Issue With Cold Starts-Low Rpm mode

[Trenchshop]

Recently found a couple issues going on with my 05 Tb Ls EXT. just had my tranny rebuilt ($1700 I was not accounting on...) and now I find myself having some weird startup issues. I have the later end of 05 trailblazer and the non serviceable fuel sock attached to the fuel pump in the gas tank. The other day I was getting some weird rpms (cold morning startup usually 12-1500 rpms) and it went straight to 600 rpms from 1200 rpms after about five seconds. Ran kinda rough so turned it off and tried to turn it back on again. Nothing. Engine turned over repeatedly but no start. 3-5 minutes went by and finally started by pumping accelerator repeatedly. Worked fine after that, but this has happened before. I have about 180k miles on this guy now, and I am wondering if maybe the fuel sock is clogged or pump is failing, (hasn’t been replaced at all as far as I know). Anyone have any thoughts on what might be causing this? (Fuel gauge just went past full and is staying put too, heard it’s in the same place as the pump). The issue of the TB running in low-rpm mode is becoming more consistent. Used to be every so often with cold start, then every cold start, now even when warm too. Misfired here and there too, all with no Check Engine Light. Idles now warmed up in low-rpm mode under 0 rpm still running and no engine roughness. Pretty baffled by all of this. Read somewhere it might be the Cat, but anyone else have any thoughts?

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[littleblazer]

So is it just dropping to 600 rpms or is it going into reduced engine power mode? If you drive it when it's running rough does it drive normal or does it behave strange, Speed limited?

 

[mrrsm]

Here Are Four suggestions: (BUT.... It will be much easier for you with less guess work from the GMTN Members ...if you can obtain the PXXX Codes from the PCM and post the information back here...)

(1) If you installed the Replacement Transmission yourself... in the absence of CASE Re-Learn... The Rough Start might happen as a result of the PCM needing a New Setting for the Crankshaft Angle. If a Jobber did the work and they did not specify performing the CASE Re-Learn... give them a call ...and ask. You also may have a GMTN Member living nearby who will perform the job with a GM Tech2 Clone and treat you right. The Dealership(s) will charge around $100.00 for the few minutes it takes to perform this task. The Average Mechanic may not even know what a CASE Re-Learn means... so if they give you any Dead Air Time during your call... then it might not have been done... and they won't say otherwise..

(2) Take Live Fuel Pressure Reading(s) from the Schrader Valve Fitting on your Fuel Rail (keep a rag or hand towel ready...and Run the Engine down by Pulling the Fuel Pump Relay FIRST) and after starting the Engine...eyeball the FP at Idle and with the Engine RPM raised to ensure that the Fuel Pump is responding and is capable of Fuel delivery through the In-Tank Filter. 50-60 PSI would be about right on your Gauge. No sense suffering too much worry if these test pressures are all good... and this fix comes in at around a (1) on Scale of (10) for the 'Diagnostic and Repair Agony Scale'

(3) Your Upstream O2 Sensor might need changing if the engine experiences Wild Swings between running Lean and Running Fat ...and having an inexpensive OBD2 Code reader should be able to provide you with information on your Long and Short term Fuel Trim readings. If the O2 Sensor cannot go into Closed Loop and remain there... it might have bad Zirconium Innards and have to be replaced.

(4) Wild Swings between Lean and Fat Fuel Dumps into the engine can also be caused by Vacuum Leaks around the Intake Manifold that will steady themselves once the entire Engine is heated up enough to close the gaps between the Engine Head and the 'Figure 8' Rubber Gaskets. If you can tighten them because they seem loose enough to move with your fingers... that problem is easily solved ...by GENTLY snugging them all back down using no more than 87 Inch Pounds of "Twist"

 

[Mooseman]

(1) If you installed the Replacement Transmission yourself... in the absence of CASE Re-Learn... The Rough Start might happen as a result of the PCM needing a New Setting for the Crankshaft Angle.

Please ignore this. A CASE relearn is only necessary when making repairs or changes in the engine's timing system or the crank position sensor.

