my truck is a 1990 GMC sierra with a 350 with 135000 miles. It idles poorly when it is running. The problem is worse when it is in park. there is also a very long hesitation from an idle if the gas pedal is put to the floor. It takes about 2 seconds for the truck to realise the pedal is to the floor. I have tried numerous things to try to track down this problem.

Work done includes:
New spark plugs (AC delco)
New Plug wires (moroso blue max custom fit)
New coil (ac delco)
new cap and rotor (ac delco)
new O2 sensor (bosch)
new knock sensor (ac delco)
new coolant temp sensor (ac delco)
new EGR valve

The vehicle has headers, a new edlebrock camshaft with lifters, the heads were recently refirbished but are the originals. I have a one inch throttle body spacer as well. the throttle body was cleaned well when the head work and cam was installed (may 2006). It gets about 15.5-17 mpg when driven normally. Also the timing is currently set at 4 degrees BTDC.

If anyone has any ideas that could help me please let me know. I have adjusting the timing but it has not helped.

Thanks

Jon

Could be that the bushings in the distributor are wore out. Common problem.

I have replaced the pickup coil in the distributor last year. Is the pickup coil is the same as the bushings?

I have replaced the pickup coil in the distributor last year. Is the pickup coil is the same as the bushings?

No they are not the same. If you remove the cap, try to move the rotor side to side. If you can feel it move the bushings are probably bad.

I also have a 90 GMC K1500 that I bought new back in Nov 1989. I am the only person to work on this truck. I also have a rough idle only when the engine is fully warmed up. In my case, I suspect the ECM may be bad but have not replaced yet. The reason I suspect the ECM is based on the quad driver resistance. Book says 50K ohms but mine is more like 11K ohms. In my case, rough idle is the only issue. As soon as you press the gas pedal, it runs fine.

On yours, you do not mention the MAP. Make sure the vacuum source to the MAP is good.

How does the injector spray pattern look? Do you know if the fuel pressure is within specs?

Did you check the EGR solenoid? It needs to hold a vacuum. Mine was bad and needed replacing.

I also found the dist gear was worn (120K miles) and replacing it helped slightly in the idle quality.

//2000CAYukon

You mentioned that your timing was set at 4 deg btdc, I believe it is supposed to be set at 0, Thats what my 91 is supposed to be set at,
also has the cat been changed? I have many of the same mods to my truck and have no throttle lag,
My truck had a really poor idle when using Calif gas, since moving out of Calif my idle has improved dramaticly, though I still feel a bit of the same poor idle now and then.,
I installed the Edelbrock cam and lifter kit # 2101, I believe it was and have thought from the start that maybe this was a bit too much cam for the puter, the Edelbrock tech said this was the most cam I could install with the stock puter, mine seems to run a bit lean, so my next change is an after market chip designed just for the mods I have,

I just replaced the O2 sensor (wire was melted) and I took off the throttle body spacer that was on it. The idle seems to have cleared up almost completely.

And yes the timing is supposed to be set at 0 degrees, and I also have an aftermarket cat on it because the stock one wouldn't fit with the headers.