98 Tahoe - Cylinder 6 misfire - fuel or ignition problem?

[HoeLV]

Hey guys, hoping you can help me narrow down the issue instead of shooting the parts cannon.

98 Tahoe, 4WD, 222k miles

Recently, I have noticed that I have been intermittingly been getting a misfire and pulled the info, finding mainly on cylinder 6 (initially ~300) and occasionally cylinder 5 (initially ~40) along with a code regarding the B2S1 O2 sensor (forget what code was - thought I screen shot it before clearing it). I ran some MMO in the gas tank and with an oil change (per instructions on the bottle). Later, I was driving up a decent and long grade at 65+ MPH and really noticed a misfire and started monitoring the data. The cylinder 6 misfire climbed to about 3000 and cylinder 5 to about 250. The road leveled off and the misfire went away. It seemed the harder I pushed the engine up the grade, the more prominent the misfire would get.

The code for the O2 sensor seems to have stayed away since initially seeing it. After the grade that really caused the misfire to be prominent, I have driven about 150 miles and no more codes/misfires.

I figure it is most likely a fuel delivery issue (I hear the CFI spider injection has its problems) or an ignition issue (I think I've read the ignition system can be a little finicky when starting to change parts there).

Any input on what the issue may be and additional info I can provide would be appreciated.

History - as far as I am aware, the truck still has the original CFI spider, distributor, cap, rotor, etc. Spark plugs were last changed at 180k miles. I got the truck with 176k miles and for the most part, had no issues. I am unsure of any maintenance down prior to getting it. The only major issue was in the first 150 miles of buying the truck and the transmission blew as I was towing a small trailer up a hill on the freeway, however when I got it towed to the shop and they replaced it, I was told it appeared as someone put gear oil in the transmission instead of ATF, causing the problem.

Thanks.

 

[wjburken]

What do the plugs on 5 & 6 look like?

How old are your plug wires?

 

[HoeLV]

What do the plugs on 5 & 6 look like?

How old are your plug wires?

I am going to try and pull the plugs this evening. As far as the wires, I haven't changed them, so the may be original or its unknown.

Edit: here are the 2 plugs. They look as expected to me. Took a wire brush to the threads and put them back in.

Plug 5

Plug 6

 

[YukonGTmaster]

I would start by pulling off your cap and rotor. Those are like consumable items on these trucks. Cheap insurance for me is, replace cap, rotor, plugs and wires, and clean or replace air filter.

However, the last bout I had with my truck misfiring was an intake manifold leak. I did the MPFI upgrade with the new Felpro gasket kit. I was also having a lot of coolant leaking on the garage floor. The vacuum leak was apparent as a whistle, as well as an increase in idle rpm putting it into “Park”

 

[Fless]

Those plugs look severely worn (out). Consider replacing them, along with the wires and the cap & rotor.

 

[wjburken]

I am going to try and pull the plugs this evening. As far as the wires, I haven't changed them, so the may be original or its unknown.

Edit: here are the 2 plugs. They look as expected to me. Took a wire brush to the threads and put them back in.

Plug 5

View attachment 367693
View attachment 367694
Plug 6
View attachment 367695
View attachment 367696

Did you check the gap on those before you put them back in? As @Fless stated, they look used up.

 

[OR VietVet]

What has been said. Replace plugs, cap, rotor and wires, especially since you think the wires may be OE. Deposits on those plugs and would love to see all 8 plugs when compared.

 

[YukonGTmaster]

Just a Note. I have tried other spark plugs. I always go back to OEM AC Delco. I’ve got 430k Kms/267k Miles on my truck.

 

[Eman85]

Poor man's ocilliscope....Get a spray bottle with water and a few drops of dish soap. Have someone lightly power brake the truck while you mist some water around the plug wires. If you have a bad wire it will start missing. Also look at the porcelain of the plugs for any carbon tracks looking for the path of least resistance. As the plug gaps gat bigger it increases the load on the ignition and the additional voltage will jump to whatever is easier.

 

[HoeLV]

Thank you for the responses.

To the question asked, I did regap the plugs when I put them back in as they were no longer proper.

@YukonGTmaster, I did notice a few times the RPM jump when going into "Park," but I only recall it happening 2-3 times. I have not seen a drop in coolant.

It looks like I will be ordering some new plugs, wires, cap and rotor. Is there a consensus on brands to stick with or avoid? Just sticking with AC Delco? I've always heard good things about MSD when it comes to ignition, distributors, wires, etc.

Is there any benefit in making the switch from the CFI to the MPFI? Is the new system that much better?

 

[Fless]

Thank you for the responses.

To the question asked, I did regap the plugs when I put them back in as they were no longer proper.

@YukonGTmaster, I did notice a few times the RPM jump when going into "Park," but I only recall it happening 2-3 times. I have not seen a drop in coolant.

It looks like I will be ordering some new plugs, wires, cap and rotor. Is there a consensus on brands to stick with or avoid? Just sticking with AC Delco? I've always heard good things about MSD when it comes to ignition, distributors, wires, etc.

Is there any benefit in making the switch from the CFI to the MPFI? Is the new system that much better?

