New Engine =terrible gas mileage

I have a 1998 Chrysler town country with a 3.8 liter engine. I had 290,000 miles onthe original engine and decided to put in another one from an identical vehicle with 80000 miles on it. I gave a new engine a once over with new plugs and wires and water pump, etc. Even gave it a little paint. I had my local shop swap out the engine. My car was getting 21 to 22 miles a gallon on the freeway and now its getting 16 miles a gallon… Any ideas?

The first thing that I would do is to change the thermostat. Since you don’t know the maintenance history of the donor engine, it is entirely possible that the thermostat is the original one, and it could be stuck in the open position. That situation would drastically reduce your gas mileage–especially in the winter.

Did you also swap the ECM from the donor car?
I ask because ECMs vary somewhat from one model year to another, even with the same engine, and if the donor engine is from a Chrysler of a different model year, it is possible that the ECM is not compatible with the “new” engine.

The temp gauge shows a good and steady temp. I did not swap out the ECM. How can I check it?

I did put in a new thermostat with the once over

trans is going into 4th? 290k on original dodge trans?

How many miles have you driven with the new engine?

RPMs drop to 1900 at 55 so shifting seems to be fine. I have put 11,000 on the new engine.

Was the old engine burning a lot of oil? If so the catalytic converter may be partially coked up and causing problems. Restriction in the exhaust will drop manifold vacuum which will be interpreted by the ECM as a demand for more fuel.

Installed a new converter 6 months before the swap… Could have clogged?

Engine was not using much oil

You could connect a vacuum gauge to the intake and check the manifold “pressure” when cruising. If cruising at 60 mph and pressing the accelerator slightly drops vacuum significantly an exhaust restriction might be the cause.

Can I ask why the original engine was swapped? Granted high miles, but a known good runner is worth more than a “pig in a poke” (unknown used engine) in my book.

But, approach it like any poor runner…tuneup 101…plugs, wires, check compression, timing…

Losing powrr/compression… Just put new plugs & wires… Haven’t checked compression…

Did you use intake/injectors from new motor? Or swap that stuff over? Not sure I would like a junkyard motor sitting on a shelf in warehouse for 6months with no intake. Who knows if valves got rusted or seals dried out or rings got stuck. I think all newer motors have 2 piece intakes anyways. An upper and lower section

My guess would be either the engine is running at a cooler temperature for some reason, or the computer is never going into closed loop mode. By that I mean the computer is ignoring the O2 sensors for some reason, and operating the engine as it does in the cold start mode.

First order of business, you need to somehow verify the coolant temp is the same as before, and that the engine coolant temp sensor is calibrated the same as before. Next is to somehow verify whether or not the computer is leaving cold start mode and going to closed loop mode.

What the O2 sensor status? Same ones as before?

@Rod Knox… Trying t figure out how to check the manifold “pressure” when “cruisingt”…

@Cavell Can’t remember which injectors I used…I believe I used the ones in the new engine… How could I test them? As far as the valves and rings… wouldn’t a compression test help that?

@GeorgeSanJose Not sure how to test these things???

The ECM’s coolant temp sensor on most vehicles is a resistor who’s resistance varies with temperator. A thermistor in other words. The factory service manual will show the resistance vs. temp chart. Testing is just a matter of measuring the resistance of that sensor.

Coolant temperature testing can be done with a contact-less laser-guided infrared temp measuring device, pointing it at the upper radiator hose or upper part of the radiator.

Closed-loop vs open loop, you probably need a specialized scan tool, capable of showing real time parameters. And in most cases if the ECM doesn’t get the readings to allow it to go into closed loop mode after a certain number of minutes, it will set a code.