350 tbi rough idle no power

[jacobdavis1979]

Guys please help me. About a week ago i bought an 88 gmc stepside dirt cheap brand new paint beautifful aftermarket wheels and tires tinted windows the works with only 121000 miles for 1000 bucks. I thought it was a good buy anyway now i dont feel that way. The guy i bought it from said he pulled the motor ( i forgot to ask why) had the headgaskets intake gaskets and valve cover gaskets at a shop then he put the motor back in but didnt know how to time it. Simple enough right. I brought it home i timed it yes it was off but not much. I pulled the tan wire tried to set it at 0 degrees but it wont idle there i have it set about 6 to 8 degrees so it would run. I plugged in the obd scanner i dont remember the code numbers but map sensor o2 sensor and i think it was ecm or ecu i dont remember whats its called. But i assume that was the module in the distributor i bought a new one at napa there was little to no change in how it ran and i did not change the o2 or map. I replaced all the vacuum hoses they were rotted as hell but still no change. I took the throttle body off and cleaned it no change i put on a new fuel filter no change. Dizzy cap rotor bug no change and yes firing order is correct. I even cut off the catalytic converter no change now its noisyer. The obd still shows those same codes ecm map and o2 sensor.
The thing runs perfect for like 20 minutes then the next 20 its like it revs itself up and down then the next 20 min it lopes around like its got a giant cam then back to perfect. And the transmission slams into 2nd and dont kick down when goin up a hill. I know that problems not related but with this computer junk nowadays i dont know anymore. And the thing has no power at all. It can rile up and scream but thats it. Guys please help me

 

[pontiacgp]

I'd be looking at the map sensor, code 38 or 39. Make sure the vacuum line to the map sensor in not pinched anywhere. When it is running rough unplug the map sensor and see if it runs any different.

and by the sounds of it even with the problems you got a good deal, those stepsiders are nice

 

[horners85regal]

un plug the o2 sensor, first one out from the engine and see if it runs good. That should put the truck into open loop. It will not care about any sensor inputs when in open loop thus if running good you know its relative to sensor input. Ive had a 91 5.7 tbi run like a bag of sh*t only after it got warm from a lazy o2 sensor. It would over fuel, then under fuel because the sensor was reading incorrectly and the ecm was tryn to compensate for fuel issues that were not ever there. but according to the o2 sensor ....they were.
the ecm or ecu if its the same as 91 is behind the glove box its the computer for the vehicle.

 

[86judge]

These set ups are very sensitive to vaccume. The map sensor is the main one that determines this. If you have a vaccume gauge put a t in line with the map and check how much vaccume your reading at its best idle. Also check the cool and temp sensor. If it's goes bad on the cold end of the reading the truck will usually idle half way descent cold but as the motor warms up it will run more and more rich cause the ECM thinks the motor is cold

 

[horners85regal]

the best thing i did with the one i was working on was borrow an obd 1 scan tool that i could actually graph and moniter the various sensor inputs with once i determined if it was only happening when in closed loop operation....

 

[MUELLERBU]

had a 94 305 TBI which on the odd occasion would start running rough, did the same cap rotor swap still ran rough, checked codes, reset timing, finally by fluke I noticed one of the vaccum hoses came off of the egr valve (Not sure if its a valve as it was a plastic box on the passenger side of manifold next to TB), plugged it back in nice idle, same condition would happen every month or so I swapped out the hard brittle vaccum lines with rubber haven't had that problem since, my two cents hoping the simple solution is there

 

[ITh4n4tosI]

the transmission I believe is a 700r4, so that may be another thing entirely but if the engine was pulled then the tv cable probably just needs to be adjusted again.

 

[jacobdavis1979]

As of now I now have 9 codes and I cant help but think this is an electrical problem now.
Il list them in order they appear on the scan tool
41 Cam sensor failure, cylinder select error, electronic spark timing circuit open or shorted during engine run, ignition fault loss of x1 reference pulse signal.
35 Idle air control system problem can not set desired rpm or idle speed actuator.
33 manifold absolute pressure sensor signal voltage is high during engine idle.
32 manifold absolute pressure sensor signal voltage is low. Exhaust gas recirculation valve switch closed during start up or open when egr flow requested by ecm, Electronic vacuum regulator valve error.
31 Park/Neutral swich circuit problems, CAM sensor circuit problems ( this code also brings up turbocharger problems but I don't have one obviously but it says its shorted to ground)
55 Electronic control module fault or oxygen sensor circuit fault or egr solenoid #3 failure
54 Fuel pump circuit fault or mixture control solenoid fault or ECM fault, Fuel pump relay.
52 Calibration package chip or ECM fault or oil temp sensor fault/low engine temp, faulty or missing, System voltage high for a long period of time.
43 Electronic spark timing circuit low voltage detected, Electronic spark control circuit problems.

Im not an expert with these scan tools but a part of me doesn't believe that all this stuff is bad. I know there is a wiring short somewhere I hit a bump while driving it and radio and gauges quit working. I pulled those fuses to be safe but its not helping it run. But I know for a fact there is a short somewhere could that be the problem or am I crazy?

Thanks for the help guys I really like this truck and I wanna register it soon

 

[tc1959]

I had a truck in our shop a few years ago with similar symptoms .They had pinched the wiring between the trans bell housing and the back of the engine. They had also left the ground strap off the back of the engine.

 

[jacobdavis1979]

I noticed no ground from the block to firewall so I made one. I added a 16 gauge wire grounded it on the intake then ran that wire to the ground where the hood light went. If there is a short in the truck I know for sure I eliminated it by pulling those fuses but its not changing anything. Knowing my luck I probly blew up this new 100 dollar ecm I bought I hope I didn't.

Im like so close to throwing a quadrajet and hei on it! :x