The dreaded flashing Check Engine Light

[Motojoe86]

Apologize in advance as this is now posted in 2 forums as I was unsure where to post this at. Anyhow, hello guys, I am new to this forum as a member however have been on here a ton to read on issues. Always seems to be a great source for all things Tahoe related. I have read several of your forums regarding check enginelight and am unfortunately creating a new one with hopes there might be more insight. Here goes:

Vehicle:
2003 Chevy Tahoe 4wd z71 with 5.3l flex fuel engine 160k miles

Symptoms:
I am getting a flashing cel whenever i am on the highway over about 70 mph. If i rev it in neutral it will flash being held over 2400 rpms as well. Also, whenever i have it in tow/haul mode it flashes almost all the time. Lastly, towing in regular mode it will flash about 50mph+. It seems to run good minus a very minor stumble in idle (unnoticeable rpmfluctuation) and terrible gas mileage. 2 side notes- i have an srs light on and my alternatoris going out which I am replacing this weekend. I cant imagine these would have any affiliation with cel though would it? It usually doesnt store a code however I have been able to pull random misfire code twice.

Work done to no avail:
-New iridium plugs gapped at .40
-New plug wires
-New coils
-New air filter
-New pcv valve
-Cleaned MAF
-Cleaned throttle body
-Fuel pressure tested within spec
-Seafoam through motor oil
-Crankshaft sensor tested within spec

I am running out of ideas on what to replace and the costs are beginning to add up. Any thoughts or ideas on the culprit would begreatly appreciated. Thanks in advance!!

 

[iamdub]

With what you've spent on it already, you could've had a shop with a scan tool perform some diagnostics and probably had it fixed. There are just too many things that could cause random misfire. Until a knowledgeable tech can watch a real-time data stream while the truck is being driven, you're just gonna continue to throw parts at it and likely waste more money.

The SRS light is unrelated but a faulty alternator could be related. Left to test are MAP sensor, O2 sensors, cat converter(s) and all related wiring.

 

[Motojoe86]

Appreciate the reply. I should have noted that I HAVE had it in to a few different shops with diagnostic checks and hooked up to a tech 2 amd none of them see to have a specific idea of whats causing it. They all have pretty much said they'd throw random parts at it with no guarantee of a fix... Was hoping i might get lucky on here with someone that found a solitary fix. Aside from alternator that im doing this weekend, where would you start first? Thanks for the help!

 

[Scottydoggs]

typically only two things make the cel flash is, needing a tune up real bad, but you got that covered, the other is clogged cats. clogged cats cause? yup you got it, random misfires.

it likely also feels down on power too when wot.

 

[Motojoe86]

Thanks! That was one issue I was afraid of lol. What is tye best way to test cats? Ive heard of a pressure test, temperature test and one other kind... Also, any idea of what a fair price for a shop to test them might me? Thanks!

 

[HiHoeSilver]

Thanks! That was one issue I was afraid of lol. What is tye best way to test cats? Ive heard of a pressure test, temperature test and one other kind... Also, any idea of what a fair price for a shop to test them might me? Thanks!

Pull an O2 sensor and shove a camera down there.

 

[iamdub]

Appreciate the reply. I should have noted that I HAVE had it in to a few different shops with diagnostic checks and hooked up to a tech 2 amd none of them see to have a specific idea of whats causing it. They all have pretty much said they'd throw random parts at it with no guarantee of a fix... Was hoping i might get lucky on here with someone that found a solitary fix. Aside from alternator that im doing this weekend, where would you start first? Thanks for the help!

At least you brought it to get scanned. So many DIY-ers avoid paying a professional their diagnostics fees because they feel they're too high, yet spend hundreds or thousands trying to get lucky.

Honestly, I still feel that seeing a live data graph would really help. But, it has to be read by someone that knows what they're looking at and what to look for- not just scan for code(s) and replace whatever sensor the code relates to. A random misfire can be a nightmare to track down, as you're experiencing, and I've seen shops dismiss such cases because it's not worth their time. If they have a flat diagnostics rate of $X, they know they're screwed getting involved with such a problem because they could be on your vehicle for hours and hours when they could be knocking out and getting paid for repairs on multiple other vehicles in that time. I can't say this is what you've gotten, but I had to mention it.

Anyway, there HAS to be something that happens that causes it and a sharp/well-trained tech should be able to pick up on something being amiss (pun intended) in the data stream right before or during those misfires. A MAF reporting some outlandish amount of air (or lack of) at certain RPM, a MAP sensor reporting odd vacuum amounts at certain RPM, a clogged cat converter stopping up the system (although this should cause catalyst efficiency codes), upstream O2 sensor(s) reading awry, the fuel supply being insufficient at the higher RPM... All should present numbers that don't "add up" in the data stream. They may not cause codes to be set, but if the tech knows that, for example, the MAF should not be reading 1,000 g/s at 2,000 RPM, then they would investigate the MAF. Same scenario for the MAP sensor, which is primarily used to calculate engine load by intake manifold vacuum. The PCM adds or removes fuel and timing based on this input in conjunction with the input from the other sensors. If it's erroneously adding a bunch of fuel because it thinks the engine is under 100% load when it's really only at 50% load, that would cause excessive fuel consumption and misfires. It should also cause the O2s to report a rich exhaust and, there again, throw code(s). The same goes for the opposite of perceived load versus actual load and lean exhaust.

