Runs rough when engine is warm

Look for a vacuum leak first here is a guide to show you how.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Also the MAF sensor can cause the engine to run rough, I would try cleaning it first and if that doesn't help replace it, here are some guides on that.

https://www.2carpros.com/articles/mass-air-flow-service

and

https://www.2carpros.com/articles/how-to-replace-a-mass-air-flow-sensor-maf

Please let us know what happens.

Best, Ken

[quote:1812cb5954="chevy350guy"]start by checking the MAF[/quote:1812cb5954]

If its the MAF will it set a code?

it may but it may not. it depends how far the sensor is out of range. but because it happens after it warms up i would start there.

I replaced the MAF sensor. The "service engine soon" light went out. I let it warm up to about 200 degrees or so, drove it for about 15 minutes and it was driving a lot better than before. The light came back on though, 'system too lean, bank 1.' It only seems to do this when I give it a lot of throttle. I put some fuel system cleaner in the fuel when this whole thign started last week, you think this might be causing it to be sluggish? Or what about another fuel filter, or even fuel pump? Are there more advanced ways of diagnosing this truck sinece there are no codes if I take it into a shop? I thought I would save money by doign it this way, but I have spent about $400 so far, and its still not fixed. Thanks- Eric.

this should be hooked up to a scanner so you can see live data as it happens to better diagnose the problem. before throwing more money at it.

Do you have any idea how much a shop might charge me for something like that?

I tested the fuel pressure tonight. I get 18-20psi with the key on-off a few times. When running I get between 15-22psi. What next, new fuel pump or regulator? Thanks, Eric.

I have a 99 Silverado 5.3 as well. I recently had it in the shop to have them check a couple of diagnostic trouble codes and was charged $84. (North Dallas)

Have you cleaned the inside of the intake port on your throttle body?

Pressure is definitely low. Should be about 60 psi I think. Low pressure points to fuel pump rather than regulator. Can you hear the pump whine when you turn the key over without starting engine?

[quote:ebcf1e889a="winddilla"]Have you cleaned the inside of the intake port on your throttle body?[/quote:ebcf1e889a]

No, but I will.

[quote:dade444d33="winddilla"]Pressure is definitely low. Should be about 60 psi I think. Low pressure points to fuel pump rather than regulator. Can you hear the pump whine when you turn the key over without starting engine?[/quote:dade444d33]

My book says it should be 55-62 psi for key on-key off. And should drop 3-10 psi with th eengine running. It fluctuates between 15-22 when running. I can hear the fuel pump with key-on for about a second or so. I doesnt seem as powerful as it used to be (could be a mental thing though).

How did it run after you cleaned the throttle body? You should hear the fuel pump whine as long as motor is running. It shouldnt stop.

[quote:12159e9e0f="winddilla"]How did it run after you cleaned the throttle body? You should hear the fuel pump whine as long as motor is running. It shouldnt stop.[/quote:12159e9e0f]

Its sort of weird, I dont hear anything now. I dont even hear the 'whine' from the fuel pump, even with key on-off. I really think its the fuel pump now. Thanks, Eric.

Thats what it sounds like. Let me know how it goes. I havent changed mine but I have a fuel level sensor thats flakey and I think you have to take out the tank to change it. Either way it looks like a major pain.
Done any mod's to it?

Does it even start anymore? If it does then starts sputtering, check the PCV valve before you start the daunting task of the fuel pump. Their cheap and easy to replace. It'll make your truck cough and hack if it has un-seated itself or gets clogged.

[quote:210a5c6655="winddilla"]Does it even start anymore? If it does then starts sputtering, check the PCV valve before you start the daunting task of the fuel pump. Their cheap and easy to replace. It'll make your truck cough and hack if it has un-seated itself or gets clogged.[/quote:210a5c6655]

Already replaced the PCV.

I dont have any mods on the truck. It has about 137k miles, so I'll save my money to put into a new 2500 HD Sierra when I get the money in a few years.

Does anyone know how to tell if my fuel pump is a TCU or TCF without pulling the tank or bed, and what the difference is? I think its something to do with emissions. It was originally sold in CA, butI dont have to smog it anymore. Is it going to matter?

