Chevy P/U 305 to 350 Swap Problems


Supporting Member
May 26, 2001
Hi all,

A guy here at work has approached me after hearing that I know things about cars.

The 305 in his '88 Silverado P/U died and he wanted a 350. So he paid the dealer to install a new Goodwrench 350. He says it's down on power (less than his 305) and won't run when it's cold. He has to sit in it with his foot on the gas until it gets up to operating temp before he can drive it.

It turns out all they really did was bolt the 305 parts to the 350 engine. He's been fixing things a little at at time, like upgrading the cooling system, swapped out the 305 injectors for the 350 injectors, etc. It's a TBI engine and the dealer's contention is that the ECM and chip are the same for the 350 as the 305. I find that hard to believe, especially after they acknowledged that the injectors are different. There's no way to cross reference the 350 because a 350 wasn't available in whatever model he has in 1988.

So this thing is all f'd up and he's got $5k invested in it. I'd like to help him out but am coming up empty handed with decent web resources. Anyone know of someone he can talk to or somewhere on the web he can look?

88 Chevy pickups were available with 350's. The chip would be different too. Shouldn't be to hard to find one in a junk yard. The calpack maybe different too.
Ditto on what Steve said..

Did he have a short block swapped in or a long block? Reason I ask is that the 305 combustion chambers are smaller than the 350s so the ECM may be pulling timing out due to detonation from 305 heads on the 350. Also, while Im on the detonation subject if the installer of the engine tightened the knock sensor too tight making it more sensitive to normal engine noises (aka false knock) the ECM could be pulling timing out. The knock sensor is suposed to be installed at a specific torque (I dunno what it is right off as it varies depending on the engine). Those two things might explain his lack of power.

FYI... The chip and injectors are the only different parts that affect power levels between the 305 and 350 assuming that everything else is installed correctly...which from the sounds of things is possibly NOT the case. Ive swapped TBI 350s into trucks that originally had 305s and those two things are the only issues that need to be addressed.

If it wont run correctly when cold once the chip is swapped Id look REALLY hard at the coolant temp sensor as the problem (located on the intake next to the T stat housing), This part does interchange between engine sizes but may very well be bad or gotten damaged during the swap.

If it were me Id check base timing too. Should be 0deg with the set timing connector unplugged.

Lastly, being as the truck has some age on it you should prolly verify good connections at things like the TPS, IAC, MAP, coolant temp sensor, and the engine grounds. These connections deteriorate due to age/heat and can cause drivability and performance issues. If the engine installer treated these now fragile connectors with anything less than care their condition and function should be looked at.

Thanks for the replies. I don't know a lot about them other than what he's told me, and I don't get the feeling he knows a lot about it either. He claims that whatever model he has (may not actually be a Silverado but it is a full size) didn't have a 350 option. I would think that crossreferencing any 350 available in a full size truck that year would work for a chip.

I'll relay the info to him.....

Just some info to pass along to the owner.. You CAN get a 350 in any truck from the 1/2 ton short bed regular cab 2wd all the way up to a 4 door 1 ton dually 2wd in 1988. If his truck is a 4wd then you could get the 350 all the way up to 3/4 ton (Ive never personally seen a 4wd 1 ton with a 350 but it may have been available)
Have him double check the base timing. Assuming since this was a dealer installation they probally swapped long blocks and 305s use different balancers so the timing marks are way off and it sounds like the tech tried to time it by ear. We do this all the time and the crate 350s run just fine with the 305 injectors and chips. Also check the obvious like bad ignition parts and make sure the ECM isn't running in open loup.

Eric Fisher