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Why Does My Engine "Lope"?

Updated on October 30, 2017
Dan Ferrell profile image

Dan Ferrell writes about do-it-yourself car maintenance and repair. He has certifications in automation and control technology.

Engine lopes at idle.
Engine lopes at idle. | Source

We say an engine "lopes" or "surges" at idle when RPMs cycle unsteadily up and down, for example between 600 and 1000 rpm.

When your engine lopes, there could be a problem in one or more systems or components:

  • Fuel system
  • Ignition system
  • PCV system
  • EGR system
  • Throttle body
  • Idle air control valve
  • Ignition system
  • Compression problems
  • Vacuum or pressure leaks

Troubleshooting for a loping engine can be hard, since one or more systems may be behind the fault.

So you need a practical way to approach your diagnostic. In this guide, systems or components as potential sources of problems are laid out in the following sections starting with the most common and, often, troubleshooting procedures are presented to check the system.

As you go through the following sections, keep in mind those systems you haven't paid attention to in the last few months, or those systems that gave you signs of problems in the near past. They can point you to the potential failing system or component.

And don't forget your Check Engine Light. If it is on, scan the computer memory for diagnostic trouble codes, and check those flagged systems or components first.

Index
I If Your Engine Lopes When Cold, Check These Things
II. If Your Engine Lopes When Warm, Check These Things
III. Check the Fuel System
IV. Check for Bad Ignition Timing
V. Check for a Clogged PCV Valve
VI. Check the EGR Valve for Leaks
VII. Check for Throttle Body Problems
VIII. Check for Incorrect Idle Speed
IX. Check for Ignition System Problems
X. Check for Compression Leaks
XI. Check for a Leaking Intake Manifold Gasket
XII. Check for Vacuum Leaks
XIII. Check for a Clogged Air Filter
XIV. Check for Worn Camshaft Lobes
XV. Check for Leaking Valves

I. If Your Engine Lopes When Cold

You may notice that your engine lopes only when cold. Temperature plays a key role in engine performance. That's why engines use a number of sensors to increase temperature on a cold engine as soon as possible.

A few components, found in some models, can cause an engine to lopes or idles erratically when cold, either because they become faulty or wear out after miles of service:

  • Check for a bad Heat Control Valve (HCV), also called Power Heat Control Valve (PHCV) or Early Fuel Evaporation (EFE) heater. You may find the EFE heater near the intake manifold.
  • Inspect the Thermostatic Air Intake (Thermac), usually located around the air cleaner assembly.

These systems are used to improve engine efficiency and decrease emissions during engine warmup.

  • Check for a faulty or worn-out fuel pump.

See whether your particular model has one or more of these components. Consult the vehicle repair manual for your particular make and model. Amazon has aftermarket Haynes manuals that can help you locate and troubleshoot and maintain almost any component in your vehicle.

When bad, some components can affect engine performance only at certain temperatures.
When bad, some components can affect engine performance only at certain temperatures. | Source

II. If Your Engine Lopes When Warm

In the same way, an engine that lopes when warmed up is associated with problems in some specific components:

  • Check the Throttle Position Sensor (TPS) or its circuit. On some models, you can adjust the TPS.
  • Make sure the Heat Control Valve is opening.
  • Inspect the Mass Air Flow (MAF) sensor and circuit. Often, the MAF only needs a little cleaning.

Check the fuel system for pressure issues causing engine performance problems.
Check the fuel system for pressure issues causing engine performance problems. | Source

III. Check the Fuel System

Over time, components in the fuel system wear out or fail, causing fuel pressure to decrease. The problem may include the fuel pump, fuel pressure regulator, fuel filter, or fuel injectors.

You can do the following fuel system checks, using a fuel pressure gauge:

1. Relieve fuel system pressure. If your system has a Schrader or test valve, you can depress the valve using a small screwdriver while covering the valve with a shop rag to catch the squirt of fuel. Do this with the engine off. Then connect a fuel pressure gauge to the fuel pressure valve or test port.

2. Start the engine and read the fuel pressure gauge. Make a note of the gauge reading.

3. Then, disconnect the vacuum hose from the fuel pressure regulator and make a note of the pressure. Compare your readings to the specifications listed in your vehicle repair manual. Pressure shouldn't rise considerably.

4. If pressure is low in step 2, shut the fuel return line by placing a rag over the line and pinching the line with a pair of pliers. Use the rag to prevent damaging the return line.

