GMT 800 (Sierra 4.8L) Injector sources??

rchalmers3

Original poster
Platinum Donor
Jul 11, 2020
190
Irmo, SC
Gentlemen,
I have my finger on the trigger of the parts cannon.

My truck has random miss-fires on cylinders 1,3 and 4. I have felt it drop cylinders at idle for years now, but the miss-fire counts would clear before setting a code. Otherwise the truck ran good and gave me 19MPG over the highway, and 17.5 MPG around town.

Last year I replaced the spark plugs and wires as routine maintenance, with no impact on the issue. I measured the compression and found significant (about 20%) pressure variation, as can be expected on an old, tired engine. However, the low pressure cylinders did not correlate to the cylinders having miss-fire counts.

As summer here in the northern hemisphere approaches, the warm weather is exacerbating the issue. Yesterday while idling in traffic the engine recorded sufficient miss-fires to trigger the P0300 random miss-fire code. It stumbled through light acceleration and the CEL started blinking until the engine cleared and then the CEL went on to steady. My poor old horse!

I want to replace the injectors. They are stock units with over 370k on the clock. I invite you to pepper me with alternative potential sources of causation, but my main question is where I can get my best bang for the buck on replacement injectors. There are a gazillion options out there. Some are so cheap it's ridiculous! New ones from the dealer are obscene! How about some proven alternatives that won't break the bank?

RockAuto offers some new Standard Product injectors for $70usd. I'm wondering if that is a good option???

Thanks in advance for your opinions,
Rick
 

TollKeeper

Guru
Dec 3, 2011
6,886
Brighton, CO
I would not do the injectors. That would give a entirely different code. P0200 and up to P0208 (just like a misfire).

You replaced the wires.. I just recently went thru this where I had a bad plug wire. Yes they were new.

When you replaced the spark plugs, what are the plugs you used?
have you checked for quality of spark thru the coils?
How are the LTFT and STFT looking?

Edit: This isnt to say you dont have a bad injector, but there are other areas I would check first. The injectors on my Envoy 5.3 have 210k miles, and another 18 months of Police style idle time. Effectively near 180k miles in my estimation. (6-10 hours idle time a day, 5 days a week, for 18 months).

Edit2: A leak down test to see how the Valve seats are, and rings, might also be in order.
 
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rchalmers3

Original poster
Platinum Donor
Jul 11, 2020
190
Irmo, SC
@TollKeeper, Thanks for watching over me!

Yes, I replaced the plug wires along with the spark plugs. It ran the same before and after the replacement. I bought and installed AC plugs, according to the application. I do not remember the part number. I can look at fuel trim numbers this weekend.

The P020X codes relate to injector circuit anomalies. If the vehicle displayed those codes, the injector replacement question would be a much shorter diagnostic tree! I'm supposing the injectors are suffering from wear or deposits affecting the spray pattern at low duty cycle, but producing adequate spray and atomization at partial throttle. That type of failure won't set an injector code, but would rack up random miss-fire counts.

See my first post about engine mechanical condition. It is not perfect, but has no dead holes nor a dead miss.

Rick
 

TollKeeper

Guru
Dec 3, 2011
6,886
Brighton, CO
Surprisingly, the 4.8 LR4 injectors are actually pretty cheap!

These show to be OEM, but from a small business..

I am guessing you have the 2010 LR4, thats what I searched for. You didnt mention your year or specific engine model.
 
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TollKeeper

Guru
Dec 3, 2011
6,886
Brighton, CO
@TollKeeper, Thanks for watching over me!

Yes, I replaced the plug wires along with the spark plugs. It ran the same before and after the replacement. I bought and installed AC plugs, according to the application. I do not remember the part number. I can look at fuel trim numbers this weekend.

The P020X codes relate to injector circuit anomalies. If the vehicle displayed those codes, the injector replacement question would be a much shorter diagnostic tree! I'm supposing the injectors are suffering from wear or deposits affecting the spray pattern at low duty cycle, but producing adequate spray and atomization at partial throttle. That type of failure won't set an injector code, but would rack up random miss-fire counts.

See my first post about engine mechanical condition. It is not perfect, but has no dead holes nor a dead miss.

Rick
Agreed, which is why I suggested the leak down test.

On the plugs, do you remember if they were the 11ish bucks per plug, 9ish bucks per plug, or 4ish?

Edit: and were they ACDelco plugs?
 
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flyboy2610

Registered Member
Aug 24, 2021
235
Lincoln, Ne.
Have you checked the coils? A bad spark plug can eventually cause it to fail. I went through this just yesterday with my '04 TB. Had a P0302 code and flashing MIL for cylinder #2. Swapped coils between cylinders 1 and 2, then got a P0300 code for multiple misfires. Replaced the plugs in 1 and 2 (planning to do the rest on Saturday), put a new coil on #2 ditched the bad coil. No more codes, runs like new!
 
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Mooseman

Master Blaster
Moderator
Dec 4, 2011
22,548
Ottawa, ON
To rule out injectors, you'd have to do an injector balance test but you'd need a scanner capable of doing it. As mentioned, I'd be looking more at the coils. At that mileage, they could be tired.
 
