High LTFT under load/ Bad MPG

Chevyevan · January 3, 2021

Hey guys, I swapped my 1995 montero with a 5.3/4l60e about 7 months ago. The engine/trans/pcm is out of a 2001 suburban. Completely stock. My montero is on 35s with 4.63 gears. It is very close gearing to a stock suburban.

Now here is my problem. Under 20-30% load both LTFTs go to 17.3. I know this is not normal because I have a couple other rigs that I can test off of (2002 silverado, 2002 yukon xl). They both have over 250k miles and their LTFT stay +-4 under load. I dont think it is fuel pump/filter related because i can mash it to 6k rpm and it doesnt break up at all it has 50psi at idle and jumps up if i give it gas. . It has no codes either. My Montero feels like it has no power in OD at all, it struggles pulling up hills at 65mph 2k rpm. Where my moms yukon will pull up the hill at 1600rpm. My torque converter is locked at 65 for sure. I replaced the o2 sensors with denso ones, cleaned maf, and new delco spark plugs. I am thinking a vacuum leak maybe? I sprayed starting fluid everywhere and didnt find one.

I get a consistent 12mpg with the best being 12.5mpg full freeway driving at 65-70. I think it should get closer to 16-17mpg. I am sure the issue is related to my LTFTs being high. Its either a sensor is giving a false lean and the pcm is dumping fuel in and giving be crap mileage. Or its actually running lean and struggling to keep itself at speed. According to my scangauge, the montero gets better mpg at 80mph than it does at 65. I think thats just because it is almost bogging at 65 and I have to give it more throttle to keep at that speed. I am going on a 1500 mile road trip in 2 weeks and I would love to get better than 12mpg. Thanks in advance guys.

CamGTP · January 4, 2021

Does the computer know what gears and tire size is on the truck?

If you are positive fuel pressure is not an issue start looking for vacuum leaks or just go ahead and replace a few gaskets/o-rings. Intake gaskets are cheap and often times the fuel injector o-rings can leak in air when under load. Of course make sure you have no exhaust leak at the manifold and connection point at the collector.

Chevyevan · January 4, 2021

3 hours ago, CamGTP said:

Does the computer know what gears and tire size is on the truck?

If you are positive fuel pressure is not an issue start looking for vacuum leaks or just go ahead and replace a few gaskets/o-rings. Intake gaskets are cheap and often times the fuel injector o-rings can leak in air when under load. Of course make sure you have no exhaust leak at the manifold and connection point at the collector.

Yes the computer was calibrated for the gears and tires. MPH reads dead on. I rebuilt the whole engine so it has new gaskets all around. Just today I swapped my MAF with one off my moms yukon and the fuel trims went down significantly. from 17 down to 8.6. And it seemed that the STFTs were compensating for that 8.6. So maybe more time and it would have went back down. I went ahead and ordered a delphi maf today so hopefully that solves the problem. Its also interesting that you say the injectors can allow air under load. It seems as if my fuel rails are bent or something. the rear injector on the drivers side looks like it isnt seated all the way like the other ones are. But there is no adjustment with them. However, now that I think about it, if the injectors themselves had a vacuum leak then that would make one cylinder lean and probably ping. Both banks read almost exactly the same so I figure its something that affects all the cylinders. The maf should be here in a few days so I'll check back then. Thanks for the suggestions!