I have a '90 Iroc-Z with a 5.0L TPI and I'm having Isues. I changed the Cap & Rotor, Ignition Module, Coil, and Spark Plugs thinking that would solve the problem. That didnt help so then I changed the Knock Sensor and MAP sensor. After that I found out that the Timing was off. Every time I set the initial timing to 6 degrees like the specs ask for the car runs beautiful with full power for about a mile or so then it gives and runs like crap all over again. The computer advances the timing for some odd reason. The computer isn't throwing out any codes NO matter how much of a rough idle it has.

 

It sounds like it might be going into "limp home mode" once the computer goes into closed loop operation. Are you absolutely sure the parts store sold you the correct knock sensor? They're different for 305 and 350 motors, and the 89-older MAF knock sensors are also different from the 90-92 SD sensors. You might want to try putting your old knock sensor back in and see if it fixes the problem. If it does, then they sold you the wrong knock sensor for your application. The ecm doesn't use the knock sensor in open loop, and driving about a mile is long enough to go into closed loop operation. It's worth a shot.

 

The knock sensor came from the GM Dealer.... They took my Vin # to make sure they were giving me the right part.... Plus it ran that same way before I changed the knock sensor. I changed the knock sensor and everything else in hopes that it would solve the problem. The ECM or something is making the ECM advance the timing which is making the car idle and run erratically rough

 

Ok... So I got the engine running again... After changing MAP sensor, External Coil, Cap & Rotor, Spark Plugs, Ignition Module, Knock Sensor, and Fuel Filter, still have no luck on what causes the timing to advance after setting initial timing at 6*. The odd part is that sometimes it'll advance the timing and sometimes it'll retard the timing.... I'm stumped on why the random adjustments. I followed the instructions from the Chilton's Manual on the Timing adjustment procedure but still NO Luck... I grounded the ALDL, disconnected the EST, and set the timing. It ran good for about a mile then advanced and ran like crap. Again I grounded the ALDL, disconnected the EST, and set the timing. It ran good for about a mile then it retarded itself. I then disconnected the EST but left the ALDL alone and set the timing and it still seemed to do either advance or retard. Any Suggestions would be greatly appreciated...

 

do you have a stock cam on your car or performance one?

 

The Engine is completely stock... No Headers, No Mods, No performance upgrades at all

 

Base timing is a mechanical aspect and can only be changed by physically moving the distributor. If you set the timing at 6 degrees it should be 6 degrees everytime the est is disconnected, If not you need to tighten the distributor hold down.

 

The Distributor doesnt move by hand at all.. It's completely tightened and bolted down. I set the initial Timing and it doesnt stay at all.

 

You may not be able to turn it by hand, but something is causing the distributor to move after each timing reset.
Reset the timing again, with a wide tip black marker put a mark down the side of the hold down flange and onto the manifold then scribe a line on both along the center of the black mark (makes it easier to see the thin scribed line.) When it starts to run a foul check the scribed line to see if the distributor has moved.

 

I have a Problem....
At start up it idles great, as soon as it warms up, it idles rough.

At a normal take off from a stop light, it hesitates and feels like it has a major miss. As soon as I get up to desired speed you it runs decent to where you couldn't even tell if something was wrong with the car. On the freeway and at higher speeds there nothing with the car at all.

Another problem is when idleing for too long or after being driven for over 20 minutes or so there's a strong fuel smell but there's no leaks anywhere.

Within the past 2 months I have changed External Coil, Ignition Module, Cap & Rotor, Knock Sensor, Fuel Filter, Spark Plugs, MAP Sensor, Oil Change, Radiator Flush, EGR Valve, Fuel Pressure Regulator, Oxygen Sensor, Engine Coolant Temperature Sensor, Air Intake Temperature Sensor, and Fuel Injectors (Bosch III's). All parts replaced where OE replacement AC Delco from the Dealership.

Man I'm so Lost right now and so close to being broke

 

the same thing is happening to me but i have a 87 trans am with 350tpi.

need help!!!!!!!!!!!!!!!!!!!!

 

might be worth it to check to see if the rubber ring on the harmonic balancer is shifting

 

I am going throw this out there. This happen to me aways back. Pull the dist completely out marking the rotor location. What I found was that the Dist shaft bushing was worn and at high speed changing the timing. But at idle it was fine due to no load on the dist shaft. Its worth a look. Good luck, you will fine the issue