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

east302

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GM’s fuel pressure spec is 60-66 psi, so you’re technically there and any variance within a pound or so could just be an inaccurate gauge.

If you can hear it run and have no further codes or issues when starting, then I’d probably leave the pump alone unless something else comes up.

Besides, those things only fail when you’ve just filled the tank up. Murphy’s Law.
 
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HoeLV

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UPDATE:

I got the new distributor and cap installed. I verified the CMP retard is at -1.4°, so it's within the +- 2° allowance. Still have the misfire on cylinder 2.

In trying to determine the cause of the misfire, so far have done compression check, fuel pressure check (results a few posts back), swapped the cylinder 2 plug with cylinder 8, swapped the cylinder 2 plug wire with one from cylinder 5 with no changes, replaced distributor/cap (partially because it was the original and the screw tab had broken so the cap couldn't sit tight). The fuel injection system had recently been converted from CFI to MPFI. Everything ran great for the first 400+ miles, until the cylinder 2 issue popped up.

Thoughts on what to look at next?
 
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HoeLV

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GM’s fuel pressure spec is 60-66 psi, so you’re technically there and any variance within a pound or so could just be an inaccurate gauge.

If you can hear it run and have no further codes or issues when starting, then I’d probably leave the pump alone unless something else comes up.

Besides, those things only fail when you’ve just filled the tank up. Murphy’s Law.
Its putting out P0452 – Evaporative emission (EVAP) pressure sensor - low input, bad fuel tank pressure sensor for probably the last year. I've been putting it off since I'd have to drop the tank. From my understanding, the sensor being out shouldn't have any impact other than a CEL on the dash.
 

exp500

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Check your intake manifold for vacuum leaks. It's a small one if there so hard to find.
Next mechanical check for me would be a cooling system pressure check if you have lost any coolant.
What is your "normal" #2cyl misfire count? Might have to set a monitor or forget what it's called on your scanner.
Last is When were valves last adjusted?
My experience with fuel pressure on these is 60 is a "may run". Only one of the trucks I work with/have seen is reliably going at 60#. I have a couple guages, all match. I've changed a few FPR's (after pump change)to get 62# and thats why I recommend it(62#).
On your Evap - I have blown air thru vent to charcoal can & can to engine (gently if something is in there), just to be sure before trying anything else.Good luck.
 
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HoeLV

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Check your intake manifold for vacuum leaks. It's a small one if there so hard to find.
Next mechanical check for me would be a cooling system pressure check if you have lost any coolant.
What is your "normal" #2cyl misfire count? Might have to set a monitor or forget what it's called on your scanner.
Last is When were valves last adjusted?
My experience with fuel pressure on these is 60 is a "may run". Only one of the trucks I work with/have seen is reliably going at 60#. I have a couple guages, all match. I've changed a few FPR's (after pump change)to get 62# and thats why I recommend it(62#).
On your Evap - I have blown air thru vent to charcoal can & can to engine (gently if something is in there), just to be sure before trying anything else.Good luck.
If there is an intake manifold leak, I'd have to assume it would be in the vicinity of cylinder 2, right? The P0302 code is the only misfire Code so far and whenever I've checked the misfire counts, it's solely coming from 2. I just set for a log to show the misfires and going to go on a quick drive.

I don't mind replacing most items, especially when the truck has 223,500 miles on it and when it comes to alot of things, they appear to still be original. With that, ideally, would like to get the misfire resolved and then start replacing items as preventitive/necessary, ie. the plan I mentioned for suspension/steering next. I say that, regarding the fuel pump, if it is caring the misfire, I'd for sure swap it now. If that's not likely, I'd put it off until a bit later or it became necessary. I am guessing the FPR is fuel press. regulator. Are there additional tests to run and either rule out/show cause for the fuel pump/ regulator?
As for the valves, I haven't done them since I bought it at 176,000 miles. Before that, I assume it is still original as I've not come across much that wasn't and I have zero records to tell me different.

I'll post those misfire counts later this evening.

EDIT: The log showed just shy of 6000 (history only) in cylinder 2, however that number gradually grew from under 1000 the longer the truck was running and the "current" was zero. You can feel the misfire as it is running rough, not just triggering the CEL.

Thanks for your help!
 
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exp500

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Is the #2 Spark plug wire fully seated in the cap? Dielectric grease on boots? These trucks are right at the limit of what a distributor can do reliably. Have you tried a late night spark test with a spray water bottle mist looking for sparks? Trying to think of easy things to look for. Intake leak with carb cleaner, small dribble along seams or likely spots. Check the heater/AC vacuum line at firewall.
What are your Fuel trims when steady cruising? Fuel trims way hi= fuel pump.
Not knowing the history, fuel pump moves up the list for me.
Also check your front 2 O2 sensors with scanner, that they follow what the engine is doing and that voltage has a full up/down range.
I kinda feel your desire for a definate fix. On these trucks Ignition before Fuel is normal sequence.

You could figure out a way to pinch off or valve off fuel return line to tank to see if fuel pressure responds accordingly.
 
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HoeLV

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Is the #2 Spark plug wire fully seated in the cap? Dielectric grease on boots? These trucks are right at the limit of what a distributor can do reliably. Have you tried a late night spark test with a spray water bottle mist looking for sparks? Trying to think of easy things to look for. Intake leak with carb cleaner, small dribble along seams or likely spots. Check the heater/AC vacuum line at firewall.
What are your Fuel trims when steady cruising? Fuel trims way hi= fuel pump.
Not knowing the history, fuel pump moves up the list for me.
Also check your front 2 O2 sensors with scanner, that they follow what the engine is doing and that voltage has a full up/down range.
I kinda feel your desire for a definate fix. On these trucks Ignition before Fuel is normal sequence.

You could figure out a way to pinch off or valve off fuel return line to tank to see if fuel pressure responds accordingly.
I've checked the wires, moved them around, made sure they were on good, same with plugs. Dielectric grease on both ends of each wire.

I am going to seem if I can get a buddy to help today/tonight to check for vacuum leak and will do the water bottle test as well.

The more I think about it, I am tending to lean towards fuel pump as well. If it's already just barely at the suggested range, that fuel trims.jpgis the furthest point to push fuel.
I happened to have the fuel trims log last night as well. Here is a quick graph of the data.
 

exp500

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With those fuel trims, fuel pump should cure it.
 
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HoeLV

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With those fuel trims, fuel pump should cure it.
Fuel pump and not the regulator?
I am genuinely asking why it would be one and not the other? I'm not a mechanic obviously, it's not my field of expertise. I understand the basics and trying to understand a bit more.
 

exp500

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Regulator bleeds off pressure, it seems yours has trouble getting there, so=pump.

P.S- Charting fuel trims is best done on hiway steady state cruise.
 
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