Request help: L31 5.7 Cylinder 5 Misfire

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

Joined
Nov 14, 2022
Messages
7
Reaction score
4
Location
Quantico, Virginia
No offense taken at any questions or suggestions because I am clearly missing something…and I hope it’s obvious and easy.

I agree that number 5 does not look like it’s firing.

Truck definitely runs like it has a misfire. Rough idle and rough running throughout operating range when cold. Idle gets smoother as the engine warms, but not like it should be. Rev it when warm or take in on the road and the misfire is immediately discernible.

The misfire counter on the scan tool backs up the seat-of-the-pants feel by logging misfires on cylinder 5, and cylinder 5 only, throughout the engine’s operating range. Interestingly, the DTC for a misfire specific to cylinder 5 does not set. I only get the P0300 DTC.

The misfire was present from the first start up with with new engine. It was also present in the old engine. The crack in cylinder 5 on the old engine led me to believe that was my problem. Clearly, the crack was not my only problem. Maybe it wasn’t even contributing to the misfire.

There are no visually observable mechanical issues with the valve train when cranking the new engine. Copy on resetting valve lash.

Visual inspections of the intake runner and fuel injector hole before assembly and installation indicate no debris or obstructions of any type.

No coolant or carbon build up in the exhaust runner. The cylinder heads are new, too, and were inspected before installing the intake manifold.
 

b454rat

I'm Awesome
Joined
Jul 25, 2019
Messages
1,759
Reaction score
1,482
Location
Windsor NY
When I first got my Yukon, did the usual, plugs, wires, cap n rotor. When I put the wires on, it looked like #4 wire wasn't on the cap all the way. Make sure the electrode is in the boot, not half way out. I thought that was my fix to the misfire, but never (hardly ever) that easy....
 

Komet

I'm Awesome
Joined
Nov 12, 2022
Messages
742
Reaction score
1,938
Location
Skagit Valley, WA
Seems like you've been everywhere, which means something you believe to be true, is not true. Two bad distributors?
 
Joined
Nov 14, 2022
Messages
7
Reaction score
4
Location
Quantico, Virginia
I don’t think two distributors are bad because I put the second one back in the truck I ”borrowed” it from, set the cam retard offset to zero, and that truck runs fine.

I’ve had the ignition wires on and off the plugs and cap about four dozen times between swapping plugs and distributor changes. Those electrodes and boots can be a pain, but I don’t think I got number 5, and number 5 only, unseated each time.

All that said, I got something wrong. Going back to the beginning with troubleshooting, including all of the suggestions you guys provided.

Standing by for any more thoughts.
 

HotWheelsBurban

Gotta have 4 doors..... Rawhide, TOTY 2023!
Joined
Sep 18, 2019
Messages
10,171
Reaction score
18,933
Location
Houston, Texas
I had a misfire issue on my Burb last year('99 L31 Vortec 5.7) and I noticed when I changed the plugs, cap and rotor, that #6 wire was loose on the plug end. So I made sure it was back on properly when I finished, and the truck did run better after the work. I checked that wire once or twice since then, and it was staying on.
We've been driving the crew cab since the Burb is in the shop, and it's been running well. If you have to sit at a long light, or in a drive thru, or a lot of stop'n'go traffic, it begins to idle a little roughly. But as soon as it gets to move some, and get the rpms up, it smooths out. While we're having fun December like weather, I'd like to get the cap, rotor and plugs changed, since I have the parts. But I almost hate to mess with it, when it is running great.....
 

blackdeathmessenger

OBS Enthusiast
Joined
Aug 31, 2020
Messages
63
Reaction score
43
Location
Denver, CO
Back to the fact that you put another engine in the vehicle, did you put in a used engine, did you build it, etc? I believe Vortec heads are prone to cracking, so that would be a possibility if there's no issues with this block
 

Schurkey

Supporting Member
Supporting Member
Joined
Jan 14, 2018
Messages
11,682
Reaction score
14,856
Location
The Seasonally Frozen Wastelands
Symptom - Cylinder 5 misfire at all engine speeds - P0300 DTC; no other DTCs. Diagnosed with Snap On Solus Pro scan tool.

New GM L31 replacement engine (old engine had a crack in cylinder 5 and the same P0300 misfire in cylinder 5 only)

New GM multi-port fuel injection (MPFI) meter replaced central sequential port fuel injection (CSFI) meter
Is the mult-port injection "new" to the new engine, or was it carried-over from the "old" engine?


