Vortec 8.1 Popping or Misfire Under Load

[garyb1st]

The old Pace has more than 84,000 miles on the clock. It has performed flawlessly since we got it 9 years ago. However, for the last 1,000 + miles, it started backfiring. Initially it was almost unnoticeable. During the last 1000 miles, it got louder. But only noticeable on grades when the engine RPMs got close to 4,500.

I have the original owners service records. One record indicated the engine was "missing" from time to time, (owners description). The solution was to install upgraded spark plug wires. That was about 70,000 miles ago. So likely due for replacement.

Plug wires were apparently a known problem with the Vortec 8.1. Google says the problem could be wires, coils or possibly an exhaust manifold. In any event, I'll let my Heavy Duty Truck Mechanic diagnose the problem. I just hope he is familiar enough with the 8.1 engine to get it right.

 

[Gary RV_Wizard]

An actual "backfire" is rare in a computer managed, fuel-injected engine, so I'm guessing you are hearing/feeling a misfire. You would definitely notice a backfire! Faulty plugs or plug wires would be a likely explanation. Your mechanic will probably change spark plugs and wires as a first step, eliminating the sources for a bunch of potential & intermittent problems. Then see if the issue persists.

 

[garyb1st]

An actual "backfire" is rare in a computer managed, fuel-injected engine, so I'm guessing you are hearing/feeling a misfire. You would definitely notice a backfire!

The last few under load were quite noticeable. I've never heard of a misfire. How does that differ from a backfire and what is the cause?

Faulty plugs or plug wires would be a likely explanation. Your mechanic will probably change spark plugs and wires as a first step, eliminating the sources for a bunch of potential & intermittent problems. Then see if the issue persists.

While I'm 90% sure the old Pace is going to have a for sale sign in the window before long, I don't want to spend a lot of money trying this and that.

There was one time on the last trip when we had to pull over to minimize a major overheat problem. Long grade with no place flat enough to pull over and disconnect the toad and engine temps at 230º. After pulling over to let the engine cool the temps initially increased to almost 250º before cooling. Not sure how to diagnose for possible heat damage. While we'll likely sell the Pace, I don't want to sell it with undisclosed engine issues.

 

[Mr Lars]

4,500 rpm? High probability the engine is getting ‘oversped’. As for the overheating? Slow down, downshift, take your time. On prolonged grades, turn off the driver cabin a/c.

 

[Arch Hoagland]

Had a similair probblem with my 2004 8.1 and replaced the sperk plug wires along with the spark plugs which cured the problem.

Have you ever cleaned the MAF?

Are you getting any Check Engine Lights?

 

[Lou Schneider]

The last few under load were quite noticeable. I've never heard of a misfire. How does that differ from a backfire and what is the cause?

A misfire is failure to ignite the air-fuel mixture in the cylinder so you get a miss, or lack of power from that compression stroke.

A backfire is igniting the fuel mixture in the cylinder at the wrong time, while the intake valve is still open. The flame front propagates backwards through the intake manifold and "pops" through the carburetor. Sometime you'll see actual flames if the air cleaner is removed.

Or the spark can be delayed until the exhaust valve opens and the flame propagates through the unburned air and fuel into the exhaust, popping there. But a more common cause of exhaust popping is unburned air and fuel from a misfire reaching a very hot exhaust like during a hard pull and igniting there.

 

[garyb1st]

4,500 rpm? High probability the engine is getting ‘oversped’. As for the overheating? Slow down, downshift, take your time. On prolonged grades, turn off the driver cabin a/c.

Overheating only occurred once on a long grade that I wasn't prepared for. At the time we had slowed to 30 mph. Pretty sure the engine AC was off.

 

[garyb1st]

Have you ever cleaned the MAF?

It was looked at more than a year ago.

Are you getting any Check Engine Lights?

No.

 

[garyb1st]

A misfire is failure to ignite the air-fuel mixture in the cylinder so you get a miss, or lack of power from that compression stroke.

Hard to tell on some of the rougher roads because of the noise and bouncing. On smoother sections of the highway, I don't recall a lack of power (what I think of as a miss) occurring when I heard the noise.

 

[Mark_K5LXP]

I've seen references of some codes for the partial chassis 8.1L engine computer not setting the engine light. Specifically as it relates to mixture, but before I started throwing parts at it I would pull codes. Any problem significant enough to cause noticeable issues while running should leave a clue in the codes or running parameters.

Mark B.
Albuquerque, NM

 

[garyb1st]

I'll hook up my code reader and see if anything shows. Thanks for the suggestion Mark.

 

[TheBar]

I've never heard of a misfire. How does that differ from a backfire and what is the cause?

