Bad gas mileage


neighbor has a 4 cyl. truck 2001, rebuild engine from 2000 put in. 2200 engine.

mileage went from 20 to 6 per gallon. no codes and dealer nor local mechanic find anything wrong. any suggestions where to look? No visible leaks.


This is probably the most common subject on this forum. We get at least one such example every week.

The most common reason for such incredible drops in fuel economy is owner error. That is, he is not calculating his mpg correctly or is using an inaccurate method. Sometimes a person supplies us with method & data and we can point out his mistake. In your example we have nothing to go on. Let another person drive his truck for a couple of weeks and we will get a different verdict – 20 mpg.

This is not what your neighbor wants to hear. They never do. Maybe you ought not mention my reply at all. But YOU know. Handle the situation as you see fit.


Or? They don’t even tell us the type of vehicle?



Info might help a bit.
Type of truck, EXACT replacement engine, any CEL on, runs smooth or rough, etc. Any black smoke out the back?

That word “rebuild” could use some clarification also. If this engine was purchased out of a wreck and the seller said it was “just rebuilt” that might be a bad sign.


Did the people who checked the truck KNOW that the engine is not original? It could even be from a different year, with different sensors, and controlled devices (idle airflow control valve, egr valve, etc.). Sensors can look the same, but, not BE the same. The basic 2200 engine might be successfully married to the original engine management computer; but, it could take a GOOD mechanic to do it.


was out of wreck-put in under used car warranty by dealer. is a S10 pickup 2001-repacement eng.2200 4 cyl from yr. 2000. blew black smoke for a time after a cleaner was added to gas.


I still don’t know since it’s unknown how many miles on this engine or any story behind it. It’s highly unlikely that a dealer is going to install a "new/rebuilt engine on a used vehicle warranty.

Since the engine was out of a wreck no one knows what’s going on with it. I sold a good running, non-oil burning engine out of a wrecked personal car of mine one time and that engine burned a quart of oil every 5 miles after installation. It appears the impact of the collision jolted the crank forward, this multiplied the force of the crank end play, and broke a few pistons and rings.

About all I can suggest is pulling codes. If the mileage is that bad then there should be something present in there to look at.
You might take a look at the fuel pressure regulator as a ruptured diaphragm can cause rough running, black smoke, and lousy mileage.


hellokit has it right…In this day and age of government certified electronically controlled engines, doing a quick engine swap is a thing of the past. EVERY component must match perfectly. Check the ECM part numbers for 2000 and 2001 S-10 trucks. If they are different, you can’t expect a year 2000 engine to talk to a 2001 control board…They don’t speak exactly the same language…


An excellent point has been made that when an engine is pulled from a wreck the ECU should come with it. Another excellent point has been made that wreck engines are a total crap shoot.

However, I’m confused…the engine was pulled from a wreck and installed under a used car dealer warranty? I’m sensing that there’s more to the story than has thus far been told.

I’ll close with two suggestions. (1) find out if the used car dealer can get the ECU from the wreck and install it. (2) start with the basics, a compression check of the engine, a “reading” of the spark plugs, a check of the filters and ignition system function, stuff like that.


Mountainbike; hellokit; caddyman:
While you are correct about the need for the engine sensors and the ECM to have the ability to communicate with each other, you do not need to change ECUs. Any qualified tech will remove all sensors from a used engine and install the sensors from the bad motor. In this particular case the sensors are exactly the same. In some cases the sensors will look the same, plug in the same, but not work same. Also when dealing with a wrecking yard motor you have no idea of the condition of the sensors installed in that engine. The previous owner of the wrecked vehicle could have been driving it with the check engine light on for years. Since the vehicle in question has been scanned and does run most likely the sensors are operating within parameters and communicating with the ECM.

OP: Did the mileage drop appear right after the motor swap? Or is it something that happened days, weeks, months later? How does the truck run? Is performance where it should be? Was it just checked for codes or was it connected to a scan tool so the data stream could be monitored? If the truck runs properly (no misfire, lack of power, black smoke) I would look for a fuel leak or a gas thief. If the truck is running poorly take it to a qualified mechanic who can scan it for codes and monitor the data stream from the ECM looking for any strange readings.


you may be right. I did not outright mention your reply but suggested he mark down the mileage and gallons for a time. He said It started about 2 weeks after the exchange and that there was no loss of power or rough running.
He may have gotten stressed out from getting layed off of work. Starts a new position soon and his transit can help him track his usage.
Everyone on this site has been very kind, informative and helpful.
Thanks, Don