Very poor gas mileage.
Thread Starter
1st Gear
Joined: Jan 2018
Posts: 1
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From: LITTLETON Colorado
Very poor gas mileage.
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.
Drifting
Joined: Jun 2013
Posts: 1,590
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From: ft myers FL/Munising MI
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.
Melting Slicks
Joined: Dec 2008
Posts: 2,772
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From: Mystic CT
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?
Last edited by ddahlgren; Jan 4, 2018 at 06:28 PM.
Team Owner
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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.
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.
Le Mans Master
Joined: Jan 2006
Posts: 5,621
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From: Orlando FL
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.
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.
Safety Car
Joined: Aug 2009
Posts: 3,752
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From: Melbourne, Victoria, Australia
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.
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
Last edited by gerardvg; Jan 5, 2018 at 11:04 PM.
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From: moraga ca
I always start with a basic tuneup.
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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.
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.