Lately my truck have been giving me a headache. The week before the truck ran beautifully. I mean she would idle good , warm up good, good throttle response and all. She was a dream. But now that's all gone. My truck idles like shyt, horrible mpg, an several other problems. one problem that I am having is when I drive the truck an let off the gas a bit to coast or help the trans shift into another gear, the truck backfired ALOT. Also age I slow down approaching a stop sign when I do stop the truck lunges forward. Even when I am hard holding the brakes the truck lunged forward. It does this:
Approach stop sign..backfires...stop...engine idles down LOW then LUNGES FORWARD...idles badly low like it will die ..then idles up fast and pushes the truck forward. I really have to be on the brakes. It is pudding me off. So tomorrow am a look at the resistance in my oxygen sensors and see if they are bad. My truck is 1986 Ford f150 300 engine 3.08 gears. Warm up time doesn't help anything. Any suggestions? I will try to pull codes tomorrow I am a bit nervous. Anyway any help is appreciated thanks. Oh and I smell fuel like its running extremely rich. I have my power probe , vacuum gauge, test light and dvom with me. Where is the best place to read vacuum on our engines?

 

Check for codes first.

Then, I would check for a bad water temp sender, and check the O2 sensors. I suspect your water temp sensor is bad, making your computer think the engine is stone cold all the time. Does it run fine cold, and the trouble starts a few minutes later as it warms up?

 

Sounds like you definitely have some sort of fuel problem.

Like he said, pull the codes first and see what the computer says is going on.

I'm probably going to bet that you're not getting enough fuel and catching a lean backfire or two. Then when you slow down it gets flooded with gas and almost dies. Once it lights off though, is when all that extra gas makes it surge forwards. Either that or you're slowing down and it's running so lean that there's not enough gas to keep it running, then when it finally fills the bowl up the truck surges back to life.

I think your fuel pump is going out, personally.

This is an 86 with the feedback carburetor and all of the emissions equipment is still present, right?

Should be a mechanical pump on the 300-6 from 86. Around 84 to 86/87 I think the 460's and some of the other motors were getting electric pumps and stuff.

Maybe if you ran the gas down almost empty and filled it back up you picked some trash up in the fuel lines/carburetor?

Let us know what you find.

If the codes check out fine, feel free to come to the 80 to 86 forum and seek help there.

 

No smog equipment is not equipped. And I was told by a buddy a lot of the computer stuff has been removed. How true that is I am not shore. But when I tried to pull codes the computer didn't respond. As in my test light didn't blink and I made absolutely shore I have everything hooked up. How can I go about checking fuel pressure? I took my vacuum gauge an hooked it to an open vacuum port on the tree and I got no steady readings. I got 15 in hg to 19in hg. The needle constantly bounced around in t range. I was a dark needle means its drifting. If u hit to throttle the vacuum would jump and drop extremely low. According to the chilton book that means bent or burnt valve. So i ran a compression test and everything checked out great there and my spark plugs looked good too. So I am at a dead end. Thanks for your help guys

 

This truck originally had computer-controlled ignition timing and a carb with a computer-controlled mixture solenoid. If "a lot of the computer stuff has been removed" then it will likely not be advancing the timing with engine speed as it should. You will need to determine what kind of ignition system is operating on this truck. It also sounds like you have a vacuum leak.

 

What Galendor said X2.

 

How to Use and Interpret a Vacuum Gauge

There's a good web page with a good guide.

Check at the bottom and click on the scenarios and stuff to learn about what the gauge indicates. It's very helpful.

You can check fuel pressure by t-ing into the fuel line with a plastic fitting, and if your vacuum gauge is a "dual-use" gauge that can read in pressure and is safe for reading fuel pressure, then plug it into the fuel line when you're done. You can leave the fitting in the line to test it later or you can take it out and put new rubber line on. If your gauge can't read fuel pressure, you'll have to get a fuel pressure gauge of some sort at the auto parts store.

Setting Idle Mixture The Right Way

There's also a link to tuning fuel mixture with a vacuum gauge so after you get your issues sorted out you can make sure your carburetor is dialed in.

If a lot of the stuff has been taken out of the engine bay, you may want to put it back like it originally was. Photos would be beneficial.

 

Thanks everyone and 1983f1503004x4 for your advice. If the computer is setting the air fuel mixture how much better would I get a tune with the vacuum gauge if the computer is goin to change everything all over again? An I will try to load pics shortly .