How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask Mike S. Your Own Question
Mike S.
Mike S., Chevy Mechanic
Category: Chevy
Satisfied Customers: 7499
Experience:  ASE Certified Master Technician
Type Your Chevy Question Here...
Mike S. is online now
A new question is answered every 9 seconds

Chevy: rough idling and the engine would cut..stop light..gas tank

Resolved Question:

Hi, I am having troubles with my Chevy silverado 350. I had been experiencing rough idling and the engine would cut out if I was at a stop light for awhile and then once I pushed on the gas the engine shut off. The other day I put seafoam in the gas tank and changed spark plugs and spark plug cords. After I did this I drove around the block and once I made it back to my house I experienced a very rough ide and loud engine sound. I noticed that there is no oil pressure and now when the truck is started it dies out or puts at a consistent level. When I put the truck in gear to drive there is no movement. I would appreciate any advice. Thanks!
Submitted: 8 years ago.
Category: Chevy
Expert:  Mike S. replied 8 years ago.

No oil pressure? Well, that comes first. Is the oil level ok? Any leaks? Is the oil filter tight?

Did all these problems start all at once? Did any of them come first, if so can you list the number they came in and how long between problems untill the next problem occured. have you checked the ignition timing? Checked the air filter? Checked for any engine codes? Is the check engine light on now or have you ever seen it on? Is it an automatic or manual tranny? Is the loud noise a knock, bang, tap, clunk, etc?

Expert:  Mike S. replied 8 years ago.
Why did you use sealfoam? Did you suspect carbon buildup in the combustion chamber or fuel system? Was there smoke coming out tailpipe and if so how much and what color?
Expert:  Mike S. replied 8 years ago.
What is the mileage? Have you checked fuel pressure? When was the last time a new fuel filter was installed?
Customer: replied 8 years ago.
The oil level is ok and rhe filter is tight. The problems all came within two to three weeks. I drive the vehicle a few times a week. I first noticed the rough idle at stop lights and engine would die when I'd hit the gas. I also noticed a small coolant leak. I applied a no leak soluyion to correct the antifreez leak and that has been good. Although a few weeks after I applied the solution I noticed the truck running rougher. I have not checked the timing or engine codes b/c I don't know how??? The filter is a K&N system and seems to be clean. The service engine light will come on and go away. Sometimes it remains on. The truck is a automatic. The loud noise is more of a screech with a rough bang. I added the seafoam b/c I though it would help with the problem of the truck dying at stop lights. The fuel filter is pretty old. I have purchased a new one and want to install but the fuel filter on now is rusted and really difficult to get loose (suggestions
Expert:  Mike S. replied 8 years ago.

To check for codes,

The OBD Diagnostic Link Connector is located:

Front seating area, driver side, behind instrument panel, mounted on driver side of steering column .



If a fault occurs intermittently, such as a loose connector pin breaking contact as the vehicle hits a bump, the ECM will note the fault as it occurs and energize the dash warning lamp. If the problem self-corrects, as with the terminal pin again making contact, the dash lamp will extinguish after 10 seconds but a code will remain stored in the computer control module's memory.

When an unexpected code appears during diagnostics, it may have been set during an intermittent failure that self-corrected; the codes are still useful in diagnosis and should not be discounted.


Since the control module is programmed to recognize the presence and value of electrical inputs, it will also note the lack of a signal or a radical change in values. It will, for example, react to the loss of signal from the vehicle speed sensor or note that engine coolant temperature has risen beyond acceptable (programmed) limits. Once a fault is recognized, a numeric code is assigned and held in memory. The dashboard warning lamp: CHECK ENGINE or SERVICE ENGINE SOON (SES), will illuminate to advise the operator that the system has detected a fault. This lamp is also known as the Malfunction Indicator Lamp (MIL).

More than one code may be stored. Keep in mind not every engine uses every code. Additionally, the same code may carry different meanings relative to each engine or engine family.

In the event of an computer control module failure, the system will default to a pre-programmed set of values. These are compromise values which allow the engine to operate, although possibly at reduced efficiency. This is variously known as the default, limp-in or back-up mode. Driveability is almost always affected when the ECM enters this mode.


Reading Codes

1988-95 MODELS

See Figures 1 and 2

Listings of the trouble for the various engine control system covered here are located in this section. Remember that a code only points to the faulty circuit NOT necessarily to a faulty component. Loose, damaged or corroded connections may contribute to a fault code on a circuit when the sensor or component is operating properly. Be sure that the components are faulty before replacing them, especially the expensive ones.



