As the subject says I am having a problem with a rough idle and setting code 45. This only happens when the truck is warm, when cold it idles fine. This is an '87 GMC with a 350 tbi.

If, after the truck warms up, I sit at a light the idle gets very rough and throws a code 45. I have diacom data which shows the O2 sensor reading (constantly) above 900mv during this time, and of course it goes in to open loop. All other sensors read normally. Immediately after hitting the gas a little the ses light goes off and it goes back to closed loop. The truck accelerates and runs smoothly, just problematic at idle.

I have tried all of the following (only ac delco parts used):

Replaced EGR valve
Replaced all plug wires
Replaced dis. cap and rotor
Replace all plugs (they looked normal though, nice tan/white)
Replaced O2 sensor (that also looked normal, no carbon build up)
Replaced air filter
Replaced evap. can. filter
Replaced fuel filter
Replaced fuel pressure regulator
Replaced both fuel injectors
Replaced MAP sensor

Fuel pressure is normal
Return line is not clogged
There is no fuel in the evap. can. line
PCV system is working normally
The gas mileage is normal
I triple checked and there are no vacuum leaks
Vacuum at idle is normal, until code is thrown, at which time the vacuum drops (as if under load)

Any comments/suggestions would be GREATLY appreciated.

Thanks

 

350,

You've done a lot to maintain your truck, but one thing conspicuous by its absence is adjusting the TPS and throttle minimum air position. Try cleaning the TB and IAC and its air passages, then set the minimum air, then the TPS voltage. Reinitialize the ECM and try it again. You may find that you are lacking adequate idle air for the rate of fuel flow. That could be from blocked air passages, or from too much idle fuel due to an improperly adjusted TPS. If that doesn't solve it, there are some other possibilities.

I had a very similar problem on an '88 TBI truck engine. After doing all you have and more, I finally determined that the PCV FLOW rate was incorrect. The brand new (Fram) replacement PCV valve I had installed a month before was not properly calibrated to the engine (in case you didn't know, the springs and pintle are calibrated to flow a specified rate at various vacuum levels). Amazingly, I cleaned and reinstalled the old AC PCV valve (still had it in the garage) and all issues were solved.

Another problem I had on an older TBI was a rich idle, but normal-to-lean operation through the rest of the range in closed-loop. After a similar bout of changing parts, tuning, etc., I tested the fuel pressure and found it to be 7 PSIG instead of the required 11-13 PSIG. That should have caused a lean condition to my way of thinking. However, it seems that the ECM programming tried to compensate for the lean idle (caused by low pressure) by incrementing the fuel injector pulse widths in steps. Once it had made some rather large steps, the O² sensor was satisfied. Unfortunately, that last step made it actually too rich, to the point where I could smell fuel and get a DTC 45. Once it was running off idle and cruising, I would get a DTC 44 (lean exhaust) because the ECM couldn't compensate enough for the fuel demand under load. After the fuel pressure issue was resolved, the problems cleared completely.

Just another couple possibilities.

 

Vader,

Thanks for the response. I did remove the IAC and cleaned out the passage by spraying carb cleaner through it while the engine was running. I also tried a different IAC (from my friends truck with the same exact motor and tbi) and that didn't help. I forgot to mention this in my long list of things I tried. I am fortunate that he lets me "swap" these things to try I also tried a new throttle body gasket (torqued correctly with a torque wrench).

I did not try a new PCV valve yet, man I hope that's it, but I will. I have a link here to my data collected from Diacom, if you have Diacom and you would be interested in taking a look at it. It shows the TPS is set and working correctly, as is the CTS but maybe you might see something that I overlooked.

http://members.aol.com/tunedport350/public/gmctrbl.gdf

 

Hey I’m new but I done the same thing changed all those parts I even changed the ECM new one still rich and still rough idle

 

You might start a new thread this one died in 02.