  • If pressure still doesn't rise, the fuel feed line or the fuel filter may be clogged, or the fuel pump may be bad.
  • If pressure rises considerably you have a bad pressure regulator.

5. If fuel pressure is too high in step 2, disconnect the return line and blow through it.

  • If the line is blocked, check the return line and connectors for the blockage. If there's no blockage, replace the regulator.
  • If the line is clear (no blockage), confirm that the regulator is working:

6. If the pressure in steps 2 and three matches the specification listed in your manual, your fault may be with one or more fuel injectors. You can have a shop do an injector balance test to check fuel flow and amount of fuel through the injectors and injectors operation.

However, a quick test might help you here: After warming up the engine, disconnect the idle air control (IAC) valve and, with the engine at idle, unplug a fuel injector.

You should notice a change in idling RPMs if that injector is working. Plug back the injector electrical connector and proceed to the next injector. Any injector that doesn't produce a noticeable change in engine idle when unplugged, is a suspect of failure.

Remember that this is a quick and dirty injector balance test. If you suspect one or more injectors, troubleshoot that injector with the help of your vehicle repair manual or have a shop confirm your findings.

Troubleshoot the regulator:

1. Hook up a hand-held vacuum pump to the fuel pressure regulator and leave the fuel pressure gauge connected.

2. Gradually apply 4 in-Hg of vacuum to the regulator while watching the fuel pressure gauge. Pressure should decrease as you apply vacuum and decrease when vacuum is removed.

3. Now, hook up a vacuum gauge to the vacuum hose that connects to the pressure regulator. Check for vacuum at idle. If there's no vacuum, check for a clogged vacuum hose or port. If there is adequate vacuum, replace the regulator.

4. Now, disconnect the vacuum gauge and reconnect the vacuum line to the fuel pressure regulator, but leave the fuel pressure gauge connected.

5. Turn the engine off and wait for about 5 minutes and check the fuel pressure gauge. Pressure should hold. If it doesn't, you may have a:

  • bleeding pressure regulator
  • leaking fuel injector
  • bad fuel pump
  • leaking fuel line

Consult your vehicle repair manual for specifications and particular test procedures for your model.

Worn timing sprockets will disrupt ignition timing.
Worn timing sprockets will disrupt ignition timing. | Source

IV. Check for Bad Ignition Timing

Another cause for an engine to lope may come from a bad sensor related to ignition timing. Several sensors provide information to the ECU to adjust timing:

  • knock sensor
  • camshaft sensor
  • crankshaft sensors
  • manifold absolute pressure sensor
  • throttle position sensor
  • engine coolant temperature
  • intake air temperature sensors

On the other hand, ignition timing problems on high-mileage engines may be caused by a worn or skipped timing belt or chain, or worn-out timing gears.

In most modern vehicles, ignition timing is controlled by the car computer, and can't be adjusted. On older models equipped with a distributor, timing can be adjusted.

Whether ignition timing can be adjusted or not in your particular model, you still can check timing using a timing light to make sure that it is within specs:

  1. Warm up the engine and then turn it off.
  2. Locate the spout connector in your vehicle and remove the plastic block. You may find the spout near the distributor, ignition coils, ignition control module, or computer. Consult your repair manual, if necessary.
  3. Connect the timing light clamps to the battery and the inductive clamp to the spark plug wire on cylinder number one, the one closest to the front of the engine.
  4. Start the engine, aim the light on the crankshaft pulley and check the timing marks on the pulley against the timing pointer above the pulley or timing chain cover. Compare your reading to the specifications.
  5. If the timing is not correct and you have a distributor, loosen the distributor mounting bolt and rotate it left or right to adjust to the correct timing mark. If your vehicle doesn't have a distributor, check the those components the computer relies on for timing adjustment.

Check for a clogged pcv valve or vacuum hose.
Check for a clogged pcv valve or vacuum hose. | Source

Check Engine Light

...scan the computer memory for diagnostic trouble codes, and check those flagged systems or components first.

V. Check for a Clogged PCV Valve

Is not uncommon for sludge to fill and clog hoses and other components in the positive crankcase ventilation (PCV) system. Other times, a hose or some other component may begin to leak and cause the engine to lope.

