Sluggish, weak 350 tbi that no one can figure out

[87burb4x4]

I have an '87 Scottsdale Suburban 4x4 w/a 350 tbi... it's running very sluggish and weak. Everything I do to it makes it better but doesn't fix it. I have changed plugs, wires, cap, rotor, fuel filter (twice, clean on second change), rebuilt the tbi, and gutted the catalytic converter and had new exhaust put on it. The TPS was new when I bought it, but haven't changed that. The previous owner had a crate engine put in about 20k miles ago, along with a rebuilt 700R tranny, and went through the front and rear ends, which are .373 gears. I'm running 33 inch tires, so there should be no problem there. Trucks cranks, idles, and revs smoothly and sounds awesome. Once I get it out on the road, it's like I have something trying to pull me backwards and the exhaust sounds horrible. It doesn't pop, and has never smoked. Even after sitting for months this truck doesn't have the infamous Chevy valve seat leak. So I don't think there is anything wrong internally. Next on the list is to check fuel pressure. If that's not it, what else could it be? Oh, and I also OHMed the coil, and it checks out fine.

[rich weyand]

What's your base timing set at?

[87burb4x4]

Well that's tricky lol. I spoke to the man that built the engine and he said he set it at 8 degrees out and 25 degrees full advance. I'm by no means a mechanic and I understand how timing works, but nothing past that. I tried to check timing but someone told me that I needed to unplug something before I check/adjust timing

[87burb4x4]

Without unplugging anything, the timing wasn't even on the tab

[rich weyand]

I'm not sure what you do on an 87 with the computer stuff to set base timing.  On mine, you just unhook the vacuum advance.  Without unhooking the vacuum advance, it's way off the mark.  Maybe someone can chime in on that.

With that mechanical advance, you should probably go to a base timing of 11-12* BTDC.  That will give you 36* all in, which is the target, and will improve performance.

[87burb4x4]

Ok I got a buddy that's gonna check timing on it tomorrow. About an hour ago I checked codes and it came back EGR circuit fault... so I'm gonna change that tomorrow and see if that works along with timing

[jetmech]

Disconnect the tan wire firewall where the brake booster meets the firewall, start engine, set to zero degrees, stop engine, re-connect wire.

[78BIG-TEN]

Like Jet said,wire breaks out of harness near booster,single connector but I believe its tan with black tracer.Check timing,I always try to give a couple extra degrees,but if egr is not function it could ping.Fuel pressure should be about 12,snap throttle and look for a drop,it shouldn't.Seen them with 8-9 psi start and idle and rev up fine,but under load be real dogs.Disconnect map and and engine should go way rich.Check ground at t\stat housing.Pul bolt\stud and look for corrosion on threads.If all is good then might look at injectors being restricted.Had a suburban doing just the same thing and it was injectors.Found it using a strobe type timing light

[87burb4x4]

It was timing... After unplugging the wire, having the light set at 8 degrees advance... The harmonic balancer mark wasn't even on the tab. But now I'm wondering how much advance to give it. I've heard anywhere from 2-8 degrees

[Engineer]

"0" degrees on a TBI with the timing wire disconnected.

The computer needs the base timing set at 0 to function properly. These engines are different than the mechanical advance ignitions they replaced.

You will need to clear the service engine soon light before driving it.

[87burb4x4]

So I got a hold of a mechanic buddy of my dad's. He said it should be timed at 2 degrees out according to factory specs. I just got through timing it and it drives like a new truck. Runs great now. But now my question is this... why does it seem that everyone has a different opinion on where to set timing? I've heard anywhere from 2-16 degrees. I realize that if using a timing light with a preset advance you should set the advance to the desired degree and then zero it in according to the harmonic balancer mark on the deepest "V" groove on the tab...

[rich weyand]

Separate in your head the ECM trucks from the mechanical advance trucks. 

For a mechanical advance HEI setup, base timing ranges from factory spec (4* BTDC I think) which is set to accommodate the pollution controls, all the way to 16* BTDC (my favorite) or even a bit beyond, if the pollution controls went missing (oops) or were not required by law (C20, C30, and all K before 1979).

The ECM setup on the 87 builds in the factory specs if you disconnect the wire and just tell the computer where "0" (TDC) is.  You can finesse the factory timing a bit by setting the "0" at 2* BTDC as your buddy says, which will add 2 degrees of advance to the factory specs across the board.

Glad you found the problem.  Hope that little explanation helps you.

[87burb4x4]

That definitely made it all make sense. Got her dialed in at 2^o and couldn't be happier. And the good thing (now that it's all said and done) is that a LOT of things have been gone through and changed on the old girl while trying to find the problem. Thanks to all that helped out!

[78BIG-TEN]

Glad you got it worked out.