Engine monitors: Drive with safety – and frugality


By Greg Illes

In 1996, the U.S. auto manufacturers began adhering to (another) new standard. This time it was the On-Board Diagnostic port, or OBD interface to the engine electronics. This port (since evolved to OBD-II) allowed access to a host of engine parameters, from water temperature to fuel flow and other critical operating numbers. The OBD also reads the CEL (check engine light) fault codes that help mechanics diagnose engine troubles.

These units are sometimes referred to as “engine monitors,” and they do exactly that, with programmable displays for parameters of concern. One such product is the ScanGauge, available in a variety of models and capabilities for about $160. But why would anyone want to look at OBD information as long as their engine was in good health? There are a few good reasons.

Fuel consumption
Perhaps the most useful information available from OBD is fuel mileage. The ScanGauge unit provides both instantaneous and average mile-per-gallon numbers, so you can immediately see the effect of your driving techniques on fuel economy. Rather than waiting for a tank refill to get an idea of your average consumption, you can watch the numbers rise or plummet as you negotiate hills and headwinds. It’s also possible to watch the gauge and seek out which throttle and gearing combinations provide the best fuel economy.

Non-gauged information
Today’s “idiot light” panels often won’t tell you about an engine parameter until it is out-of-bounds. Furthermore, some truly important information, for example transmission oil temperature, is not gauged or error-lit in most vehicles. The engine monitor can provide information which could only be had by installing an entire separate dash gauge.

Trends and precision
With numerical information on water temperature, transmission temperature or what-have-you, it’s possible to see a negative trend and take appropriate action before you’re stranded. You can also see “197F” instead of the too-simplistic L — H indicator.

Installation is simplicity itself. All information and power come from the OBD port. Plug in the unit, find a place to mount it with its Velcro stickies, and that’s it.

The bottom line is that an engine monitor will let you get the best mileage from your rig and will enable you to see and manage the health of your engine operations. Why would you NOT want to do this?

[Editor: Here are some ScanGauges available at Amazon.com.]

Greg Illes is a retired systems engineer who loves thinking up RV upgrades and modifications. When he’s not working on his motorhome, he’s traveling in it. You can follow his blog at www.divver-city.com/blog.


Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.

newest oldest most voted
Notify of

I use a Bluetooth obdii adapter that links to my cell phone or tablet. You can set up custom gauges and change then as order as you need to. Best player of this setup is cost… Pretty low as phone or tablet do the transfer work.

Just my 2 cents.

Tommy Molnar

Yeah, I’m wondering how much info my OBD port divulges on my 1997 F-350 diesel. I use the port for my Edge programmer and it reads error codes easily, and eliminates them. But in terms of all the other stuff (tranny temp, water, etc), I’m not sure these older OBD thingies handle as much information as say, a 2010 vehicle. I’m not willing to put out a bunch of $$$ and not be satisfied. I’m sure they’re not quick to refund money on ‘electrical’ type stuff.


I too was interested in knowing more about my unit’s internal readings, but was put off by the cost of the scan gauges. So, I bought a BAFX scan tool ($22 @ Amazon) for Android which plugs into the OBDII outlet. It transmits readings to any Android device by Bluetooth, so no wires. I use the Torque Pro ($5) Android app on my phone to receive the readouts. I added another BAFX to our Jeep toad and use the same phone setup in it. Since I use the same phone with Google Maps to Navigate there is a IOttie phone holder/wireless charger on the dash of both our Motorhome and toad. I can run both the mapping and Torque Pro apps and monitor where I am going and how the vehicle is doing for lots less than a dedicated scan tool and GPS. I do have to switch back and forth between apps, but for $49 for two vehicles I think it is a GREAT deal. Plus


I have a scan gauge in both the motorhome and tow but I’ve never been able to program it beyond the gauges that are preprogrammed like water temperature, RPM’s Voltage, fuel economy, etc. Thing like transmission temperature and other parameters have to be programmed and that is not easy and hit and miss for your vehicle. It would be nice if there was a source of the codes, I believe hexadecimal, were listed for particular engines and then could be entered into the gauge. But even that can be trying. Any suggestions


I have had an $80 ScanGuage-E (features a trending graph on the left) for several years, and “generally” love it. I originally bought it for a 1999 Suburban that had no dashboard computer/very little useful information displayed, and the SG did that job perfectly.

I moved it to my car, and it was another story — for some reason, it refuses to ever shut off when installed on an 07 Subaru. Since OBDII has constant power (which this uses instead of the switched power), it kills my battery every couple days of non-driving. I spoke to the manufacturer and they have no idea why it’s doing this so diligently since on other cars it normally does turn off and save SOME power (still a parasitic load that kills a battery in two weeks instead of two days).

My unit is a few years old now, so maybe there’s some updates in the software, but my SG-E also has no idea what the heck my MDS-equipped RAM truck is doing — when half the cylinders shut down, the gauge gives absurdly high or low fuel readings alternately until all the cylinders re-engage. So, although I loved the SG originally, I now have two cars that it doesn’t work for — from 100% loved to 100% useless to me.