[Greenwave]

The Facts:
1986 D150, 2bb Holley, 2WD, Automatic, 117,500mi, Long Bed, 5th owner
Problem:
Engine doesn't run smoothly. In fact it sometimes spits, sputters, hiccups, stalls and just die starting off from a stand still. Other times, it runs like a champ! No sputter, no sluggish starts, no hiccups. Then a day later, back to the same old, same old.
Let me be more specific:
When driving (at a constant speed... any constant speed: 25, 35, 55mph, etc...) the engine "hiccups." I'm gonna use the letter "M" below to describe the sound/feel of the engine at a constant speed when hiccuping is in process (sound it out to yourself):
MMMMMMM MM M MMMM MMM MM MMM MMMMM MMMMMMMMMMMM
As you can see/hear, there is no real rhyme or reason. No "pulse" or consistency. And though the truck doesn't have a tack, I'd be fairly certain, the hiccups are a drop of 5-600 RPM's.
There is another problem that may be the same or related. While sitting at a stop with the truck in drive (haven't sat still in reverse long enough to know if it happens in reverse too), it almost feels as if the truck lurches and then there is a drop in RPM's.
New stuff list: These are all the updates in the last 2 months/1000 miles that offered no change in performance:

Gas (even tried high octanes and octane boosters)
Voltage regulator
Interstate Battery
Fuel Filter
Air Filter
Rebuilt Holley Carburetor
NGK Spark Plugs
Wires
Distributer
Rotor
Front brakes & rotors (obviously not engine related... I'm just saying)
also, 4000 miles ago, I replaced the radiator and put on new tires
I also check the timing and it was good too

I need HELP!

 

[DiverDwnPowrRam]

i would ask...last time you put in new:
cap
rotor
plugs
wires
After these I would of course if it was timed correctly, look at rebuilding the carb.

 

[Speed Dragon]

He says he's changed the whole ignition in the last 2 months. He hasn't said whether or not he has the Lean Burn computer on the air cleaner or inside the left front fender behind the battery.

 

[Delray Dude]

1st thing I would suspect would be the carb. Sounds like the float level is set a tad to high or the needle valve that shuts off the gas flow from the pump flow isn't completely closing. Either condidtion, the gas might be overflowing and giving the hiccup you describe - especially if it goes away at high RPM - as the high RPM requires more gas.



If you have an electric pump - you might need to regulate the pressure down.
It could be crud that is caught in the needle valve holding it open, or just the float level is high.

This is where I would start looking.

 

[Greenwave]

Thanks for responding so quickly.
1. All plugs, wires, cap, dist, rotor: New
2. Timing: Good (checked by ASE pro mechanic)
3. Carburetor: Installed rebuilt carb from guaranteedcarbs.com 4 weeks ago

The "Lean Burn" mentioned... If that's the same as the Electronic Fuel Control System (found attached to the air cleaner) or Spark Control Computer (attached to the inside fender wall), no, I do not have either.

And again, it doesn't matter how slow or fast I'm driving, its just obvious when I'm maintaining one continuous speed. And, then also (maybe related, maybe not) "the lurch" (see first post) when in gear and sitting at a stop sign, driveway, etc...

 

[3834B]

I might be suspicious of the fuel supply. From the pick up in the tank forward could be suspect.

You could do a flow test on the fuel pump. Maybe run a line into a container and crank the engine for (X) ammount of time and see what volume of fuel was delivered.

Make sure to exercise all safety precautions.

 

[dodgensince74]

could be a bad/weak coil or dirty connections to it. Do you still have your old carb, if you do put it back on and see what happens

 

[Speed Dragon]

Hmm, sounds like a vacuum leak maybe, spray around the intake where it bolts to the heads w/ carb spray, starting fluid, etc and see if the idle changes. If so, you have an intake leak.

How do the brakes feel? Hard pedal at all? Sometimes the boosters can spring a vacuum leak also.

 

[Greenwave]

I replaced the old carb with the rebuilt one thinking Okay, I replaced all the stuff that I thought could be causing this, maybe it's the carb? The engine runs exactly the same with both new carb or the old carb. No change.

I failed to mention I did search for a vacuum leak. I searched long and hard with a gas torch (not lit of course). And there was not increase in RPMs

Do you think it could be the fuel pump? It's about the only part I haven't thrown at this thing.

 

[Delray Dude]

i'd try the fuel pump...heck it isn't too expensive....and then you know it's good after that.

 

[3834B]

Try a flow test with your current pump. If you have a problem (restriction/blockage) on the (pick up/inlet side) the best pump in the world will not help you.

Generally speaking a bad fuel pump will leave you walking. They are a diaphram style pump. When the diaphram gets a hole/tear it is usually over.

I suppose a mechanical failure with the pump could be possible i.e. a damaged/bent arm or the eccentric on the cam gear.

Check your fuel lines where the rubber joins with the steel portion. Any cracks will allow the entry of air.

Good luck.

 

[3834B]

There's two check valves in that pump that could be suspect as well. If either is held open or (partially open) flow will be interrupted.

Try a flow test on the (outlet) side of the pump. Should it fail I would add a (Dumby Line) on the inlet side of the pump going to a container of fresh fuel. If that test passes you now would know the problem exists down stream.

 

[doodlebug]

3. Carburetor: Installed rebuilt carb from guaranteedcarbs.com 4 weeks ago

I bought a rebuilt carb from them a couple of years ago. Ran extremely rich. I had to take it down and re-check the float level which turned out to be way high. All I can say about their carbs are that they are "guaranteed" to need rechecking. :cuss:

 

[DiverDwnPowrRam]

nice.

 

[sgillett]

Does it run good at full throttle?
If so, it doesn't sound like a fuel delivery issue to me.
My $.02

 

[Greenwave]

No, it doesn't run better at full throttle or otherwise. It pretty much runs the same at any continuous speed.

Today I was replacing the stock stereo with a unit from an '88 I picked up over the weekend at a U-Pull it yard (they charged me $5 for the "Infinity" all digital from Chrysler). After I made the switch I fired up the truck to let it run a while. While idling in the driveway for 4-5 minutes, the engine went from a full rage idle (maybe 2-2200rpm for about 45 seconds) to a more normal idle to nearly stalling out at the 4+ minute mark. It was just sputtering and gasping to stay alive. UGH!

Thanks to everyone that's responded. Once I get the chance I'm gonna try the different ideas. And if anyone has any other thoughts, I'm certainly open cuz I'm beating my head against the wall.