It has to do with the VIN or you can do like I did this past weekend. Take a picture of the top of your fuel pump to see how many fuel lines connect to it and what the electrical connector looks like. Also, Autozone and O'Rielly's sell the same type fuel pumps but (I think it was an AVTEX brand), the correct fuel pump may have a new oval connector instead of the factory square connector. The new pump may have an adapter kit that fixes connector problems. the new style is oval in shape and is easy to swapp. just stagger cut the old wires/connector off and crimp on the new oval style connector. I didn't have a transmissioin jack so I took out the bed bolts and jacked up the bed about a foot, secured it with jack stands, and squeezed in between the bed and tank to replace it. It was a tough way to go about doing it.

Hope this helps.

check engine light came on, scan said mass air flow sensor, replaced still ran real bad with misfire's. I changed plugs still runs bad. It will not idle and runs rough?

Do you still have the maf sensor code? If so, unplug it, see if it runs any better.

No the p102 code is gone but p300 now there because of multp- misfires

i unpluged the old one before and still ran bad, that.s why i replaced it

I see, check for vacuum leaks around the throttle-body and intake manifold. What about your plug wires? Run the vehicle in the dark, raise the hood, do you see arch running anywhere? I'm not sure of engine litre size?

I've been having computer trouble. When you look at one of my posts, what do you see, to the left of,"answered by", I see a box with a red x and the word, "tiny". thanks

First off it's 5.3, When i was coming back from up north from hunting it ran fine over 250mi. no issue's. then it started to run bad after stopping at stop light. you know what else i already did, the throttle body was replaced last year. so now i.m leaning towards a vacume leak or pcm? thanks for your help on this issue. (:

So here i am in cold garage, sprayed carb cleaner all around intake no change at all. re-scan again get code for map sensor so replace it same thing! po106, while it's sorta running pul coil connectors and #1-2-3-4-5 are dead no change in running so swap coil pack from a good hole and still dead! so are we looking at pcm or other area?

Did you check for primary voltage at the ignition coil? Pink wire goes hot with the key on.

http://postimage.org/image/522beg0q5/

checked the voltage on coils and there hot w/key on

Use an led testlite between voltage and control wire, crank engine, is there a pulse? Check the grounds, also.

Could be a computer issue,but, if a coil or something caused computer to go bad, you have to fix it before installing a new computer.
Click on the link for wiring info.

http://postimage.org/image/y568b7ne3/

Use an led testlite between voltage and control, crank, is there a pulse? Probably not the best way to test. Do you have a digital multimeter with a min/max setting?

Could be computer issue,but, if something caused the computer to go bad, you have to fix that first, or the new computer may be effected.

JDL- With christmas upon us here the truck is going to have wait until wend. then i'll have a multimeter to check it out, ? could the ground be out on them coil pack's and could i use a jumper to ground one to see if would fire that cyl ? Thanks for all your help today as my head is pounding so off for holiday cheer! Merry-Christmas

just out of curiosity, unplug the o2 sensor on the drivers side and see if it changes how the motor runs.

Roy

I tried to post twice after your last post. My messages wouldn't post. I don't know why?

crankshaft position sensor

So got better scanner now #1 & #3 have mise fire, shows lean or - fuel trim on #1 and #2 bank disconnect bank 1 sensor 2 with no change at all, check spark on # which is good and also check for pulse on injectors which are good, seems like it's dumping fuel into them two clyinders on idle but when i pick up rpm's still runs bad but not as badwith lot of raw gas coming out. I'M VERY CLOSE CLOSE TO TAKING IT TO CHEVY GARAGE TO FIND OUT WHAT THE HELL IS GOING ON!!!!

Sounds like a problem with pcm.

So JDL is there any way to test a pcm? any thing short of taken to a dealer, some one suggested the crank sensor but i'm thinking it would not run at all because if it does not pick up #1 cyl it would not cycle the firing for the other cyl? I sure don't wont to keep trowing parts at it to see if thats the problem thanks again real stumper!

Did you check compression on the cylinders with problem?

So today i spent some time with it, did compression test and have 180psi in #1 and #3 replaced the fuel injectors on bank 1 no change. put a vacume gauge to pvc line has vacume but gauge is bounching around. check with scanner and tp sensor shows nothing or should i say no change with throttle, this was replaced 2yrs. ago,i don't know if there was an issue with the intake or the gaskets on the 5.3 was thinking egr valve don't think it has one or i could not find it as check for it. there was a bullitin for misfire cause by bad ground but where it say's the ground is not there ie right front of head.