99 Tahoe w/ P0171, P0174, and P0305

[Chevychick]

I know pre cat sensors on both sides were replaced, and 1 post cat, I'm trying to remember which one. Also, the Mass Air Flow was replaced this year

 

[OR VietVet]

Then you may have a failed/failing MAF and maybe poor quality MAF or there is a vacuum leak someplace. Try the water bottle squirt and stethoscope search method.

 

[Chevychick]

Then you may have a failed/failing MAF and maybe poor quality MAF or there is a vacuum leak someplace. Try the water bottle squirt and stethoscope search method.

The pre cat sensors were replaced approximately 3 years ago, they should still be good right? I will try the water bottle and stethoscope thing next time my Dad has a chance and I'll let you know! Thanks!

 

[OR VietVet]

Can you supply pics of the plugs, especially #5.

 

[Chevychick]

I will the next time I get a chance, I also had a code drop one time for #4, so I'll pull that one too

 

[OR VietVet]

Best to pull all and pics of them. Keep track of cylinder they come out of. Close up pics of the tips.

 

[Chevychick]

Ok, will do!

 

[OR VietVet]

One other thing, did you inspect for ANY exhaust leaks in front of the cats? Check where the pipes tighten at the manifolds. False air can get in there and cause a lean condition as well. Likely is ok but should be checked. If under there, have someone use shop towels and clog up the tailpipe to force the exhaust leak to show easier. Can only clog the tailpipe 2-4 seconds at a time.

 

[drakon543]

as everyone has been suggesting the intake manifold is a likely cause of alot of your issues. the ignition coil was replaced are you aware of the symptom or was this just an attempt at fixing your current issue. your truck has 3 components up top that handle your ignition and timing. cam sensor which is under the rotor button in the distributor. generally if those start going bad your symptoms and cel's would be worse. the ignition coil which is a potential as they like to overheat and swell especially with age but yours was replaced assuming delco? the icm which is right next to the coil is a tricky bugger sometimes but again that usually causes a complete not running symptom. now back to the distributor, cap, rotor. the distributors cam gear can get worn and will throw the timing off. if the shop doesn't think to check this its also a potential. twisting the rotor by hand it should only have minor movement before the angle of the cam gear will push the rotor upwards. excessive movement without that upward push is a sign your dizzy gear is worn. next up caps and rotors are hella fickle on these engines. could be running 100% one day and the very next day it wont start or will run like crap. if the distributor was ever replaced and they didn't use a delco dizzy its a crap shoot to find a cap that will actually work and last. easy visual is take a look inside the cap for a greenish splatter thats a sign the cap isnt venting correctly and trapping moisture in there. also ive seen the cap to rotor contact button get eaten away pretty quickly by that. now back to your intake manifold has anyone trying a can of aerosol carb cleaner around all the edges of your intake manifold at an idle? if you have a leak causing your misfires you should find a specific spot the idle changes suddenly when you spray that spot.

 

[drakon543]

sorry i didn't get to see the original @ mention work has been super busy then i been dealing with covid.