94 C2500 5.7 Stumbles

New to the board and looking for advice. I've got a '94 5.7 TBI with 120,000 miles on it. I've noticed an off idle stumble after lightly stepping on the gas, or at heavy acceleration it will bog after which I have to let off the pedal and step back on. This happens during driving or at idle in park, hot or cold. Here in Il. we have to go through the emmisions test which I miserably failed, they said it was having a problem burning fuel. Now weather it's running too rich or that meant unburned gas is getting through, I don't know. In the mean time I did a complete tune-up w/wires, fuel filter, cat. convertor and oxy sensor, still no improvement. I tend to think it's somewhere in the throttle body, but what would cause this, bad injectors? Thanks for any help.

Number off things can cause this, also failed emissions test even after a major tune-up. In my case the coolant sensor (1 of 2 to the left side of engine block) had a bad connection, and timing was way off. I also removed the air valve (1 1/4" wrench) and sprayed cleaner inside throttle body and cleaned up the value tip itself. You don't specify whether you failed on hydrocarbons, CO2 or NOx. After I went nuts going through all the posibilities I passed emissions

According to the diagnostic report I failed on the hydrocarbons, 1.60 is standard and my reading was 5.55.

While running at idle, try pulling the egr diaphram up, engine should stumble or even stall- if it don't egr passages are clogged.Vacuum leaks, lack of tune-up, O2 sensor, map sensor all can cause hydrocarbons to rise, but your way off the scale. wish you luck.

I have this same problem with my 94' Sierra K2500 5.7 only my HC's are in the 7's! Here's what I've done so far:

tune-up
Replaced MAP sensor and TPS sensor
Cleaned IAC pintle and seat
Replaced distributor that had questionable pickup coil

Next on the list are the coolant temp sensors, but I replaced those when I changed out the engine 80k/3yrs ago along with the knock and O2 sensor.

I also noticed that the timing is SUPER advanced (12'oclock) and it stumbles horribly if set correctly. It runs very rich at all times according to the emissions graph.

The emissions label says "disconnect EST connector" when setting the timing, but there is just a 1 wire connector back there not connected to anything and nothing for it to connect to. I have asked a GMC tech about it and he seemed baffled and told me to ground the wire, which shut off the engine. Therefore I have given up trying to time it by normal means. I just set it by ear and road tests. This is totally unrelated to the problem I'm having now, but just thought I would throw that in there.

Crab Spirits, If the EST is disconnected then the ECM is not controlling the advancing or retarding of the timing as it's supposed to. That in itself could be causing you some of the grief your having. The EST should be the Brown/Black wire under the black plastic cover on the passenger side firewall. It's a straight thru wire with a connector there thats goes on to the ECM. Someone would have had to disconnect it at some time. Has it always run this way since you've had it?

I have a similar problem but with 281k its more than likely another problem like valve guide seals and piston rings.

you have to disconnect the EST to eliminate any ecm interference with static timing. The EST functions like a vacuum advance, and will throw your initial timing off if connected while trying to adjust it. Make sure you re-connect it after setting your initial timing or your truck will run like a pig. The wire your mechanic told you to ground was the fuel pump test lead, so it's no surprise your truck died when you grounded it.

Ok, I've made some progress now. The reason I could not find the EST connector is because THERE WASN'T ONE. The connector I was looking at is actually the "tach test lead" by the coil. After taking apart the wiring harness for the 3rd time, I traced the tan/black wire all the way back to the firewall. I just cut the wire and set timing ,correctly this time, then put a connector on it. I still have the same problem. I have also replaced the coolant temp sensor and it's connector. No change. While it was running, I could just barely wiggle the MAP sensor (which i've already replaced) and get it to stall every time. I need to do some more investigating there but the boss needed the truck. I also got a map sensor high code, but I was getting a few codes since the wires were all over the place and I was getting interference from the plug wires while the truck was running. Wish I had access to a scan tool so I can see what the hell is going on. 300,000 miles, time for a new truck.

OK, I found somebody with a scan tool and hooked it up. All the data seemed normal but I noticed the problem occurred right when the EGR activated. I had checked it earlier, but only see if the valve was moving, which it was. Get this, I applied vaccum to it and it moves but if more than 5 in/hg was applied, the EGR valve would leak severely. It sounds like the diaphram is crumbled inside it if you shake it. Replaced the valve and all is now well. Thanks for all the replies, this was a tough one.

Well after removing the intake manifold to replace gaskets because of an oil leak, I found the 2 ports in the manifold going to the EGR valve were 100% clogged with carbon. Cleaned everything up, put it back together along with a new EGR valve and it runs like brand new. Good for another 123k miles (lol). Thanks to all who responded!