ljobbins

1988 manual trans stock everything except slightly larger tires(installed pre bad mpgs)

So I have steadily been losing mpgs since February and I haven't modified the car or changed anything driving habit wise at all.

1. Last summer I loaded my jeep with probably 800 lbs of relief food and water and drove 1000 miles round trip and averaged 18 mpg.

2. February I drove a 800 miles round trip mostly freeway and averaged 23 mpg.

3. Started noticing a drop in mpgs around the end of Feb. I was averaging 16-19 combined city/highway depending on how much traffic and how hard I was driving.

4. 3 weeks ago I was got 12 combined.

5. This week I got 10 mpg for 2 tanks in a row mostly freeway.

I did order a new o2 sensor because the exhaust smells pretty rich and bad and I have to get my car smogged (stupid CA). Hopefully that helps a little but I can't imagine that an o2 sensor can cause that much of a drop in mpgs. Any ideas of things to check or what could be causing the fast drop would be appreciated. Thanks

SuperRA

I would check all the sensors if you haven't already. Ones to keep an eye on would be the coolant temp sensor (CTS) and maybe the intake air temp (IAT).

Have you done any of Cruiser's mostly Renix tips? If not, I'd do those as well.

Cheddarnut

^^cruisers tips...and change the O2 sensor, you only have one so if it's bad you will get poor performance.

cruiser54

Tips 1 through 5 and 27 is highly recommended as an alternative to 2.

ljobbins

i've done most of cruisers tips except i haven't gotten around to the cps one yet.

how do I check the sensors? how does the coolant sensor or iat sensor work in conjunction with the computer to deal with mpgs?
I mean I have the temp sensor in back of the head that shows me accurate coolant temp on my gauge but it doesn't tie into the ecu at all and I have the coolant temp sensor on the radiator that kicks on the electrical fan when it gets hot so I can only assume it's working ok. And I have no idea how to test the IAC.
thanks guys.

Cheddarnut

Probably worth putting in the O2 sensor before you start chasing every thing under the hood. After that see if it gets better and go from there. Post up what happens when you swap it.

SuperRA

Yea like mentioned before, try swapping the O2. As for MPGs, for example, if the coolant sensor is reporting that the engine is cold all the time, the computer may be stuck in cold start enrichment mode. Same for IAT, cold air needs more fuel, but some here have found that it doesnt affect it that much. The CTS is seperate from the gauge sensor and the fan sensor. It's only job is to report engine temp to the ECU. It's located under the exhaust manifold, screwed into the block.

AKghandi

check the vacuum line to the fuel pressure regulator, mine was clogged.

BlueRidgeMark

Yes, it most definitely can!

You may also have a vacuum leak somewhere.

ljobbins

just checked my vacuum components and pressure a few weeks ago and it was all good, but i'll take another look at it all.

as for the other sensors, how to I test them? I replaced my o2 sensor but then got really sick so I haven't driven it yet.

Thanks everyone.

cruiser54

Here.

BlueRidgeMark

I went through the same drill. Everything checked okay.

Turned out to be an O-ring on an injector. Only showed up when I sprayed brake cleaner around the injectors while the engine was running.

It's worth a look.

cruiser54

I remember that!!

BlueRidgeMark

Yeah, I thought I had eliminated vacuum leaks when I built my own smoke injector and proved that there were no leaks. I was pretty pleased with my 'invention', using a cheap HF heat gun and some pipe and some tubing to make a very effective smoke generator.

But it didn't test the fuel system!

So maybe the old-fashioned brake cleaner spray is a better way after all....

ljobbins

Ok, so I installed new O2 sensor with 1/4 tank left and it was all city driving and got 12mpg when I filled up. So it did do some good but won't know how much until I use up this tank.

And I checked my coolant sensor under the exhaust man and it checked good according to this, http://www.lunghd.com/Tech_Articles/...iagnostics.htm

Also checked the iat sensor but I had relocated to the air box before and when the engine had cooled down for 5 min while I checked the other sensor it was probably at about 130-140 deg and it measured 1.7 ohms. Not sure if the Airbus cools down way faster so I got a different reading or if it is bad but I can put it back in the intake man and check again.

Any thoughts about the iat being relocated to the air box on my renix? Good bad no difference? I "heard" it improved power but I would sacrifice any power for mpgs at this point.