Has the throttle body ever been cleaned? Idle issues such as this are sometimes caused by a dirty tb and the battery having been disconnected, which would have happened when the tranny was replaced.
How to clean your throttle body

Failing that, getting fuel pressure readings and live data about what is going on, like O2 sensor voltages and fuel trims, would be more valuable than just shotgunning parts although the O2 sensor is relatively inexpensive.

 

[mrrsm]

Thank You for the Correction, @Mooseman... This PDF lists all of the conditions that require a CASE Re-Learn... and has the Proper Procedures to Follow using an SPS Device (GM Tech2) as outlined:

http://my.cardone.com/techdocs/PT 70-77-0003.pdf

 

[Trenchshop]

Checked out codes, nothing being thrown. Checked out fuel pressure and it runs about 59 psi, starts about 52, though just now tried to prime and test and it only went up to 32 psi after turning ignition on after bleeding system. After I turned ignition on a couple times got about 55psi. Have a worse problem now though, tried to start at low pressure and engine kicked over but now sounds horrible, idles and runs rough, not sure what to do here... Throttle body just cleaned.

 

[Trenchshop]

I haven’t replaced 02 sensor yet though.

Oh, and those issues initially reported have been going on since before the tranny rebuild.

 

[littleblazer]

Hmmm. May be time for an injector power balance to see what's going on.

 

[Trenchshop]

Pretty concerned about engine running right now, whole vehicle shakes while running

Can I post a short vid of what it’s doing?

Found out I can't...

 

[littleblazer]

Just shoot it to youtube then link it here. It sounds like you may have a heavy misfire on a cylinder but I'm not certain.

 

[mrrsm]

If you can see a 20 PSI drop in Fuel Pressure with no other explanation at hand... Perhaps your FPR (Fuel Pressure Regulator) located at the top front Driver's Side of the Engine is the problem... and if needed... this is a fairly easy R&R. The results your FP Test show this to be the logical component to replace. Know that you can locate the FPR by searching that area adjacent the Air Chamber for a slender Vacuum Line that attaches to the small tube at the tip of FPR. If THAT tube connection is cracked and fails to seal... It might obviate the need to R&R the FPR if you can use some Emergency Black Silicone Tape often used to repair heater hose leaks and allow for a better vacuum by sealing around the small tube end of the line.

As the intake of ambient air occurs... the increased Piston activity draws a strong suction, thus pulling on the diaphragm inside of the FPR and allowing for a greater passage of fuel into the Rail. This is not an expensive component and very worthwhile doing at this point. To help you along with this and many other repairs... visit any post made by @Mooseman and look at his lower information line and follow the instructions to Download a Service Manual for your specific vehicle. His links to these manuals are something worth remembering and all worth acquiring, when time permits.

Also ...If you pull your Spark Plugs and you can smell or see the presence of a fresh wetting of Gasoline on any of them... Some of your Fuel Injectors might need a thorough dose of Shell Techron Fuel Treatment to cleanse them... through the Tank... at least a little bit; perhaps enough to help the engine run better without having to pull the Fuel Rail to get at them. When EFIs become clogged... they tend to spray a straight stream of fuel into the Cylinders instead of an Atomized mist that easily mixes well with air and burns more efficiently and with power. Liquid Gasoline never burns well and can pass the rings and ruin the lubricity of the Engine Motor Oil. It also does more harm than good when it passes out of the cylinders unburned and enters the CAT ... where in in some cases, it can be doing permanent damage.

 

[Trenchshop]

So check engine light flickers on and off when revving in idle.

Going to get the code pulled from it. To my knowledge, the fuel pressure regulator is in the fuel pump assembly on the 05 TB 4.2 i6

Been keeping an eye on the pressure gauge too through all of this, even priming a few times at one point was in 40s. Wondering if I need to replace the pump regulator assembly.

Tried resetting PCM leaving battery unplugged for a little while, but no cigar.

