'87 Chevrolet TBI 350 runs rough under load

rough
chevrolet
2500

#1

A few weeks ago, I made the mistake of selling off my old, and simple, '66 chevrolet pickup project and bought a newer('87) truck that would quickly turn into an even bigger project.

I bought the truck two weeks ago, drove it home from a neighboring city, two highway hours away, no problems. The next day, I ran a few errands, highway, traffic and stoplight driving; again, no problems. After returning home for the afternoon, I got back in the truck a few hours later to make one last short drive across town. No start. The engine cranked over and over and over and over but no start.

I talked to the previous owner who, after saying it was running great, finally shared the info about powerwashing the engine bay the morning before he sold it to me.

The next morning I started wrenching. That was 2 weeks ago.

I found out, the cap and rotor were dry, the plugs were getting a spark, fuel was flowing all the way to the throttle body (TB), but not getting past the TB. Admitadley, I know little about throttle body injection(TBI). So, I took the truck to the local shop where I worked a few years ago as a full service attendant while in highschool.



The mechanic found a shorted wire that was preventing the injectors from firing. The truck is now running but very rough. It starts and idles and even revs smoothly BUT only when the truck is in park. The rough-running issue only comes around when the truck is in drive and the engine is under a load. It feels starved for fuel.



new parts include,

cap, rotor, plugs and wires, fuel filter, oxygen sensor, coolant temp. sensor, and I spent a few hours replacing shoddy wire connections as per my mechanics recommendations.



Again, runs smooth at idle and even Revs smoothly but only when the truck is in park. runs rough and feels starved for fuel when in drive and under any sort of load, barely drivable.



I’ve quite a bit of experience with old cars and trucks but the newer ones (yes, even 1987) give me trouble. I’m not used to excessive wires and different electrical sensors. I don’t even know where to look for the problem.



any ideas confident ideas?


#2

Air flow sensor, computer, come to mind. The does the throttle have a TPS throttle position sensor? I am also thinking bad mix (ie too lean).


#3

You’re describing an engine that’s lacking fuel under load.

If you can rev the engine with no load, but the engine stumbles and falls on it’s face when a load is imposed, the fuel pressure is the first thing to check.

Tester


#4

The first thing the mechanic mentioned was the fuel pressure. I assume it checked out just fine since he didn’t mention it again. I can confirm in the morning.

thanks for the help.


#5

I didn’t see MAP sensor on the list. That’s about the only sensor working outside of idle and cruise. The MAP indexes the pre-mapped fuel curve for open loop. I don’t think a vacuum leak would be the cause. It would give you an abnormally high idle …unless it had a fixed MAP signal ;^)


#6

I finally found a bad wire connection at the throttle position sensor. The sensor is new but the connection is suspect. After messing with the connection, the truck’s power has mostly been restored. HOWEVER, there is still a slight hesitation at initial acceleration. AND, it’s still idling quite high, maybe 1200rpms; a bit higher when hot. Does this problem now point point to the MAP sensor? And, will a new MAP sensor bring the idle down to a normal rpm?

thanks, all, for the help. it’s very much appreciated.