Misfire, 60-70mph at Cruise

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jackrabbitZ71

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Hi all, relatively new to the forum but spend a good amount of time lurking. I have a 1999 Tahoe Z71 5.7 Vortec. I bought the truck last February, and around September it developed a misfire at highway speeds, but only when ascending a hill trying to maintain 65-75mph. The truck had roughly 183k on it when the problem first occurred, and I wanted to do a full refresh anyway. Since then, I've replaced:
Plugs, ACDelco Rapidfire 14
Wires, SMP Blue Streak 8.8
Cap+Rotor, UMP Vented/Brass contacts
Distributor, UMP Aluminum
MPFI injectors, Delphi
Fuel Pres. Regulator, Delphi
Ignition Coil, Delphi
ICM, Delphi
Fuel Pump, Delphi
Air Filter, Wix
Fuel Filter, Wix
Alternator, ACDelco
Upstream O2 Sensors, Denso
MAF Sensor, ACDelco
MAP Sensor, Delphi
IAT Sensor, ACDelco

I have also tested for Vacuum leaks using the propane method AND a proper smoke test, no issues there. All Cylinders hold compression at roughly 180 cold, tested twice with two different gauges. No codes for this misfire and I'm absolutely stumped. It runs AMAZING at anything below 70mph but the moment I try to maintain highway speed going uphill, it misses like crazy. If I give it more throttle and let it kick down, it runs fine, but only when ACCELERATING up the hill. I for some reason can't simply maintain my speeds without the truck misfiring. My thoughts would either be a weak spark, or maybe some sort of EGR issue? Any help is appreciated.
 

jackrabbitZ71

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Edit, I forgot to mention that the truck also has a new Delphi TPS, properly adjusted. The truck HAS thrown a P0305 ONCE. That was when I started checking compression and tested for vacuum leaks.
 

L31MaxExpress

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Time to do some scanning with software that allows for logging good data and see what the PCM is seeing to make sense of it. When I am not using HP Tuners, I use OBDLink. It scans anything with Generic OBD2 data and I have GM, Dodge and Nissan advanced parameters loaded for it. I have the mobile app for it that runs with an Android device and a MX+ bluetooth adaper. The PC version runs either bluetooth or through an OBDLinkSX or EX cable.

Just because it is a new part does not make oy good, especially the ignition parts. The only cap and rotor I will use on these engines is a Mercruiser marine unit. Those BlueStreak wires have been working great for me. When you installed the MPFI spider, did you cross the fuel tubes as required. I have seen two of these trucks with the spider installed incorrectly now and having issues related to the wrong injector feeding the wrong cylinder and spraying at the wrong time. I have had to shim distributors right out of the box even a GM one to prevent the CMR and thus rotor from jumping around. I am assuming the CMR is set correctly and you performed a crank relearn after.
 
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jackrabbitZ71

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Time to do some scanning with software that allows for logging good data and see what the PCM is seeing to make sense of it. When I am not using HP Tuners, I use OBDLink. It scans anything with Generic OBD2 data and I have GM, Dodge and Nissan advanced parameters loaded for it. I have the mobile app for it that runs with an Android device and a MX+ bluetooth adaper. The PC version runs either bluetooth or through an OBDLinkSX or EX cable.

Just because it is a new part does not make oy good, especially the ignition parts. The only cap and rotor I will use on these engines is a Mercruiser marine unit. Those BlueStreak wires have been working great for me. When you installed the MPFI spider, did you cross the fuel tubes as required. I have seen two of these trucks with the spider installed incorrectly now and having issues related to the wrong injector feeding the wrong cylinder and spraying at the wrong time. I have had to shim distributors right out of the box even a GM one to prevent the CMR and thus rotor from jumping around. I am assuming the CMR is set correctly and you performed a crank relearn after.
I've definitely learned the hard way that new doesn't mean good with my ignition. First Delphi ICM was dead out of the box, and a Hitachi coil that died in a week. I actually just pulled off my upper intake again to check my injector tube routing, all is well. CMR has been learned and is at 0. A customer of mine and mechanic is letting me borrow his Autel to properly gather some data, though just about all professional opinions I've gotten have been to replace the timing chain.
 

Schurkey

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just about all professional opinions I've gotten have been to replace the timing chain.
If the cam sensor/crank sensor synchronization is within spec, and the cranking compression test is reasonable, why would you suspect the timing set?

At least CHECK the timing set by rolling the crank back 'n' forth while watching the rotor movement. If the rotor doesn't move with ~5 degrees crank rotation, either the timing chain has slack or the drive gear/driven gear on the cam/distributor are worn.
 

Road Trip

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A customer of mine and mechanic is letting me borrow his Autel to properly gather some data, though just about all professional opinions I've gotten have been to replace the timing chain.

1) If the timing chain was the root cause, then I think that you would be able to see this via a semirandom varying
CMR value when the engine is held at the RPM where the misfire occurs? (What is your vehicle's RPM at 70+?)
Note: In the old days we would use a timing light to see the spark timing jumping around due to the cam-driven
distributor being jerked back & forth by the flailing timing chain.

