1988 305 -350 major problems wont even start!!!!

[aaron600rr]

I just converted a 305 to a 350 in a 1988 1500 and replaced everything under the hood and the entire fuel system and the truck will start and idle for about 5 seconds and dies and it backfires alot. What could it be? Looking for any suggestions.

[mr_mushroom]

with a total replacement?
that could be anything.
check the obvious stuff first.
timing, fuel flow, MAF sensor (if it's an '88 engine, it should have one. the '86 and before used carbs)
iono, fuel filter?
just all the normal problem spots.

let us know.

[silicon212]

Did you pay close attention to your cam/valve timing when you assembled the engine?

Check your firing order, should be 1-8-4-3-6-5-7-2 in clockwise rotation. Make sure the static timing is set to cylinder 1 TDC. You can adjust this once the engine is running, but what you're shooting for to get it started is ~0 degrees TDC. Make sure the rotor is pointing to the #1 plug wire when the #1 is at TDC with the (important) rocker arms on cylinder 6 rocking - both of them. This is TDC on the #1 compression/power stroke.

Make sure you use injectors rated for a 350 and not the ones from the 305. Make certain all of the electrical (i.e. oil pressure switch and fuel pump relay) is hooked up and that you're using a PROM for a 350. The 305 PROM won't quite cut it here. It may start and run with the factory PROM, but you won't have power. If this were a carbureted engine, you could use a 305 PROM and it would run decent once the carb was retuned - in fact better because a 305 PROM has more agressive spark tables - but a 305 EFI PROM won't run the injectors hard enough for a 350.

[aaron600rr]

Ok the motor was assembled by a professional motor shop. The truck was timed by a mechanic shop this past weekend the fuel system was replaced last week. So i need 350 injectors and a new prom but will either of those cause it to drowned out and overload. Im taking it to have the computer run this week and see what comes up.

[2000CAYukon]

Is the engine coolant temp sensor connected (intake manifold near the tstat housing)? If so, the sensor could be bad. Try connecting a jumper to the sensor connector. This will tell the ECM that the engine is hot. If that does not help the problem, you should check the fuel pressure.

//2000CAYukon

[aaron600rr]

I will check that but the fuel pressure is like 15-16 i just had the entire fuel system replaced.

[2000CAYukon]

Originally Posted by aaron600rr

I will check that but the fuel pressure is like 15-16 i just had the entire fuel system replaced.

Specs are 9 - 13 PSI but I doubt 15 - 16 is causing your issue.

//2000CAYukon

[mbumgua]

If you replaced the distributer with a rebuilt one you could have gotten a bad one. I have seen two in a row from Carquest be bad. Will start and run until dist heats up internally and dies. This usually takes 3 to 5 minutes. Are you sure about the engine flooding? Do you see raw gas in the throttle body? Injecters dripping? Good luck.

[aaron600rr]

I can watch the fuel freely flowing from the injectors and it is backfire and overloading so bad that it blew up the muffler. The distributor wasnt replaced just the cap and point and module. The truck will start for like 2 seconds and then start to choke out and die.

[777stickman]

I'm thinking valve timing or valve lash. Some valves could be adjusted too tight meaning that they are not closing all the way. Need to do a compression check on all cylinders to check for this. I've seen fuel flooding (from carbureted engines) do the same thing. A quick check would be to disconnect the distrubutor and crank the motor. Any momentary increase in cranking speed would indicate one or more weak compression cylinders..............Steve

[silicon212]

Originally Posted by aaron600rr

I can watch the fuel freely flowing from the injectors and it is backfire and overloading so bad that it blew up the muffler. The distributor wasnt replaced just the cap and point and module. The truck will start for like 2 seconds and then start to choke out and die.

You say you replaced the module in the distributor. Did you properly apply the thermal heatsink compound on the metal side of the module that mounts on the distributor base? If you didn't, I suggest you do.

[Telco_Tech]

I completed this swap to my '89 K1500 EC back in '93. The original 305 had a factory defect with a crack in the lifter valley. The only critical item in this swap is the computer chip. The truck will run fine with 305 injectors. It will have a slight lean surge, but I ran mine for years like that before I finally replaced the 305 injectors with 350 injectors. Every thing else is bolt in and run. If the engine won't run there is a mistake somewhere; whether it could be a pinched wire or something is incorrectly plugged in (?). It's hard to say when we're remotely trying to diagnose a problem.

I can say I still have my truck...I get anywhere from 12 to 17 mpg depending on conditions. My 350 has about 150K miles since my swap with absolutely no trouble. I just turned over 197K (chassis miles).

I just thought of an additional issue...are you sure the distributor is installed correctly? It is very easy to get the dist. out of phase 180 degrees. It almost sounds like thats your problem.

Get the 350 chip for your computer! It's very easy to change, but pay close attention to handling electronic components since a static charge will destroy it. At the very minimum, touch metal parts of the truck to release any static charge you might build up before handling your vehicle computer.

[2000CAYukon]

Is the MAP vacuum line connected right? No vacuum to the MAP is like running under a load and the ECM will give the engine a lot more fuel.

If you have access to a scan tool, it would help to see what the sensors are reading while it is trying to run.

//2000CAYukon

[aaron600rr]

The distributor is not out 180 because it wont start when facing the other way. Where would i get a chip for the computer? The vaccum lines should be connected correctly because an engine shop hooked them up. Where can I get that Chip?

[J-Mech]

A PROM can be purchased from your local chevy dealer about $80. But i think the guys who said valve lash and distributor timing are right on. get the motor running before buying the prom.