Possible vac leak / lean issue?

Hello I have a 2005 Chevy Colorado i5 and I'm having an initial start up and occasional "misfire" issue on acceleration. This almost feels like I hit the brake quick while driving.

My initial start up is basically a surge. Its a manual transmission so if I blip the gas to get going it will cut out, rev up a bit, drop down to almost stalling and rise back to an idle. It has never stalled on me before though. After about 30 second give or take it will rev fine. No cut out.

If I start driving and let off the gas immediately after taking off the whole truck will jerk as if I slammed the brake and then keep going. Real weird.

Unfortunately I was no able to record any data upon startup but I have some at idle. Holding the rpm at 2000 and a short driving clip. There are a few high + and -.

I hope what I have for now is good enough to get some sort of clue.

These were recorded during driving. I scrolled quick through the frames and these caught my eye because of the STFT numbers. The low negative was coasting to a stop in gear I believe. Not hitting the gas.

Unfortunately it's taking way too long to insert the video files. The last two I posted above didn't work so I will try again tomorrow

Check out this video for finding vacuum leaks





Also clean your throttle body real good use a can of carb cleaner and have someone turn the key on and hold the throttle pedal down while you spray a rag and in the throttle body and clean the area that the throttle blade closes at real good

Sent from my SM-N920P using Tapatalk

Sent from my SM-N920P using Tapatalk

You have given lots of data here, and, unfortunately, it is hard for me to assimilate it.

But, since you indicated something about braking and drivability, I suggest using pliers to carefully pinch off hose going from intake to brake booster. If idle smoothes out, you have a vacuum leak in the booster.

For some reason i am not having any luck putting an attachment video in here so i am trying to figure out how to share the video on here.

I was able to get a good video on the surge.

still trying to get this to post.

Made a rookie move and didn't read the post about how to add videos. I ended up getting vimeo just to share here.

So finally lol here is the video.

vimeo.com/209146447


I caught a pretty good one. It is best to listen with headphones because i had the door shut. The first rev i did was usually what happens. The isecond one was decent and the third one it lingered for a few seconds before returning to idle.

I got a few more videos of an idle reading the trims and o2's and some playback readings of a short drive.

vimeo.com/209295530

vimeo.com/209296202

Nice uploads

Could you also track the IAC? What I'm looking for is the PCM requesting way open or way closed.

I don't know off hand if my scanner reads the IAC. I will check tomorrow.
Unfortunately the throttle body on this truck acts as the IAC. It's built in non serviceable. I actually immediately thought of the IAC right after I got the truck and it had this problem.

From the info I've given what did it seem like it's doing? Is it running lean, rich?

I should add that I've cleaned the throttle body thoroughly weeks ago. New fuel filter as well. My o2 sensor 1 is new. It does have an oiled k&n air filter from the previous owner. New plugs also. And I sea foamed and have run fuel system treatment a few times after I got the truck a few months back.

I think I might pull my maf tomorrow and just look at if it's dirty. I'm not sure what the reading needs to be but it's worth a check. Easy to get to.

Good stuff...

On some fly-by-wire throttles, I've had to force a relearn after cleaning the them. I was lucky in that I was able to use bi-directional control to do this. Many times they have a manually triggered procedure for doing it as well; you might need to look that up.

Do you think the rooming trim values depend on whether the engine is warm and cold? Are the trim values different when the engine is warm vs cold? (if so, it could be metal changing shape causing or closing up a vacuum leak)

I also suggest you try the brake-torqueing technique to see if you can get the misfire to occur while you aren't moving. This will allow you to determine if load is causing your misfire. If you can get it to misfire on cue, I would next test your fuel system if it has a Schrader valve on the fuel rail; otherwise I would test your ignition system. ScannerDanner has tons of videos demonstrating these two tests using a variety of tools and techniques, from scopes to test lights to scan tools to pressure gauges.

Does your scan tool allow you to see misfire information? Does your scan tool allow you to get at Mode 6 information? If so, look up your Mode 6 information to see if it can give you a hint on which cylinder(s) are misfiring. What readiness monitors have not completed successfully?

