'87 4x4 5.7L running rough

[Blake_H]

So, I bought an 1987 Chevy 4x4 1/2 ton with a 350 tbi engine.  It ran great driving it home from Idaho, about 9 hours.  It ran great running around town trying to get it smogged.  It ran great going to pick-n-pull and pulling an entire smog system off a wrecked unit.  It ran great for the smog check.  That was a week ago.  It is now running like crap.  The service engine light comes on and off with no consistency.  I was under the assumption that if there was a problem, that the light would stay on until you fix it.

What it does:
Difficult to start.  I mean really difficult.  I have to crank it for a minute or 2 and then it labors to fire, but does.
Once it's fired up, it idles normal for 30 to 45 seconds, and then gets really rough, like it's trying to die, then comes back to life.
When I pull out of the driveway, it stumbles and gags, and then comes to life and goes.  Same at any stop light or stop sign.  If I don't keep my foot on the pedal while waiting for the light to turn green, it dies.  It easier to start after that, but annoying none the less.  On the freeway out to our house it runs great.  Just low rpm and seems to me that 30 mph and under is where the trouble is.

What I did.
I had to unplug the harness across the top of the engine to get the smog equipment on.  I had to disconnect everything from the throttle body and the distributor.  I had to take the fuel lines off and out, in order to run the air pipe around the back of the engine.  I installed the smog pump in the factory location and connected everything.  I put in new spark plugs, cap, rotor and wires while I was doing all this.  Reversed the process and I had taken pictures to make sure I got it all back in the correct places.  I had to install different exhaust manifolds, as the original ones, were destroyed where the air pipes went into them.  I swapped my O2 sensor over to them and reconnected.

Other than the tuneup, the only thing new is the smog system which was connected to the factory pigtail.  I have checked and re-checked all the wiring.  Bought a fuel filter, but haven't put it on yet (that's tomorrows project).

I have no experience with the TBI engines.  I also have limited knowledge on troubleshooting, but am willing to try anything.

Does anybody have any thoughts.  Any help is greatly appreciated.  Thank you!

[Irish_Alley]

i would check the psi of the fuel first it should be around 14 i think. and i thought the cel would only stay on if the problem was consistant in the older vehicles with obd.

[Irish_Alley]

could be a split/crack/pinhole in the short section of hose that attaches the pump to the hard line on the sending unit if your psi is low. now it that way on a 94 i know, but not sure about yours it should be tho.

[Blake_H]

Thanks for the replies!  I will check psi before I swap the fuel filter.  Luckily it's not my daily driver, so I will check it when time allows and report back.

[dumbucket1]

Sounds alot like the syptoms an unplugged or broken coolant temp sensor would do

[Irish_Alley]

thats what i was thinking or even a tps  but thought test the fuel first see what thats like

[Blake_H]

Are you guys referring to the single wire going to the driver's side cylinder head?  Reason I ask, is the temp has not been reading the same as it did before I did all this.

[Irish_Alley]

i would say the one on the intake next to the rad hose is the sensor to look at the one in the head does controll the gauge tho

[Captkaos]

Coolant Temp is the one next to the thermostat housing, My Crew cabs went bad and wouldn't stay running. 
You should be able to pull the codes and tell what it is saying is wrong.

[Blake_H]

Okay, so I bought an actron code scanner at AutoZone tonight.  I went through the book after reading the codes, which came up 12 and 21.  According to the book, it is a TPS signal voltage high.  Which is great, and I'm glad I didn't throw a bunch of parts at it, but what do I do now?

[Captkaos]

You only needed a paper clip to get the codes out, but.....

21 = TPS voltage was above 2.5 volts for 3 seconds when the MAP sensor signal showed manifold vacuum to be 15 pounds or more, or TPS was above 4.5 volts at any time.

Check your TPS voltage, if it is higher than around .55V at idle you need to adjust your TPS.  You will either need WINALDL or just backprobe the sensor for the voltage loosen the mounting screws and adjust accordingly.

[Blake_H]

Thanks, Captain! 

I back probed the TPS and found it to have 4.6 to 5 volts, no matter what I did with it.  So, I ran down to the parts house and got a new one.  Put it on, fired right up, and it ran better instantly.  But there was still some power loss and it seemed to not be firing on all 8.  Checked the plug wires over carefully, and found one of my new plug wires, #2, had come to rest against the exhaust.  Argh!

So, I taped it up, and checked it with the meter.  Same resistance as #1.  Plugged it back in, and it ran a little better, but it is still running rough under load.  Not as rough, but still.  It's like it has a miss.  Not a dead hole, but a miss. 

Any thoughts??

[Blake_H]

Finally figured out what was causing the miss I thought it had after all the items I had fixed or looked at wouldn't quite bring it back to normal.  Yesterday, while the wife and I were on the way home from the home improvement store it started to smell funny.  When we got in the driveway, it was poring smoke out of the drivers side exhaust.

Turns out I had a bad head gasket that was filling #4 with fluid from the radiator.  I guess it was getting progessively worse during this time.  At least now I know what it was.  Now it moves to the project section, for an overhaul.

Thanks for all the help and input.  I really appreciate it. 

[Captkaos]

Eww! that isn't a nice discovery.