Hey All,

I just picked up my '89 XLT Lariat with 5.0L today. I drove it home ~120 miles.

Immediately I noticed that the engine temp gauge on the dash did not work. I then also noticed that I got very poor gas milage on my return trip home (all highway): ~12-13mpg.

I think I need to replace the ECT. Do I also need to replace the sensor switch? What does ECT and sensor switch do?

The CEL never came on.

Anything else I need to replace? Also thinking about replacing the O2.

Thanks

 

Depending on your trans and axle ratio, 12-13 mpg is normal. The gauge receives it's signal from a temp switch on the driver side of the intake manifold, not to be confused with the temp sensor closer to the thermostat housing. The temp switch only controls the gauge, while the temp sensor goes to the ecu, and is what the ecu uses to determine when it should be in the warm up cycle (closed loop) and when it should switch to an open loop, once the engine warms up. I wouldn't change out any sensors unless you know they are bad. But you can still attempt to pull codes, even if the check engine light isn't on.

 

That's not much info for us to go on. You should put ALL the truck's details & history (as much as you know) into your signature so it shows with each post, as this page explains:

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Phone apps don't always show signatures, so you may need to switch to a real browser in desktop mode on your phone, or just use a desktop/laptop computer. Put your location (nearest city) in your profile & upload an avatar of the truck. The more pics you post of the truck, engine, wiring, labels, & undercarriage, the more likely we can help you. Not all its details are relevant to these issues, but you don't necessarily know which ones are relevant, so just put everything in now.That's not far enough to get a useful MPG calculation. You need to 1) fill the tank and either note the odometer reading or reset the trip odometer (if equipped), 2) drive most of the tank out, and then 3) refill to get an accurate measure of fuel quantity used. Divide the elapsed miles by the gallons used to get MPG. Read this page:

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How do you know? Did you ever measure the engine temperature with a non-contact (IR) thermometer to make sure it was actually warming up? Read this page:

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That's not very poor for these trucks - it's about normal.What diagnostic test did it fail?What sensor or switch? A sensor is not a switch, nor vice versa.If you don't even know what they do, why would you think they need replacement? Follow the links on this page:

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Then the bulb is probably burned out. It should come on for ~1sec each time the key is turned from OFF to RUN. The instrument cluster must be removed (disassembly of the cluster is NOT necessary, but it's probably advisable since it probably needs a thorough cleaning) to change the bulbs. Be careful popping the dash bezels off because the plastic is getting old & brittle. Start with the 2 lower panels beside the steering column. There's a slot in the headlight **** where a small tool presses a spring catch rearward to release the **** from the shaft. Do not touch or clean the odometer wheels - their paint is barely adhered so that tampering is evident.

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Anything that's due on the maintenance schedule, or that fails a published test, just like always. Buy only top-quality parts & chemicals, and follow published service & diagnostic procedures.Why? What test did it fail? It is NOT on the maintenance schedule. Read this page & the NEXT several:

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Okay, how do you know what your fuel mileage is? Did you leave out the part where you filled up before driving home, reset the trip meter then filled up again at home?
Not trying to sound rude, just there are a thousand Post #1 with reports of poor MPG and no data to support their claim.

I don't know if an '89 would be the same, however if the vehicle is not getting to normal operating temp it could be affecting the fuel metering. The computer could be stuck in open loop (I think open loop is the cold circuit).

Depending on how well the engine runs, the tire size and how fast you drove the mpg may be correct.