Fig. Fig. 1: ALDL connector-1988-92 models

The Assembly Line Diagnostic Link (ALDL) connector or Data Link Connector (DLC) may be located under the dash and sometimes covered with a plastic cover labeled DIAGNOSTIC CONNECTOR.

  1. The diagnostic trouble codes can be read by grounding test terminal B. The terminal is most easily grounded by connecting it to terminal A (internal ECM ground). This is the terminal to the right of terminal B on the top row of the ALDL connector.
  2. Once the terminals have been connected, the ignition switch must be moved to the ON position with the engine not running.
  3. The Service Engine Soon or Check Engine light should be flashing. If it isn't, turn the ignition OFF and remove the jumper wire. Turn the ignition ON and confirm that light is now on. If it is not, replace the bulb and try again. If the bulb still will not light, or if it does not flash with the test terminal grounded, the system should be diagnosed by an experienced driveability technician. If the light is OK, proceed as follows.
  4. The code(s) stored in memory may be read through counting the flashes of the dashboard warning lamp. The dash warning lamp should begin to flash Code 12. The code will display as one flash, a pause and two flashes. Code 12 is not a fault code. It is used as a system acknowledgment or handshake code; its presence indicates that the ECM can communicate as requested. Code 12 is used to begin every diagnostic sequence. Some vehicles also use Code 12 after all diagnostic codes have been sent.

Expert:  Mike S. replied 8 years ago.


Fig. Fig. 2: ALDL connector-1993-95 models

  1. After Code 12 has been transmitted 3 times, the fault codes, if any, will each be transmitted 3 times. The codes are stored and transmitted in numeric order from lowest to highest.

The order of codes in the memory does not indicate the order of occurrence.

  1. If there are no codes stored, but a driveability or emissions problem is evident, the system should be diagnosed by an experienced driveability technician.
  2. If one or more codes are stored, record them. Refer to the applicable Diagnostic Code chart in this section.
  3. Switch the ignition OFF when finished with code retrieval or scan tool readings.

After making repairs, clear the trouble codes and operate the vehicle to see if it will reset, indicating further problems.


Trouble Codes


Fig. Fig. 1: Carbureted engine trouble codes

If that is fuel injected, those codes are here, too.

Expert:  Mike S. replied 8 years ago.

You said, When I put the truck in gear to drive there is no movement.

Question: Did the engine rev up at all? Would it rev up in neutral?


As far as the rough idling and cutting out, I would check for vacuum leaks, check the air filter, if the distributor is equipped with a vacuum advance check that and check the ignition timing.

You may also want to do a cylinder balance test to make sure all cylinders are firing.


First of all you will need a 12-volt test light and about 8-12 inches of neoprene vacuum hose. The vacuum hose will conduct electricity cause it is carbon based, use an ohmmeter if you aren't sure and see if the vacuum hose you have will conduct.

Now, cut off the same # XXXXX cylinders you have in small equal lengths of the vacuum hose. 1-2 inches will do fine. The small diameter kind like to carburetors, etc will work as long as they fit over the distributor cap tower connection.

Now, mark all your spark plug wires at the cap and remove them all. Put those short pieces of vacuum hose on the distributor cap connections and shove the other end of the vacuum hose into the spark plus wire boot until it makes a good connection.

Now connect your 12 volt test ground clip to a ground and start the vehicle. With the engine running touch the 12 volt test light to each of the vacuum hoses 1 at a time and listen for the cylinder to short out and die and drop in r.p.m. They should all be about equal. If 1 or a few don't drop or do anything than you have your dead cylinder there.

Expert:  Mike S. replied 8 years ago.

Vacuum leaks can be checked with a can of starting fluid or either. Spray small amounts around the carb or throttle body base gasket and listen for rpm increase indicating that the starting fluid got sucked into a bad gasket and got burned in the combustion chamber. Also, take both hands and grab the whole carb or throttle body and try to move it forward and backward, right and left and listen for rpm change indicating the bolts may be loose.

Also spray starting fluid around the intake manifold gasket and all vacuum hose connections while wiggling them and listen for rpm increase.


If the distributor has a vacuum advance you can either reach down there and move the advance arm by hand and listen for rpm to change as you move timing, or pull hose off and on while reving engine a little or if the diaphram is actually busted you can blow smoke into the hose disconnected from the vacuum advance at the carb end and smoke with come out of and all around distributor cap.


