16 MPG before engine replacement, now...

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ADDgolfer

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Feb 17, 2016
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Ram Year
2003
Engine
4.7
2003 4.7L
Could my ECM/ECU not be playing nice with the replaced crate engine?
No codes
Rich gas exhaust smell before Cat replacement
Still smells rich, but not as much

Blew it up in 2015 on a rare below freezing Florida morning. Stressed it a few times before that by overheating from a bad radiator. Was using flush in the radiator before I replaced it when that cold snap hit. No antifreeze.
It sat in the driveway for a year. Drove wife's car as her new work schedule coincided with mine, and her work was on the way.

Replaced the 4.7L engine in Jan 2016 (crate engine) when I retired. Ran smooth except for no 'passing gear' It dogged in WOT, and the mileage went from 16 mpg to like 12. Sat some more, only 3,000 miles since 2016 until last month when wife's low mileage, one owner, 09 Lexus got totaled

The catalytic wound up being the reason for the ******* and serious poor gas mileage, but still not getting anywhere near 16 mpg average
No codes but replaced the O2 sensors (just 2 on my truck) the platinum plugs back to copper core, replaced the stock air intake with a cold air intake. Throttle response is sharp and love the growl

I have spent a few $$ on one idiot mechanic then one old timer who told me it was the Cat and he was right, but he is just guessing at the poor fuel mileage, after taking it back when the Cat was replaced, hoping he would hook up his $5,000 diagnostic equipment to read live data, (he boasted about it), but just gave me a list of things to replace.
Are professional OBD readers and a capable tech able to decipher data of timing, or air/fuel mixture even tho codes aren't thrown?

Back to that replacement engine. The company asked for VIN and were careful to match the replacement to the one my truck called for.
 
Last edited:

Oliver Closehauf

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Cincinnati
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2005 Laramie
Engine
Cummins 5.9
If it smells rich, it probably is and your new cat is doing its best.

I think I would disconnect the O2 sensors and see if you get any codes. Just to make just the ECU is working like you expect. And use a volt meter to read the voltage of the O2 sensors and see where it is.

Also with all of the sitting its been doing, check for leaky injectors and gas in the oil.
 
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ADDgolfer

ADDgolfer

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4.7
Thank you, I have been away and just now opened the forum.
Mileage has gotten worse. Doing the ciphering the old way comes out the same as the digital readout. Normal driving, in suburb type of routes, 45 mph roads with no stop and go traffic, without a tow load, I am now getting 8.4 mpg. The readout did get to 10 mpg after 90 miles, but the average was 8.4

I was questioning the injectors, yet the engine is idling smoothly at 600 rpm with no surging or miss. My mechanic agrees, a poor performing or stuck injector would likely have a that telltale 'miss'. My old plugs showed no burnt or dirty electrodes.

Could a dyno give up any information from their readout? I have no problem spending the $$ if I could find the type of pro who knows what he's looking for
 

LoneWolf3574

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Arizona
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2006
Engine
Hemi 5.7
Pardon me if I state anything obvious here, it's just a quick checklist that I've made for myself over the years. Check your tire pressure, underinflated tires are a common cause of poor fuel economy. I'm assuming new plugs, air filter, fresh fuel (if it has sat for a while), and fresh oil (5W30 for yours) because you have a reman/rebuilt engine. That's it for my quick checklist.

I read that you had new cats put in, are the O2 sensors new as well? Have you checked your fuel trims? If you can't interpret them, have a mechanic you know and trust do so. I personally may not know how to interpret them, but there are several mechanics that I know and trust that can.

Transmission issues can also cause a loss in fuel economy. Have you had your transmission serviced recently? Are your fluid levels good? Was the correct transmission fluid used? I do know that ATF+4 is correct for the 545RFE, and I would assume the same for the 48RFE (I'm assuming that is what you have).

Last year, I replaced the timing set, oil pump and pickup, and the connecting rod bearings and bolts as part of replacing my timing cover gasket. All part of a "well, I'm already here, what else can I do". About week after all that, I had the muffler removed so that I was running just the resonators (basically open exhaust). In doing all this, I managed to lose ~0.8-1.2mpg of fuel economy, which has irritated me ever since because I have no idea what did it out of everything that I did.

I'm sure I missed plenty and welcome more input, it may help myself and others as well as you.

You may want to check out this article by EricTheCarGuy, he's pretty good at stating common sense troubleshooting and solutions on his YouTube channel. As you can see, the list is rather long. -
https://www.ericthecarguy.com/finding-and-fixing-the-causes-of-poor-mpg/
https://www.youtube.com/channel/UCa7guRnhniICnS0mJbSDmMg

The same goes for this guy on YouTube, South Main Auto Repair -
https://www.youtube.com/channel/UCtAGzm9e_liY7ko1PBhzTHA
 

EdGs

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2015
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Hemi 5.7L
I don't know if this will help, but I had a 2001 Saturn that got 30+ MPG. All of a sudden, mpg dropped to like 20, and you could smell gas big time.

I called the dealership and spoke to someone in service, explained I could not afford to bring it in, could they point me in the right direction as to what to check?

He said coolant temp sensor. Said they fail, and usually such that the computer thinks it's -30°, and in return, dumps massive fuel.

Sure enough, that did it. I did not notice until aftet he said that, that my temp gauge read lower than normal. Replaced the sensor with OE, and she was back to her old self after that.

Not sure if it helps your situation, I hope you get it sorted.
 
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