Two entirely different things. Generally you feel a misfire\hesitation which is a cylinder(s) failing to fire. But if you feel and hear a bang it is a backfire. Which is an explosion either in the intake manifold or exhaust system somewhere from the manifold to the muffler. Note the exhaust manifold is where the explosion happens but will not be the cause.

As said above new plugs and wires or fuel mixtures are the usual solution for a misfire. But there can be many other causes. Usually a short term issue doesn't hurt the engine.

A backfire "can" cause serious engine damage. There is a long list of causes. It can be as simple as carbon buildup in the cylinder. The carbon stays hot enough to ignite the fuel on the next intake stroke before the valve closes. There are several chemical solutions to help clear a carbon buildup. Years ago Chevy recommended slowly pouring water down the carb. But that can be dangerous and I would google that option first.

A fuel mixture that is too rich can also cause a backfire. Unburnt fuel can ignite in the exhaust system. That should show up in the OBDII codes and will most likely be caused by the oxygen sensor.

But a backfire can be caused by sticking or failing valve lifters. Changing the oil and/or using chemical additives may fix a sticking lifter.

Sticking lifters are more common on RVs that sit idle for long periods. But it can also be a failing lifter which would have to be replaced. Failing lifters are often caused by rust or congealed oil (which is why I start my engine once a month).

 

[Kirk]

I'll hook up my code reader and see if anything shows.

If you have a code reader, I would make that my first step with any suspected issue.

 

[garyb1st]

Code reader indicates an Oxygen Sensor problem. I plan on bringing the Pace in for an oil change and I'll have the shop verify that and do what's necessary.

 

[Arch Hoagland]

A dirty MAF can cause that reading. Happened on mine too and changing the O2 sensors did not cure the problem. Cleaning the MAF cured the problem.

Code reader indicates an Oxygen Sensor problem. I plan on bringing the Pace in for an oil change and I'll have the shop verify that and do what's necessary.

 

[Gary RV_Wizard]

4,500 rpm? High probability the engine is getting ‘oversped’.

Nah. The GM 8.1L is rated to produce it's max HP at 4200 RPMs and has a "red line" of 5000 RPMs.

 

[Mark_K5LXP]

5K RPM sounds right. Odds are high it has a rev limiter. I think about what RPMs and gears I see (8.1/4L85E) and I don't think I've ever seen it break 4K. 70 on a flat road in top gear is like 2600. If a grade starts and it drops to third, I get what I get for speed. I guess I could put it in 2nd and let it scream but I wouldn't end up going much faster, it's out of RPM range.

Mark B.
Albuquerque, NM

 

[RayIN]

A misfire is failure to ignite the air-fuel mixture in the cylinder so you get a miss, or lack of power from that compression stroke.

A backfire is igniting the fuel mixture in the cylinder at the wrong time, while the intake valve is still open. The flame front propagates backwards through the intake manifold and "pops" through the carburetor. Sometime you'll see actual flames if the air cleaner is removed.

Or the spark can be delayed until the exhaust valve opens and the flame propagates through the unburned air and fuel into the exhaust, popping there. But a more common cause of exhaust popping is unburned air and fuel from a misfire reaching a very hot exhaust like during a hard pull and igniting there.

I once had a 1976 Allegro MH on a Dodge chassis. The engine would backfire and blow out an exhaust manifold gasket.
I "fixed" that problem with a can of gumout carb cleaner. I slowly trickled about ½ the can down the carb to clean the intake, then suddenly dumped the remainder down the carb and purposely stalled the engine.
Next morning I started the engine, which popped and smoked, then ran rough for a couple minutes, then smoothed out. That removed the carbon from the valves and allowed them to seal properly. Mechanic quoted nearly $500 to just replace exhaust gaskets without eliminating the root cause. I replaced the gaskets myself after fixing the cause.

 

[garyb1st]

Current plugs and wires installed in 2008. They have 67,000 miles of wear on them. $785 for new AC Delco plugs and wires. Not sure I will have them installed since I'm not sure that is the problem. Also not sure I'm keeping the Pace.

Mechanic doesn't know if they will fix the problem. However since they've been on the motorhome for 16 years and have worked fine for 67,000 miles maybe it makes sense to have them replaced.

But a backfire can be caused by sticking or failing valve lifters. Changing the oil and/or using chemical additives may fix a sticking lifter.

While there was less than 4,000 miles on the oil, the dash was saying it was time for a change. Not sure if that's simply because the mechanic decided that's when it should be changed or if the engine brain can make that determination. In any event, the oil has been changed.

 

[dktool]

$785 for plugs and wires !!
You need a new mechanic.