But by the same token, a flailing timing chain would *not* affect the spark timing on a Vortec V8. This is because
the Initial (Base) timing is set (ahead of the timing chain) by the CKP sensor and the 4x reluctor on the crankshaft's
front snout.

Then again, a loose timing chain would cause the exhaust valve closing / intake valve opening points to be jumping
around...but I don't know if a handful of cam degrees would give you the misfires that you are experiencing. Again, I would be
interested in the CMR readings when the problem is occurring.

2) Valve spring surging. Sometimes with older engines with a lot of miles on them, the valve springs get weaker over time,
and the valves bouncing on their seats allows dynamic compression to bleed away (closing intake) or unplanned EGR backflow. (closing exhaust)

The problem with this theory is that weak valve springs rarely go on to perform *better* at high rpm.

3) Possibly the misfire is load based? For example, let's say that in overdrive, any time you attempt to cruise
over, say 3200 rpm, the misfire occurs? What if you were to shift into 3rd and continue to drive at the same speed?
Does the misfire continue despite a higher engine RPM? (ie: Is the misfire primarily a function of how much HP it takes
to push the frontal area of your truck through the atmosphere at 70+ mph?)

Or, let's test to see if it's more a specific RPM issue vs a load issue:

* Get the misfire to show in Overdrive. 3000 rpm?
* What if I then downshift to Drive and stay at the same RPM. Does the same misfire still occur?
* What if I downshift again, & now I am in 2nd at 3000 RPM? Does the same misfire still occur?
* What if I downshift again, and now I am in 1st gear at 3000 RPM? Does the same misfire still occur?
* Lastly, with the truck stationary in the driveway, can I hold the throttle open at 3000 rpm & the same misfire still occur?

Note: Between these test strategies (the first set = speed/load as a constant, RPM as the variable, whereas
the 2nd set of tests are keeping the engine RPM a constant, with the vehicle speed a variable) ...the results
are going to point me in different directions.

NOTE: A rule that's been proven to me countless times over my career is that you can't fix an
intermittent until you can first figure out how to Break It At Will. If I'm reading your original post correctly,
you can indeed break your GMT400 at will by attempting to drive it at 70+ in Overdrive.

So what I'm actually looking for is increased detail surrounding the failure. Is it load based? Or RPM based?

If it's load based, then L31MaxExpress's live data capture/analysis is going to be the key to success.
(Misfire counts, Long Term Fuel Trims, sensor readings, etc.)

If it's RPM based, now it's time to take a closer look at the timing chain, surging valve springs, or other
dynamic mechanical malfunctions.

NOTE: It could even be something downstream of the engine. (Think TCC slipping/'Chuggling' under load.)

There's also a GM TSB written about the exhaust valve guides in the center cylinders of the GMT400-era small
block being just a bit too tight, causing the exhaust valves to start to hang open, but only when towing or
climbing hills at Interstate speeds?

****

I know that I covered a lot of ground, and probably generated more questions than answers for you. But if you
could come back with some additional observations tightening the problem description (if the misfire is load based vs
engine RPM based) , this would allow us to start the process of elimination. Or at least put together a short list of
possibilities, organized in order of descending probability.

I have no doubt that with enough focus and shared data that the members of this community will be able to tease
out what has reached the end of it's service life and is letting you know via the irritating misfiring at speed.

But I'd hate to see you speculatively change the timing chain at this point, only to have it not be the root cause of
your problem. More data is needed.

Best of luck with the hunt. Let us know what you discover.

Cheers --
 
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jackrabbitZ71

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1) If the timing chain was the root cause, then I think that you would be able to see this via a semirandom varying
CMR value when the engine is held at the RPM where the misfire occurs? (What is your vehicle's RPM at 70+?)
Note: In the old days we would use a timing light to see the spark timing jumping around due to the cam-driven
distributor being jerked back & forth by the flailing timing chain.

But by the same token, a flailing timing chain would *not* affect the spark timing on a Vortec V8. This is because
the Initial (Base) timing is set (ahead of the timing chain) by the CKP sensor and the 4x reluctor on the crankshaft's
front snout.

Then again, a loose timing chain would cause the exhaust valve closing / intake valve opening points to be jumping
around...but I don't know if a handful of cam degrees would give you the misfires that you are experiencing. Again, I would be
interested in the CMR readings when the problem is occurring.

2) Valve spring surging. Sometimes with older engines with a lot of miles on them, the valve springs get weaker over time,
and the valves bouncing on their seats allows dynamic compression to bleed away (closing intake) or unplanned EGR backflow. (closing exhaust)

The problem with this theory is that weak valve springs rarely go on to perform *better* at high rpm.

3) Possibly the misfire is load based? For example, let's say that in overdrive, any time you attempt to cruise
over, say 3200 rpm, the misfire occurs? What if you were to shift into 3rd and continue to drive at the same speed?
Does the misfire continue despite a higher engine RPM? (ie: Is the misfire primarily a function of how much HP it takes
to push the frontal area of your truck through the atmosphere at 70+ mph?)

