1991 gmc 5.7 surging while driving

[roonie]

this truck has 180km on it and runs fine exept as of lately i have developed a surge while at constant speed. When i step on the gas it does not seem to happen.....it only happens when I reach a constant speed. The odd time it will surge at idle and the engine will rev up and down on its own also. When at hyway speed it seems to not do this. The truck never dies completely....just surges slightly. Any clues guys?

[maxwedge]

Many things can cause this, a full scan will look at O2 inputs as well as other sensors, also the egr could be staying partly open at low speeds.

[roonie]

ok...so best to do a scan then....is there somewhere on here that tells how to bring up codes? Can i bring up codes on this 1991 truck with the key or do I need a storebought scanner? The truck actually accellerates fine ...the surging is worst when i am at cruising speed with no accelleration or no let off. thanks

[maxwedge]

Codes is not really, the answer you could jump A and B terminal at the adl connector and count the flashes, but a bad o2 may not set a code and only a scanner will pick this up.

[roonie]

Sooo......i made up a wire and jumped A and B on the code connector. I came up with a code of 12. I then went to this page http://www.troublecodes.net/GM/ and found that most the codes for my 1991 gmc with a 5.7L engine, start at 13.
However.....i forgot to mention that there is more often than not, a small pool of oil that lays on the top edge of my intake manifold.The oil is almost always there but the surging just started. Is it possible to have oil come up from a leaking intake manifold or is this oil coming from my valve cover? What to do next?

[maxwedge]

The oil could come from the corner of the intake more likely, unless this caused a vacuum leak it is not related to the surging, a full scan may be required to check the sensor inputs, they don't always set a code in the older vehicles.

[roonie]

just thought i would post back with what i found. I know i dont like when others never post back with findings. The truck needs a head gasket. I crawled under today to find water leaking from the front of the head. Just soooos ya know.

[crxlvr]

im having a similar problem with my 5.7 motor, i dont see any leaks though

[roonie]

Well.....i still have the same issue after changing out the gaskets in the head and intake manifold. I have also changed plug wires/cap and rotor. I have also changed the fuel filter, checked the EGR valve and EGR selenoid function.
Checked and changed any cracked or questionalble vacuum lines. Runs somewhat better but still has that erratic engine when the accellerator is held at one rpm. I doubt it is the fuel pump as when on the hyway she goes along just fine and when punched she performs great. It seems to be just around town when i reach the speed i require and then held at that speed....the truck then is shudders intermittantly. Any more thoughts folks.

[2000CAYukon]

On my 90, I found that the vacuum hose to the MAP sensor was leaking and that caused some issues for me. Also make sure the MAP sensor will hold a vacuum using a vacuum pump.

//2000CAYukon