Hey All,

We just did an engine swap on my truck. Swapped out an old burnt 305TBI for a 350TBI. The 350TBI was running great in the old donor truck, now that its in my truck its acting weird. At cold start it starts up and runs fine. Once the ECM goes into closed loop it starts surging and tries to die. Putting the truck into gear causes it to drop in rpm or stall right out.

Now we didn't swap out any sensors because we got this engine complete. We had to replace the coolant temp sensor because the old one broke at the terminal and I replaced the o2 sensor because the old one was coated in burning oil.

We did a vac leak test with a bottle of propaine and that didn't show any problems. The only thing we haven't changed out was the ECM from the old truck but I wasnt' sure if that was needed as my sierra has a 7747 and the ECM from the old truck also has a 7747.

I will swap out teh ECM tomorrow and see if that makes any difference..if not is there anything else I should be looking at?

 

The ecm might be the same but the prom for the 305 will be different than the 350 so you could just try changing that. The prom holds the tune for the engine if you do not know.

 

would the wrong eprom cause surging like this?

 

Well it's prom is burned for the way a 305 drinks and breathes, so I'd guess the 350 prom would help a lot if not fix it.

 

Surging at times can be troublesome but it's my understanding mostly from a lean condition. So if the tune [what is on the prom] is for the smaller engine that means less fuel. [lean] Get that chip changed to the one the engine was running on. One other thing to consider is what fuel pressure its running on. It's a little late now to check the truck it came from to have a comparison though. Did you leave the same injectors and tb unit on the engine?

 

ya we left the engine complete. It came with a TBI unit, injectors, knock sensor and all that other stuff. It even had a 700R4 transmission still bolted to it. We literally pulled it from one truck and dropped it in my truck.

After we swapped out the ECM the problem went away so thats good except durring our test drive we sorta ran into a new problem. Went to shut the truck down, turned the key to the off position yet the engine still ran. It wasn't deiseling it was still getting power from somewhere as the spark plugs were still firing and there was no knock. I hit the throttle and it died right then but yeah...that was definately something I have never experienced before. After the restart we went for another rip and this time the engine shut down properly. There is no stereo not even stock stereo installed in this vehicle and the rest of the components are completely stock. Any advice would be greatly apreciated.