slngsht727

1986 C4 with automatic transmission. Getting about 7.5 mpg or less around the neighborhood. Slightly rough idle. Starts fine. runs up to about 1500 rpm then settles to around 900 rpm. drops to 700 in gear. Was idling around 1000 rpm while driving yesterday. When the temp gauge reaches about 185 I start to hear a clicking noise. Particularly when accelerating. No check engine light or codes.

playsdixie

sounds like it could be your iac....not sure if that throws a code....get an fsm and set your idle according to the manual...it would be good to inspect the iac and/or clean it. Or replace it....could be a stuck egr valve too, also, make sure you don't smell gas around the fuel pressure regulator....that's bad.....spray tb cleaner around the vac lines to make sure you don't have a leak.....just some things to get you started....hope you have an fsm....good luck

ddahlgren

When was the last tuneup a complete one cap rotor wires plugs filters etc. Oil change and what oil? How many miles on the car? Are you going by the dash display for MPG or actual measured use?

JrRifleCoach

You didnt mention how many miles. Might have the cats checked.
Or you can do it yourself by banging on the cats and listen for rattling.
If they rattle its time for new cat/s. 86 has pre-cats at each bank and one before the diff.

pcolt94

You may have bad O2 sensors. It may not throw a code possibly.

I would get a scanner on it and check the voltage swings, fuel trim numbers and see if it goes in to closed loop. If it is running open loop, that would radically affect mileage. I don’t think a tune up or plugs would cut the mileage more than in half.

John A. Marker

I would check O2 sensors then the cats next. You can check the pressure in the exhust pipe by pulling a O2 sensor and mesuring pressure.

gerardvg

Check the simple things first,

1 timing double check the no 1 piston is at TDC when timing mark is at TDC, the outer ring on harmonic balancer can slip. retarded ignition will use more gas.

2 check TPS voltage should be 0.54 volts DC with ignition on or idling, i find 0.60 volts helps keep idle smoother.

3 600 rpm is what she should idle at, 580 i think is when hot higher when cold till she warms up. Should start run at 1200 rpm then drop to 600 rpm, if it doesn't check your EGR is not sticking open should spring closed. you may have to set the base idle if the idle screw has been adjusted by someone.

4 exhaust check for clogged cat or muffler.

5 does the torque converter lock, when going up hills do the rpm stay or go up then down when over the crest of a hill. Should only affect highway mileage.

6 does your transmission slip??????

7 Check your IAC valve has not come loose

Good luck

ex-x-fire

OP, are you doing the math on the fuel mileage or just reading the numbers on the dash?

383vett

I always start with a basic tuneup.

vader86

Check the mileage yourself, without relying on the digidash to calculate. Its probably correct, but its worth checking.

Personally, I have DataMaster for my laptop and a handheld scanner and I can read the information they give. If I see the O2 giving funny readings, car is staying in open loop, or always reading lean in general, then its telling me the sensor is compensating by richening up the mix. That means the sensor itself is probably bad. O2s generally do not throw codes until they are completely toast, which will explain why yours isnt setting that code.

I would then start with the standard tune up. Change plugs, wires, coil/cap/rotor, fuel filter, and then the O2 sensor.

Cat problems are possible, but it can be cause/effect. A bad O2 will richen up things so bad you clog the cats.