 

[mrrsm]

Yes... Sorry about that... and ...'Welcome to the Genius of GM Regressive Engineering' done expressly with the idea of discouraging and excluding as many of their Loyal Buyers as possible from ever working on their own vehicles with Woolly Notions of being "Shade Tree Mechanics" performing even the most mundane of tasks. Here is an interesting video on this R&R... albeit on a Silverado ...but helpful to watch, nonetheless:

 

[Trenchshop]

I have officially upgraded to a P0300 multiple misfires code.

This same code appeared twice

 

[mrrsm]

Please know that if you are tempted to purchase an inexpensive Complete Fuel Pump Kit... your troubles will re-double if there is a premature failure at the most inconvenient of times. Whether AcDelco, Delphi or Bosch.,.. Choose wisely amongst them and avoid the $89.00 Chinese Special as this is definitely an OEM repair... if ever there was one. Here is a list of what causes the P0300 Random Misfires... taking care to add in your Low Fuel Pressure findings into the calculus of whatever you decide to do next:

http://www.myautorepairadvice.com/p0300.html

 

[Trenchshop]

So I’m gathering then that I have a few options to go from here. I have not replaced any 02 sensors as of yet, maybe upstream a good place to start? I was looking into the Airtex Fuel Pump system through rockauto.com I have a can of BG 44k and some people recommend Seafoam in the vacuum lines to help with possible clogged injectors. Would these be a good place to work towards next?

Else wise I’m not quite sure how to check out the ignition coils, I don’t immediately have access to specialized equipment. I can check out the spark plugs

 

[littleblazer]

As it sits drive it more the truck will identify what cylinder is misfiring.

 

[mrrsm]

There is no point in imagining that you can have the astronomically rare possibility that ALL of your Ignition Coils would go 'T*TS UP" at the very same time as the very 'randomness' of the problem would indicate against that possibility. Working the problem from the least expensive and most likely to work approach is always best. So the BG-44K is a good idea...but only if it is applied just as they indicate on their site... and using the right equipment to perform this up close Carbon and Fuel Gum Clean-out. Their Through-The-Tank stuff can help a little... but unfortunately... the solvent needs to be introduced in a much more concentrated manner to have any immediate effect on the EFIs and on any Carbon Build-Up inside of the Combustion Chambers.

I cannot object to you changing out the Upstream O2 Sensor... but it would be nice to have the supporting data logs that include some Long and Short Term Fuel Trims to analyze what has been going on before saying yes to doing that work just yet. If you have not done this yet... try to Tighten Down the Intake Manifold Fasteners (Don't Exceed 87 Inch Pounds)... as it will be better to do that to isolate any Vacuum Leaks... even as difficult as some of them are to access, rather than just assuming out of hand that the O2 Sensor is The Enemy.

 

[Trenchshop]

So I’m digging in and checking out coils, plugs, and compression. Also going to check out intake manifold bolts. Will keep y’all updated



Update: plugs are shot, oil leaking into 3/6 cylinders. Ran compression tests on each, dry is as follows:
Cylinder 1 60-120 psi
Cylinder 2 80-130 psi
Cylinder 3 60-125psi
Cylinder 4 60-130 psi
Cylinder 5 130 psi max
Cylinder 6 120 psi max

All pressures built up. Will do wet soon. Going to replace plugs and valve cover gasket. Going to run some seaform through vacuum and maybe BG 44k through gas tank as well. Decided Bosch 4-prong iridium fusion spark plugs may not have been best idea.

 

[mrrsm]

The GM Atlas LL8 Engine Platform seem to be distinctly 'finicky' about which Spark Plugs it enjoys in-dwelling... and so this is the favorite flavor you will hear the most about from the majority of the GMTN Membership as being the best one to use in the 4.2L Aluminum Engine Head:

https://www.amazon.com/ACDelco-41-103-Professional-Iridium-Spark/dp/B000IZ2AVE/ref=au_as_r?_encoding=UTF8&Make=Chevrolet|47&Model=Trailblazer|489&Year=2005|2005&ie=UTF8&n=15684181&newVehicle=1&s=automotive&vehicleId=1&vehicleType=automotive

