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Random cylinder misfire due to lean air/fuel ratio

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Check for vacuum leaks, unmetered air entering the combustion chamber. Check for bad hose between MAF sensor, and the throttle body. Possible a leaking intake manifold.

Posted on Jun 04, 2009

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Posted on Jan 02, 2017

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System to lean bank, manufacturer control fuel air metering, cylinder misfire detected


Intake or vacum leak will be the first codes 174,171, try idle and spray propane around intake if car idles up, then intake is leaking, otherwise could be a broken vacum line, check with a gauge

Jan 18, 2014 | 2004 Hyundai Tiburon GT V6

1 Answer

2007 kia rio losing power when going up hills


Random misfires could be gas related- low fuel pressure, vacuum leaks or air leaks upsetting the air/fuel mixture ratio, clogged air filter or fuel filter. Could be ignition related- worn spark plugs, possibly. Or could be an internal mechanical problem-possible valve train malfunction.
Start with a fuel pressure test. check vacuum hoses for leaks, check intake manifold and throttle body assembly for leaks. If you have an exhaust leak at or near the engine, can cause misfires. Air going into the engine is metered by the mass air flow sensor in the big air boot on the throttle body. If extra air gets into the engine through a vacuum leak or a loose hose, the engine computer won't know it, so the air/fuel mixture will be too lean, resulting in misfiring. Several things to check-good luck.



Dec 28, 2013 | 2007 Kia Rio 1.6

2 Answers

98 Pontiac GRAND am


The plugs and crank sensor could have done it, but the TPS and idle air control would have nothing to do with a random miss.
A more likely cause is the ignition module or a fuel problem. You may have to wait until the problem is bad enough to kill the engine or you have more codes to check out.

May 20, 2013 | Cars & Trucks

1 Answer

I have a 95 camaro with a 3.8l. It had a #1 cylinder missfire code, I found out it had a bad fuel injuector so i replaced it. Now, it's showing a random misfire code. What could cause this change


An intake vacuum leak can cause this as well as other issues, read about it below.
Check Engine Light On, P0300 Random Misfire Code A random misfire means your engine is misfiring, but that the problem is not isolated to one or two cylinders. It is jumping around in a random way from one cylinder to another. A random misfire code usually means the air/fuel mixture is running lean. But the cause might be anything from a hard-to-find vacuum leak to dirty fuel injectors, low fuel pressure, a weak ignition coil, bad plug wires, or compression problems. Even a dirty MAF sensor can cause a lean code and/or misfire to occur. The engine may be stalling because it isn't getting enough throttle opening. The cause is often a problem in the idle air control system. The first thing to check is the intake vacuum with a vacuum gauge. On most vehicles a normal reading is 17 to 21 inches Hg. If the needle is lower, is jumping up and down or steadily dropping, you have a vacuum problem. Look for possible vacuum leaks by checking vacuum hose connections, the throttle body and manifold, and PVC valve and plumbing. An EGR valve that is leaking can also act like a vacuum leak and cause a random misfire. The next thing you should check is the fuel pressure with a gauge. If it is not within specifications (refer to a service manual for specifics because fuel pressure is critical for proper engine performance), the problem may be a weak fuel pump, low voltage to the pump (check the relay and wiring), or obstructions in the fuel line (like a plugged filter). A bad fuel pressure relay can also leak pressure and prevent an otherwise good fuel pump from delivering full pressure to the injectors. Dirty injectors can also restrict fuel delivery and cause a lean fuel condition. Many regular grades of gasoline do not contain adequate levels of detergent to keep the injectors clean. Frequent short trip driving accelerates the buildup of injector deposits. Cleaning the injectors with a good quality fuel tank additive (or having them professionally cleaned) can solve this problem. Look at short term fuel trim (STFT) and long term fuel trim (LTFT) with your AutoTap Express DIY. If the numbers are high, it tells you the engine is running lean

Jul 27, 2017 | 1995 Chevrolet Camaro

1 Answer

My 99 grand prix bucks and jumps in reverse and when idling in drive or park


The codes pretty much explain the bucking and jumping. The engine is obviously misfiring on several cylinders.

With the P0171 code, the lean condition (and therefore, the misfiring) could very possibly be caused by a bad Mass Airflow (MAF) sensor, a faulty fuel pump, a clogged fuel filter, or a HUGE vacuum leak.

However, the P0171 code can also be caused by excess unburned air/fuel mixture going down the exhaust due to cylinders misfiring for other reasons like a faulty ignition coil, ignition module or bad spark plugs (as well as other possibilities).

