Sputtering at cruising speed

[Bigd_099]

Hi I'm new to the site and there's so much info on here do if this has been discussed sorry. But anyways I have an 87 scottsdale 3/4 ton with the 350tbi. The truck runs fine when around town, starts fine even when cold, idles fine, and accelerates fine. The only time I have a problem is when I am crusung along about 50 55 mph with hardly any pressure in the throttle. It will start to spit and carry on and act like it is gonna die but as soon as you go up a hill or speed up a bit it will straighten out and there's no hesitation to speed up either. It does seem to run fine at thus speed when it first starts but when it warms up it starts this and like I said if its around town and your crusung at say 25mph it doesn't do it either. Any suggestions as to where to start? Thanks guys

[rich weyand]

Let's start with timing.  What is your base timing set at?  Is your vacuum advance connected?  Where is it connected (manifold vacuum or ported vacuum)?  Are all the vacuum gewgaws and pollution control stuff still on it, or has that been removed?

[Bigd_099]

I'm not sure what the timing is. All the vacuum lines are right under the throttle body. I just bought the truck not to long ago and I'm new to these motors. I seem where they had in hooked the line that runs to the canister up front to the tank and I hooked up and the lines that run to the egr vavle are hooked up but I believe some removed the other 2 gadgets the diagram under the hood showed. If these need replaced  that shouldn't be a problem or can you eliminate all that.

[Bigd_099]

I just went out and looked yes someone removed the air pump.

[rich weyand]

You need to get a timing light and set the timing.  You need to check the vacuum line to the distributor.  I think your truck has a distributor delay valve, and that is connected to ported vacuum.  Can you take a pic of the vacuum diagram and post it?

[bd]

Except for base timing, the factory timing control on TBI engines is completely electronic.


@ Bigd_099

What happens if you very slightly step on the brake pedal while driving at 50-55 MPH?  Do the symptoms disappear?

[rich weyand]

Except for base timing, the factory timing control on TBI engines is completely electronic.

OK, wasn't sure.  That's why I wanted to see the vacuum diagram.

My thought was that he is way retarded at speed with high vacuum. 

[Bigd_099]

Here's the diagram

[Bigd_099]

@bd

Ill try the brake thing tomarrow and let you know

[bd]

Which transmission does it have?

[VileZambonie]

Have you tuned it up (plugs, wires, cap & rotor)? Sounds like a misfire.

[454Man]

What carb do you have?

What's under the hood??? If you have to ask maybe we shouldn't race...

[rich weyand]

454Man: Back in first post, it's 350tbi.

[454Man]

Oh lol well then IM thinking your throttle position sensor is bad

What's under the hood??? If you have to ask maybe we shouldn't race...

[darmentle]

Could Also try a new fuel filter there like 5-10 Bucks