Or, let's test to see if it's more a specific RPM issue vs a load issue:

* Get the misfire to show in Overdrive. 3000 rpm?
* What if I then downshift to Drive and stay at the same RPM. Does the same misfire still occur?
* What if I downshift again, & now I am in 2nd at 3000 RPM? Does the same misfire still occur?
* What if I downshift again, and now I am in 1st gear at 3000 RPM? Does the same misfire still occur?
* Lastly, with the truck stationary in the driveway, can I hold the throttle open at 3000 rpm & the same misfire still occur?

Note: Between these test strategies (the first set = speed/load as a constant, RPM as the variable, whereas
the 2nd set of tests are keeping the engine RPM a constant, with the vehicle speed a variable) ...the results
are going to point me in different directions.

NOTE: A rule that's been proven to me countless times over my career is that you can't fix an
intermittent until you can first figure out how to Break It At Will. If I'm reading your original post correctly,
you can indeed break your GMT400 at will by attempting to drive it at 70+ in Overdrive.

So what I'm actually looking for is increased detail surrounding the failure. Is it load based? Or RPM based?

If it's load based, then L31MaxExpress's live data capture/analysis is going to be the key to success.
(Misfire counts, Long Term Fuel Trims, sensor readings, etc.)

If it's RPM based, now it's time to take a closer look at the timing chain, surging valve springs, or other
dynamic mechanical malfunctions.

NOTE: It could even be something downstream of the engine. (Think TCC slipping/'Chuggling' under load.)

There's also a GM TSB written about the exhaust valve guides in the center cylinders of the GMT400-era small
block being just a bit too tight, causing the exhaust valves to start to hang open, but only when towing or
climbing hills at Interstate speeds?

****

I know that I covered a lot of ground, and probably generated more questions than answers for you. But if you
could come back with some additional observations tightening the problem description (if the misfire is load based vs
engine RPM based) , this would allow us to start the process of elimination. Or at least put together a short list of
possibilities, organized in order of descending probability.

I have no doubt that with enough focus and shared data that the members of this community will be able to tease
out what has reached the end of it's service life and is letting you know via the irritating misfiring at speed.

But I'd hate to see you speculatively change the timing chain at this point, only to have it not be the root cause of
your problem. More data is needed.

Best of luck with the hunt. Let us know what you discover.

Cheers --
Thank you for such a thorough response. I don't want to do anything else based on speculation, as the parts cannon has already been fired. I do believe my misfire is load based rather than RPM based. I just drove a few miles on the freeway to replicate the issue with my fuel pressure gauge attached and sticking up from the cowl. The misfire manifests when I reach 65-70 MPH and overdrive kicks in. Just now, I let the problem persist for roughly 20 seconds, and the truck kicked me out of O/D. It then ran fine, but I can still hear the misfire (almost like a dead cylinder, maybe pinging?) especially when accelerating from a stop. Fuel pressure stays solid all the time its running, no drop offs. I'd like to check the exhaust valve guides, as I suspect some sort of valve issue at this point, or some sort of intake gasket leak. I'm really at a loss here. Going to try to collect some data, I'll come back with fuel trims in a moment. Maybe just get a professional diagnosis. This is my only vehicle and I don't have time to tear too deep into it.

EDIT: Drove it around in closed loop, here are my fuel trims from a freeze-frame after getting the p0305 code to manifest:
STFT 1: 3.9
LTFT 1: 7.8
STFT 2: 3.9
LTFT 2: 4.7

VSS: 61MPH
RPM: 2046
LOAD: 23.5%
ETC: 193F
TP%: 27.5
MAF g/S: 54.89

Im not too sure what this could be telling me other than it's running leaner than it should be. I think it may be Intake Manifold gaskets, as I am unable to find any visible leaks with a smoke test. Would the variation between my TPS % and the Load % have anything to do with it?
 
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JohnZ71

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I had this issue and chased it for a while. Turned out to be the crankshaft positioning sensor at the lower front of the block. I replaced it with a Delco one and used the shim that was included. The instructions said the shim may or may not be necessary so I used it just in case.

Problem was solved. I had to do a crank relearn of course but that was easy

But same issue, only misfired under load around 70mph. Check engine light would start flashing
 

jackrabbitZ71

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I had this issue and chased it for a while. Turned out to be the crankshaft positioning sensor at the lower front of the block. I replaced it with a Delco one and used the shim that was included. The instructions said the shim may or may not be necessary so I used it just in case.

Problem was solved. I had to do a crank relearn of course but that was easy

But same issue, only misfired under load around 70mph. Check engine light would start flashing
I actually did throw in a new Crank Position Sensor this morning, no difference unfortunately. Haven't done a crank relearn though.
 

JohnZ71

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I actually did throw in a new Crank Position Sensor this morning, no difference unfortunately. Haven't done a crank relearn though.
Do you recall if you used a shim or not? It’s possible the sensor is being hit by the crankshaft if the shim isn’t used which also causes misfiring.
 
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