@exp500 you usually have some recommendations. Any now?

 

[Eman85]

As far as ignition components go I've had the best luck with Standard Motor Products.

 

[exp500]

AC Delco/Delphi or United DCR 820X cap /rotor , Packard or GM wires, NGK or AC plugs Bare minimum.
Coil, ICM, CPS only delco/delphi/GM.
There is a substantial improvement to upgraded injectors in driveability and engine life.
You guys covered it well! He should still do a driving fuel pressure check when missing on #6, If you change injectors do lower intake gaskets also.

Eman- Which grade of Standard Motor Parts? And how do you determine which is which?

 

[YukonGTmaster]

Thank you for the responses.

To the question asked, I did regap the plugs when I put them back in as they were no longer proper.

@YukonGTmaster, I did notice a few times the RPM jump when going into "Park," but I only recall it happening 2-3 times. I have not seen a drop in coolant.

It looks like I will be ordering some new plugs, wires, cap and rotor. Is there a consensus on brands to stick with or avoid? Just sticking with AC Delco? I've always heard good things about MSD when it comes to ignition, distributors, wires, etc.

Is there any benefit in making the switch from the CFI to the MPFI? Is the new system that much better?

MSD makes good stuff. I used to run their fat 10mm plug wires. I went back to the factory 8mm AC Delco wires and plugs. I’ve never had a problem with AC Delco cap and rotors.

The MPFI is a real upgrade in fuel delivery. The CFI is a weak link, as are the original lower intake gaskets. The upgrade is a long weekend project, IF you don’t find other stuff wrong. Here are some of my adventures below.

5.7L Vortec Problems

I was having a rough idle, and the Amber engine light would flash (Random misfire) when I started up in the morning. I would cycle the key off for ten seconds, the light would go out and I would drive to work. It was not latching any codes. I had a bad coolant leak as well. Then I started to...

www.tahoeyukonforum.com

 

[HoeLV]

AC Delco/Delphi or United DCR 820X cap /rotor , Packard or GM wires, NGK or AC plugs Bare minimum.
Coil, ICM, CPS only delco/delphi/GM.
There is a substantial improvement to upgraded injectors in driveability and engine life.
You guys covered it well! He should still do a driving fuel pressure check when missing on #6, If you change injectors do lower intake gaskets also.

Eman- Which grade of Standard Motor Parts? And how do you determine which is which?

Thanks. I placed an order prior to suing you reply.
Here is what I ordered:

AC Delco D465 rotor
AC Delco D329A cap
AC Delco 9718Q wires
AC Delco 41-993 plugs
Standard FJ504 injectors w/ gaskets and bracket

 

[BG1988]

How is the fuel trims?

 

[HoeLV]

MSD makes good stuff. I used to run their fat 10mm plug wires. I went back to the factory 8mm AC Delco wires and plugs. I’ve never had a problem with AC Delco cap and rotors.

The MPFI is a real upgrade in fuel delivery. The CFI is a weak link, as are the original lower intake gaskets. The upgrade is a long weekend project, IF you don’t find other stuff wrong. Here are some of my adventures below.

https://www.tahoeyukonforum.com/threads/5-7l-vortec-problems.125550/[/URL

http://

MSD makes good stuff. I used to run their fat 10mm plug wires. I went back to the factory 8mm AC Delco wires and plugs. I’ve never had a problem with AC Delco cap and rotors.

The MPFI is a real upgrade in fuel delivery. The CFI is a weak link, as are the original lower intake gaskets. The upgrade is a long weekend project, IF you don’t find other stuff wrong. Here are some of my adventures below.

5.7L Vortec Problems

I was having a rough idle, and the Amber engine light would flash (Random misfire) when I started up in the morning. I would cycle the key off for ten seconds, the light would go out and I would drive to work. It was not latching any codes. I had a bad coolant leak as well. Then I started to...

www.tahoeyukonforum.com

Well shit, I don't have time to deal with all that. Hopefully what I have goes somewhat smoothly. While I do enjoy riding the motorcycle, there is a reason its not my only mode of transportation any longer.

 

[HoeLV]

How is the fuel trims?

Let me see if I can find the logs on my phone. I believe i set it up to log those along with the O2 sensors.

 

[HoeLV]

UPDATE:

I went ahead and replaced the following:

AC Delco D465 rotor
AC Delco D329A cap
AC Delco 9718Q wires
AC Delco 41-993 plugs
Standard Motor Products FJ504 injectors w/ gaskets and bracket
Fuel Filter

While at it, I put in a new AC Delco A1301C air filter and did an oil change.

Everything ran great, drove about 400 miles with everything smooth as silk, no misfires, etc.

Then out of nowhere, I started getting pretty consistent misfires on cylinder 2 once it warms up. Any suggestions to look for? I checked the plug wire and it's not loose. I am going to try and pull the plug this evening (work hours have been crazy the last few days).



And since you asked, here are the 8 plugs when I pulled them out.

What has been said. Replace plugs, cap, rotor and wires, especially since you think the wires may be OE. Deposits on those plugs and would love to see all 8 plugs when compared.

 

[exp500]

What is your plug gap? I don't see any anti-sieze on the threads either.