2003 Chev 2500 HD Allison transmission stuttering (80 mph)

[toadgarten]

I have a 2003 Chevrolet 2500HD with 242,000 miles. I have recently been experiencing a stutter/hiccup in what I believe is a transmission issue.

There is a slight hesitation once or a couple in a row, but not consistently going down the road, just at random times. It happens at high speed without much incline/decline. The truck is loaded with a 2,000lbs truck camper and sits for several weeks to a month at time. I also recently experienced a lack of reverse when it had set for an extended period of time. The truck finally responded after short time. The hesitation of slight motion going down the Interstate can really keep a guy on edge.

I changed both filters and fluid at 232,000 miles. I also change the external filter once a year with about 10,000 miles between changes. The fluid has been checked hot and is full.

Any thoughts? Thank you.

[DmaxMaverick]

Easy one first. "lack of reverse when it had set for an extended period of time"
Most likely torque converter drain-back. It happens on many early models after parked for a while. There is a fix for it, but the cost and effort isn't worth it. When it happens, let it idle for a bit, turn off for a few seconds, then restart. It should work normally. It may or may not cause the SES lamp, but it will clear after a few restarts. If it doesn't correct itself or occurs more often (repeats after parked for less than a couple days), then dig further.

Regarding the "stutter/hiccup", check for stored codes, first. Don't wait too long, as minor codes disappear quickly from history. You may have to visit a capable shop to get the TCM codes if you don't have a capable scanner. Typical code scanners/readers will not capture the TCM codes, other than P0700, which is only the PCM code that the TCM requested the MIL.

Transmission stutter/shutter under load would certainly be accompanied by several other symptoms. First, the TCM will detect a speed differential (slippage), set the SES lamp, and very likely enter limp mode. If the SES lamp isn't on, with an accompanying P0700 code, it is very unlikely to have been a tranny issue. Almost any serious tranny faults will instantly cause limp mode.

At high speeds with a significant load (such as toting a camper), headwinds alone can cause a very significant load increase/decrease. Wind resistance is a much greater factor than the actual load mass. Load mass is constant, while wind resistance force increases exponentially as speed increases. If not in T/H mode (Tow/Haul), the torque converter may unlock from time to time with more throttle input, and you should notice a corresponding engine speed increase. Also, 80 MPH, if the truck is in stock trim with stock tires, places the RPM's near the pilot injection threshold, which can be 2500-2800, depending on throttle position and load. If you remain on the edge of that envelop, the pilot engage/disengage can seem like a "hiccup".

Check for codes. If there are none, the cause was much more likely an environmental effect.

[toadgarten]

Here are the codes:
P0089 - Fuel pressure regulator performance
P0101 - Mass Air Flow sensor performance

I just changed the fuel filters (factory and inline Racor). Any other quick check/changes to eliminate issues? Next steps?

[DmaxMaverick]

In no specific order. Do what's easiest or most convenient at the time:

Check for fuel filter/system leaks. Too much air or a restriction (kinked hose, etc.) can cause fuel pressure issues.

Check the entire air intake tract. Long parked periods allows critters to move in. Don't be surprised if you find the intake nearly completely blocked with rodent nesting material, and/or a damaged air filter or plumbing. I've seen this many times, including my own more than a few. I've had a car burn down in my driveway due to rodent damage.

Check the entire electrical harness. Look for any damage or harness/connectors out of place.

Replace the fuel filler cap. Make sure it's "Diesel Fuel Only", or it won't vent properly. They're cheap consumables.

Clear the codes and retest under similar conditions.

[More Power]

Here are the codes:
P0089 - Fuel pressure regulator performance
P0101 - Mass Air Flow sensor performance

I just changed the fuel filters (factory and inline Racor). Any other quick check/changes to eliminate issues? Next steps?

The P0089 code indicates that there is a beyond spec mismatch between actual and requested fuel rail pressures. The problem could be due to a weak FPR (Fuel Pressure Regulator), but the more likely is a fuel problem like what DmaxMaverick mentioned. It could also mean a problem with worn fuel injectors leaking more fuel than the high pressure pump can overcome. These sorts of problems (leaking injectors) happen more often in the summer than in winter due to hot thin fuel and low viscosity.

[toadgarten]

The truck was connected with a monitor and put under some load going up a hill with slide-in camper. We could not recreate the same stutter I was hoping for. The monitor did show the actual fuel pressure was very close or same to the needed pressure (23,000). The problem did arise in very hot weather (95+degrees) and the test was done in about 85 degrees.

After the ride the injectors rates were:
Cylinder 1 0.16
Cylinder 2 -0.84
Cylinder 3 0.84
Cylinder 4 0.3
Cylinder 5 -0.44
Cylinder 6 0.53
Cylinder 7 0.41
Cylinder 8 -1.05

One code was noted before and after the ride: P0380 - Glow Plug feedback circuit

I did tighten the Mass Air Flow sensor screws, replaced the fuel tank cap and as said before, changed the fuel filters. Not sure what to think, but maybe all or one of the suggestions might have fixed it.

[More Power]

The injector fuel rates look really-really good.

FYI - my 2001 GMC will stutter a bit when under mostly full pedal - and running unloaded. I suspect it's related to the FPR. It's done this for a long time, but it's not interfered with my use of the truck (code for "not worth the cost of replacement"). And, a GM tech said the requested/actual rail pressures were in spec during a recent service visit.