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

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.

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.

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.

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.

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

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

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

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.

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.

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

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.

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.

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

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?

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.

Aaron600rr had a question about the computer chip I'm talking about commonly called the PROM. I figure I would share this with the board as well. I had bad experiences with the dealer regarding the early ownership of my truck and I wound up sorting the problems myself. I don't have much confidence in some of the work I've seen done in some shops. That's why I try to do as much as possible myself.

I'm amazed that someone that calls themself a mechanic doesn't understand computer controls on vehicles these days. All the equipment that works with a 305 will work with a 350, except for the PROM computer chip. A 350 will run badly with a 305 computer chip. If you've ever driven a vehicle with a carburetor with a faulty accelerator pump, that's about how I can describe it. You have to feather the throttle to barely get the vehicle moving. That's the best description I can give you. One of the guy's on the F-body lists in Orange county (Calif.) programs chips and we had a discussion about this at one time. He programs chips for TPI engines and the 305/350 engines aren't as critical with the programming as the 305/350 TBI engines are.

I have a Hypertech PROM chip in my vehicle computer. I bought mine from a speed shop many,many years ago. I did my engine swap back in '93. I had some very bad experiences with people who called themselves mechanics and couldn't back it up with skills. I complained to management and made a list of all the parts that were broken and mishandled when my 305 was at the dealer being replaced by another 305. The work was so bad I gave up on that dealer and I became well known at the local Pontiac dealer who treated me better. I ordered a 350 over the counter at the Pontiac dealer and they delivered my engine to me on the street next to my apartment. I was working graveyard shift at the time so when I got home from work in the morning I slept for a few hours and then worked on my truck out in the street. It was a quiet street so no one gave me any problems. It took about a week for my chip to arrive after I ordered it. In the meantime I worked cleaning up the engine compartment and painting the firewall where the previous grease monkeys scarred the paint up. I went down to the Borla factory and bought a set of headers to install on my truck. It's weird; the cast iron exhaust manifolds "shrunk" and I couldn't bolt them up to the heads. It was a good enough excuse to install headers.

About the chip; I would check with Hypertech to see if they're still available. If not, there are tuners out there that program PROM chips. If the person that swapped your engine out doesn't understand computer controls I would definitely find someone else. The truck will run and start with the 305 chip but drivability will be very bad. I would strongly recommend not going out in traffic until this gets sorted out.

For everyone that reads this a prom for a a conversion from a 305-350 is not necc at all my truck drives beautifully with the 305 prom. I have spoke to several shops that build race cars and they said all the parts are the same and the fuel diffrence can be solved by a mod to the engine.