As Scott mentioned, check the cat(s). They can be visually scoped through the O2 sensor bungs, have the inlet and outlet temps compared or have the system pressure checked. I know you said the fuel pressure was in spec, but was this with key on/engine off, engine idling, etc.? What about while driving with engine under load? I've had fuel pressures pass tests with the car sitting in the shop, but read low when driving. I'd secure the gauge to the windshield under the wiper with the hose going between the hood and cowl to the fuel rail and road test it.

 

[Motojoe86]

Thank you, a lot of good information. I think I will try the scope of cat and fuel pressure check while driving tomorrow. I imagine a cheap harbor freight fuel pressure gauge would work for the time being? I guess I will go soak my sensors in pb blaster now lol

 

[iamdub]

Sure, I'd use a cheap Hazard Fraught gauge for this. The pressures it reports may not be accurate, but you're looking more for an excessive drop rather than actual numbers.

 

[Motojoe86]

Sure, I'd use a cheap Hazard Fraught gauge for this. The pressures it reports may not be accurate, but you're looking more for an excessive drop rather than actual numbers.

Awesome, im going to give this a try tomorrow. Thanks!

 

[swathdiver]

Do the GMT800s have 4 O2 sensors? Regarding the Tech2 or a scan tool that can record, did they have the Tech2 hooked up when the CEL was flashing while driving down the highway? They could record that and review later against the trouble shooting diagram for your truck.

I suggest you get a subscription to ALLDataDIY for access to this material.

Regarding the 02 sensors, if a cat is going bad it will throw a cat specific code, telling the computer which one. The downstream O2 sensors measure the health of the cats. That is of course, if you have them.

Ohh and btw, welcome aboard Joe!

 

[mizzouguy]

Does your exhaust smell like sulfur? I'd also think cats but which cel codes are on would help....could also be intake leak or injectors.

 

[Scottydoggs]

you can also remove the o2 sensors pre cat and go for a ride, yes its loud and sounds like crap, but if the cel dont flash when wot, you just confirmed the cats are bad.

you can find high flow cats for about 50 bucks each on ebay, would hope a shop is less then a 100 to r and r the cats.

 

[HiHoeSilver]

you can also remove the o2 sensors pre cat and go for a ride, yes its loud and sounds like crap, but if the cel dont flash when wot, you just confirmed the cats are bad.

you can find high flow cats for about 50 bucks each on ebay, would hope a shop is less then a 100 to r and r the cats.

@Tonyrodz has some I think.

 

[Tonyrodz]

@Tonyrodz has some I think.

I do.

 

[rockola1971]

Id slow down a little here. You mentioned the alternator needed replaced. Why? Has it been replaced yet? If the alternator has an internal short in the diode pack or its rotor then it can load down your battery and cause every sensor you have to be supplied with low enough voltage that the whole system as a whole will not operate correctly and throw false codes or you could very well be getting valid misfire codes...because at higher rpm the ignition coils require higher amounts of current (higher duty cycle) to supply the engine with spark but cant because the whole charging system is loaded down from a defective alternator.

 

[Motojoe86]

Id slow down a little here. You mentioned the alternator needed replaced. Why? Has it been replaced yet? If the alternator has an internal short in the diode pack or its rotor then it can load down your battery and cause every sensor you have to be supplied with low enough voltage that the whole system as a whole will not operate correctly and throw false codes or you could very well be getting valid misfire codes...because at higher rpm the ignition coils require higher amounts of current (higher duty cycle) to supply the engine with spark but cant because the whole charging system is loaded down from a defective alternator.

 

[Motojoe86]

Lots of great information from everyone here. I appreciate all the input. Rockola, I am actually really hoping that the alternator is in fact the problem. I didnt realize it could cause flashing cel. I actually just finished changing that out, new belt and a second clean of maf and throttle body. Im beat for the night but im gonna hop on highway tomorrow and try a couple minute run at 80 or so and see if it comes back on. I will update you guys tomorrow. Thanks again everyone for all the help. This site is awesome!

 

[M1Gunner]

Also if you ever think you have a cat issue all you need to do is hook up a vacuum gauge to your intake manifold. And if the cats were restricting airflow out of your engine, aka a big ass air pump, at higher rpms then the vacuum gauge would be an immediate tattle tell.

 

[swathdiver]

What might it read for a restricted cat?