When I first crank my truck in the morning, outside temp is around 70 degrees, the truck run pretty rough at idle, then when I start out on the road the truck doesn't want to go. Its like something is holding it down. the more gas I give it the the lower the rpms go. I have to barely give it gas just to make it go 15 - 20 mph. If I keep going approx. 1 mile, its like all of a sudden a switch is turned on, the truck makes a slight jerk and it runs fine. Anyone have an idea what this could be? Its an 1986 4x4 with the 4.9, 4 speed. Thanks for your help

 

Mine runs similar. Was worse in colder weather. I leaned my mixture out a bit, seems to have helped. Im thinkin carb rebuild. Same truck. But im interested in the answers too.

 

A 1986 6-banger is computer controlled, it kinda sounds like it's putting you into limp-home mode at times.

Pull the codes, best way to diagnose it.

Fuel Injection Technical Library » How To Run a Self-Test

 

Listen to Chris. He is setting you on the best path.

I have a 1986 4.9L, which has been converted to Duraspark. But, before the conversion process, my truck did the same as you describe. It was due to the temperature sensor for the computer, which is mounted in the thermostat housing. It failed such that the computer "thought" that the coolant was hot, even when cold.

 

My check engine light isn't on. I'm pretty sure it is in limp mode though. Im getting like 6 mpg. I gutted the cats because I thought they were clogged and it couldn't breathe due to carbon build up but that wasn't it. Is there a way to bypass the limp mode or a way to manually lean it so that it isn't running so rich other than to do the DSII thing? As far as the water temp sensor, what would happen if I just unplugged that?

 

That's because there isn't a check engine light; the first year of a CEL connected to a computer is 1988.

Dude, you are making this FAR harder than it needs to be! Just go pull your codes, let the computer HELP YOU in figuring out what is wrong instead of taking all these shots in the dark at it!

Auto parts stores will usually pull your codes for free (bring the vehicle to them, they do it in the parking lot) but only some people know what they're doing there, and even fewer know how to do this on OBD-I computer you have.

Nevertheless, it is not hard, you can do it at home with a test light (or volt meter) and a paper clip (or a jumper wire with alligator clips on the ends).

 

Oh ok. Thanks. My dash bezel (sp?) has a light and spot for a check engine and when I hold it up to light I see the words Check Engine. Maybe I dont have the OEM bezel, thats why I figured I had one. hahaha

I planned on eventually doing the DSII but until I can find the parts and save up enough for the carb and stuff, I was hoping of a way to rig it so that I can at least get better than 6 mpg. All the smog stuff had been removed on the truck, ie pump, hoses and electrical connections, which I'm sure is part if not all the issue. I was just hoping for a temporary work around.

 

I wonder if something like in this link can be done until I can afford to do the DSII conversation? Im really reaching here guys...hahahaha

The "Nutter" Bypass

 

Is the light you see in your bezel in the top-right-hand corner? If so, that is for EMISSIONS and not a true CEL... no 1980-86 have a CEL, and the dash bezels changed in 1987.

 

So, the true problem comes out... somebody who had no idea what they were doing tried to "make it run better" by "ripping out all the smog crap" or "cleaning up the engine bay" and now it runs like crap and gets 6 MPG.

Yes, best thing you can do at this point is the DSII conversion.

 

Update: I unplugged the temperature sensor thing and now it runs MUCH better. No more bogging down. Runs so smooth now. Now about another day or so of driving to see if that also improves the gas mileage. Thanks everyone

 

MPG is up to around 12. Still not what I hear it can be, but much better than before.

 

Starting with 6 mpg and now getting 12 mpg... that represents a 100% improvement!!!

 

Sorry to hi-jack this thread.

My 86 F150 with the fuel injected 5.0L is kind of doing the same thing.

Background,
It sit's all winter, I did put fuel stabilizer in last Fall. Engine wouldn't turn over until the weather warmed up mid April. First warm day I let the fuel pump prime, it fired right up. The "emissions" light stayed on for the first start, but a week later it didn't stay on after the engine started. It has started each time since like it has for years.

Last Sept I put new plugs, cap and rotor on.

The water temp sending unit in the front of the intake manifold hasn't worked in years, the pins are corroded off.

Truck has run great for years. Until last Saturday. I got about 4 miles from the house and started "running like crap" bucking, it sounded like the muffler was clogged. I could hear exhaust trying to blow out of any joint in the exhaust system.

As I turned around and headed home as I hit a down hill section and lifted of the gas the truck began to run better, but when I had to give it gas again it started bogging, bucking, etc.

I have removed the muffler which was old and I could see pieces of cat in the holes of the muffler.

Sunday with no muffler it started fine, I drove it up the highway one exit, put $20. worth of 89 octane in the tank, which had may be 5 gallons of gas from last Fall. Drove back to the house on back roads, may be a total of 5 miles, truck ran fine until the last 100ft. Started bucking, bogging, etc.

I finished removing the rest of the inside of the cat. Still ran like crap in the driveway, I could get it to rev to 3000rpm but you could hear a slight miss.

Today it starts right up and rev's fine. But I have not let it get fully warmed up or driven it today.

I will put a new muffler on tomorrow.

I tried that link to the "how to run a self test" but only have a 120v test light, not a 12v test light.

Truck is stock.

Any body have any thought's ?

Thanks

 

My 82 F150 with a 302 V8 is doing the same thing Ive been correcting issues one at a time and have got it better still got a bot if the AC is on or when leaving a redlight. I hope fixing the split negative batter cable connector will resolve this. If not then I will pull the plugs and check that out since I had a modulator leaking for who knows how long.

If that turns up nothing then I guess the stock rebuilt 302 I have with a C6 transmission with a 2.75:1 axle ratio and going from a 28.9" tire to a 31" tire raising my gear ratio to 2.56:1 is my problem then as the bog is only when the engine is under an extreme load such as leaving a redlight or stepping on the throttle while moving with the AC on.