Bad mileage and rough idle when warm

[WRDeaton_88]

because this post is awfully long, the short version is: really bad gas mileage, rough idle when warm, I suspect fuel/air mix is way too rich. suggestions? solutions?

So recently I bought a 1988 ford ranger, 4wd with a 5-speed and the 2.9 V6. when I bought the truck, I was told that it had a new motor (still under warranty) from autozone, but the fuel injection system was from the old motor. My best guess was that just about everything from the valve covers down was new, or pretty close to it. Now, I was told that it could use some tuning, but I'll get to that in a minute.

The truck had a rough idle since the day I bought it - after some research and talking to a mechanic, I determined that I couldn't find a vacuum leak, and so replaced the Idle air control, which helped a fair bit with the idle, but it was still a bit rough. the same day that I did that, I got the old muffler cut out and a glasspack + tailpipe installed.

a couple days later, I take it to another shop, and they tell me that the distributor cap was loose, and the timing was about as bad as it gets - both were fixed, and new plugs were installed - cap, rotor, and wires were pretty new. at the same time, they tested the fuel pump and found it to be in spec.

so, the issue(s) is this - and I'm thinking/hoping they are related. right now, I am getting terrible gas mileage - between 7 and 10 MPG. I have 235/75/15's on right now, but I don't imagine they are affecting it that much. after the tune-up at the shop, it runs a whole lot better, and I feel like I have more power, but the mileage is still just as bad.
With a cold engine, it runs pretty smooth, maybe between 900-1100 rpms or so. but once it gets warmed up, it will idle erratically - between 500 and 1200 rpms. at night, I can actually see the headlights dimming when the rpms dip, and it feels like it's about to stall out. also, once it is warm, it can be a pain to try to fire it up again right after I cut it off (or stall out). when I was messing with it today, I left it running for about half an hour - at the end of that time, there was a visible line of soot on the ground from the tailpipe. also while running it, if I jabbed the pedal, it would spit out black smoke. holding it in didn't, though.

from doing some research, I would guess that the fuel mix to the engine is WAY too rich. since I know just enough to get myself in trouble, I'm unwilling to tear apart the engine looking for something. I'm willing to take it to a shop, but I would like to know some possible causes before I just drop it off.

My "check engine light" isn't lit up, so I don't think I would find any codes. I did check the PCV valve today, and it rattles like it should. The previous owner did put on an aftermarket cone filter - it might be messing with the idle speed, but I can't imagine it would screw up the gas mileage.

If you actually took the time to read all this, thanks! I tried to give all the info I could think of, since I'm a firm believer that too much info is better than too little. If you have any advice, tips, suggestions, or whatever, let me know! the sooner I fix this issue, the happier my wallet will be.

 

[Tedybear]

We have the same type truck, and we also have 235 tires on it. Our current miles per gallon runs about 16-20. (It needs a tune up, so it's been a bit towards the 16 lately)

Runs decent when cold...let it warm up and it runs bad. Sounds like a sensor issue. The engine computer runs off "presets" when cold before it goes into "closed loop" mode. (fancy term for using the computer and sensors/servos to their full extent).

Pull the codes from the computer. Just because the check engine light isn't 'on'? Doesn't mean the computer didn't store anything. But if I had to guess with the listed issue? And this is ONLY a guess? O2 sensor.

S-

 

[RonD]

When fuel injection was added to engines the Choke was lost for cold engine operation.

A choke worked by limiting air flow at the intake which increased vacuum, this sucked extra fuel out of the jets(running rich), the extra vacuum also advanced the timing(vacuum advance distributors)

With Fuel injection there are no jets so extra vacuum won't suck extra fuel.
So an engine temp sensor was added, a 2 wire sensor, different than the 1 wire sensor for the dashboard temp gauge.
This 2 wire sensor is connected to the computer(PCM) it tells the computer to run engine rich with advanced timing until engine warms up.
If the 2 wire sensor never tells the computer the engine is warm then you are stuck in "choke mode", so rough running and a fuel hog.

The two engine temp sensors are usually located near each other, by the t-stat housing.
You can check them with an OHM meter, check with engine cold and engine hot.
OHMs should change from cold to hot, not sure of the specific numbers.

 

[WRDeaton_88]

thanks for the suggestions! Unfortunately, I got in a pretty good wreck last night while driving home from work, and I dont know if the truck will make it. if it does, I'll be sure to try to fix it up.... if not, I suppose I'll be listing it for sale for parts or something. I'm alright, but I'm pissed about the truck. Bought the damn thing two weeks ago today.

 

[RonD]

That's really too bad, no injuries I hope.

 

[WRDeaton_88]

no, no injuries. everyone but the vehicles was alright. Here's hoping the ranger will live again, though.

 

[Tedybear]

thanks for the suggestions! Unfortunately, I got in a pretty good wreck last night while driving home from work, and I dont know if the truck will make it. if it does, I'll be sure to try to fix it up.... if not, I suppose I'll be listing it for sale for parts or something. I'm alright, but I'm pissed about the truck. Bought the damn thing two weeks ago today.

ghess that blows monkey chunks.

Keep us up to date on what you decide to do.

S-