. 

There are only two options when this problem occurs. The intake manifold would need to be removed and the valves blasted with a walnut shell media. The second treatment would be to spray intake valve cleaner into the engine air intake while running engine at a high RPM.

But even if you try either of these solutions success depends on how long the carbon has been on the valve surface. There is a point where carbon has been there so long, it cannot be chemically removed because it has polymerized and turned into varnish that is impervious to chemicals.  So blasting the carbon off would be the only workable solution.

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This post was published on April 21, 2015 and was updated on November 21, 2018.

Topics: Lubrication, Car Care