Engine running like crap

[Isaiahtrent4]

So I have a 1989 Silverado c1500 and it had the stock motor which was a 305. I took it out and put a brand new 350 K (https://www.summitracing.com/parts/nal-12568758) in it. I reused everything off the 305 that I could and I also put a mild cam in it (https://www.summitracing.com/parts/cca-12-249-4) and the engine is running awful. It is a TBI unit and I put the bigger injectors in it, but when I turn the motor on and let it idle, and it idles good for about 5 - 10 seconds. It then drops the idle down to the point where it almost stalls and then the revs immediately spike really high and spews plumes of grey smoke, then it drops the revs and repeats the whole cycle over again. I have no idea what is going on, could my 305 computer be the culprit to the crap idle? Is the 350 making the 305 computer freak out and dump massive amounts of fuel into it?

 

[1952Chevy]

I would think it's a tuning issue. The 350 is going to need more fuel at idle than the 305. Plus the cam swap changes things up. My understanding is anytime you do a cam swap you're going to need a tune. I do not know anything about tuning those trucks though.

 

[PlayingWithTBI]

I have no idea what is going on, could my 305 computer be the culprit to the crap idle? Is the 350 making the 305 computer freak out and dump massive amounts of fuel into it?

Yes, it sounds like your "stall saver" is kicking in when your RPM drops too low. What kind of fuel pressure are you getting? Even with those restrictive 193 swirl port heads, and that cam, you're probably starving it but, you really can't tell until you put a scanner on it and data log. I would think you need closer to 15 PSI fuel pressure than the stock 10 - 13. Then you should tune your VE and SA tables at least.

 

[Erik the Awful]

It's not your cam, although that might be an issue in the future. I have a bigger cam (211/219 @.050, .507/.515 lift) and mine fired up, idled, and ran just fine with the stock TBI. It bogged on the street if I gave it more than half throttle.

Also, going from a 305 to a 350 shouldn't affect your idle much. It doesn't take a lot of fuel to keep the engine running.

Was the truck running fine with the 305 before you did the swap? Mine was not, and one of my exhaust pipes was full of mud. You don't know what surprises a previous owner might have left for you.

Run through all the normal stuff you would for a tune-up and rough idle diagnosis.

 

[PlayingWithTBI]

I'm going on the assumption (I know) that you set your initial timing with the EST wire disconnected. Has anyone messed with the throttle plate min stop screw?

 

[Schurkey]

As said...verify fuel pressure, and connect a scan tool so you can find out what the computer is trying to tell you. Verify all the sensors, and all the outputs using the scan tool.

Did you degree the cam when it went in? What is the cranking compression?

How old are the various "tune-up" items? Plugs, plug wires, cap 'n' rotor, PCV valve. Will the coil fire a spark-tester calibrated for HEI? Is the flapper on the snorkel of the air cleaner working right?

 

[Isaiahtrent4]

It's not your cam, although that might be an issue in the future. I have a bigger cam (211/219 @.050, .507/.515 lift) and mine fired up, idled, and ran just fine with the stock TBI. It bogged on the street if I gave it more than half throttle.

Also, going from a 305 to a 350 shouldn't affect your idle much. It doesn't take a lot of fuel to keep the engine running.

Was the truck running fine with the 305 before you did the swap? Mine was not, and one of my exhaust pipes was full of mud. You don't know what surprises a previous owner might have left for you.

Run through all the normal stuff you would for a tune-up and rough idle diagnosis.

The engine ran fine despite its old age and I drove on the 305 for maybe a year.

 

[Isaiahtrent4]

i haven’t messed with the the throttle plate screw at all aswell. I’ll look into reading through a scan tool. I didn’t degree the cam when it went in, and I have a new 02 sensor, knock sensor, spark plugs, wires, new distributor cap and rotor, radiator and thermostat.I also tried a “stage two chip” from Jet Performance and it seems to have given me a check engine light as well (code 51 which is an ecm or prom fault) The Jet chip seems to have solved my “stall saver” issue but it has led me down another hole with the check engine light, and the overheating at idle. Can the overheating be a cause of my problems so far?