1990 C-1500 350 to 305 engine swap problems

I replaced my wore out 350 with a rebuilt 305. The truck now has all of the 305 sensors and ecm but the 350 injectors. On cold starts it idles up and down from 400 to 500 rpm with a slilght rich smell from the exhaust. The 305 injectors were leeking a lot so i can't use them. It has a new EGR and there are no codes. The truck runs fine in closed loop and even driving while cold. I'm thinking maybe the pre-programmed PWM for open loop with the 350 injectors may be the cause but I'm looking for some ideas. Any would be of great help Thanks.

Dewey, If you look at the post "305/350 combo" I am having close to the same issues, here is what I have found out through allot of research and talking to several Chevy guys. The injectors for the 350 are just to much for the .7 liter difference, if you look at the ratings on the two sets there is a 15 lb/hr difference in the ratings. I just bought the upper part of the TBI with both injectors and the pressure regulators for $50 today from a local junk yard, some of the U- Wrench it yards have them for $5, if they have them. That swap went allong ways to helping my problems. Next thing is you must use the intake gasket for the year and motor size that the TBI came off of. The back two water jackets have different size holes to regulate the water for the intake. I was also told that ANY leak at all in the intake gasket will result in a rich burn and black smoke. I have now narrowed my problem from extreme over fueling and lots of black smoke to where you are. Just swapped the injectors and put all new gaskets in the TBI. The two gaskets of most concern are the one in the middle of the TBI and the O-rings on the bottom of the injectors themselves, any leaks here will let the injectors drip and over fuel. The center gasket isnt bad, just undo fuel lines, take the two fittings off (3/4" open end) and take the three bigger Torx screws out to pull the top half off and replace. I ran out of time tonight so I will redo my intake gasket tomorrow night, I know its leaking because of the stream of oil down the bellhousing. Will let you know if it fixes it. Good luck!!!!! Oh by the way the TBI bodies are the same, all the sensors are the same and the computer will not care about the difference between the two motors, only diffence is the injectors. Only other thing that can cause this is the temp sensor in the front of the intake however that will only cause rich mixture after it reaches temp, found that out the hard way on the 350.

Dewey, good stuff from Chvsuburban. I'm also having similar type issues with a 305 to 350 engine change. For your case I've also heard, but don't know for sure if true, that the 350 chip keeps the injectors open longer than the 305 chip does. I was also told, (they do have different part numbers) that the knock sensors are different but I doubt thats the cause of your trouble. Good Luck.

Yep your right the computer holds the burst for a few milliseconds longer. Was told that was ok as the major difference was the volume of the burst. I just walked in from doing the intake gasket, will test drive in am and let you know how it went. My next thing was to swap the ecm with one from a bone yard. However, can remember the web site, there is a guy that burns chips that says as long as you stay within 10% on engine displacement computer would'nt care.?

I think I'm gonna have a look at the 305 injectors and see if i can get them to quit leaking because they are the only thing not originally off of the 305. I used the ecm out of the 305 truck because the prom had a diffrent number than the 350 truck. As far as I can tell the only diffrence between 305 and 350 tbi engines is the Knock sensor, EGR vavle orface size, and injectors, and prom in the ECM. I will wrok on it again today and let You know how it went. Thanks for the ideas so far and maybe I can think of 1 for You.

Dewey, How did it go? After the before mentioned fixes my is drivable now, however you can still smell thats it's rich. Got rid of the black smoke, but still seems to be loading up and lower RPM, idle's great, drives good between 35 and 55. Still playing with it above that, think timing is maybe off. Low RPM like take off from light is when it's most noticable. Was thinking about swapping ecm's with one for 305 however I dont think I can. I have the 4l80e which runs off the computer so I think my only route would be to burn the chip to the new specs, and then when the rebuild is done burn it back to the 350. Let me know how you have faired!!!!

I swapped from a 350 to a 305 and I am having idle problems and to fix that I was wondering if someone who has successfully did this swap knows what chip to use to fix the problem do I need a performance chip can I use stock chips please elaborate I would appreciate if someone would take time out of their day to help thank you

305 prom, 305 injectors, 305 EGR, maybe 305 knock sensor.

The more you bring over from the 305 donor truck, the more likely it is to run right--assuming the donor parts aren't damaged or worn-out.