Ignition system inspections and tests

  • Ignition wires and coil wire passed resistance test.
  • Tested for arcing, cross-firing, and electro-magnetic interference with no issues found.
Run engine in the DARK, open hood, (remove under-hood light bulb) look at plug wires for "corona" glow.


  • Tested spark on cylinders 1, 3, 5, and 7. Adequate spark on all cylinders.
HOW are you testing for "adequate" spark?

Other
  • Long term fuel trim on Bank 1 was 154 counts. Typical value is 127 counts.
  • Heated oxygen sensor Bank 1, Sensor 2 voltage varied widely just like Sensor 1. Bank 2, Sensor 2 was steady as expected for the H02S downstream from the catalytic converter. This variation indicates to me that there is too much unburned fuel in the exhaust in Bank 1, probably from the misfiring cylinder 5.
Long-term fuel trim in rich-command is likely due to the Bank 1 upstream O2 sensor showing false "lean" response due to misfire. Fix the misfire, no more false lean signal from the O2 sensor, and the fuel trim should return to normal. That means the other three cylinders don't get over-fueled.

The downstream O2 sensor voltage range indicates that perhaps the catalyst for Bank 1 has failed. When you fix the misfire, MAYBE the downstream O2 will show the catalyst is still functional...you can hope, anyway.


Check the ceramic for cracks. I've chased a misfire before to discover that one new plug had cracked ceramic resulting in weak spark and noticable misfire.
My '97 K2500 had a dead #8, and no torque converter clutch. Seller discounted the price. I figured maybe I'd have to pop the head to fix valves on #8, or maybe it had injector issues. And I was guessing a TCC solenoid replacement for the transmission.

I decided to do a compression test on #8; when I got the plug loose and reached down to unscrew it by hand...I felt the cracked porcelain. Compression test showed adequate compression. I grabbed a used spark plug from my boat engine; misfire gone.

And the computer locks-out the TCC when it detects misfire. So that "used-but-usable" spark plug fixed the engine AND the transmission.

The misfire counter on the scan tool backs up the seat-of-the-pants feel by logging misfires on cylinder 5, and cylinder 5 only, throughout the engine’s operating range. Interestingly, the DTC for a misfire specific to cylinder 5 does not set. I only get the P0300 DTC.
I don't understand that. That tool is capable of reporting cylinder-by-cylinder misfire codes.

The misfire was present from the first start up with with new engine. It was also present in the old engine. The crack in cylinder 5 on the old engine led me to believe that was my problem. Clearly, the crack was not my only problem. Maybe it wasn’t even contributing to the misfire.
I'm concerned that the crack was a result of the root problem, and it's gonna do the same to the new engine unless you get it found and fixed.

2 engines with cyl 5 miss fire seems to defy odds.
Not at all. My suspicion is that something failed on the first engine, which caused the cylinder to crack. The same part was transferred to the new engine, and is still causing problems with Cyl 5.

I suspect issues with the intake manifold. But I can't know that for sure.

In MY driveway, that engine would get some testing you haven't done.
1. Cranking compression test of all eight.
2. Cylinder leakdown test of #5, and at least one other cylinder (the one with highest cranking compression.)
3. Connect engine to an ignition oscilloscope. View spark-voltage patterns for all eight cylinders on the 'scope. Good luck finding someone with an ignition oscilloscope. They're essentially extinct tools. Used to be every gas-station service shop had one. Now, at best, automotive service shops have hand-held 'scopes that do one cylinder at a time.

But as I said...my guts say you've got an intake manifold problem. (I've been wrong before.) My suspicion regarding the intake manifold may be due to having a couple of failed intake gasket sets on my '88 TBI 5.7L causing certain cylinders to misfire (mostly at idle, though.) These gaskets are a year old, they replaced gaskets that looked exactly like them that were also a year or two old. I think the manifold is warped.

You must be registered for see images attach
 
Last edited:

Orpedcrow

I don’t know what I’m doing
Supporting Member
Joined
Aug 20, 2022
Messages
2,612
Reaction score
6,662
Location
East Texas
I think it’s gotta be a part used on both engines, leading to the intake.

My upper had a handful of cracks in it.

The lower gaskets aren’t the best design either.

Do you have a bore scope so you can look inside cylinder 5?
 
Top