You may want to take off the distributor cap and inspect that for worn terminals, cracks and inspect the rotor for wear or a burnt button,etc.


If your check engine code shows any codes then you will have to inspect that circuit or part.


Also, if equipped with an EGR, reach down while the engine is idling and push in on the diaphram and plunger on the inside of that EGR, the engine should die or almost die. If not and there is vacuum to that EGR either it is bad or the gasket is bad. Rev engine a little and pull off and push on EGR vacuum hose and watch to see if the plunger moves.

Expert:  Mike S. replied 8 years ago.

Another cause of engine running rough and cutting out may be too lean or too rich of a fuel mixture.

One good way to check for a too lean fuel mixture is to take a shop rag and completely cover the air horn of the carb or throttle body completely cutting off all air going into it. The engine should die. If not it is getting air from somewhere else such as a vacuum leak.

To check for too rich a mixture look down the air horn with a mirror while engine is idling and look to see if any gas is dripping on the throttle valves. Or in the case of fuel injectors they are dripping.

You may also want to go around to the rear while engine is idling and smell the exhaust. too rich and it will smell like gas and maybe burn black smoke. Too lean and it may burn you eyes and have a strong smell. Also, feel the exhaust with your hand, it should be hot, not warm and should have constant puffs not one small or big puff every once in a while.

Expert:  Mike S. replied 8 years ago.
As far as checking those engine codes, the auto-parts stores sell a small tool that fits on a keychain to connect to that "A" and "B", terminal. They cost probably about $0.99 Or you can just use a bent up paper clip. Connect A and B turn ket to run and it will flash once, pause the flash twice, that is 12, which is normal. It will flash that 3 times, then it will start flashing any codes and will do each code 3 times, then return to 12 when done.
Expert:  Mike S. replied 8 years ago.

Is the fuel filter the inline kind under the frame drivers side looks about the same size of a soda can?

You will need line wrenches for that.


As far as the screeching and loud bang goes, you are going to have to determine where that is coming from.

A good way to determine if it is the engine or not is once you get that vehicle running is get it up to speed to where the noise occurs and shift into neutral and turn key back 1 click to run as to not lock steering and shut off engine for a few seconds. If an automatic just make sure the engine did infact die and then just turn key to start while in neutral, if a clutch then shift into a medium gear and pop clutch with key in run. If the noise goes away with engine killed then it was most likely the engine, engine accessories or exhaust or front half of tranny making noise. If not, the second half of tranny, the rear, wheels, brakes, etc.

Mike S. and 6 other Chevy Specialists are ready to help you
Expert:  Mike S. replied 8 years ago.
Screeching sounds like it may be a belt, though, check them. A bang, maybe the idle is too high.
Expert:  Mike S. replied 8 years ago.

Component locations.



Air Injector Reactor (AIR) System


The A.I.R. system is not completely silent under normal conditions. Noises will rise in pitch as engine speed increases. If the noise is excessive, eliminate the air pump itself by disconnecting the drive belt. If the noise disappears, the air pump is not at fault.

Check Valve

To test the check valve, disconnect the hose at the diverter valve. Place your hand over the check valve and check for exhaust pulses. If exhaust pulses are present, the check valve must be replaced.

Diverter Valve

Pull off the vacuum line to the top of the valve with the engine running. There should be vacuum in the line, if not replace the line. No air should be escaping with the engine running at a steady idle. Open and quickly close the throttle. A blast of air should come out of the valve muffler for at least one second.

Air Pump

Disconnect the hose from the diverter valve. Start the engine and accelerate it to about 1500 rpm. The air flow should increase as the engine is accelerated. If no air flow is noted or it remains constant, check the following:

  1. Drive belt tension.
  2. Listen for a leaking pressure relief valve. If it is defective, replace the whole relief/diverter valve.
  3. Foreign matter in pump filter openings. If the pump is defective or excessively noisy, it must be replaced.

Exhaust Gas Recirculation (EGR)


See Figures 3, 4, 5, 6, 7 and 8

Refer to the appropriate chart for diagnosis the EGR system. On linear EGR systems, an OBD-II compliant scan tool will be needed.