  1. Find the PCV valve in your vehicle. It's usually located somewhere around the valve cover. If necessary, consult your vehicle repair manual.
  2. Disconnect the hose(s), check their condition, and remove any foreign matter from inside the hoses.
  3. Many PCV valves can be checked by shaking them in your hand and listening for a rattle. If you can hear the valve rattle, the valve is in good condition. If you can't hear it rattle, try cleaning the valve with solvent or just replace it with a new one.
  4. Also, check the grommets and any other connectors in the PCV system for damage. Make sure they are in good condition and replace components as necessary.
  5. Some models may use a breather filter as part of the PCV system. You may find the filter underneath the cover of the air filter housing. You may need to replace the filter as well. Consult your repair manual.
  6. Start the engine and apply the parking brakes and set the transmission to Neutral (manual) or Park (automatic).
  7. Disconnect the vacuum hose from the PCV valve or valve cover and place your finger at the hose opening. You should feel a slight pull from vacuum on your finger. If not, check the hose for a split, and check for a clogged hose, connector or valve. Also, inspect and make sure that the oil filler cap, valve cover gasket, and grommets are not leaking.

Replace the EGR valve if the diaphragm isn't working.
Replace the EGR valve if the diaphragm isn't working. | Source

VI. Check the EGR Valve for Leaks

On many vehicle models, the exhaust gas recirculation valve (EGR) operates through vacuum and is controlled by a vacuum regulator.

These type of valves can develop leaks in the vacuum hose, or the valve itself may get stuck open. Depending on the type of valve, this problem may cause the engine to lope at idle or even at cruising speeds.

  1. Check the vacuum hoses that are part of the EGR system for wear, tears, and bad connections.
  2. Test the EGR valve by connecting a hand-held vacuum pump to the valve, and apply 5 in-Hg of vacuum to the valve. Wait for a minute or so. The valve should hold vacuum. Otherwise, the valve is leaking and needs to be replaced.
  3. On some models, you can touch the valve's diaphragm with your finger from underneath the valve. If you apply vacuum to the valve, you should feel the diaphragm moving; if not, the diaphragm isn't working and you need to replace the valve.
  4. On many models, the EGR valve is accessible and you can remove it using a few common tools. If possible, remove the valve and inspect the valve and intake ports.
  5. You may need to remove carbon deposits using a wire brush and scraper. Sometimes, it is necessary to use a solvent like carburetor cleaner to thoroughly clean the valve. If so, be careful not to allow solvent to reach the diaphragm or any electrical components that are part of the valve or you'll ruin the component.
  6. Replace the valve using a new gasket, if necessary.

Remove buildup from the throttle body and plate.
Remove buildup from the throttle body and plate. | Source

VII. Check for Throttle Body Problems

Engine lope problems can also come from throttle plate or bore issues.

After miles of operation, carbon accumulates around the throttle bore and around the plate (valve), creating engine performance issues. But you can remove buildup using carburetor cleaner:

  • Remove the air duct or air filter housing, depending on your vehicle model, and examine the throttle bore and under the plate.
  • Remove any carbon deposits. Spray carburetor cleaner on a clean shop rag and wipe clean the bore and plate.

Watch the video below so that you know how throttle body issues can cause an engine to lope.

Look for the carburetor idle speed adjusting screw.
Look for the carburetor idle speed adjusting screw. | Source

VIII. Check for Incorrect Idle Speed

On some older vehicle models, engine lope can be caused by the wrong idle speed adjustment.

  • You can adjust the idle speed according to your manufacturer specifications. Check the Vehicle Emissions Control Information (VECI) label located in the engine compartment for the adjustment procedure or your vehicle repair manual.

On modern vehicles, the idle solenoid or valve is also a common failure after a few years of operation. Carbon deposits accumulate inside the valve or along the ports, preventing proper operation of the valve. Other times, the valve just reaches the end of its service life and stops working.

  • Usually, the IAC is readily available and can be removed for inspection or diagnosis. You can troubleshoot the valve with the help of your vehicle repair manual using a digital multimeter (DMM).

Check ignition system components for a possible failure.
Check ignition system components for a possible failure. | Source

IX. Check for Ignition System Problems

The ignition system is also a common source of engine performance issues. Bad spark plugs, wires, ignition coils, distributor and distributor components can all contribute to engine performance issues.

  1. Check the spark plugs' gaps and readjust it if necessary.
  2. Make sure the plug tips are not covered in carbon deposits. If so, find the source of the problems. The plug may need replacement, you may have an ignition system problem, or something may be causing the fuel system to run rich.
  3. Your vehicle repair manual may also give you the resistance value for the spark plug wires. Check the wires using a digital multimeter. Commonly, wires have around 10K Ohms of resistance per foot. Consult your repair manual.
  4. Check ignition coils and distributor for proper operation. Consult your repair manual, if necessary.