Incidentally... After 150,000 Miles or so... these Motors are notorious for building up a lot of Greasy Carbon Deposits that will actually form substantial Black Carbon "O" Rings around the edges of the Valves and inside of the upper Combustion Chambers. That BG-44K should knock Hell of that problem and likewise allow the valves to close and seal better inside the ports... once all of that the crap is well cleaned out. The added Carbon also displaces and reduces the volume inside of the upper Cylinder Head Combustion Chambers, often causing pre-ignition by reducing the "quench" and naturally increasing compression ...and that can add to the causes of pre-ignition problems. The Top Compression Rings on these engines with high mileage can also get stuck inside of their grooves ...held tightly by Carbon and Fuel Gum inside of the Ring Grooves and a loss of Compression ensues when this occurs. So in addition to cleaning out any gummed up EFIs.... the action of the BG-44K Treatment(s) will kill a multitude of these Carbon and Gummy Birds... with One Stone... so to speak.

 

[Trenchshop]

So final update for the night, decided fuel regulator assembly is giving me the headaches on the fuel issue that started all of this. Going to replace it later. As of now, the resonator, serpentine, alternator, pcm, air intake and filter, every coil and spark plug, and about twenty wires are disconnected or removed. Been a nice 12 1/2 hours since I started this morning... replacing valve cover gasket and spark plugs with platinum acdelco. Going to probably run seagoam through brake booster vacuum line and see about the 44k treatments. Will update further when all is said and done. To those who have ever had to remove the intake manifold, I salute you. You have earned a salute.

 

[Mooseman]

Been there a couple of times. Good times. Just be sure to use ONLY ACDelco Iridium 41-103 plugs. This is the only one that is known and confirmed to work correctly with this engine.

Although we haven't seen too many, if any at all, failures of the fuel pressure regulator, it is possible but the majority of the time, the fuel pump is to blame. Check for fuel leaking out the vacuum port of the FPR. Fuel pump is a known failure part.

 

[Trenchshop]

My understanding is that the fuel pump, regulator, sock, etc is in one assembly in fuel tank in 05

 

[Mooseman]

Ah yes, of course. Thinking like my old 02 which had the FPR at the rail. So that would mean you need to replace the whole fuel pump module.

 

[Trenchshop]

Yeah... that’s gonna be another day XD

 

[Trenchshop]

So, update time! Finally finished replacing spark plugs (thanks Moose) and valve cover gasket, cleaned throttle body, ran some sea foam through and... still misfiring. Ran across this post online though talked about p0300 random misfire code. It mentioned running the misfiring engine and pulling the cable out on each of the ignition coils one at a time. If the misfire gets worse when pulled, then you know that coil is working. If the engine idle doesn’t change when pulled, you know that coil wasn’t working anyways, ie bad coil. Found out my #1 and #4 coils were not firing, so going to replace them. Thanks everyone for your help, hopefully post final update tonight after coils are replaced, God willing will be Good news.

Last update: Coils 1 and 4 replaced and she runs like a dream. Going to change our air filter and fuel pump too, but for now, the misfiring is gone, and startup is a little smoother too. Thanks again everyone!

 

[Mooseman]

I guess with more than one cylinder misfiring, it can't determine which one. Good catch!

 

[littleblazer]

I'd have thought with two bad the thing wouldn't even run. That's actually impressive. Good luck and hopefully this fixes it.

 

[thebat]

So I’m digging in and checking out coils, plugs, and compression. Also going to check out intake manifold bolts. Will keep y’all updated



Update: plugs are shot, oil leaking into 3/6 cylinders. Ran compression tests on each, dry is as follows:
Cylinder 1 60-120 psi
Cylinder 2 80-130 psi
Cylinder 3 60-125psi
Cylinder 4 60-130 psi
Cylinder 5 130 psi max
Cylinder 6 120 psi max

All pressures built up. Will do wet soon. Going to replace plugs and valve cover gasket. Going to run some seaform through vacuum and maybe BG 44k through gas tank as well. Decided Bosch 4-prong iridium fusion spark plugs may not have been best idea.

These numbers are very low. Good compression on these engines is 180+