rough idle?

[setandreel]

Hey all,
need a little help please. I have a 87 silverado 4x4 with a 350  tbi in it. It idles very rough almost a surge. Man I have tried everything IAC, idle sensor, o2 sensor,coolant sensor,EGR valuve, I have checked all over and no vacumn leaks. what else could it be? I am trying to keep from taking it to the chevy dealer cause of the cost but if I keep this up I will have spent enough to buy a new truck. I know it is probley some thing simple. But its got me stumped.
Thanks
Mike

[setandreel]

Also did new plugs,cap,rotor still stumped?
thanks
Mike

[VileZambonie]

Rough idle is your only problem? Check the TBI base gasket. Check your fuel pressure. Does it idle rough when hot, cold or both? (hot/cold meaning engine temp)

[GMCTed]

i have an 1987 gmc suburban with a 350 tbi and i had the same thing wrong with mine and replaced alot of the same parts you did. then a friend suggested that i try lucas fuel system treatment, i never thought it would fix my problem but after about a half tank of gas it was idling like it was new. i hope this can cure your problem too.


www.cardomain.com/ride/2387678

www.cardomain.com/ride/2387717

[setandreel]

Yes sir idle is the only problem above idle it runs like a new one
I replaced the tbi gasket and then sprayed carb cleaner around base plate, no change.Also tried lucas in both tanks twice no change.

[VileZambonie]

Does it idle better when the engine is first started (cold) This is when the computer is in open loop and is not looking at your O2 sensor. Or does it always have a poor idle?

[setandreel]

soon as you start it it starts its surgeing some times worse than other times but always present seams its worst when it is very hot 90's  

[VileZambonie]

Well the EFI systems on these trucks were very simple.

ECT-engine coolant temp
TPS-Throttle position sensor
MAP-Manifold Absolute pressure
Distributor reference pulse

All of these = speed density in which calibrate your a/f ratio.

02 voltage is the best means of calculating the A/f ratio in closed loop but not until the engine run temp and run time get you there.

Have you tried misting a little carb clean over the throttle body to see if it smooths out the idle? If it does at least we know you are looking at a lean condition and we can start to narrow it down in that direction. If it makes it worse you have an overly rich condition. Since you proabaly don't have access to the essential tools to look at your datastream start with that and report back. From there we can help pinpoint your exact problem.

[setandreel]

Vile,
Went out this morn. Cranked my truck and it stumbled like always, then did as you said with the carb. cleaner barely misting over the tbi with the air cleaner off no change but I Sprayed it down the tbi to the left toward where the IAC is (down the tbi not on the outside) and the motor almost died so I did 5 or 6 bursts in the same spot and the motor would idle up after almost stalling and ran well so I cut it off let it set 20 min. went out started it up same stumbling, did the carb cleaner as above ran well? man I am confussed, anyway I am going to drive it to church and see how she does. When she cleared up after the carb. cleaner I thought I had it, oh well I will keep on tring. I will keep on reporting and I do thank you for all this help it has been a blessing
Mike

[VileZambonie]

Well the idle speed changing tells you that the ECM is working controlling the IAC motor.

Do you have access to a digital volt meter?

You said you tuned it up right? gapped all the plugs correctly, new cap and rotor, fuel filter, etc?

The fact that it does it cold and it runs fine off idle eliminates the O2 sensor as a probable cause too.

Make sure the vacuum hose to the map sensor is not cracked.

If you have a meter I'm going to have you test a few things pretty easily.

[setandreel]

plugs are right .35, inspected the vacumn lines, look good. I don't have access to a volt meter, to be honest I would not know how to use one

[VileZambonie]

It's pretty easy so if you get one I can walk you through it.

[setandreel]

Well think I got the problem fixed. Took it to a shop a friend has and he checked the timing and it was good, then he saw the new EGR and said that was it I said man ain't know way, well he unpluged the vacumn line to the EGR had me ride around the block and man what a differance didn't know my truck had that much power. So I put a BB in the line to block it and rode around and the differance is amazing I don't have to worry about emmisions my truck is to old so I am good to go. Vile I do thank you Sir for your time and help.
Mike  

[VileZambonie]

OK glad you figured it out but it's not the EGR valve. It's the EGR solenoid, it's stuck open.