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Category: Chevy
Satisfied Customers: 26642
Experience:  ASE certified tech ADVANCED LEVEL SPECIALIST. Wisconsin certified emissions state inspector
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Hi. 1999 chev tahoe, 5.7 138k. rough idle, check engine light,codes

Customer Question

Hi. 1999 chev tahoe, 5.7 138k. rough idle, check engine light,codes say #3 misfire, lean bank, both sides. smooths out under load, but lacks power on hills. replaced spider injectors and regulator, newer fuel pump, replace all plugs, #3 was very fouled, new wires, cap, rotor. advised here to replace ecm, done, and flashed to improvement. clean mass air sensor. compression all within 10%, and over 150psi. new front o2 sensors, rear ones 2 years ago. now lean bank code still on. still has a bad vibration rough idle. all code #'s po171, po174, p303, po1336
Submitted: 7 years ago.
Category: Chevy
Expert:  AMEDEE replied 7 years ago.


1. Install a scanner and look in the function test heading to see if it has a crankshaft relearn option.

2. If it has the option, click on it and follow the directions from the scanner.
Potential Causes:
Crankshaft Relearn
Information Only — Was not hitting rev limiter when doing the test.


This procedure has to be done with a scanner. If the scanner does not offer it, then borrow a scanner that does. The engine has to hit the rev limiter for the relearn to work.
AMEDEE and 4 other Chevy Specialists are ready to help you
Customer: replied 7 years ago.
should I just replace sensor. was told they go bad at high mileage. if new one is installed, does it also need to be relearned? di dI mention engine pings even on better gas?. thanks.
Expert:  AMEDEE replied 7 years ago.
No... this is just a relearn procedure. No parts need to be replaced when doing this.

After this is done, then we can re evaluate it and see if we need to go further. But with this code in the computer, I would start with a relearn procedure.
Customer: replied 7 years ago.
I will try re learn, and get back to you if needed.
Expert:  AMEDEE replied 7 years ago.
Sure not a problem!

Let me know if you have any further questions!
Customer: replied 7 years ago.
Hi. I had the dealer re-learn the crank position sensor. also replaced distributor. still shows lean bank on both sides, and misfire on #3. fuel pump 3 yrs. old, mechanic said pressure is good, but leaks down 20lbs in 10 seconds when pump is off, and may be causing lean condition. as for misfire, good compression, new plug, wire, ignition good. suggested could be carbon built up around intake valve on #3. any thoughts.
Expert:  AMEDEE replied 7 years ago.

Well, the fuel leaking down is going to be a big problem. I would start with this first.

Either the fuel pump check valve has gone bad leaking fuel back into the tank or the fuel pressure regulator is leaking fuel into the engine causing your misfire.

Customer: replied 7 years ago.
Hi again. had dealer perform re-learn on crankshaft position sensor, no improvement. fuel pump repaired. codes 171 and 174 keep coming up. dealer mechanic said either maf sensor defective, or intake manifold gasket leak. is one or the other, or both more likely? was also told maf cannot be checked accuratly, was cleaned, should just replace?. runs like extreme vacuum leak. high idle on start, settles down to normal, does this every time accelerator is pushed power, oings. checked brake booster and egr.
Expert:  AMEDEE replied 7 years ago.
If the engine pings on accelerating, I would suspect more of a mass air flow sensor problem then a vacuum leak. But if the engine idles high, rough or unstable, then I would suspect the intake manifold gaskets to be leaking. Just be sure that fuel pressure id good and the exhaust is not restricted.
Customer: replied 7 years ago.
Hi. changed the mass air flow sensor. no improvement. new symptoms. added GM top engine cleaner through as suggested by local dealer mechanic. he thought maybe carbon was blocking intake valve. Now engine idles at 1500 rpm when first started, then comes back to 500 or so. also stays at high idle when letting off gas, have to fight brakes to stop truck, idle settles back to normal after about 15 seconds after stopping. still misfires, code 303, but seems to smooth out when accelerating. Loss of power on acceleration seems more like lean condition, , code 171 and 174 come up, rather than cylinder missfire. All new ignition parts, good compression all around, new injectors, crank relearned, new ecu, o2 sensors, fuel filter, fuel pump, checked egr, pcv, brake booster. considering pulling intake manifold to change gaskets. Did a leak test with carb cleaner, but I know the bottom of gasket could be leaking. no antifreeze leak, and gaskets may have been change many years ago before I owned. any thoughts?
Expert:  AMEDEE replied 7 years ago.
You might have an air or a vacuum leak. This is very common! Open the hood and listen for a hissing sound when the engine is running.
Unmetered air can enter the engine through a vacuum leak, a dirty airflow sensor that is not reading airflow accurately, an EGR valve is not closing and is leaking exhaust into the intake manifold, an EGR valve that is allowing too much flow.
If it is hard to pinpoint take some brake cleaner or starting fluid around the intake manifold and vacuum lines and see if the engine stumbles or if the idle is affected. Be extremely careful when doing this!

