About to drive into a lake...

[Unofficial]

I'm at the end of the road with this truck. I don't even remember the last time it was running right. I have an '87 with a 350 TBI that I dropped in the truck in place of the 305 TBI. It's just been running rough for the longest time. It stalls when I'm slowing down to stop or when I put it into gear/reverse. It often idles rough, but not all the time. Another thing is that when I'm cruising at around 15-30mph, the engine surges. The RPMs will fluctuate, making a very annoying ride. After trying to figure it out for the longest time, I finally took it to a shop. Before I took it to the shop, I tested the coil, fuel pressure, checked for vacuum leaks, tried to time it, put in a new EGR valve and solenoid, etc...

When I got it back from the shop they said that since there is an aftermarket cam, it's pulling too much vacuum when it shouldn't be. They told me that it's pulling the EGR open when it shouldn't be. So they plugged the EGR valve and the solenoid. They also timed the engine. It ran GREAT! (For about a day  ) Since I've gotten it back I unplugged the connector from the EGR solenoid, and I plugged the port on the actual throttle body where the EGR vacuum hose goes. Still stalls, surges, and runs rough.

The surging even occurred when I had the 305, and the only thing I really didn't check is the distributor. When I dropped in the 350, I put a new cap and rotor on, and everything seemed fine, so I guess I just ruled it out. I'm sooooo sick of working on this thing guys, it's really stressing me out and I'm just going crazy... Here's the cam I used, (recommended by Vile) http://www.summitracing.com/parts/SUM-1102/. Thanks in advance for any help, and feel free to ask questions.

[TexasRed]

I'm not sure what they are talking about. Usually cams pull LESS vacuum. Did you get a custom chip and a bigger throttle body? It may be worth swapping out your distributor for a reman, but I'm not sure.

[Grim 82]

My first guess was going to be intake manifold gaskets, but that's kind of off the table if it did the same thing with your old motor. I had similiar symptoms and I blamed carburetion but when I put on a new intake all of those problems went away.
If you are running headers, do you still have the O2 sensor hooked up? I don't know if it makes much difference, but it's just a thought. The TBI gurus on here will get you squared away, so don't take the truck for a swim just yet.

[eventhorizon66]

Larger cams will produce less idle vacuum (if all other factors remain the same).  They also allow the heads to flow more air into the combustion chamber, leaning the mixture if no tuning changes are made.  An overly lean mixture will cause surging.

Does it have a chip, or does the ECM still think it's feeding a stock 305?  What's your fuel pressure?  12 psi is what you want.  Low fuel pressure will cause a lean mixture.  Was your TBI unit rebuilt at the time of engine swap?

http://www.cfm-tech.com/gm_tbi_fuel_pressure.htm

Is your ignition timing correctly set?  I believe stock TBI engines are set to 0° BTDC (with the ECM-to-distributor wire disconnected).  If it still reads 0° BTDC after you plug the wire back in, you've got a problem.

[Unofficial]

Thanks for the replies guys...

I do have headers on the truck, and yes, I did put an o2 sensor in. However, I'm getting an o2 sensor circuit code and I did get a lean code too...I don't have a chip, so right now it thinks it's a stock 350. I'm not sure what the shop set the timing too, I'm assuming 0 degrees... And I'm getting a steady 11.5psi for fuel pressure. Is this the wire I'm supposed to unplug to time it?

[Grim 82]

Here's a post with some related info.
http://forum.73-87chevytrucks.com/smforum/index.php?topic=16688.0

There was a post not too long ago where somebody had a picture and pointed out which wire you need to unplug. It was in the same vicinity as the wire in your picture. You can hook up your timing light, let it run and unplug it and you will see the mark on the balancer move several degrees if it's the right one. Then set it to 0.

[Unofficial]

Thanks Grim, I read that thread over before, that's why I replaced my EGR system, checked fuel pressure, etc...

[Lt.Del]

http://forum.73-87chevytrucks.com/smforum/index.php?topic=9232.msg80106;topicseen#msg80106

[Grim 82]

That's the one I was looking for.

You said you are getting 11.5 psi fuel pressure, and in the post I linked to topp said
"Should be just between 13-15 for best results.....

12 or less and it'll run like crap, over 15 it'll flood"

You might want to check into that.

[Swims350]

agreed you need more fuel pressure. could be a filter or you need a new pump.

Now is the tbi unit from the 305 or 350? I think there's a difference in them and I think someone said the computer matches them, not positive on that one but could be other things, like 350 injectors needed or whole tbi and 350 computer to run it?

[eventhorizon66]

Now is the tbi unit from the 305 or 350? I think there's a difference in them and I think someone said the computer matches them, not positive on that one but could be other things, like 350 injectors needed or whole tbi and 350 computer to run it?

Throttle body same; injectors and chip different.

Unofficial: I am not sure if that's the wire, but I think so.  What you're looking for is a tan wire w/ black stripe that goes directly to the module located inside the dist.

[Swims350]

glad somebody cleared it up, so it's injectors and chip only that's different I'll have to remember it.

On my 88 the timing connector is right over the PS valve cover but there's a junction/distribution block there and it looks as the one posted below looks just like it and I thinkit is tan and black striped.

[VileZambonie]

That is the EST timing connector. Disconnect it with the engine off at operating temp. Restart it and set the timing at 0°BTDC . The cam you used which I apprently recommended  is just slightly larger than stock and shouldn't even require a chip to solve your issues unless you are running a 305 prom. Get that fuel pressure up, verify no vacuum leaks. Make sure you are using the right injectors and clear the codes after you set the timing.


PS find a better mechanic and take a systematic approach if you want to get this thing premo. Patience and a good understanding is important. We're here to help

[Layne]

Glad you got your truck back. Hope you figure it out. (I'm no help)