Since it is free to clean your MAF, I would carefully inspect it to see if it needs it. I suggest taking your time doing it so that you can ensure the re-installation doesn't add an additional vacuum leak; sorry - I just had to say this disclaimer. ScannerDanner has tons of videos showing how to inspect and clean them. I personally don't use a pocket screwdriver technique, but I'm not ScannerDanner; LOL

Unfortunately my scanner is as simple as it gets. I believe it can do EVAP and o2 tests but it doesn't have any misfire stuff or anything. It's basically a good code reader and displays basic diagnostics.

I took a recording of a cold drive to work and it looks to me like I'm still getting the same readings. My short term goes pretty rich up to 18.0 and I noticed a few times warm or cold that my o2 sensor 1 lingers rich for a few seconds. .800 to .700 as an example.

LTFT 11.7
STFT -1.6

Is an example of one of the things they caught my eye because the LTFT was higher.



Basically what I am feeling when driving is if I accelerate to merge or something under a load it feels like it's struggling a little. Not all the power is there.

Well, three things come to mind based on your response.

1. Given the data you have observed, it appears the vacuum leak is not dependent on engine temperature, so most likely metal temperature is not affecting vacuum leak. In some ways, that makes it easier to find.

2. Trims

Your trims indicate a lean condition, meaning the O2 sensor is seeing too much oxygen (not enough fuel). Positive number implies fuel is being added. In addition, look at overall fuel trims by addition together long and short for each bank.

Long term allows you to see overall adjustment for a particular load cell in the fuel map (sort of like remembered behavior from the past). ScannerDanner discusses this in a couple of his videos. So, when you find the root cause to your issue, you will see negative STFT trims that will result in the LTFT trims decreasing and resting around 0% and then the STFT should be hovering around 0%.

3. Here's a better way to look at the A/F ratio. It is a ratio, therefore, when the overall number is large, that indicates the numerator (top number) is much larger than denominator (bottom number). On the contrary, when the overall number is small, that indicates that the denominator is larger than the numerator.

So, what does that mean?:

18:1 means 18 parts air to one part fuel -> lean condition

12:1 means 12 pars air to one fuel -> rich condition

So, in summary, IMHO, you are experiencing a lean condition (like your subject indicates LOL). I suggest the following tests:

1) Physical inspection for a vacuum leak after MAF
2) Use water to look for a vacuum leak
3) Check fuel pressure
4) Check out MAF for being dirty
5) to name a few

ScannerDanner videos show you how to do all of these tests.

I hope that helped and I didn't tell you something you didn't already know. I probably did, but I just felt clarification would be helpful.

I definitely appreciate the help. Any help is good even if I know it already.

I guess I will start out with the MAF just because it's easy.
Then I will check around for a vacuum leak with the water. I watched his video on that.

My first thought was maybe it's a weak ignition coil or a weak spring inside the coil not making a strong contact on the plug. but it's looking more like a lean condition like you said.

So i got a pretty nice weather today and after work i was able to poke around and check for vacuum leaks.

Found nothing. I start spritzing water on known vacuum lines. Intake manifold. Throttle body where it mates to the intake and nothing. no change in trims. No engine speed or noise change.

I checked the MAF and i was very surprised how clean it was. That's a good thing though i guess although i was hoping to find it totally dirty.

I guess next is onto the fuel pressure check. I have a cheap vacuum/fuel pressure tester that i can see if i have the correct fittings to fit the Schroeder valve on the fuel line. My fuel pump does make a little buzzing which may be an indication of struggle. Possible the sock is dirty on it or whatever.

I should throw in also that the previous owner always ran 93 octane which is premium here. I filled up with a tank of regular (87) on the drive home from picking the truck up and the check engine light popped on which turned out to be a faulty o2 sensor 1. I forgot what the exact code was.

Well that pending code i found the other day P1133 - o2 Insfficient Switching Bank 1 Sensor 1 popped on today. I took some pictures of the freeze frame data here.