Flashing check engine light

[KRaZiE KiWiE]

I’m new to the forum so let me know if I did anything wrong.

 

I bought a 2003 Yukon xl denali to move things in and I’ve had it for 5k miles and about 4 months. The truck has 230k miles on it currently. I was driving on the freeway when suddenly I started to lose power. It was accelerating but not as fast as it normally does. After some time it would not accelerate and I pulled over. I turned it off and back on and it have me a check engine light. It was also idling very rough. The next morning I turned it back on and the check engine light was flashing and was still Idling rough. I will pull codes tomorrow and post them when I do. Any suggestions on what you guys think it could be?

[davester]

It could be anything.  O2 sensors, knock sensors, exhaust or intake leaks, fuel pump failing or fuel filter is plugged for a start.

 

I would suggest actually figuring out what the problem is before throwing parts at it.  It tends to be cheaper that way.

[Jsdirt]

^^^ What Dave said.

 

A flashing check engine light is an indicator of a cat-melting misfire. In the old days, a misfire meant ignition problems 99.9% of the time. Not anymore. These days just about anything can be the cause a misfire, from a failed injector (or it's wiring), vacuum leaks, valve train / compression issues, to individual cap coils, and their controls and wiring, to bad powers or grounds to the engine computer.

 

As with anything, start with the easy stuff, and go from there. What's the exhaust doing? Shooting black smoke out? Burning your eyes and smelling super rich? Or, no smell at all? That's where I start. If there's no black smoke or rich indicators, I check fuel pressure next. A failing fuel pump, clogged fuel filter, or failing regulator will cause misfires as well. Actually before I do anything with fuel pressure I'll watch the o2's on a scan tool to see if they're pegged rich (.8 -1v) or lean (.1-.2v). At full throttle, they should ALWAYS go rich. If not, you either have a bad o2 or a fuel delivery issue. If it starts out one way and goes the other with RPM, you could have a MAF issue. 

 

As you can see, testing is 100% needed here.

[KRaZiE KiWiE]

41 minutes ago, Jsdirt said:

^^^ What Dave said.

 

A flashing check engine light is an indicator of a cat-melting misfire. In the old days, a misfire meant ignition problems 99.9% of the time. Not anymore. These days just about anything can be the cause a misfire, from a failed injector (or it's wiring), vacuum leaks, valve train / compression issues, to individual cap coils, and their controls and wiring, to bad powers or grounds to the engine computer.

 

As with anything, start with the easy stuff, and go from there. What's the exhaust doing? Shooting black smoke out? Burning your eyes and smelling super rich? Or, no smell at all? That's where I start. If there's no black smoke or rich indicators, I check fuel pressure next. A failing fuel pump, clogged fuel filter, or failing regulator will cause misfires as well. Actually before I do anything with fuel pressure I'll watch the o2's on a scan tool to see if they're pegged rich (.8 -1v) or lean (.1-.2v). At full throttle, they should ALWAYS go rich. If not, you either have a bad o2 or a fuel delivery issue. If it starts out one way and goes the other with RPM, you could have a MAF issue. 

 

As you can see, testing is 100% needed here.

I got a p0300 code and the exhaust is white. I do smell some gas but not that rich. There is black liquid drops coming out of it.

[Jsdirt]

Sounds and looks like a dead miss from here. Should be very easy to track down. If you don't have a scan tool, when you first fire it up cold get right under the hood (watch the belts and fan) and feel each exhaust manifold outlet on all 8 cylinders.  The cold one, or ones are your dead cylinder(s). From there you can swap coils around and see if the problem follows the coil - if so, it's just a dead coil pack, or the short secondary wire is bad.

 

As far as the white steam and black liquid dripping, that's normal this time of year anywhere in the northern states.

[KRaZiE KiWiE]

I also noticed that behind the rear driver tire, there was something that used to rattle (I think apart of the fuel system) that doesn’t rattle anymore since the misfire.

[KRaZiE KiWiE]

I attached a file containing the scan tool live data and the liquid dropping out of the exhaust.

[KRaZiE KiWiE]

 

 

[Jsdirt]

First thing that jumped out at me is your MAF reading. You should be reading in grams per second close to what the engine is in displacement in liters. So a 5.3 should be at or near 5.3 g/s. You're at almost 15!

 

Your MAF is F'ed. Your fuel trims were pig rich at idle, then went flat lean when you hit the gas. Classic MAF issue.

 

Before you go winging a MAF at it, check out the wiring. Watch ScannerDanner on YouTube for all his MAF videos - he'll show you how to track down wiring problems there. If that all checks out good, try cleaning it. Got nothing to lose at this stage. 

 

Even a pin fitment problem right at the MAF wiring connector could do this. I've seen ones where the rubber seal in the plug was damaged or missing, so road-salt water got in and destroyed the pins and the connector itself - was all green crust.

[KRaZiE KiWiE]

I just checked the pins and they seem fine. Any way you could link me to a YouTube video of how to check the wires? There are so many I don’t know which one to choose. Also do you think I should get some mass air flow cleaner and clean the whole maf with it? When I took it out black gunk got on my hands

[KRaZiE KiWiE]

[Jsdirt]

Any one of these vids are great. Different vehicles, but same principles apply: https://www.youtube.com/user/ScannerDanner/search?query=MAF

 

Pins look good from here. What you'll be cleaning is actually on the inside. Black hands from touching the MAF itself is normal underhood grime, or old rubber coming off.

 

The inside of the MAF is what you need to clean, and you need to be EXTREMELY CAREFUL doing so. The little wires in there are EXTREMELY fragile, and hard to get to. You'll need a brush, preferably on a wire so that it can be bent 90° to reach the front of the wires (facing the incoming air). Also make sure that unit goes back in EXACTLY the way it came out (unless that was wrong to begin with). You can install it backwards There's usually an arrow on the rubber indicating direction of airflow.

 

Here's a GM specific vid: 

 

And here's a great misfire training video: 

 

 

[KRaZiE KiWiE]

Ok thank you so much I appreciate it. I’m going to head to autozone and just get a new maf sensor and I’ll update in about 2 hours

 

[CamGTP]

I would rather have watched each cylinder misfire on the scan tool before going straight to the MAF.

 

The easy solution would have been to just unplug the MAF and if the truck was still misfiring guess what, perhaps it's deeper than the MAF.

[Jsdirt]

If you're going to wing parts at it, at least find a better name brand one. RockAuto is your best (cheapest) bet there.

 

I refuse to install parts from any chain store except for NAPA on customer's vehicles. There's a reason for that - I don't like working TWICE for the same pay.