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rchalmers3

Original poster
Platinum Donor
Jul 11, 2020
190
Irmo, SC
Have you checked the coils? A bad spark plug can eventually cause it to fail.
Thanks flyboy, As I mentioned, this issue was present before replacing the spark plugs and wires.
I have been monitoring the occasional miss-fire at idle for 6 months now. As I mentioned previously, it is worse at idle and non present when cylinder pressures rise at cruise. That's why I have been discounting the ignition: The issue presents when combustion pressures are low and the coil energy required to fire the plugs is least. Maybe I'm not seeing things straight, but that's my current opinion!

Rick
 
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rchalmers3

Original poster
Platinum Donor
Jul 11, 2020
190
Irmo, SC
To rule out injectors, you'd have to do an injector balance test but you'd need a scanner capable of doing it. As mentioned, I'd be looking more at the coils. At that mileage, they could be tired.
After performing two injector balance tests, I can state there is no correlation between the specific cylinders (1, 3 and 4) producing miss-fire counts and injectors that produce either high or low comparative pressure drops compared to it's companions. However, I'll also admit that I don't know what I'm looking for, so I'll pass on that both tests were fair duplicates with the following results: the fuel rail base starting pressure was 52lbs. The pressure drops ranged from 20 lbs (32psi) to 16lbs (36psi). One note, on both tests, the even side bank cylinders (2,4,6 and 8) produced the majority of the lower pressure drop readings. Please provide your opinion about whether those results are suitable or out of range.

Fellas, I'm not ruling out ignition woes, but I'm not aware of any easy way to monitor the primary and secondary ignition patterns. Is there some trick or an affordable breakout box that will save me from back pinning the ignition coils individually?

Rick
 

TollKeeper

Guru
Dec 3, 2011
6,886
Brighton, CO
Cylinder 3 and 4 are easy to get to, and ones you are having problems with. A cylinder compression test, and leak down test.

Also pull the coil off cylinders 5 and 7, and swap those coils to 1 and 3. See if the misfire follows the coils.
 

Mooseman

Master Blaster
Moderator
Dec 4, 2011
22,548
Ottawa, ON
You could do swapnostics. Swap the injectors to others that aren't giving misfires. Ruling that out, try the same with the coils.

Good idea to do a compression test.

I had a very similar issue with my 4.2.

Very Slight Miss at Idle

 
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rchalmers3

Original poster
Platinum Donor
Jul 11, 2020
190
Irmo, SC
You could do swapnostics.
Good idea to do a compression test.
Compression test revealed two cylinders at 150psi, two at 125psi, and most other cylinders somewhere in between. Again, no correlation of pressures to the random miss-fires accumulating in cylinders 1,3 and 4. I did the compression testing months ago, when I started to notice the issue. I failed to mention this in the first post.

Swap-nostics is an option for sure. The effort requires no expense, and may be the option I'll undertake. My current thought (pun intended) is to compare the secondary ignition patterns. That's an appropriate window into determining whether the issue is compression, fuel, or ignition related. Back in the day (that would be Ronnie Regan's first and second administration) it was easy to clamp a probe onto the coil wire leading to the distributor, and see the entire parade of cylinders firing on a single display. Today, ignition systems are different! All I can find now is a COP probe that lays over over a single coil.

I could purchase a single COP probe, ('bout $80) then test and record each cylinder individually, followed by a one-by-one comparison. That seems a bit cumbersome, and before I go that route, I'd like to hear from those of you who are up to date with waveform diagnostics and tools. BTW, I'm using a Snap On Verus 4 channel.

Thanks to all,
Rick
 

rchalmers3

Original poster
Platinum Donor
Jul 11, 2020
190
Irmo, SC
Fellas, I have chosen to make a small tool investment in order to satisfy my desire to diagnose this issue in a manner that pleases me. I have purchased from Snap On the required probes for use on my scope: one is an inductive clamp to establish a #1 cylinder trigger, the other is a daisy chain of COP probes that can be plugged together to monitor up to eight cylinders.

With these probes, I hope to produce secondary ignition patterns that can be measured and compared. My old Verus scope has the capability to produce parade, raster and super-imposed patterns. From those waveform patterns, I hope to be able to make a correlation of any measurable discrepancies that exist in the affected cylinders, leading to a deeper understanding of the causes.

I'll happily share the results with this group, good, bad or inconclusive. What I'll learn remains to be seen, and in the end, I may take up your good advise and perform swapnostics. But I gotta try to do it this way first.

Thank you,
Rick
 
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mrrsm

Lifetime VIP Donor
Oct 22, 2015
5,984
Tampa Bay Area
Concur 100%...

The ONLY way to understand what is REALLY happening INSIDE of the Combustion Chamber of each Cylinder based upon prevailing Electrical and Combustion Artifacts is to See Them In Real Time on an Oscilloscope Screen.

For 'The Curious ...and The Doubters' alike... These images testify to what the Average Normal Ignition Wave Form should look like in Modern 4 Cycle Combustion Engines for the purposes of making a Deeper Analysis of A-F Problems and Errant Ignition Coil Behaviors:

CLASSICIGNWAVEFORMPATTERN1.jpgCLASSICIGNWAVEFORMPATTERN2.jpgCLASSICIGNWAVEFORMPATTERN3.jpg

Want to know more... or get your OWN Inexpensive Oscilloscope and Equipment, Too? GMT Nation has you covered at THIS Link:

 
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