Check the service interval for the ignition system components as suggested by your car manufacturer. You may find the service schedule in the car owner's manual or vehicle repair manual. Replace components as necessary.

Check and replace the head gasket if blown.
Check and replace the head gasket if blown. | Source

X. Check for Compression Leaks

A compression leak may come from a cylinder head gasket and cause the engine to lope sometimes. This usually happens because the engine has high mileage, or a particular failure, like overheating, caused the gasket to blow.

To check for a leaking head gasket, do a cylinder compression test using a vacuum gauge. Consult your vehicle repair manual for this.

Inspect the intake manifold gasket for leaks.
Inspect the intake manifold gasket for leaks. | Source

XI. Check for a Leaking Intake Manifold Gasket

Just like a head gasket, an intake manifold gasket may leak from excessive wear or damage, allowing vacuum to escape.

You can use a piece of fuel or vacuum hose to check for vacuum leaks around the intake manifold.

  • Put one end of the hose against your ear and use the other end to trace around the intake manifold gasket area to listen for hissing sounds with the engine at idle.
  • Another way to check for gasket leaks is to spray the gasket area with carburetor cleaner. If you hear a change in idle speed, you've found the leaking area.

Check vacuum hoses for leaks.
Check vacuum hoses for leaks. | Source

XII. Check for Vacuum Leaks

Vacuum leaks are a frequent source of engine performance trouble.

Vacuum hoses, especially, are subject to tears, deformity, rot, and other damage due to the high temperatures, chemicals, and vibration in the environment they operate.

Sometimes, just opening the hood and paying close attention to engine sounds can help you discover a vacuum leak from the hissing sound it produces. Other times the sound is not as obvious.

Still, you can use a trick to help you uncover a potential leak.

  1. First, inspect each vacuum hose visually while tracing the length of the hose with your fingers.
  2. Try to feel for uneven surface areas left by tears, soft, hardened, and rough spots. Pay special attention to those places where hoses bend. Any of these can be an indication of a potential vacuum problem.

Replace the air filter at regular intervals or as necessary.
Replace the air filter at regular intervals or as necessary. | Source

XIII. Check for a Clogged Air Filter

Air filters clog as they trap and accumulate dirt and foreign matter as air flows into the engine.

An air filter will cause performance problems if not replaced at regular intervals.

To check the filter:

  1. Remove the air filter from the air cleaner box. If the filter is dirty, just replace it. Some models use a reusable filter that can be cleaned. Check your car owner's manual or repair manual, if necessary.
  2. Also, clean the air filter housing of debris, dirt and dust. Use a damp rag for this.
  3. After installing or replacing the air filter, close the lid properly, and make sure any air intake tubes and ducts are properly connected to avoid air leaks.

Measure camshaft lobe if you diagnosed mechanical problems.
Measure camshaft lobe if you diagnosed mechanical problems. | Source

XIV. Check for Worn Camshaft Lobes

If your engine has high mileage, it's possible that the camshaft lobes are worn and causing the engine to lope.

You may be able to measure lobe wear using a dial indicator. This is a general camshaft lobe lift check:

  1. Remove the valve cover(s) and move the rocker arms to the side or remove them. Keep track of each rocker arm's place so that you reinstall each rocker arm into its correct place.
  2. Set up a dial indicator so that it sits on the end of the push rod or lifter for the cam lobe you want to measure first.
  3. Turn the camshaft clockwise by slowly rotating the vibration damper on the crankshaft two complete revolutions. Use a ratchet, ratchet extension and the correct socket for this.
  4. Note the high and low dial indicator readings. Subtract the low from the high reading. This is the lobe lift measurement.
  5. Repeat the procedure for each intake and exhaust lobe. Compare your readings to your repair manual specifications.

Inoperative valves can also cause your engine to lope.
Inoperative valves can also cause your engine to lope. | Source

XV. Check for Leaking Valves

Another source of problems is a leaking valve, usually from carbon buildup or another problem that prevents the valve from properly closing.

Usually, you can remove carbon buildup around the valves and cylinders with a decarbonizer like Seafom.

Properly diagnosing valve closing problems can be tricky. Still, you can use a vacuum or compression gauge to diagnose this type of mechanical problem. Consult your vehicle repair manual.

Recap

When your car lopes, remember to start your diagnostic with those systems or components that may have given you some trouble recently, or those that may have neglected lately. The strategy presented here can help you find the source of the problem faster and, often, help you make the repair yourself.

© 2017 Dan Ferrell

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