Fuel Temp sensor

[LarryM]

Took my pickup into the dealer for some warrenty work today. Needed to fix loose weather strip under hood, put new pads on the rear overload leafs, fix leaking rear axle seal, and I wanted the latest reflash for the "Trussman Syndrom".

Got the call from the service guy and he tells me all is being fixed except I have the latest PCM/TCM programing (which I know I don't), but my fuel temperature sensor is defective. He claims this will effect transmission performance due to fluctuating fuel pressure. I guess if I'm short on power due to low fuel delivery, it would cause early downshift. Sure didn't seem like I was short on power though. There was never a SES light.

Anyone got an idea what this is really all about?

[mark45678]

sounds like the TEC doesnt want to do a reflash? Its really hard to tell those guys that every one on the internet know more then they do..... BE CAREFULL HOW YOU WORD THINGS these guys have a lot of pride in what they do and most of the time it backfires if you tell them there job!!!! my.02 ;)

[Kennedy]

Sounds goofy to me. Maybe there was a history code (bogus) for the fuel temp fault. Now you mentioned pressure and temp sensors in the same paragraph, so keep that in mind.

Anyhow, the latest updates address false codes, and I am told that when one digs deeper, the update involves the bogus fuel tmp sensor codes. I would twist his arm to update, and then see what reoccurs...

[LarryM]

I must admit that I am a bit confused.

The service writer indicated that a faulty fuel temperature sensor could cause fluctuations in the fuel pressure. He did not elaborate. I suspect because that's all the service tech told him.

When I checked the GM VCI web site for my VIN, the number that SteveO posted last month as being the updated calibration addressing 2-1 shift, shift business, and OD lockout did not appear. As I understand it, this would mean it is not available for my truck.

When I last had the thing in, they uploaded all the latest stuff, but that was back in November or December. (Paperwork in truck at dealers, can't check date) Since there are no dates listed on the VCI page, I have no idea what, if anything, I'm missing. The service writer was polite, but adamant that they checked my current revision levels and there were no updates available for my truck.

[mdrag]

LarryM,

TCM updates are not listed on the GM website, only ECM updates are there. I've not heard of a site for the TCM updates...

IIRC, the 2-1 shift and OD lock-out are TCM updates.

[LarryM]

I think I knew that at one time. CRS is a terrible thing, but I can't remember why!

If I recall the discussion about that update, there was a PCM calibration as well as a TCM calibration that needed to be done. I could only find a number for the TCM when I did a search. Looks like I'm going to have to do battle at the dealer again today.

[LarryM]

Update:

Got truck back from dealer this afternoon. Here's the results:

Wobbling belt tensioner is normal. First bearing with 0.125 runout I've ever seen that was "normal".

Overlaod pads replaced with new pads plus slip on cushions and spring leafs re-aligned.

New rear shocks ordered.

Under hood weather strip re-secured.

Excessive transmission downshifting and hunting determined to be possibly caused by defective fuel temp sensor. (Code PO181) Sensor replaced and tested OK. They still insist that there are no available software updates for my truck. This discussion wil be continued when I go in for the new shocks.

For what ever it's worth, it seems to have smoother power delivery and better response at the low end. Just seat-of-the-pants observations. I'll see what happens next week when I hook up the trailer and run 1500 miles at 16K GCW.

[pinehill]

LarryM,

Ask your tech to do a printout of the current flash. They can do this for you in about 5 minutes. Then compare it to what you see on the GM site.

[Kennedy]

While it is definitely possible the sensor was bad, i kinda doubt it. Best way to tell would be to overnight it in the shop and key up to compare the reading to the other sensors.

The update for false codes MAY be only for the 2002 models.

[D-max Man]

LarryM,

On the 2001 model year, the fuel temp. is compared to the inlet air temp. (IAT) and if the difference between the two is too great, it will set the P0181.

The IAT is part of the Airflow Sensor, which is mounted in the air filter box. The problem is that it is right on top next to the hood. When the truck is parked and the sun is beating down on the hood, it will warm up the IAT so when you first start the truck, you will have a false IAT reading until the airflow cools it down.

This is what causes the P0181 code to set on the 2001 models even though there isn't anything wrong with the fuel temp sensor.

On the 2002 models, they reference coolant temp instead of inlet air temp to solve this problem

[ 05-22-2002: Message edited by: D-max Man ]</p>

[LarryM]

D-max Man:

Thanks for the info. That explains the PO181 code. I wonder why that never flashed the SES light? Not important enough to bother?

What does the computer do with this information? Why is the relationship between inlet air and fuel temperatures important enough to measure? Is this just another funky emissions thing?

In spite of the code, the service people insisted that the fuel temp sensor was indeed defective. Time will tell if it matters or not.

[LarryM]

Went back to the dealer to have the new rear shocks installed this morning. I showed the service writer the information on the latest reflash provided by SteveO. He said that they would be able to find it and do the flash with that info.

The service order states it was reprogrammed and checked. I can't really tel any difference yet, but have not pulled the big trailer up any hills yet. I did make a dump run with the little trailer (2500lbs.) and did the first gear re-train from 55. It did not jerk me through the windshield this time, however it did pull down very smooth and quickly.

Next week I'll put on about 3000 miles with a 7000 lb. trailer and will report back on the preformance when I get back.

[Diesel Crat]

This post is old but I've just experienced how a bad Fuel Temp Sensor will affect the fuel delivery and transmission shifts. Thought replying to this post would help keep the info. all in one place. I have an 02 CC SB.

Here it goes, truck on 4 instances since January has had issues upshifting with a noticable loss of power and throttle response causing HI RPM's for extended periods of time before shifts.

No SES light occurred in any of the incidents.
The RPM's come up and hold too long before each shift, then each shift is noticably harder and it just dumps to the next gear. Very rough shift that jolts the truck enough to feel the feedback through the drivetrain. Dealer checked the code and it showed a failed Fuel Temp Sensor which they said would affect the fuel delivery and transmission shifts.

At any rate, some of the other posts talked about a software gliche falsely setting the P0181 code when truck heaters were used. In my case the dealer said they verified the sensor is bad and that they reset the P0181 code and are going to reflash the updates for both the ECM and TCM. The truck still didn't run right this morning before I dropped it off to the dealer although the heater was plugged in and timer set at 2 hours. On one other incident though the truck wasn't plugged in before it happened so with the intermittent failures and the inconsistency of being plugged in, I guess I tend to agree that it in fact is bad.

Thing is, the code has never been reset on any of the other instances because the truck was always fine by the next day. In fact, I had the truck in a month ago for a Service 4x4 SES light that would blip during startup. No codes were found then and I know the FTS problem happened twice before that.

The part is 3 to 4 days out so I'll let you all now how it turns out. It appears the cost for my GMPP is going to pay off on this truck as well!!!

Any other info. or thoughts before then would be great.

Here's a link to another post for reference on this topic.

http://forum.thedieselpage.com/ubb/ultimatebb.php?ubb=get_topic;f=3;t=004346#000000