So, now you are faced with trying to determine if the lean condition is caused by the misfires or if the misfires are being caused by a lean condition. (PROPER DIAGNOSIS REQUIRED)

Mar 24, 2012 | Cars & Trucks

1 Answer

My check engine light keeps coming on and everytime we check the computer it keeps saying the same thing: random misfire. is there a reason why this keeps happening?


Possibly there is a vacuum leak into the intake manifold downstream of the throttle body. This is unmetered or unsensed air that the computer doesn't know about. This air is leaning out the fuel mixture to several cylinders but at random due to the swirling nature of the air inside the manifold. The computer can't compensate for it. The EGR valve sometimes has a leaky diaphragm and this can be a possible source of the leak.

A fuel pump that is growing weak will not send the proper pressure to the injectors and they will at random not squirt enough fuel when called for. Again this will lead to lean fuel mixtures and cause missing. As a possibility, check the fuel pump pressure output. A faulty fuel pressure regulator that is stuck on the idle pressure setting (lower than cruise setting) will also cause this.

Feb 11, 2011 | 2003 Toyota Corolla

2 Answers

P0300 engine code


Random cylinder misfire, you will need an OBDII Scanner with MODE 6 to see what the miss count is to help find the offending cylinder.

You may get lucky and get a P0301 though P0304 code to tell you which cyling but then you may not...

Then after you find the offending cylinder you will have to figure out why its running bad... Plug, coil, injector, compression or vavle problem.

May 15, 2010 | 1998 Toyota Corolla

3 Answers

2003 Ford Ranger 4x4 extended cab- diagnostics checked and got these codes- P0171, P0174, P0300, P0316. Can you tell me what they mean?


P0171------ system too lean bank#1
Po174------ system too lean bank#2
For these 2 codes, try cleaning the maf sensor (mass air flow), that is mounted in the big black hose that goes from the air filter housing to the throttle plate, if it is dirty, that will cause those 02 sensors to send a signal to the computor to lean the air fuel mixture. If cleaning it doesn't work, the sensors will probably need changing.
P0300---- Random/multiple cylinder misfire
P0316---- Cylinder misfire at start up---- first 100 revolutions.
On those 2 codes, you have to do some testing to find out why there is a misfire.
Possible causes:
dirty/malfunctioning injectors
weak coil pack
fowling spark plugs/wires
Low fuel pressure

May 11, 2010 | 2003 Ford Ranger Regular Cab

2 Answers

98 camry engine light on w/misfires & system too lean


take sample of your fuel into clear plastic bottle from fuel line
is there any water present?
this can cause these codes and then not go on storing codes for other bank even though that bank will be missfiring also due to water & P0300 Random/Multiple Cylinder Misfire Detected being logged
check all connections to coils/leads & wiring connectors
due to miss fire codes for cyl 1,3,5 all on the same bank
also look and check for air leaks on that bank (hose knocked off or splithose)inlet manifold gasket,brake booster etc...
3 injectors being blocked slightly on same bank posible!!!
check wires and signal

was it running like this b4 tune up & repairs?
this is what codes are
P0170 Fuel Trim Malfunction (Bank 1)
P0300 Random/Multiple Cylinder Misfire Detected
P0301 Cylinder 1 Misfire Detected
P0303 Cylinder 3 Misfire Detected
P0305 Cylinder 5 Misfire Detected

the last code is for trans due to misfire/running lean on bank 1 probably a torque problem code
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May 29, 2009 | 1998 Toyota Camry

1 Answer

2000 CIVIC System too lean Cylinder 3 and 4 misfire Deteced


  1. P0171: This means that the oxygen sensor has detected a lean air-fuel ratio. This can be caused due to a vaccuum leak or a problem in the HO2S circuit or the HO2S (Heated Oxygen Sensor) itself. (Bank one Sensor one). This is commonly located along the exhaust pipe. It could also be a bad Mass Air Flow sensor or a bad Engine Coolant Temperature sensor. Diagnostic checks will be needed to be carried out to be verified though
  2. Well as for the other codes, it means #3&4 cylinders are misfiring. It is a possibility that the P0171 code mite have caused the others to show up. I can be wrong though. But it is a strong possibility. If the air fuel mixtue is too lean indeed, mis-firing will occur, unless it is an actual electronic and not a mechnical problem. By the way, when these codes appear, how is the engine's performance. Does it remain the same or behaves poorly?

Feb 14, 2009 | 1999 Honda Civic

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