Reader Question: 1991 Ford Ranger Engine Light

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Here’s the latest reader question, along with my reply!

Cyril asks: Engine light started coming on and fuel mileage dropped to 14 MPG from 22 MPG. Local shop replaced the oxygen sensor and one other sensor but not the throttle position sensor. Engine light comes on intermittently. Should the TPS be replaced?

My reply: Before you have your mechanic replace anything else – and charge you for the parts and labor – I’d ask him to find out what’s wrong with your truck! It sounds as though he’s been guessing because if the light was coming on due to a problem with the 02 sensor (and the “one other” sensor you mention) replacing those items ought to have cured the problem. I would not replaced the TPS unless it needs to be replaced.

Proper diagnostics are in order. Your truck may be pre-OBD II (On Board Diagnostics). OBD II-equipped cars have a universal plug-in port, usually located under the dash, on the driver’s side, to the left of the steering wheel. This is where you plug in the code reader to obtain the trouble codes stored in the computer, which help you trace the problem. Pre-OBD II cars work similarly, but they do not have the universal port; you’ll need a shop that has the necessary equipment to deal with the Ford-specific system.

As an aside: Has your mechanic checked the catalytic converter? Given your truck is almost 30 years old, I’d be looking at that unless it has been replaced and the one on the truck right now is less than five years old. And I’d still check it.

A bad (partially plugged) cat could easily cause your mileage to dump as it has and also would trigger the check engine light.

Keep us posted!

. . .

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  1. EEK 4, as GTC says is the worst of both worlds. More complicated and failure prone than a carbureted and electronic distributor car, but with really poor diagnostics and the scanners are hard to use and interpret.

    Rangers of that vintage have lots of bugs, I had a few and liked the concept of a capable pocket pickup, but the engines, trannys, and electronics are atrocious.

    In regards to your specific problem: I dont think that a 91 has a mass flow sensor, but if it does try pulling it and cleaning it with some spray electronics contact cleaner or isopropyl alcohol. Test the coolant temperature sensor. Then O2 sensor and catalytic converter. Also the analog fuel pressure regulator can really kill your mileage (and performance).

    • Yeah, it’s almost easier to tell you the things to try by trial and error than to follow the Ford Diagnostic Trail. Things like the fuel pump and the regulator are not even monitored by the emissions control system at all.
      Even a Bad ignition coil, wires and plugs can kill the fuel mileage, especially the Ranger with the 8-Plug 4-Cylinder. Sometimes your just fortunate if it runs on all 4, fuel mileage notwithstanding. Just a simple case of low A-Freeze due to a water pump (or god forbid a timing cover) leak, will send the temp. sensor offline which will cause the ECU to run the fuel mixture so rich it spews black smoke (unburned fuel). The root cause could be so remote that you should just start by making sure all the basic systems are in good working order.

  2. 1991 is definitely pre-OBDII. It is the Ford EEC-IV system. One extremely aggravating feature of this system is that often you have to respond to each code, and run the code test after each item is addressed. Not all of the issues will be displayed from the start. The system does not allow the mechanic to prioritize the system repairs, rather, the system give out code information as it sees fit to prioritize. Then again the codes may only lead a mechanic in the general direction of the cause, and the solution will only be arrived at through trial and error. The bottom line is, you will have to decide at what point to stop throwing money at it, and replace it with something more dependable or affordable.


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