I have a 94 gmc sierra with a 5.7L. am having trouble with a rough idle than stall, however sometimes it runs perfect. Timing is at 4 deg. btdc. I have changed cap and rotor, ignition mod., coil, plugs and wires, and fuel pump relay. It ran good for about a week, now is acting up again. last set of plugs were fouled out and carboned up. Any thoughts?

Did you change the ECM to match the newer engine?

https://www.google.com/search?q=1992+GMC+Sierra+ST%2C+5.7+TBI+runs+rough

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Did you disconnect the battery? If so, the computer has to "relearn" all the "adaptive values." That can sometimes take up to 10 starts (from cold to full operating temp).

Idle speed 900 RPM
Fast Idle 1500 RPM
Timing (I prefer ) 6 DEG BTDC (book) 4 Deg BTDC
Spark Plug gap 35
Points Gap. I switched to Elect Ing so got to go get the book ok 0.019
Dwell 28 to 32 Deg

Let me know if you need anything else Mike

set it at 12 degrees btdc where it should be
If it has electronic timing it will advance /retard from that point and not as you expect to go to the correct setting

Sounds like you could have an aux air regulator valve, that is not working when it will not work; there is no fast idle cold. Should be around 1500 rpm cold, the car will stall out and pop through the intake when bad. You have a digital volt meter to do some testing with to make sure the mass air flow is working correctly.

Also, check the plugs for how they're burning, check the air filter for good flow, do a compression test and judge from there. Intake backfire is usually caused by running lean or improper timing.

For the 79 model zx, ignition timing was 10 degrees BTDC @ 700rpm
For 80 and up zx, non turbo - 8 degrees BTDC +- 2 degrees @ 700rpm
turbo - 20 degrees BTDC +- 3 degrees @ 700 rpm

Also done with distributor vacuum advance hose removed and plugged.
NOTE:
* BTDC = Before Top Dead Center
* + - = Plus or minus
Hope thos helps; keep us updated.

correct ignition timing should be at 10 degrees btdc at idle ( 850 rpms )
use a timing light connected to no 1 piston ht lead to set the timingby adjusting the distributor to get the light to flash on the timing mark
the vacuum hose is probably to increase idle speed when then ac is on and the surginging indicates a leak in a diaphragm

basic tune up... plugs, wires, cap, rotor theres a start

throttle position sensor

firing order for 5l is 1-8-4-3-6-5-7-2
timing is 0 deg btc at idle speed

Hi mnfchen, here is a little info for you,enjoy, on the 272-292 and312 cu.in the firing order is:

1-5-4-8-6-3-7-2 (from front to rear right: 1-2-3-4 left:5-6-7-8

spark plugs should be gapped @.035

timing is 3 deg BTDC for manual trans

6 deg BTDC on ford-o-matic

valve lash hot and @idle .019 in/ex

idle speed for manual trans is 475 rpm

idle speed ford-o-matic is 450 rpm

hope this helps you good luck :)