Click image to see an enlarged view

Fig. Fig. 3: Ported EGR system wiring diagram

Click image to see an enlarged view

Fig. Fig. 4: Ported EGR system check

Click image to see an enlarged view

Fig. Fig. 5: Negative backpressure EGR system test

Click image to see an enlarged view

Fig. Fig. 6: Negative EGR system wiring diagram

Click image to see an enlarged view

Fig. Fig. 7: Linear EGR system test

Click image to see an enlarged view

Fig. Fig. 8: Linear EGR system test (continued


Manifold Absolute Pressure (MAP) Sensor


See Figure 4

  1. Backprobe with a high impedance voltmeter at MAP sensor terminals A and C.
  2. With the key ON and engine off, the voltmeter reading should be approximately 5.0 volts.
  3. If the voltage is not as specified, either the wiring to the MAP sensor or the ECM may be faulty. Correct any wiring or ECM faults before continuing test.
  4. Backprobe with a high impotence voltmeter at MAP sensor terminals B and A.
  5. Verify that the sensor voltage is approximately 0.5 volts with the engine not running.
  6. Start the vehicle.
  7. Verify that the sensor voltage is greater than 1.5 volts at idle.
  8. Verify that the sensor voltage increases to approximately 4.5. volts at Wide Open Throttle (WOT).
  9. If the sensor voltage is as specified, the sensor is functioning properly.
  10. If the sensor voltage is not as specified, check the sensor and the sensor vacuum source for a leak or a restriction. If no leaks or restrictions are found, the sensor may be defective and should be replaced.

Click image to see an enlarged view

Fig. Fig. 4: Manifold Absolute Pressure (MAP) Sensor wiring diagram


Oxygen Sensor


See Figures 1 and 2

Click image to see an enlarged view

Fig. Fig. 1: Inspect the oxygen sensor tip for abnormal deposits

  1. Perform a visual inspection on the sensor as follows:


Remove the sensor from the exhaust. If the sensor tip has a black/sooty deposit, this may indicate a rich fuel mixture. If the sensor tip has a white gritty deposit, this may indicate an internal anti-freeze leak. If the sensor tip has a brown deposit, this could indicate oil consumption.

All these contaminates can destroy the sensor, if the problem is not repaired the new sensor will also be damaged.

  1. Reinstall the sensor.
  2. Start the engine and bring it to normal operating temperature, then run the engine above 1200 rpm for two minutes.
  3. Backprobe with a high impedance averaging voltmeter (set to the DC voltage scale) between the oxygen sensor (02S) and battery ground.
  4. Verify that the 02S voltage fluctuates rapidly between 0.40-0.60 volts.
  5. If the 02S voltage is stabilized at the middle of the specified range (approximately 0.45-0.55 volts) or if the 02S voltage fluctuates very slowly between the specified range (02S signal crosses 0.5 volts less than 5 times in ten seconds), the 02S may be faulty.
  6. If the 02S voltage stabilizes at either end of the specified range, the ECM is probably not able to compensate for a mechanical problem such as a vacuum leak or a high float level. These types of mechanical problems will cause the 02S to sense a constant lean or constant rich mixture. The mechanical problem will first have to be repaired and then the 02S test repeated.
  7. Pull a vacuum hose located after the throttle plate. Voltage should drop to approximately 0.12 volts (while still fluctuating rapidly). This tests the ability of the 02S to detect a lean mixture condition. Reattach the vacuum hose.
  8. Richen the mixture using a propane enrichment tool. Voltage should rise to approximately 0.90 volts (while still fluctuating rapidly). This tests the ability of the 02S to detect a rich mixture condition.
  9. If the 02S voltage is above or below the specified range, the 02S and/or the O2S wiring may be faulty. Check the wiring for any breaks, repair as necessary and repeat the test.

Click image to see an enlarged view

Fig. Fig. 2: Oxygen sensor (02S) wiring diagram


Throttle Position Sensor (TPS)


See Figure 1

  1. Backprobe with a high impedance voltmeter at TPS terminals A and B.
  2. With the key ON and engine off, the voltmeter reading should be approximately 5.0 volts.
  3. If the voltage is not as specified, either the wiring to the TPS or the ECM may be faulty. Correct any wiring or ECM faults before continuing test.
  4. Backprobe with a high impedance voltmeter at terminals C and B.
  5. With the key ON and engine off and the throttle closed, the TPS voltage should be approximately 0.5-1.2 volts.
  6. Verify that the TPS voltage increases or decreases smoothly as the throttle is opened or closed. Make sure to open and close the throttle very slowly in order to detect any abnormalities in the TPS voltage reading.
  7. If the sensor voltage is not as specified, replace the sensor.

Click image to see an enlarged view

Fig. Fig. 1: Throttle Position Sensor (TPS) sensor wiring diagram