HELP- 305 to 350 conversion

Did you use the 305 TBI unit? You need the 350 TBI unit.

If I remember correctly, you will need the 350 computer too.

interesting... the part numbers for the injectors and the idle air control valve are the same for the 305 and the 350. I just discovered that the numbers for the throttle body itself are different... so I wonder what's the deal there... now I wonder about the intake too.

Also the computer (ECM) numbers are identical, just the proms are different, so I don't plan to replace the computer. There's also a chip in there called a 'cal pak' that I know nothing about.

Compare intake gaskets from the year of 305 and the year of 350. I don't think there is any difference in intakes. 305 injectors are originally

40lb/hr, and 55lb/hr for the 350. If the 305 TBI has 350 injectors in it then chances are that it might have been monkeyed with or that it might have been off an HO-305. Get a standard issue 350 TBI. Swap in the PROM that you have for the 350.

The IAC, TPS, MAP, CTS, etc will be the same. Its the TBI, injectors, PROM, and I think some computers that are mildly different..

Thanks for this valuable info... but I'm still confused about the injectors having the same part number for both engines if they are calibrated differently. any thoughts??

I don't know why they would be the same part #. Ask them what the rating is(lb/hr). Have them look up TBI injectors for an '87 454(they run 68lb/hr(I think)...If that part # is the same, find a different parts store!

All the injectors are interchangeable, but will definatly change the way the engine runs, performs, economizes...

I think you guys are getting carried away with this.... Did you set the timing while the EST is disconnected?(very important) Did you try setting the minimum air?(important after a big mod) Is the IAC working ?(controls idle speed) If you really feel the need to change the ECM, it is the same but has a different Prom (calibration pack). You MIGHT be able to still order one from a GM dealer. They are about $40. I would ask for one for the same year and just tell them it is for a 350. The prom is under that little cover on the ECM (2 screws)But I really think you can make it run good by adjustments. I am assuming by "wont idle" you mean the idle is too slow.?.

I don't know what you mean by "setting the minimum air", how do you do that?????

I set the timing correctly, in fact, rebuilt the distributor (pick up coil, etc.). I replaced IAC, the knox sensor, the spark plugs and wires... still misses at idle and stalls (worse when hot).

I don't plan to replace the ECM, but Chevy says the eprom and calpak chips in it are different for a 350 than a 305. If the injectors are different I guess I'll need to change them too. Someone else suggested the EGR as well. Damn, this is frustrating.

"dtrmn8r" wrote in message news:yannf.536$Zu5.74@fed1read03...

Try that(link above) It's been a long time since I've done it but I'm pretty sure it will apply to you truck. EGR is a good possibility too-stuck open-. pull the vac hose off while idling and see if it getting vacuum, it should not at idle. It could also have carbon stuck in it too that holds it open. you can remove it and check/clean it. The throttle body gasket is a common problem on that truck too. look for a leak under the throttle body with carb cleaner. Check your intake too. You can use a piece of hose- one end in your ear- to listen for vac leaks. Make sure your vacuum hoses are on correctly too. check it over good BEFORE setting the minimum air. Look to see if the IAC is working by watching it. It should open way up when your idle is low. I think you mentioned that the fuel injectors are the same part number on the 305 and 350? That tells me that isn't the problem....know what I mean? Check the basics first. You did set the timing with the ESC disconnected right? At or very near idle? I wonder if the harmonic balancer/front cover is right (timing marks) You could advance the timing a little to see it it runs better. Advancing the timing will raise the idle. I have no idea if the balancer/front cover gauge is the same on the engine you put in....it seams like it would be. Did you use you old balancer and a new cover, or vic-versa? Just a thought. Balancers can slip too causing your timing mark to be off. It wouldn't hurt to crank up the timing a little at a time and drive it just to make sure (stop if it starts pinging). You better let us know what it was :)Good luck.

Thanks for the link! I'll keep you posted on my progress. (I now know my cat is not the problem, some local yahoo thought it must be plugged up...) I'm going back to basics again, going to make doubly sure its not a simple vacuum leak...

don't know if anyone's still paying attention here... but I finally got fed up with just throwing parts at it and decided to bite the bullet and take it in to a real diagnostician. All the sensors checked out fine with his scanner; he confirmed there was no vacuum leak; but, when he put the motor on the scope the cylinder efficiency was way down. Not just one or two but all cylinders! His theory is I have 'tight' valves. Tight, as in the rocker arms were over tightened during final valve adjustment. OK, so now its back to the engine shop to let them re-adjust my valves. (I almost hate to, but I really want to keep that 50k mile warranty in effect.)

So, the valve adjustment didn't help... in fact loosening them up has generated some lifter noise, so a readjustment is in order. I'm going to go with a new EGR valve next. I'm told that a sticky egr can generate low efficiency readings on the scope along with poor idle when coming to a stop, which is my main problem... I also think I'll replace my 305 EPROM with one for a 350. This may not solve my problem, but at least my computer will know I've made an engine change, eh? What I'm not sure about is which egr valve to use (for a 305 or a 350). I've gotten conflicting advice on this one.

Read what I wrote earlier, about checking it and the other stuff I wrote too.

So, for the last two weeks I've been waiting for delivery of new EPROM and CALPAK chips from GMPartsDirect.com (at a substantial savings over the dealer... nearly 50%) and it appears to have been worth the wait. I just put the new chips in an hour ago, went for a test drive and my truck is definitely idling better. Stop and go rush hour traffic on the freeway will be a better test next week, but I'm cautiously optimistic. I suspect that it might be time to attempt to "set minimum air"...