P0128 DTC Explanation

[CCR91]

Hello,

I have a 2001 Duramax/Allison and have a question concerning DTC P0128.

After driving for about 8 hours straight with a temp of about 170 (it took forever to get to 170), I had the SES activate with a corresponding DTC P0128. Just prior to and during the this code, the engine ran loud as if the pre-injection was off, idled about 350 RPM higher; basically it acted as if it was overfueling. This makes sense because that code is set to indciate the water temp has not come up to expected levels.

I had it reset and the engine was smooth as before. I drove another 8 hours straight 2 weeks later and by the end of that trip, the engine was running as if overfueling again but with no SES light. I started it the next morning after sitting all night, still rough and overfueling.

I was able to change both thermostats and it still ran as if overfueling, still no SES, and the temp now comes up quickly to 185-190. I used a Tech II to scan for codes and a P0128 had been set again. Once cleared, it runs silky smooth. I have yet to run it for 8 hours straight.

Can anyone explain why my truck seemed to go into some sort of limp mode with this code set even though the temp was no longer a problem (either totally cold or after the thermostats had been replced and code still present).

I would have thought once the ECM received correct water temp vs. run time, the engine would run like normal.

Thanks for the explanation,
Corey

[More Power]

When I first began reading your message, I thought one or the other t-stat may have stuck open.

Since they have been replaced, what you're seeing could be the result of the codes being set along with the specific driving conditions since the t-stat replacement. I'd give it while to see if it repeats.

Is it cold where you're driving? Do you have the winter front on the truck? Is the fan-clutch disengaging soon after a cold startup?

Bascially, DTC P0128 means that the engine coolant did not reach a preset target temperature within the calculated length of allotted time.

Jim

[Mark Rinker]

Maybe you have a bad coolant temp sensor?

[CCR91]

I agree that one of the thermostats was probably sticking open. The ambient temp here in Virginia/Tennessee was in the 30s. No winter front or excessive fan clutching was noticed.

With the new thermostats installed, the temp comes up much faster; the ECT sensor appears to be working correctly.

I am just wondering why the engine continued in the overfuel mode when problem had been resolved. Continuing in a mode even though the error parameters no lpresent seems like a poor design.

On a separate note, can anyone definitively pass along which way the thermostats are to be oriented? When I removed the old ones, the rear t-stat had the senson probe up and the forward was down. The Helm's manual pictured them both with the sensor down so that is how I reinstalled them. I figure since the probe should be on the same side as the hot water and the flow goes from engine through t-stats to radiator, that is correct.

Thanks,
Corey

[More Power]

http://www.thedieselpageforums.com/photopost/data/500/thermostats.jpg

Some trouble codes persist even after resolving the problem. You may need to clear the codes or complete a prescribed number of ignition cycles before the ECM will clear them on its own.

Jim