Mechanics

WHY IS MY 4.3 TBI RUNNING ROUGH AND VERY RICH?

1992 GMC S15 • 250 miles

My 1992 GMC S15 4.3 TBI Blazer just started running rough and very rich won't accelerate no power. The engine was completely re built 18K miles ago and ran great up until a couple of weeks ago. Block was bored out new pistons, rings, main bearings, oil pump, Heads reconditioned with all new valves, new cam, lifters, timing gears and double roller timing chain. New MSD distributor and new Magnecor ignition leads and spark plugs.

I have replaced the distributor rotor and cap and spark plugs after it started running rough and rich. I have replaced the fuel filter and measure 12 PSI at the fuel filter with steady pressure key first turned on and engine running. The fuel filter was replaced with new. I have tried a different MAP sensor, a different TBI, different EGR valve. I have put a scanner on and the temp sensor. O2 sensor and TPS all have normal indications with the scanner as well as the MAP sensor. I have tried a different Ignition control module also. The CAT was melted and plugged so both the CAT and muffler were just replaced with new. The base timing has been checked and it is bang on at 0 degrees (wire under dash disconnected). The wire was reconnected and after all the above. The engine still runs rich and rough. I do not want to run this vehicle much in fear that I will burn up the new CAT. I feel I am down to the ECM which is original with 250K miles on it. I will be trying a different computer tomorrow night.

Do you have any other thoughts or suggestions?

Thanks in advance for any help,
Glenn


Avatar
Wa7spy
September 23, 2012.



Check under the tbi for th gasket being blown. Spray some choke cleaner there while running if rpm changes you found it. Also replace the hose to map sensor if soft as well as pcv hoses.


Hmac300
Sep 23, 2012.
The gasket under the TBI is fine The TBI was just off and swapped with a spare still runs rich and rough with either TBI. Hoses to MAP sensor and PVC are new.

I feel I am down to a ECM


Tiny
Wa7spy
Sep 23, 2012.
Use choke spray to check for a vacuum leak it may be a bad prom not the pcm. Good luck on finding one though. You have to change the prom in the pcm and if it does the same it's probably theprom.


Hmac300
Sep 23, 2012.
I have a friend with the same vehicle and will be trying the prom swap tomorrow. I will let you know.

Thanks


Tiny
Wa7spy
Sep 23, 2012.
It ahs to be the same numberon it or it won't work right.


Hmac300
Sep 23, 2012.
Ok thanks for the heads up on the PROm being the same. Do I need to pull the dash down in order to remove the ECM? Is it accessible from under neath the dash by removing the lower plastic pieces?


Tiny
Wa7spy
Sep 23, 2012.
It shold com eout without removing the dash.


Hmac300
Sep 24, 2012.
Ok got off work and installed my ECM with prom in another Blazer with 4.3 TBI same year 1992. The other Blazer ran great with normal power and acceleration with smooth normal idle with my ECM and PROM.

I removed the MSD distributor and tried the distributor out of the other Blazer in my Blazer with my original ECM and PROM. My Blazer ran like a champ with the other distributor. I purchased a rebuilt distributor installed it and timed the engine. All is well. Problem solved! My engine now purrs like a kitten! I will be contacting MSD about the distributor and it making the engine run very rich.

Thanks for trying to help.
Glenn


Tiny
Wa7spy
Sep 25, 2012.