Also, your throttle body may be carboned up and need to be cleaned! This can cause all sorts of idle and hesitation problems. This is caused by the throttle plate not seating properly. The First thing i would do is clean out the throttle body with some throttle plate and intake cleaner and a small brush. Another common cause would be the Idle Air Control motor. This is very common on older cars. The IAC motor gets lazy and cant keep up with the fast idle changes. Also when the IAC motor is out, I rec to check the passages for carbon build up. If they are plugged they need to be cleaned out.

Check for the following conditions:

Poor connection at PCM or IAC motor. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection.
Damaged harness. Inspect the wiring harness for damage.
Restricted air intake system. Check for a possible collapsed air intake duct, restricted air filter element, or foreign objects blocking the air intake system.
Throttle body. Check for objects blocking the IAC passage or throttle bore, excessive deposits in the IAC passage and on the IAC pintle, and excessive deposits in the throttle bore and on the throttle plate. Check for a sticking throttle plate. Also inspect the IAC passage for deposits or objects which will not allow the IAC pintle to fully extend.
Vacuum leak. Check for a condition that causes a vacuum leak, such as disconnected or damaged hoses, leaks at EGR valve and EGR pipe to intake manifold, leaks at throttle body, faulty or incorrectly installed PCV valve, leaks at intake manifold brake booster hose disconnected, oil filler cap, oil level indicator loose or missing, etc..

Possible Causes
Check for air leaks in the Air Induction system (e.g., in the intake manifold)
Check for air leaks in the PCV system and in its related hoses and/or valve
Idle air inlet passage or throttle bore is dirty or full of deposits
IAC valve has failed
MAF sensor is dirty, "skewed" or installed improperly
Throttle plate, throttle shaft or linkage is damaged or sticking
TP sensor is out-of-range or "skewed" high

Expert:  AMEDEE replied 7 years ago.
Another idea that I has just come to mind is a vacuum leak right around the intake manifold gasket around the number three cylinder. Because you have a cylinder number three misfire code along with the two lean codes, this leans towards a vacuum leak right around number three cylinder.

Also, just be sure that the fuel pressure regulator is not leaking. Wink

Edited by AMEDEE on 2/15/2010 at 1:14 AM EST
Customer: replied 7 years ago.
Thanks. . not sure how to check idle motor, will look at it tomorrow. still trying to figure out misfire in #3. like I said, all ignition parts are replaced. compression good, new injectors. replacing intake gaskets is not an easy job, but if it sounds like a good possibility of the problem, I may try it. seems like high idle, lack of power, pinging is caused from vacuum leak.. I hear no hissing, and used carb cleaner around gaskets.
Expert:  AMEDEE replied 7 years ago.

Sorry for the delay!


Is the problem more at idle or under a load? I know that you said that the idle is erratic but is it worse under a load or does it get better? If it still gets better under a load, then I would lean towards a vacuum leak.

Edited by AMEDEE on 2/15/2010 at 3:24 AM EST
Customer: replied 7 years ago.
very rough at idle. seems to smooth out as soon as some load is applied. but not alot of power, struggles to go up hill at even at slow speed. seems to run fairly smooth above 1000-1500 rpm. #3 intake leak sounds reasonable at this point. pressure reg. is new.
Expert:  AMEDEE replied 7 years ago.

Ok... I think a vacuum leak is going to be the problem. But first, I know that you said that the fuel pressure did leak down fairly fast. Did we ever correct this problem? Does the fuel pressure still leak down with the key off?


What is the fuel pressure currently at?


Verify that fuel pressure is going to 60+ PSI with engine loaded, vacuum at 0".

Edited by AMEDEE on 2/15/2010 at 1:56 PM EST